<table><caption>**Table 4.** Vegetable oils' extraction with alcohols.</caption><thead><tr><th>Material</th><th>Seeds' Pre-Treatment</th><th>Extraction Technique</th><th>Solvent</th><th>Oil Extraction (% of Total Oil)</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="3">Rapeseed</td><td rowspan="3">Coarsely ground</td><td rowspan="3">Soxhlet extraction</td><td>Ethanol</td><td>22.8</td><td rowspan="3">[65]</td></tr><tr><td>Isopropanol</td><td>83.1</td></tr><tr><td>Butanol</td><td>78.3</td></tr><tr><td rowspan="6">Soybean</td><td rowspan="6">Cracked, dehulled, flaked and expanded</td><td>Batch extraction 60 °C</td><td>Ethanol absolute</td><td>90.3–93.4</td><td rowspan="6">[62]</td></tr><tr><td>Batch extraction 90 °C</td><td>Ethanol absolute</td><td>89.0–92.5</td></tr><tr><td>Batch extraction 60 °C</td><td>Ethanol 94%</td><td>28.2–31.3</td></tr><tr><td>Batch extraction 90 °C</td><td>Ethanol 94%</td><td>92.5–96.0</td></tr><tr><td>Batch extraction 60 °C</td><td>Ethanol 88%</td><td>2.9–6.6</td></tr><tr><td>Rapeseed</td><td>Coarsely ground</td><td>Soxhlet extraction</td><td>Ethanol<br>Isopropanol</td><td>46.6 <sup>a</sup><br>45.0 <sup>a</sup></td><td>[66]</td></tr><tr><td rowspan="3">Rapeseed</td><td rowspan="3">Flaked</td><td>Batch extraction</td><td>Isopropanol 99%</td><td>63</td><td rowspan="3">[67]</td></tr><tr><td>Batch extraction with US</td><td>Isopropanol 99%</td><td>79</td></tr><tr><td>Batch extraction with US</td><td>Ethanol 96%</td><td>51</td></tr><tr><td rowspan="4">Rapeseed</td><td rowspan="4">Crushed</td><td rowspan="4">Four stage cross-current<br>extraction</td><td>Ethanol 96%</td><td>92.7</td><td rowspan="4">[68]</td></tr><tr><td>Ethanol 92%</td><td>86.5</td></tr><tr><td>Isopropanol 88%</td><td>89.3</td></tr><tr><td>Isopropanol 84%</td><td>87.1</td></tr></tbody></table>

However, several obstacles make the ethanol non-competitive with hexane. The first is the need to dry the oilseeds (water content below 2–3%) to avoid water entering the system, which is dissolved by the ethanol, reducing its affinity to the oil. Conventional drying is associated with high energy costs. Secondly, the affinity of the matrix for ethanol is much higher than for hexane, which means that a higher quantity of solvent is retained and must be evaporated from the meal. Ethanol extraction also results in the denaturation of proteins, which lose their solubility on prolonged contact with this solvent. The displacement of structural water results in a conformational change due to the merging of domains with opposite electrostatic charges and the formation of new bonds [13]. However, this loss of solubility is reversible after some hydration treatments, unlike that occurring when the proteins are exposed to heat. Thirdly, there is a significant risk of protein degradation in the meal due to the mass of solvent to be evaporated and the higher boiling point of ethanol. In addition, the use of direct steam should be avoided in order not to rectify the solvent at each extraction cycle. Desolventisation technologies other than those used in the existing plants should be used. These include the use of larger heating surfaces to work only with indirect steam or vacuum desolventiser. Both solutions are associated with higher capital investments. Fourth, the affinity of the solvent to the matrix, the difficulty of using direct steam and the need to maintain temperatures compatible with protein stability result in a much higher solvent content in the meal than in the case of hexane. This can lead to increased flammability of the meals.

Carré [13] proposed a process in which the dehulled seeds are first cold pressed to mechanically extract a high-quality oil and then extracted with azeotropic ethanol to complete the defatting. Subsequently, a washing step with aqueous ethanol removes soluble carbohydrates, phenolic compounds and most glucosinolates. The meal is pressed to mechanically remove as much of the retained solvent before a gentle desolventisation. This model is more expensive than the current one, but the different products obtained (pressed oil, solvent-extracted oil, meal and an ethanol extract of lecithin, phenolic compounds, etc.) could make it economically attractive. Carré et al. [69] proposed a cost estimation to evaluate the feasibility of a large-scale hot ethanol extraction process. The simulated extractor involved extraction at a temperature above the boiling point (90–95 °C) in a module consisting of hydrocyclones operating in partial countercurrent. The total cost of processing was estimated at EUR 47.4 per tonne of seed processed, which was higher

<footnote><sup>a</sup> Oil yield (g/100 g DM seed).</footnote>
<table><caption>**Table 4.** Vegetable oils' extraction with alcohols.</caption><thead><tr><th>Material</th><th>Seeds' Pre-Treatment</th><th>Extraction Technique</th><th>Solvent</th><th>Oil Extraction (% of Total Oil)</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="3">Rapeseed</td><td rowspan="3">Coarsely ground</td><td rowspan="3">Soxhlet extraction</td><td>Ethanol</td><td>22.8</td><td rowspan="3">[65]</td></tr><tr><td>Isopropanol</td><td>83.1</td></tr><tr><td>Butanol</td><td>78.3</td></tr><tr><td rowspan="6">Soybean</td><td rowspan="6">Cracked, dehulled, flaked and expanded</td><td>Batch extraction 60 °C</td><td>Ethanol absolute</td><td>90.3–93.4</td><td rowspan="6">[62]</td></tr><tr><td>Batch extraction 90 °C</td><td>Ethanol absolute</td><td>89.0–92.5</td></tr><tr><td>Batch extraction 60 °C</td><td>Ethanol 94%</td><td>28.2–31.3</td></tr><tr><td>Batch extraction 90 °C</td><td>Ethanol 94%</td><td>92.5–96.0</td></tr><tr><td>Batch extraction 60 °C</td><td>Ethanol 88%</td><td>2.9–6.6</td></tr><tr><td>Rapeseed</td><td>Coarsely ground</td><td>Soxhlet extraction</td><td>Ethanol<br>Isopropanol</td><td>46.6 <sup>a</sup><br>45.0 <sup>a</sup></td><td>[66]</td></tr><tr><td rowspan="3">Rapeseed</td><td rowspan="3">Flaked</td><td>Batch extraction</td><td>Isopropanol 99%</td><td>63</td><td rowspan="3">[67]</td></tr><tr><td>Batch extraction with US</td><td>Isopropanol 99%</td><td>79</td></tr><tr><td>Batch extraction with US</td><td>Ethanol 96%</td><td>51</td></tr><tr><td rowspan="4">Rapeseed</td><td rowspan="4">Crushed</td><td rowspan="4">Four stage cross-current extraction</td><td>Ethanol 96%</td><td>92.7</td><td rowspan="4">[68]</td></tr><tr><td>Ethanol 92%</td><td>86.5</td></tr><tr><td>Isopropanol 88%</td><td>89.3</td></tr><tr><td>Isopropanol 84%</td><td>87.1</td></tr></tbody></table>

However, several obstacles make the ethanol non-competitive with hexane. The first is the need to dry the oilseeds (water content below 2–3%) to avoid water entering the system, which is dissolved by the ethanol, reducing its affinity to the oil. Conventional drying is associated with high energy costs. Secondly, the affinity of the matrix for ethanol is much higher than for hexane, which means that a higher quantity of solvent is retained and must be evaporated from the meal. Ethanol extraction also results in the denaturation of proteins, which lose their solubility on prolonged contact with this solvent. The displacement of structural water results in a conformational change due to the merging of domains with opposite electrostatic charges and the formation of new bonds [13]. However, this loss of solubility is reversible after some hydration treatments, unlike that occurring when the proteins are exposed to heat. Thirdly, there is a significant risk of protein degradation in the meal due to the mass of solvent to be evaporated and the higher boiling point of ethanol. In addition, the use of direct steam should be avoided in order not to rectify the solvent at each extraction cycle. Desolventisation technologies other than those used in the existing plants should be used. These include the use of larger heating surfaces to work only with indirect steam or vacuum desolventiser. Both solutions are associated with higher capital investments. Fourth, the affinity of the solvent to the matrix, the difficulty of using direct steam and the need to maintain temperatures compatible with protein stability result in a much higher solvent content in the meal than in the case of hexane. This can lead to increased flammability of the meals.

Carré [13] proposed a process in which the dehulled seeds are first cold pressed to mechanically extract a high-quality oil and then extracted with azeotropic ethanol to complete the defatting. Subsequently, a washing step with aqueous ethanol removes soluble carbohydrates, phenolic compounds and most glucosinolates. The meal is pressed to mechanically remove as much of the retained solvent before a gentle desolventisation. This model is more expensive than the current one, but the different products obtained (pressed oil, solvent-extracted oil, meal and an ethanol extract of lecithin, phenolic compounds, etc.) could make it economically attractive. Carré et al. [69] proposed a cost estimation to evaluate the feasibility of a large-scale hot ethanol extraction process. The simulated extractor involved extraction at a temperature above the boiling point (90–95 °C) in a module consisting of hydrocyclones operating in partial countercurrent. The total cost of processing was estimated at EUR 47.4 per tonne of seed processed, which was higher

<footnote><sup>a</sup> Oil yield (g/100 g DM seed).</footnote>
and ethanol at 95.6% (92.7%). A small increase in water content significantly reduced the extraction yield of alcohol solvents. Due to the variability in extraction conditions (extraction technique, temperature, liquid/solid ratio, agitation, etc.), sampling conditions (moisture content, drying efficiency, oilseeds' pre-treatment) and oilseed composition, it is difficult to compare the different authors' experimental work.

## 5.2. 2-Methyloxolane

2-Methyltetrahydrofuran, also known as 2-methyloxolane (2-MeOx), is a biodegradable solvent derived from levulinic acid or furfural obtained from lignocellulosic biomass conversion (e.g., corncobs and sugarcane bagasse). 2-MeOx shows interesting properties that are technically comparable to those of hexane and could easily be transported on an industrial scale [72]. 2-MeOx is predominantly lipophilic (log *p* = 1.85) and can therefore dissolve both fatty molecules, such as hexane (log *p* = 4.00), and more polar molecules, due to the presence of an oxygen atom (dipole moment = 1.38 D). Its boiling point (80 °C) is high enough to allow a good extraction temperature but low enough to be easily removed from the final products and recycled. In addition, its density (0.855) and viscosity (0.6 cP) are close to those of hexane and within an acceptable range for efficient diffusion through solid particles [73]. Moreover, 2-MeOx has a much safer toxicological profile than hexane, and the use of 2-MeOx in industrial production could lead to a 97% reduction in CO₂ emissions compared to petroleum-based solvents [72].

2-MeOx has already been approved for the extraction of organic and natural cosmetic ingredients (COSMOS label) and pharmaceutical products. On 20 March 2022, EFSA declared 2-MeOx a safe solvent for food applications, based on a comprehensive review of scientific studies. Methylloxolane (EcoXtract®) was added to the list of permitted solvents for food and feed production in Europe on 26 January 2023 (Directive 2009/32/EC) [74,75]. The main studies on the extraction of vegetable oils with 2-MeOx are listed in Table 5.

<table><caption>**Table 5.** Vegetable oils' extraction with 2-MeOx.</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Oil Yield (g/100 g DM Seed)</th><th>Ref.</th></tr></thead><tbody><tr><td>Rapeseed</td><td>Coarsely ground</td><td>Soxhlet extraction</td><td>47.2</td><td>[66]</td></tr><tr><td rowspan="2">Rapeseed</td><td>Finely ground</td><td>Soxhlet extraction</td><td>46.0</td><td rowspan="2">[76]</td></tr><tr><td>Rapeseed cake</td><td>Pilot Soxhlet extraction</td><td>95.6 <sup>a</sup></td></tr><tr><td>Soybean</td><td>Finely ground</td><td>Soxhlet extraction</td><td>23.5</td><td>[77]</td></tr></tbody></table>

The partial miscibility of 2-MeOx with water can lead to better diffusion in cases where the solids to be extracted contain moisture. 2-MeOx forms an azeotrope with water (10.6% water and 89.4% 2-MeOx) and a second distillation is required to obtain the dry solvent. In the recovery of 2-MeOx in industrial extraction plants, the recovered organic phase is saturated with water after liquid/liquid separation. The 2-MeOx/H₂O mixture (95.5/4.5% at 55 °C) must be fed to another distillation step to recover the dry 2-MeOx. However, studies have shown that 2-MeOx 95.5% can be used directly for oil extraction, showing a similar efficiency to the dry solvent [77,78]. Using COSMO-RS and Hansen solubility predictions, Sicaire et al. [66] showed that 2-MeOx can be considered the best alternative to n-hexane among all solvents tested, as it can efficiently dissolve the desired compounds. The extraction yield with 2-MeOx was comparable to that with hexane on a laboratory scale (45.96 and 46.34 g/100 g DM, respectively).

Using the 6-litre pilot percolation extractor, the percentage of residual oil in the rape-seed meal was 1.8% for hexane and 0.8% for 2-MeOx [76]. In addition, the extraction with 2-MeOx was faster, as three washing cycles allowed the extraction of almost 95% of the total oil, while five washes were required to extract 96% of the total oil for hexane. The energy evaluation of the industrial extraction process showed that the total amount of energy

<footnote><sup>a</sup> Oil extraction (% of total oil).</footnote>
and ethanol at 95.6% (92.7%). A small increase in water content significantly reduced the extraction yield of alcohol solvents. Due to the variability in extraction conditions (extraction technique, temperature, liquid/solid ratio, agitation, etc.), sampling conditions (moisture content, drying efficiency, oilseeds' pre-treatment) and oilseed composition, it is difficult to compare the different authors' experimental work.

## 5.2. 2-Methyloxolane

2-Methyltetrahydrofuran, also known as 2-methyloxolane (2-MeOx), is a biodegradable solvent derived from levulinic acid or furfural obtained from lignocellulosic biomass conversion (e.g., corncobs and sugarcane bagasse). 2-MeOx shows interesting properties that are technically comparable to those of hexane and could easily be transported on an industrial scale [72]. 2-MeOx is predominantly lipophilic (log *p* = 1.85) and can therefore dissolve both fatty molecules, such as hexane (log *p* = 4.00), and more polar molecules, due to the presence of an oxygen atom (dipole moment = 1.38 D). Its boiling point (80 °C) is high enough to allow a good extraction temperature but low enough to be easily removed from the final products and recycled. In addition, its density (0.855) and viscosity (0.6 cP) are close to those of hexane and within an acceptable range for efficient diffusion through solid particles [73]. Moreover, 2-MeOx has a much safer toxicological profile than hexane, and the use of 2-MeOx in industrial production could lead to a 97% reduction in CO₂ emissions compared to petroleum-based solvents [72].

2-MeOx has already been approved for the extraction of organic and natural cosmetic ingredients (COSMOS label) and pharmaceutical products. On 20 March 2022, EFSA declared 2-MeOx a safe solvent for food applications, based on a comprehensive review of scientific studies. Methylloxolane (EcoXtract®) was added to the list of permitted solvents for food and feed production in Europe on 26 January 2023 (Directive 2009/32/EC) [74,75]. The main studies on the extraction of vegetable oils with 2-MeOx are listed in Table 5.

<table><caption>**Table 5.** Vegetable oils' extraction with 2-MeOx.</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Oil Yield (g/100 g DM Seed)</th><th>Ref.</th></tr></thead><tbody><tr><td>Rapeseed</td><td>Coarsely ground</td><td>Soxhlet extraction</td><td>47.2</td><td>[66]</td></tr><tr><td rowspan="2">Rapeseed</td><td>Finely ground</td><td>Soxhlet extraction</td><td>46.0</td><td rowspan="2">[76]</td></tr><tr><td>Rapeseed cake</td><td>Pilot Soxhlet extraction</td><td>95.6 <sup>a</sup></td></tr><tr><td>Soybean</td><td>Finely ground</td><td>Soxhlet extraction</td><td>23.5</td><td>[77]</td></tr></tbody></table>

The partial miscibility of 2-MeOx with water can lead to better diffusion in cases where the solids to be extracted contain moisture. 2-MeOx forms an azeotrope with water (10.6% water and 89.4% 2-MeOx) and a second distillation is required to obtain the dry solvent. In the recovery of 2-MeOx in industrial extraction plants, the recovered organic phase is saturated with water after liquid/liquid separation. The 2-MeOx/H₂O mixture (95.5/4.5% at 55 °C) must be fed to another distillation step to recover the dry 2-MeOx. However, studies have shown that 2-MeOx 95.5% can be used directly for oil extraction, showing a similar efficiency to the dry solvent [77,78]. Using COSMO-RS and Hansen solubility predictions, Sicaire et al. [66] showed that 2-MeOx can be considered the best alternative to n-hexane among all solvents tested, as it can efficiently dissolve the desired compounds. The extraction yield with 2-MeOx was comparable to that with hexane on a laboratory scale (45.96 and 46.34 g/100 g DM, respectively).

Using the 6-litre pilot percolation extractor, the percentage of residual oil in the rape-seed meal was 1.8% for hexane and 0.8% for 2-MeOx [76]. In addition, the extraction with 2-MeOx was faster, as three washing cycles allowed the extraction of almost 95% of the total oil, while five washes were required to extract 96% of the total oil for hexane. The energy evaluation of the industrial extraction process showed that the total amount of energy

<footnote><sup>a</sup> Oil extraction (% of total oil).</footnote>
was slightly higher with 2-MeOx (365 MJ/t seed) than with hexane (284 MJ/t seed) [76]. Sicaire et al. [66] obtained a rapeseed oil yield of 47.19% with 2-MeOx and 46.7% with hexane, but the extract with 2-MeOx had a lower percentage of TAGs (92.83% and 99.12%, respectively) due to the extraction of more polar lipids [66]. Claux et al. [77] investigated the use of dry 2-MeOx and 2-MeOx 95.5% as alternatives for the extraction of soybean oil. Due to their higher polarity, dry 2-MeOx (23.5%) and 2-MeOx 95.5% (23.7%) gave higher oil yields than hexane (18.8%). The yield differences were ascribed to the co-extraction of more polar additional compounds such as phospholipids and isoflavones. However, the protein dispersibility index (PDI) and KOH protein solubility were slightly lower for meal extracted with dry 2-MeOx and 2-MeOx 95.5%. The concentrations of antinutritional factors in the meals were the same after extraction with hexane and 2-MeOx.

Annual production of 2-MeOx is estimated at about 4500 tonnes, which is far below the amount needed for its continuous large-scale use. 2-MeOx trades at EUR 8.0/kg for purchases over 100 tonnes from Pennakem (Memphis, Tennessee), and hexane at EUR 0.9/kg [79]. A price reduction would increase the competitiveness of this solvent. The second economic disadvantage is the higher energy consumption for evaporation of the solvent from the miscella and desolventisation of the meal compared to extraction with hexane. However, the yield with 2-MeOx is higher, and with further optimisation of recycling and the reduction in losses, the estimated total cost of the process increases by only EUR 0.47 per tonne of extracted seed. This difference is minimal and could easily be compensated by a slight increase in the retail price of 2-MeOx-extracted oils: one cent per kilo, or 1 per cent of the premium, could actually bring more profit than hexane [72].

Among the various solvents here presented, 2-MeOx, ethanol and SFE-CO₂ are the ones at the most advanced stage of study. Rapinel et al. [72] reported industrial-scale studies that demonstrated the scalability of the process. The extraction of 46 tonnes of canola press cake (22.8% oil) at 340 kg/h in an industrial immersion extractor allowed a very good extraction performance (0.3% residual oil). The replacement of hexane by 2-MeOx does not require any significant plant changes. The only adjustments encompass the replacement of incompatible polymeric materials, the replacement of the wastewater boiler with a distillation column to maximise the recovery of the solvent present in the aqueous phase and the installation of a thermostatically controlled settling tank to reduce the amount of 2-MeOx in the aqueous phase. An advantage is also the possible conversion of the mineral oil absorption system replaced by cold water.

Further studies on the extraction of secondary metabolites, antinutrients and protein quality with 2-MeOx are needed. The refining process of crude oils extracted with 2-MeOx also requires further research.

### 5.3. Supercritical and Subcritical Fluids

Supercritical fluids are an alternative to conventional organic solvents. A fluid reaches a critical state when it is simultaneously heated and pressurised above its critical pressure. The main advantages of supercritical fluid extraction (SFE) include the absence of explosion hazards, higher selectivity in the extraction of neutral lipids, shorter extraction times, easy solvent recovery that preserves the oil and meal from thermal and oxidative degradation, and the avoidance of halogenated organic solvents. On the other hand, due to the high initial cost of equipment installation, and the energy and capital costs associated with regular maintenance, SFE is more commonly used in industrial processes that aim to obtain a product with a high economic value, resulting in a high-quality lipid extract.

Carbon dioxide (CO₂) is the most widely used solvent for SFE, being inert, non-toxic, non-flammable, inexpensive, abundant and endowed with moderate critical properties (Tc = 31.1 °C, Pc = 7.38 MPa). CO₂ is a generally recognised safe solvent (GRAS), so products containing extracts made from food-grade CO₂ are safe for human health. The use of supercritical CO₂ extraction techniques allows the efficient extraction of lipophilic compounds at generally mild temperatures (30–70 °C), without leaving traces in the extract. Since supercritical CO₂ is a non-polar solvent, its solubilising capacity is known to be
was slightly higher with 2-MeOx (365 MJ/t seed) than with hexane (284 MJ/t seed) [76]. Sicaire et al. [66] obtained a rapeseed oil yield of 47.19% with 2-MeOx and 46.7% with hexane, but the extract with 2-MeOx had a lower percentage of TAGs (92.83% and 99.12%, respectively) due to the extraction of more polar lipids [66]. Claux et al. [77] investigated the use of dry 2-MeOx and 2-MeOx 95.5% as alternatives for the extraction of soybean oil. Due to their higher polarity, dry 2-MeOx (23.5%) and 2-MeOx 95.5% (23.7%) gave higher oil yields than hexane (18.8%). The yield differences were ascribed to the co-extraction of more polar additional compounds such as phospholipids and isoflavones. However, the protein dispersibility index (PDI) and KOH protein solubility were slightly lower for meal extracted with dry 2-MeOx and 2-MeOx 95.5%. The concentrations of antinutritional factors in the meals were the same after extraction with hexane and 2-MeOx.

Annual production of 2-MeOx is estimated at about 4500 tonnes, which is far below the amount needed for its continuous large-scale use. 2-MeOx trades at EUR 8.0/kg for purchases over 100 tonnes from Pennakem (Memphis, Tennessee), and hexane at EUR 0.9/kg [79]. A price reduction would increase the competitiveness of this solvent. The second economic disadvantage is the higher energy consumption for evaporation of the solvent from the miscella and desolventisation of the meal compared to extraction with hexane. However, the yield with 2-MeOx is higher, and with further optimisation of recycling and the reduction in losses, the estimated total cost of the process increases by only EUR 0.47 per tonne of extracted seed. This difference is minimal and could easily be compensated by a slight increase in the retail price of 2-MeOx-extracted oils: one cent per kilo, or 1 per cent of the premium, could actually bring more profit than hexane [72].

Among the various solvents here presented, 2-MeOx, ethanol and SFE-CO₂ are the ones at the most advanced stage of study. Rapinel et al. [72] reported industrial-scale studies that demonstrated the scalability of the process. The extraction of 46 tonnes of canola press cake (22.8% oil) at 340 kg/h in an industrial immersion extractor allowed a very good extraction performance (0.3% residual oil). The replacement of hexane by 2-MeOx does not require any significant plant changes. The only adjustments encompass the replacement of incompatible polymeric materials, the replacement of the wastewater boiler with a distillation column to maximise the recovery of the solvent present in the aqueous phase and the installation of a thermostatically controlled settling tank to reduce the amount of 2-MeOx in the aqueous phase. An advantage is also the possible conversion of the mineral oil absorption system replaced by cold water.

Further studies on the extraction of secondary metabolites, antinutrients and protein quality with 2-MeOx are needed. The refining process of crude oils extracted with 2-MeOx also requires further research.

### 5.3. Supercritical and Subcritical Fluids

Supercritical fluids are an alternative to conventional organic solvents. A fluid reaches a critical state when it is simultaneously heated and pressurised above its critical pressure. The main advantages of supercritical fluid extraction (SFE) include the absence of explosion hazards, higher selectivity in the extraction of neutral lipids, shorter extraction times, easy solvent recovery that preserves the oil and meal from thermal and oxidative degradation, and the avoidance of halogenated organic solvents. On the other hand, due to the high initial cost of equipment installation, and the energy and capital costs associated with regular maintenance, SFE is more commonly used in industrial processes that aim to obtain a product with a high economic value, resulting in a high-quality lipid extract.

Carbon dioxide (CO₂) is the most widely used solvent for SFE, being inert, non-toxic, non-flammable, inexpensive, abundant and endowed with moderate critical properties (Tc = 31.1 °C, Pc = 7.38 MPa). CO₂ is a generally recognised safe solvent (GRAS), so products containing extracts made from food-grade CO₂ are safe for human health. The use of supercritical CO₂ extraction techniques allows the efficient extraction of lipophilic compounds at generally mild temperatures (30–70 °C), without leaving traces in the extract. Since supercritical CO₂ is a non-polar solvent, its solubilising capacity is known to be
between that of pentane and toluene [47]. The solubility of oil in CO₂ is strongly dependent on pressure and temperature. At about 300 bar, it is between 0.3 and 0.7%, with temperature having a negative effect (raising the temperature from 40 to 80 °C leads to lower oil solubility) [80]. Only above 350 bar is there a positive relationship between solubility and temperature. As a result, in today's systems (which are generally limited to 3–400 bar), a large amount of solvent must be used to extract the oil. Oil separation from the solvent should also be carried out under supercritical conditions to save energy. The solubility of triglycerides in CO₂ is so low under 160 bar that excellent separation is possible. Another advantage of this system is that there is no need for a condenser to liquefy the CO₂ ahead of the pump. Cavitation in the plunger pump due to the phase change of the CO₂ is also avoided. Specific mass flows between 10 and 50 kg CO₂/kg·h at surface velocities of the solvent between 1 and 5 mm/sec have been used for SFE of oilseeds [81].

SFE plants comprise four main components: (1) a pump to ensure the volumetric flow of the fluid; optionally, it can be preceded by a cooler to transport the gaseous components in a liquid state, (2) a heat exchanger, (3) an extractor, where static and dynamic extractions take place by modulating the pressure regulated by a valve, and (4) a separator [82].

To improve the solubility of polar compounds, a polar co-solvent such as methanol or ethanol can be added. Moreover, systems have been developed that integrate ultrasonic treatments sequentially or simultaneously with SFE. Duarte et al. [83] described a system with an ultrasound probe internally coupled into the supercritical extraction cell. To avoid the use of CO₂ at high pressures, binary gas mixtures were investigated. A mixture of CO₂ and propane allows complete miscibility of triglycerides below 300 bar and at temperatures up to 70 °C [81]. Ultra-high-pressure SFE (pressures greater than 70 MPa) severely limits the amount of solvent to be used and the extraction time. Recent studies have shown that extracts obtained under high pressure are enriched with important ingredients that can only be obtained under these conditions. Although the investment costs are higher, the operating costs are lower at higher pressure. This is due to the shorter extraction times resulting from the higher oil solubility and the extraction of high-value compounds. Depending on the application, ultra-high-pressure SFE could be competitive [79].

The use of liquid CO₂ has emerged as an innovative extraction technique that offers many of the same benefits as supercritical CO₂, but at lower pressures (≤15 MPa) and temperatures (about 25 °C), therefore reducing energy costs. In addition, due to its low polarity compared to most organic solvents, liquid CO₂ might have higher selectivity for neutral lipids, while affinity for non-neutral lipids is limited. The overall extraction yield is usually lower than that of organic solvents due to the lower polarity. The use of co-solvents such as methanol is also compatible with this technique [84].

Liquified gas extraction (LGE) is a promising alternative for the extraction of lipids and other compounds. LGE involves extraction with gaseous organic solvents under pressure, which allows the solvent to be in a condensed state. Unlike SCF, LGE requires moderate pressures (1–10 bar), which results in lower energy consumption and facilitates industrial-scale application. Depending on the pressure and temperature conditions, the dissolving power can be adjusted by changing the solvent density, which enables selective extractions. The most common liquid gases are propane, butane and dimethyl ether (the first two are listed in the Directive 2009/32/EC). The main advantages of LGE are the following: (1) they can be used at low temperatures: due to their low boiling point, liquefied gases can be vaporised at moderate temperatures, preserving oil and meal and leaving no traces of solvent residues; (2) they can be used at moderate pressures; (3) under “normal” solid-liquid extraction conditions, most liquefied gases are chemically inert [47].

In LGE, the temperature increase has a positive effect on the extraction yield up to a certain limit (e.g., 55 °C at 0.5 MPa for *n*-butane). Above these temperatures, the solubility of the oil decreases as the gasification rate of the solvent increases [2]. The main studies on the extraction of vegetable oils with supercritical and subcritical fluids are listed in Table 6. Some studies have compared SFE and LGE in the extraction of oilseeds. For example, Pederssetti et al. [85] analysed the extraction of canola seeds using SFE-CO₂ and LGE-
between that of pentane and toluene [47]. The solubility of oil in CO₂ is strongly dependent on pressure and temperature. At about 300 bar, it is between 0.3 and 0.7%, with temperature having a negative effect (raising the temperature from 40 to 80 °C leads to lower oil solubility) [80]. Only above 350 bar is there a positive relationship between solubility and temperature. As a result, in today's systems (which are generally limited to 3–400 bar), a large amount of solvent must be used to extract the oil. Oil separation from the solvent should also be carried out under supercritical conditions to save energy. The solubility of triglycerides in CO₂ is so low under 160 bar that excellent separation is possible. Another advantage of this system is that there is no need for a condenser to liquefy the CO₂ ahead of the pump. Cavitation in the plunger pump due to the phase change of the CO₂ is also avoided. Specific mass flows between 10 and 50 kg CO₂/kg·h at surface velocities of the solvent between 1 and 5 mm/sec have been used for SFE of oilseeds [81].

SFE plants comprise four main components: (1) a pump to ensure the volumetric flow of the fluid; optionally, it can be preceded by a cooler to transport the gaseous components in a liquid state, (2) a heat exchanger, (3) an extractor, where static and dynamic extractions take place by modulating the pressure regulated by a valve, and (4) a separator [82].

To improve the solubility of polar compounds, a polar co-solvent such as methanol or ethanol can be added. Moreover, systems have been developed that integrate ultrasonic treatments sequentially or simultaneously with SFE. Duarte et al. [83] described a system with an ultrasound probe internally coupled into the supercritical extraction cell. To avoid the use of CO₂ at high pressures, binary gas mixtures were investigated. A mixture of CO₂ and propane allows complete miscibility of triglycerides below 300 bar and at temperatures up to 70 °C [81]. Ultra-high-pressure SFE (pressures greater than 70 MPa) severely limits the amount of solvent to be used and the extraction time. Recent studies have shown that extracts obtained under high pressure are enriched with important ingredients that can only be obtained under these conditions. Although the investment costs are higher, the operating costs are lower at higher pressure. This is due to the shorter extraction times resulting from the higher oil solubility and the extraction of high-value compounds. Depending on the application, ultra-high-pressure SFE could be competitive [79].

The use of liquid CO₂ has emerged as an innovative extraction technique that offers many of the same benefits as supercritical CO₂, but at lower pressures (≤15 MPa) and temperatures (about 25 °C), therefore reducing energy costs. In addition, due to its low polarity compared to most organic solvents, liquid CO₂ might have higher selectivity for neutral lipids, while affinity for non-neutral lipids is limited. The overall extraction yield is usually lower than that of organic solvents due to the lower polarity. The use of co-solvents such as methanol is also compatible with this technique [84].

Liquified gas extraction (LGE) is a promising alternative for the extraction of lipids and other compounds. LGE involves extraction with gaseous organic solvents under pressure, which allows the solvent to be in a condensed state. Unlike SCF, LGE requires moderate pressures (1–10 bar), which results in lower energy consumption and facilitates industrial-scale application. Depending on the pressure and temperature conditions, the dissolving power can be adjusted by changing the solvent density, which enables selective extractions. The most common liquid gases are propane, butane and dimethyl ether (the first two are listed in the Directive 2009/32/EC). The main advantages of LGE are the following: (1) they can be used at low temperatures: due to their low boiling point, liquefied gases can be vaporised at moderate temperatures, preserving oil and meal and leaving no traces of solvent residues; (2) they can be used at moderate pressures; (3) under “normal” solid-liquid extraction conditions, most liquefied gases are chemically inert [47].

In LGE, the temperature increase has a positive effect on the extraction yield up to a certain limit (e.g., 55 °C at 0.5 MPa for *n*-butane). Above these temperatures, the solubility of the oil decreases as the gasification rate of the solvent increases [2]. The main studies on the extraction of vegetable oils with supercritical and subcritical fluids are listed in Table 6. Some studies have compared SFE and LGE in the extraction of oilseeds. For example, Pederssetti et al. [85] analysed the extraction of canola seeds using SFE-CO₂ and LGE-
propane. For SFE, increasing pressure had a positive effect, while increasing temperature had a negative effect on the extraction yield. For LGE extractions, the effect of temperature was more pronounced compared to pressure due to the low variation in density with changing pressure over the experimental range examined. LGE with propane shows faster extraction kinetics than SFE since propane solubilises triacylglycerols better than CO₂. The oxidative stability and fatty acid profile of the extracted oil were similar for the two solvents. However, the oil yields obtained with SFE and LGE were only 52.7% and 64.3% of those obtained with hexane Soxhlet extraction. Due to the lower oil yield, the protein content of the meal was also lower with SFE (29.9%) and LGE (29.9%) than with hexane extraction (36.7%). Sun et al. [86] compared canola meal extracted with SFE-CO₂ with pressed cake and meal extracted with hexane. Both the hexane-extracted and SFE-CO₂-extracted meals had a higher protein content than the pressed cake. The glucosinolates content was lower in the meal extracted with SFE-CO₂, while the phosphorus content was higher than in the meal extracted with hexane. The phenolic acid contents of the meal extracted with hexane and SFE-CO₂ were similar. The addition of ethanol as a co-solvent during SFE-CO₂ reduced the phosphorus and phenolic acid content in the meal. It is well known that the addition of ethanol enhances phospholipid extraction with SFE-CO₂. Both meals extracted with hexane and SFE-CO₂ showed a similar protein solubility, high water-holding capacity, high oil absorption and high emulsifying capacity. Boutin and Badens [87] investigated the influence of different parameters on the oil extraction yield for rapeseed and sunflower seeds using SFE-CO₂. The authors found that pressure and extraction duration are the most influencing parameters together with temperature in the case of rapeseed. Under optimal conditions, SFE-CO₂ yielded 89.9% and 78.5% of oil from rapeseed and sunflower, respectively. SFE-CO₂ proved to be a selective process as no traces of phospholipids were detected in the extracted oil. This would make it possible to avoid degumming during oil refining. Nimet et al. [88] showed that LGE-propane afforded higher yields of sunflower oil than SFE-CO₂, working at a lower pressure and with a shorter extraction time. The fatty acid profile of the sunflower oil and the protein content of the meal were not significantly affected by the solvents or operating conditions. The values for the tocopherol content and oxidative stability of the oil obtained during extraction with LGE-propane and SFE-CO₂ were higher than those obtained with conventional extraction with hexane. In another study by Rapinel et al. [89], LGE of sunflower oil with *n*-butane at low pressure (0.2 to 0.4 MPa) resulted in a lower extraction yield compared to hexane extraction (36.9% vs. 53.4%).

<table><caption>**Table 6.** Vegetable oils' extraction with supercritical and subcritical fluids.</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Solvent</th><th>Oil Yield (g/100 g DM Seed)</th><th>Ref.</th></tr></thead><tbody><tr><td>Flaxseed</td><td>Ground</td><td>Laboratory-scale extractor</td><td>SFE-CO₂ 99.9%, 30 MPa, 50 °C</td><td>35.3 <sup>a</sup></td><td>[90]</td></tr><tr><td rowspan="6">Sunflower</td><td rowspan="6">Milled</td><td rowspan="6">Laboratory-scale extractor</td><td>Propane 99.5%, 8 MPa, 60 °C</td><td>92.7 <sup>b</sup></td><td rowspan="6">[88]</td></tr><tr><td>Propane 99.5%, 12 MPa, 60 °C</td><td>100 <sup>b</sup></td></tr><tr><td>Propane 99.5%, 10 MPa, 45 °C</td><td>90.2 <sup>b</sup></td></tr><tr><td>SFE-CO₂ 99.9%, 25 MPa, 40 °C</td><td>100 <sup>b</sup></td></tr><tr><td>SFE-CO₂ 99.9%, 22 MPa, 50 °C</td><td>75.6 <sup>b</sup></td></tr><tr><td>SFE-CO₂ 99.9%, 25 MPa, 60 °C</td><td>87.8 <sup>b</sup></td></tr><tr><td rowspan="6">Rapeseed</td><td rowspan="6">Milled</td><td rowspan="6">Laboratory-scale extractor</td><td>Propane 99.5%, 8 MPa, 60 °C</td><td>62.5 <sup>b</sup>, 23.1</td><td rowspan="6">[85]</td></tr><tr><td>Propane 99.5%, 12 MPa, 60 °C</td><td>64.4 <sup>b</sup>, 23.8</td></tr><tr><td>Propane 99.5%, 10 MPa, 45 °C</td><td>55.9 <sup>b</sup>, 20.7</td></tr><tr><td>SFE-CO₂ 99.9%, 25 MPa, 40 °C</td><td>52.7 <sup>b</sup>, 19.5</td></tr><tr><td>SFE-CO₂ 99.9%, 22 MPa, 50 °C</td><td>48.1 <sup>b</sup>, 17.8</td></tr><tr><td>SFE-CO₂ 99.9%, 25 MPa, 60 °C</td><td>49.2 <sup>b</sup>, 18.2</td></tr></tbody></table>
propane. For SFE, increasing pressure had a positive effect, while increasing temperature had a negative effect on the extraction yield. For LGE extractions, the effect of temperature was more pronounced compared to pressure due to the low variation in density with changing pressure over the experimental range examined. LGE with propane shows faster extraction kinetics than SFE since propane solubilises triacylglycerols better than CO₂. The oxidative stability and fatty acid profile of the extracted oil were similar for the two solvents. However, the oil yields obtained with SFE and LGE were only 52.7% and 64.3% of those obtained with hexane Soxhlet extraction. Due to the lower oil yield, the protein content of the meal was also lower with SFE (29.9%) and LGE (29.9%) than with hexane extraction (36.7%). Sun et al. [86] compared canola meal extracted with SFE-CO₂ with pressed cake and meal extracted with hexane. Both the hexane-extracted and SFE-CO₂-extracted meals had a higher protein content than the pressed cake. The glucosinolates content was lower in the meal extracted with SFE-CO₂, while the phosphorus content was higher than in the meal extracted with hexane. The phenolic acid contents of the meal extracted with hexane and SFE-CO₂ were similar. The addition of ethanol as a co-solvent during SFE-CO₂ reduced the phosphorus and phenolic acid content in the meal. It is well known that the addition of ethanol enhances phospholipid extraction with SFE-CO₂. Both meals extracted with hexane and SFE-CO₂ showed a similar protein solubility, high water-holding capacity, high oil absorption and high emulsifying capacity. Boutin and Badens [87] investigated the influence of different parameters on the oil extraction yield for rapeseed and sunflower seeds using SFE-CO₂. The authors found that pressure and extraction duration are the most influencing parameters together with temperature in the case of rapeseed. Under optimal conditions, SFE-CO₂ yielded 89.9% and 78.5% of oil from rapeseed and sunflower, respectively. SFE-CO₂ proved to be a selective process as no traces of phospholipids were detected in the extracted oil. This would make it possible to avoid degumming during oil refining. Nimet et al. [88] showed that LGE-propane afforded higher yields of sunflower oil than SFE-CO₂, working at a lower pressure and with a shorter extraction time. The fatty acid profile of the sunflower oil and the protein content of the meal were not significantly affected by the solvents or operating conditions. The values for the tocopherol content and oxidative stability of the oil obtained during extraction with LGE-propane and SFE-CO₂ were higher than those obtained with conventional extraction with hexane. In another study by Rapinel et al. [89], LGE of sunflower oil with *n*-butane at low pressure (0.2 to 0.4 MPa) resulted in a lower extraction yield compared to hexane extraction (36.9% vs. 53.4%).

<table><caption>**Table 6.** Vegetable oils' extraction with supercritical and subcritical fluids.</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Solvent</th><th>Oil Yield (g/100 g DM Seed)</th><th>Ref.</th></tr></thead><tbody><tr><td>Flaxseed</td><td>Ground</td><td>Laboratory-scale extractor</td><td>SFE-CO₂ 99.9%, 30 MPa, 50 °C</td><td>35.3 <sup>a</sup></td><td>[90]</td></tr><tr><td rowspan="6">Sunflower</td><td rowspan="6">Milled</td><td rowspan="6">Laboratory-scale extractor</td><td>Propane 99.5%, 8 MPa, 60 °C</td><td>92.7 <sup>b</sup></td><td rowspan="6">[88]</td></tr><tr><td>Propane 99.5%, 12 MPa, 60 °C</td><td>100 <sup>b</sup></td></tr><tr><td>Propane 99.5%, 10 MPa, 45 °C</td><td>90.2 <sup>b</sup></td></tr><tr><td>SFE-CO₂ 99.9%, 25 MPa, 40 °C</td><td>100 <sup>b</sup></td></tr><tr><td>SFE-CO₂ 99.9%, 22 MPa, 50 °C</td><td>75.6 <sup>b</sup></td></tr><tr><td>SFE-CO₂ 99.9%, 25 MPa, 60 °C</td><td>87.8 <sup>b</sup></td></tr><tr><td rowspan="6">Rapeseed</td><td rowspan="6">Milled</td><td rowspan="6">Laboratory-scale extractor</td><td>Propane 99.5%, 8 MPa, 60 °C</td><td>62.5 <sup>b</sup>, 23.1</td><td rowspan="6">[85]</td></tr><tr><td>Propane 99.5%, 12 MPa, 60 °C</td><td>64.4 <sup>b</sup>, 23.8</td></tr><tr><td>Propane 99.5%, 10 MPa, 45 °C</td><td>55.9 <sup>b</sup>, 20.7</td></tr><tr><td>SFE-CO₂ 99.9%, 25 MPa, 40 °C</td><td>52.7 <sup>b</sup>, 19.5</td></tr><tr><td>SFE-CO₂ 99.9%, 22 MPa, 50 °C</td><td>48.1 <sup>b</sup>, 17.8</td></tr><tr><td>SFE-CO₂ 99.9%, 25 MPa, 60 °C</td><td>49.2 <sup>b</sup>, 18.2</td></tr></tbody></table>
<table><caption>**Table 6.** *Cont.*</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Solvent</th><th>Oil Yield (g/100 g DM Seed)</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="3">Flaxseed</td><td rowspan="3">Ground</td><td rowspan="3">Laboratory-scale extractor</td><td>n-Propane 99.5%, 8 MPa, 60 °C</td><td>28.6</td><td rowspan="3">[91]</td></tr><tr><td>n-Propane 99.5%, 12 MPa, 60 °C</td><td>28.8</td></tr><tr><td>n-Propane 99.5%, 10 MPa, 45 °C</td><td>28.2</td></tr><tr><td rowspan="3">Sunflower</td><td rowspan="3">Ground</td><td rowspan="3">Pilot-scale extractor</td><td>n-Butane 95%, 0.2 MPa, 20 °C</td><td>36.6<sup>a</sup></td><td rowspan="3">[89]</td></tr><tr><td>n-Butane 95%, 0.28 MPa, 30 °C</td><td>36.7<sup>a</sup></td></tr><tr><td>n-Butane 95%, 0.37 MPa, 40 °C</td><td>36.9<sup>a</sup></td></tr><tr><td>Flaxseed</td><td>Ground</td><td>Laboratory-scale extractor</td><td>n-Butane, 99.5%, 0.5 MPa, 54 °C</td><td>28.8<sup>a</sup></td><td>[92]</td></tr></tbody></table>

In general, oils extracted with LGE have a higher quality (lower acid value), a higher content of phytosterols and carotenoids (antioxidants), a better shelf life (lower iodine value) and a better oxidation stability than oils extracted with conventional techniques. The fatty acid profiles of the oil extracted with LGE or SFE do not differ from those of the conventional methods [2]. The separation of oil from the miscella after LGE can also be performed by membrane filtration. This technique can lead to energy savings (reduced use of steam, full or partial replacement of traditional degumming, refining and bleaching steps). An ideal membrane for solvent recovery must combine specific properties such as high oil retention and permeate fluxes suitable for industrial scales, as well as heat resistance and mechanical and chemical compatibility with the process [93].

Currently, the largest SFE-CO₂ plant is in Korea and is dedicated to the extraction of sesame oil for food purposes (capacity of 2 × 2600 L/550 bar). A techno-economic analysis of the SFE-CO₂ performance for the extraction of pequi pulp oil showed that the production costs in pilot (100 L) and industrial (500 L) plants were attractive in eight scenarios tested. The most promising one showed a production cost of about EUR 33/kg pequi oil, with the purchase of pequi and other feedstocks accounting for about 85% of the cost. Moreover, the payback period was estimated as less than one year, which is attractive as the initial investment can be recovered quickly [79].

Fiori [94] developed a model to predict the economic feasibility of an industrial scale SFE-CO₂ plant (three extractors in series operating in countercurrent) designed to extract 3000 tonnes of grape seeds per year. The authors suggested that switching from a single mode to a cascade of two extractors increases the extraction efficiency from 83% to 86%. The break-even point that makes the process economically viable is a value of 5.9 EUR/kg for grape seed oil extracted with SFE-CO₂. According to these results, SFE-CO₂ proves to be a cost-effective alternative for the extraction of high-value-added specialty oils.

However, the high pressures required to reach the supercritical state limit its use to high-value-added products, as the investment costs for an industrial plant are correspondingly high. A clear disadvantage of SFE-CO₂ is also the need to extract large quantities of material in a semi-continuous process by filling and emptying the extraction vessels at atmospheric pressure. In conclusion, the use of SFE-CO₂ in the extraction of lipids for the large-scale production of biofuels and commodity vegetable oils is an uneconomic alternative, as the high costs of installation and maintenance, combined with energy costs, would significantly increase the price of the final product [79].

For liquefied gases' pilot-scale extraction, the company CELSIUS Sarl (Villette de Vienne, France) developed an isobaric LGE process (NECTACEL extractor) where the system always stays at liquid/vapor equilibrium. The liquefied gas is evaporated in the boiler at the same pressure (isobaric mode) and the vapours rise naturally to the condenser for solvent regeneration. No solvent pump or compressor is required to run the cycle, including solvent injection or recycling. The absence of mechanical equipment means lower energy consumption and maintenance costs. The extraction and separation steps are carried out at room temperature, which benefits heat-sensitive or thermo-oxidisable

<footnote><sup>a</sup> Oil yield (g/100 g initial material); <sup>b</sup> oil extraction (% of total oil).</footnote>
<table><caption>**Table 6.** *Cont.*</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Solvent</th><th>Oil Yield (g/100 g DM Seed)</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="3">Flaxseed</td><td rowspan="3">Ground</td><td rowspan="3">Laboratory-scale extractor</td><td>n-Propane 99.5%, 8 MPa, 60 °C</td><td>28.6</td><td rowspan="3">[91]</td></tr><tr><td>n-Propane 99.5%, 12 MPa, 60 °C</td><td>28.8</td></tr><tr><td>n-Propane 99.5%, 10 MPa, 45 °C</td><td>28.2</td></tr><tr><td rowspan="3">Sunflower</td><td rowspan="3">Ground</td><td rowspan="3">Pilot-scale extractor</td><td>n-Butane 95%, 0.2 MPa, 20 °C</td><td>36.6<sup>a</sup></td><td rowspan="3">[89]</td></tr><tr><td>n-Butane 95%, 0.28 MPa, 30 °C</td><td>36.7<sup>a</sup></td></tr><tr><td>n-Butane 95%, 0.37 MPa, 40 °C</td><td>36.9<sup>a</sup></td></tr><tr><td>Flaxseed</td><td>Ground</td><td>Laboratory-scale extractor</td><td>n-Butane, 99.5%, 0.5 MPa, 54 °C</td><td>28.8<sup>a</sup></td><td>[92]</td></tr></tbody></table>

In general, oils extracted with LGE have a higher quality (lower acid value), a higher content of phytosterols and carotenoids (antioxidants), a better shelf life (lower iodine value) and a better oxidation stability than oils extracted with conventional techniques. The fatty acid profiles of the oil extracted with LGE or SFE do not differ from those of the conventional methods [2]. The separation of oil from the miscella after LGE can also be performed by membrane filtration. This technique can lead to energy savings (reduced use of steam, full or partial replacement of traditional degumming, refining and bleaching steps). An ideal membrane for solvent recovery must combine specific properties such as high oil retention and permeate fluxes suitable for industrial scales, as well as heat resistance and mechanical and chemical compatibility with the process [93].

Currently, the largest SFE-CO₂ plant is in Korea and is dedicated to the extraction of sesame oil for food purposes (capacity of 2 × 2600 L/550 bar). A techno-economic analysis of the SFE-CO₂ performance for the extraction of pequi pulp oil showed that the production costs in pilot (100 L) and industrial (500 L) plants were attractive in eight scenarios tested. The most promising one showed a production cost of about EUR 33/kg pequi oil, with the purchase of pequi and other feedstocks accounting for about 85% of the cost. Moreover, the payback period was estimated as less than one year, which is attractive as the initial investment can be recovered quickly [79].

Fiori [94] developed a model to predict the economic feasibility of an industrial scale SFE-CO₂ plant (three extractors in series operating in countercurrent) designed to extract 3000 tonnes of grape seeds per year. The authors suggested that switching from a single mode to a cascade of two extractors increases the extraction efficiency from 83% to 86%. The break-even point that makes the process economically viable is a value of 5.9 EUR/kg for grape seed oil extracted with SFE-CO₂. According to these results, SFE-CO₂ proves to be a cost-effective alternative for the extraction of high-value-added specialty oils.

However, the high pressures required to reach the supercritical state limit its use to high-value-added products, as the investment costs for an industrial plant are correspondingly high. A clear disadvantage of SFE-CO₂ is also the need to extract large quantities of material in a semi-continuous process by filling and emptying the extraction vessels at atmospheric pressure. In conclusion, the use of SFE-CO₂ in the extraction of lipids for the large-scale production of biofuels and commodity vegetable oils is an uneconomic alternative, as the high costs of installation and maintenance, combined with energy costs, would significantly increase the price of the final product [79].

For liquefied gases' pilot-scale extraction, the company CELSIUS Sarl (Villette de Vienne, France) developed an isobaric LGE process (NECTACEL extractor) where the system always stays at liquid/vapor equilibrium. The liquefied gas is evaporated in the boiler at the same pressure (isobaric mode) and the vapours rise naturally to the condenser for solvent regeneration. No solvent pump or compressor is required to run the cycle, including solvent injection or recycling. The absence of mechanical equipment means lower energy consumption and maintenance costs. The extraction and separation steps are carried out at room temperature, which benefits heat-sensitive or thermo-oxidisable

<footnote><sup>a</sup> Oil yield (g/100 g initial material); <sup>b</sup> oil extraction (% of total oil).</footnote>
molecules. Solvation properties can be improved by using a co-solvent with complementary ionic character. The most used solvents are butane, fluorocarbons (HFO1234ze) and dimethyl ether. Extraction and separation take place at a relatively low pressure (less than 10 bar), with no technological capacity limit for the equipment. CELSIUS built several prototypes with 1 L, 200 L and 500 L capacities. The main applications of this system concern the extraction of molecules for pharmaceutical, cosmetic (flavours and perfumes) and nutraceutical purposes [95].

### 5.4. Terpenes

Terpenes (also called isoprenoids or terpenoids) are acyclic, bicyclic or monocyclic hydrocarbons formed biosynthetically from isoprene units (C₅H₈). Terpenes are a large and extensive family of natural products (over 30,000) with relatively different physical properties. They are generally considered biodegradable solvents with low environmental impact and low toxicity, and they are mainly extracted from tree leaves and agricultural sources. In addition, microbial production of terpenes has enormous potential as it avoids dependence on natural resources. For example, limonene, which is generally obtained from the by-products of citrus juice production by steam distillation and condensation, can be produced by microbial bioconversion of glucose by *E. coli* or *S. cerevisiae* [82]. *D*-limonene, α-pinene and *p*-limonene have been investigated as alternative solvents for the extraction of vegetable oils.

Terpenes have molecular weights and structures suitable to substitute for *n*-hexane. The solubility parameters of the solvents were investigated using Hansen's parameters and COSMO-RS (Conductor-like Screening Model for Realistic Solvents) simulations. They gave similar results for terpenes and *n*-hexane for dissolving TAGs as well as sterols and tocopherols. However, when their dielectric constants are considered, terpenes are slightly more polar and show a higher dissociation force than *n*-hexane. In terms of safety, they have a higher flash point and are therefore less flammable and dangerous. Although the results in the laboratory scale are encouraging, the scalability of the terpene extraction process is limited. The main disadvantage of using terpenes is their high viscosity and density, and the higher energy consumption in solvent recovery by evaporation due to their higher boiling points (155–176 °C) and higher enthalpies of evaporation (37–39 kJ/mol) compared to *n*-hexane (Bp = 69 °C, ΔHvap = 29.74 kJ/mol) [66,76,96].

Heteroazeotropic distillation of the binary terpene-water mixture (about 50% *v*/*v*) leads to evaporation of the terpene and separation of the terpene-water distillation product by phase decantation. With this system, the solvent can be recovered at a lower temperature (about 97–98 °C at atmospheric pressure), and a high recovery rate of terpenes can be achieved (about 90% with 98% purity for α-pinene) [97]. The main studies on the extraction of vegetable oils with terpenes are listed in Table 7.

<table><caption>**Table 7.** Vegetable oils' extraction with terpenes.</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Solvent</th><th>Oil Yield (g/100 g DM Seed)</th><th>Ref.</th></tr></thead><tbody><tr><td>Soybean Sunflower</td><td>Crushed seeds</td><td>Soxhlet extraction</td><td>α-Pinene</td><td>21.167.2</td><td>[97]</td></tr><tr><td rowspan="3">Rapeseed</td><td rowspan="3">Ground</td><td rowspan="3">Soxhlet extraction</td><td><i>p</i>-Cymene</td><td>88.9 <sup>a</sup></td><td rowspan="3">[65]</td></tr><tr><td>α-Pinene</td><td>65.5 <sup>a</sup></td></tr><tr><td>Limonene</td><td>80.8 <sup>a</sup></td></tr><tr><td rowspan="2">Rapeseed</td><td rowspan="2">Coarsely ground</td><td rowspan="2">Soxhlet extraction</td><td><i>p</i>-Cymene</td><td>39.7</td><td rowspan="2">[66]</td></tr><tr><td>Limonene</td><td>37.0</td></tr><tr><td>Rapeseed</td><td>Ground</td><td>Maceration with intense mixing</td><td><i>cis</i>-Rich pinane (<i>cis</i>/trans: 7/3)</td><td>42.5</td><td>[98]</td></tr><tr><td>Rapeseed</td><td>Ground</td><td>Maceration with intense mixing Soxhlet extraction</td><td><i>p</i>-Menthane</td><td>37.140.5</td><td>[99]</td></tr></tbody></table>

<footnote><sup>a</sup> Oil extraction (% of total oil).</footnote>
molecules. Solvation properties can be improved by using a co-solvent with complementary ionic character. The most used solvents are butane, fluorocarbons (HFO1234ze) and dimethyl ether. Extraction and separation take place at a relatively low pressure (less than 10 bar), with no technological capacity limit for the equipment. CELSIUS built several prototypes with 1 L, 200 L and 500 L capacities. The main applications of this system concern the extraction of molecules for pharmaceutical, cosmetic (flavours and perfumes) and nutraceutical purposes [95].

### 5.4. Terpenes

Terpenes (also called isoprenoids or terpenoids) are acyclic, bicyclic or monocyclic hydrocarbons formed biosynthetically from isoprene units (C₅H₈). Terpenes are a large and extensive family of natural products (over 30,000) with relatively different physical properties. They are generally considered biodegradable solvents with low environmental impact and low toxicity, and they are mainly extracted from tree leaves and agricultural sources. In addition, microbial production of terpenes has enormous potential as it avoids dependence on natural resources. For example, limonene, which is generally obtained from the by-products of citrus juice production by steam distillation and condensation, can be produced by microbial bioconversion of glucose by *E. coli* or *S. cerevisiae* [82]. *D*-limonene, α-pinene and *p*-limonene have been investigated as alternative solvents for the extraction of vegetable oils.

Terpenes have molecular weights and structures suitable to substitute for *n*-hexane. The solubility parameters of the solvents were investigated using Hansen's parameters and COSMO-RS (Conductor-like Screening Model for Realistic Solvents) simulations. They gave similar results for terpenes and *n*-hexane for dissolving TAGs as well as sterols and tocopherols. However, when their dielectric constants are considered, terpenes are slightly more polar and show a higher dissociation force than *n*-hexane. In terms of safety, they have a higher flash point and are therefore less flammable and dangerous. Although the results in the laboratory scale are encouraging, the scalability of the terpene extraction process is limited. The main disadvantage of using terpenes is their high viscosity and density, and the higher energy consumption in solvent recovery by evaporation due to their higher boiling points (155–176 °C) and higher enthalpies of evaporation (37–39 kJ/mol) compared to *n*-hexane (Bp = 69 °C, ΔHvap = 29.74 kJ/mol) [66,76,96].

Heteroazeotropic distillation of the binary terpene-water mixture (about 50% *v*/*v*) leads to evaporation of the terpene and separation of the terpene-water distillation product by phase decantation. With this system, the solvent can be recovered at a lower temperature (about 97–98 °C at atmospheric pressure), and a high recovery rate of terpenes can be achieved (about 90% with 98% purity for α-pinene) [97]. The main studies on the extraction of vegetable oils with terpenes are listed in Table 7.

<table><caption>**Table 7.** Vegetable oils' extraction with terpenes.</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Solvent</th><th>Oil Yield (g/100 g DM Seed)</th><th>Ref.</th></tr></thead><tbody><tr><td>Soybean Sunflower</td><td>Crushed seeds</td><td>Soxhlet extraction</td><td>α-Pinene</td><td>21.167.2</td><td>[97]</td></tr><tr><td rowspan="3">Rapeseed</td><td rowspan="3">Ground</td><td rowspan="3">Soxhlet extraction</td><td><i>p</i>-Cymene</td><td>88.9 <sup>a</sup></td><td rowspan="3">[65]</td></tr><tr><td>α-Pinene</td><td>65.5 <sup>a</sup></td></tr><tr><td>Limonene</td><td>80.8 <sup>a</sup></td></tr><tr><td rowspan="2">Rapeseed</td><td rowspan="2">Coarsely ground</td><td rowspan="2">Soxhlet extraction</td><td><i>p</i>-Cymene</td><td>39.7</td><td rowspan="2">[66]</td></tr><tr><td>Limonene</td><td>37.0</td></tr><tr><td>Rapeseed</td><td>Ground</td><td>Maceration with intense mixing</td><td><i>cis</i>-Rich pinane (<i>cis</i>/trans: 7/3)</td><td>42.5</td><td>[98]</td></tr><tr><td>Rapeseed</td><td>Ground</td><td>Maceration with intense mixing Soxhlet extraction</td><td><i>p</i>-Menthane</td><td>37.140.5</td><td>[99]</td></tr></tbody></table>

<footnote><sup>a</sup> Oil extraction (% of total oil).</footnote>
Bertouche et al. [97] showed a higher yield with α-pinene compared to hexane when extracting soybean oil (21.1 vs. 19.1 g/100 g DM) and sunflower oil (67.2 vs. 52.6 g/100 g DM). The fatty acids extracted from both solvents were equivalent in terms of identified compounds and relative proportions. The higher boiling point of α-pinene reduces the viscosity of the oil, which leads to better diffusion through the matrix. Li et al. [65] evaluated the use of different terpenes (p-cymene, d-limonene and α-pinene) to extract rapeseed oil. The three solvents afforded higher oil extraction yields than n-hexane (88.9%, 80.8% and 65.5%, respectively, compared to 58.2% for n-hexane). The composition of the oil extracted with p-cymene contained more FFA and DAG than n-hexane-extracted oil, and the tocopherol and tocotrienol content was lower. Sicaire et al. [66] reported lower yields with p-cymene (39.71%) and d-limonene (36.94%) compared to hexane (46.71%) for rapeseed oil extraction. The percentage of TAG in the oil extracted with p-cymene (82.03%) and d-limonene (51.31%) was significantly lower than with hexane (99.12%). D-limonene and other unsaturated terpenes are readily oxidised in air and the resulting oxidation products are labelled as allergens. p-menthane (stable saturated derivative of d-limonene) and cis/trans-pinane: 7:3 (stable saturated derivative of α-pinene and β-pinene) were investigated as new saturated terpene solvents for oil extraction. Both solvents showed promising results with comparable oil yields to n-hexane (40.5 vs. 39.5 g/100 g DM for p-menthane and n-hexane, respectively [99], and 42.5 vs. 43.2 g/100 g DM for pinane and n-hexane, respectively [98]). The encouraging lab-scale results still need further research for the development of a pilot-scale terpene extraction process.

## 5.5. Alternative Hydrocarbon Solvents

Among the alternative hydrocarbon solvents, isohexane, cyclohexane and n-heptane have been proposed to replace hexane. These solvents also originate from the petrochemical industry, and although they are considered less harmful to health than hexane, comprehensive studies are lacking, especially on their chronic toxicity [100,101]. In contrast to n-hexane, the C6 isomers without n-hexane have not shown neurotoxic effects in animal experiments. This is attributed to the fact that no neurotoxic 1,4-diketones are formed during the metabolism of these hexane isomers. However, further studies on the chronic effects of exposure to these substances are needed (e.g., lack of information on teratogenicity and genotoxicity) [102].

Wan et al. (1995) compared five hydrocarbon solvents (heptane, isohexane, neohexane, cyclohexane and cyclopentane) with hexane for the extraction of cottonseed oil. Extraction with the six hydrocarbon solvents at their respective boiling points showed that normal paraffins (hexane and heptane) were more efficient and significantly better than cyclohexane or branched paraffins in extracting oil from cottonseed flakes. Cyclohexane extracted 95.4% of the oil, with respect to normal paraffins, while cyclopentane, isohexane and neohexane extracted 91.6%, 88.8% and 88.8%, respectively. The laboratory-scale extraction study (S/L ratio of 1 to 5.5 *w/* *w*, temperature 10–45 °C below the boiling point of the solvent) showed that hexane removed 100% of the oil from the flakes at 55 °C after a one-step extraction; heptane extracted 100% at 75 °C and 95.9% at 55 °C; and isohexane extracted 93.1% at 45 °C. The extraction efficiencies of cyclopentane, cyclohexane and neohexane were lower. Based on these results, heptane and isohexane are the most suitable alternative hydrocarbon solvents to replace hexane [103].

These two solvents were therefore investigated as potential substitutes for hexane in a cottonseed processing plant with a capacity of 300 tonnes/day. The extraction efficiencies of isohexane and heptane, as measured by extraction time and residual oil in the meal, colour of refined and bleached oil and solvent loss, were comparable to that of hexane. However, isohexane appears the best solution to replace hexane with minimal equipment adjustment. The higher boiling point of heptane increased the energy consumption of the desolventiser/toaster (D/T) system, reducing the daily production by 20–30%. Heptane would require more D/T capacity to achieve the same tonne production as hexane. In addition, the solvent loss of heptane was 12% higher than the average annual loss of hexane.
Bertouche et al. [97] showed a higher yield with α-pinene compared to hexane when extracting soybean oil (21.1 vs. 19.1 g/100 g DM) and sunflower oil (67.2 vs. 52.6 g/100 g DM). The fatty acids extracted from both solvents were equivalent in terms of identified compounds and relative proportions. The higher boiling point of α-pinene reduces the viscosity of the oil, which leads to better diffusion through the matrix. Li et al. [65] evaluated the use of different terpenes (p-cymene, d-limonene and α-pinene) to extract rapeseed oil. The three solvents afforded higher oil extraction yields than n-hexane (88.9%, 80.8% and 65.5%, respectively, compared to 58.2% for n-hexane). The composition of the oil extracted with p-cymene contained more FFA and DAG than n-hexane-extracted oil, and the tocopherol and tocotrienol content was lower. Sicaire et al. [66] reported lower yields with p-cymene (39.71%) and d-limonene (36.94%) compared to hexane (46.71%) for rapeseed oil extraction. The percentage of TAG in the oil extracted with p-cymene (82.03%) and d-limonene (51.31%) was significantly lower than with hexane (99.12%). D-limonene and other unsaturated terpenes are readily oxidised in air and the resulting oxidation products are labelled as allergens. p-menthane (stable saturated derivative of d-limonene) and cis/trans-pinane: 7:3 (stable saturated derivative of α-pinene and β-pinene) were investigated as new saturated terpene solvents for oil extraction. Both solvents showed promising results with comparable oil yields to n-hexane (40.5 vs. 39.5 g/100 g DM for p-menthane and n-hexane, respectively [99], and 42.5 vs. 43.2 g/100 g DM for pinane and n-hexane, respectively [98]). The encouraging lab-scale results still need further research for the development of a pilot-scale terpene extraction process.

## 5.5. Alternative Hydrocarbon Solvents

Among the alternative hydrocarbon solvents, isohexane, cyclohexane and n-heptane have been proposed to replace hexane. These solvents also originate from the petrochemical industry, and although they are considered less harmful to health than hexane, comprehensive studies are lacking, especially on their chronic toxicity [100,101]. In contrast to n-hexane, the C6 isomers without n-hexane have not shown neurotoxic effects in animal experiments. This is attributed to the fact that no neurotoxic 1,4-diketones are formed during the metabolism of these hexane isomers. However, further studies on the chronic effects of exposure to these substances are needed (e.g., lack of information on teratogenicity and genotoxicity) [102].

Wan et al. (1995) compared five hydrocarbon solvents (heptane, isohexane, neohexane, cyclohexane and cyclopentane) with hexane for the extraction of cottonseed oil. Extraction with the six hydrocarbon solvents at their respective boiling points showed that normal paraffins (hexane and heptane) were more efficient and significantly better than cyclohexane or branched paraffins in extracting oil from cottonseed flakes. Cyclohexane extracted 95.4% of the oil, with respect to normal paraffins, while cyclopentane, isohexane and neohexane extracted 91.6%, 88.8% and 88.8%, respectively. The laboratory-scale extraction study (S/L ratio of 1 to 5.5 *w/* *w*, temperature 10–45 °C below the boiling point of the solvent) showed that hexane removed 100% of the oil from the flakes at 55 °C after a one-step extraction; heptane extracted 100% at 75 °C and 95.9% at 55 °C; and isohexane extracted 93.1% at 45 °C. The extraction efficiencies of cyclopentane, cyclohexane and neohexane were lower. Based on these results, heptane and isohexane are the most suitable alternative hydrocarbon solvents to replace hexane [103].

These two solvents were therefore investigated as potential substitutes for hexane in a cottonseed processing plant with a capacity of 300 tonnes/day. The extraction efficiencies of isohexane and heptane, as measured by extraction time and residual oil in the meal, colour of refined and bleached oil and solvent loss, were comparable to that of hexane. However, isohexane appears the best solution to replace hexane with minimal equipment adjustment. The higher boiling point of heptane increased the energy consumption of the desolventiser/toaster (D/T) system, reducing the daily production by 20–30%. Heptane would require more D/T capacity to achieve the same tonne production as hexane. In addition, the solvent loss of heptane was 12% higher than the average annual loss of hexane.
However, due to the high boiling point of heptane, it can be used in a wide temperature range from ambient to 80 °C.

For isohexane, the D/T processing rate was 10–20% higher than for hexane. Isohexane boils at 55 °C and therefore has a fairly narrow operating temperature range. This lower extraction temperature may also affect the extraction efficiency of isohexane, as shown by the slightly higher residual oil content in the meal compared to hexane. However, with isohexane, daily production increased by more than 20% and gas consumption decreased by more than 40% [104]. Isohexane is more expensive than hexane because of the additional isomerisation process to produce it. The improved production and lower energy requirements for isohexane should offset the solvent cost difference. Isohexane was shown to be a viable choice to replace hexane in oilseed extraction, but the current need for solvents that are safe and not derived from petroleum makes this solvent out of step with the new frontiers defined by green extraction.

## 5.6. Other Organic Solvents

Acetone, ethyl acetate, cyclopentyl methyl ether (CPME) and dimethyl carbonate (DMC) have also been tested as alternative solvents to hexane for the extraction of lipids from oilseeds. Of these solvents, only acetone and ethyl acetate are included in Directive 2009/32/EC.

Acetone has been used mainly for the extraction of cotton oil, but its use in the extraction of oilseeds is limited. Acetone ranks between hexane and ethanol in terms of selectivity towards lipids. This solvent is completely miscible with water and its volatility is higher than that of hexane due to its lower boiling point, which is not an advantage from a safety point of view. Acetone has a relatively low latent heat of vaporisation (534 kJ/kg) and does not form an azeotrope with water, which allows easy recovery by distillation. As with ethanol and isopropyl alcohol, the oil extracted with acetone can be recovered by simply cooling the miscella. On the other hand, phospholipids are not soluble in acetone, which facilitates refining but does not allow recovery of the lecithin. The presence of water in acetone significantly reduces its ability to extract the oil. In addition, acetone can reduce the amount of the gossypol and aflatoxins in the meal. One of the main problems with acetone extraction is that it usually imparts a very unpleasant odour (cat's urine smell) to the meal. This smell is due to the reaction of mesityl oxide (which is obtained from acetone by condensation reactions) with hydrogen sulphide (coming from the breakdown of one or more of the sulphur-containing amino acids). Further drying of the flakes and solvent before extraction can reduce these unpleasant odours but significantly increases production costs [105].

Ethyl acetate is a colourless ester of acetic acid and ethanol, characterised by low toxicity and a pleasant odour. Ethyl acetate is one of the solvents approved for food production (Directive 2009/32/EU) and is safer and cheaper (33% less) than hexane. It has a similar boiling temperature and latent heat of vaporisation, but the main disadvantages of using ethyl acetate are its higher viscosity, density and dielectric point than hexane [106]. This solvent is partially miscible with water (8.5% at 20 °C) and forms with it an azeotrope with a boiling point lower than that of the pure compound (70 °C). Ethyl acetate seems to have been relatively little studied compared to other solvents. The researchers who have made comparisons seem to prefer isopropanol. However, as the economic environment has changed, the impact of energy costs may tip the balance in favour of a solvent that is easier to distil. Lohani et al. (2015) used an accelerated pressure extractor (at 1500 psi) to extract seed oils (rapeseed, flaxseed) using ethyl acetate and hexane for comparison. Ethyl acetate shows a lower viscosity and polarity under subcritical conditions, affording an oil yield (120 °C, 90 min) close to that of Soxhlet extraction with hexane, reaching 40.38% for rapeseed and 33.33% for flaxseed, compared to 42.96% and 35.62% with hexane, respectively. These results evidenced that the quality (heating value, density, viscosity, fatty acid profile and unsaponifiable matter) of the oils extracted from different oilseeds with ethyl acetate is almost equivalent to that of the oils extracted with hexane [106]. The use of solvent mixtures such as water/ethanol/ethyl acetate was investigated for the extraction
However, due to the high boiling point of heptane, it can be used in a wide temperature range from ambient to 80 °C.

For isohexane, the D/T processing rate was 10–20% higher than for hexane. Isohexane boils at 55 °C and therefore has a fairly narrow operating temperature range. This lower extraction temperature may also affect the extraction efficiency of isohexane, as shown by the slightly higher residual oil content in the meal compared to hexane. However, with isohexane, daily production increased by more than 20% and gas consumption decreased by more than 40% [104]. Isohexane is more expensive than hexane because of the additional isomerisation process to produce it. The improved production and lower energy requirements for isohexane should offset the solvent cost difference. Isohexane was shown to be a viable choice to replace hexane in oilseed extraction, but the current need for solvents that are safe and not derived from petroleum makes this solvent out of step with the new frontiers defined by green extraction.

## 5.6. *Other Organic Solvents*

Acetone, ethyl acetate, cyclopentyl methyl ether (CPME) and dimethyl carbonate (DMC) have also been tested as alternative solvents to hexane for the extraction of lipids from oilseeds. Of these solvents, only acetone and ethyl acetate are included in Directive 2009/32/EC.

Acetone has been used mainly for the extraction of cotton oil, but its use in the extraction of oilseeds is limited. Acetone ranks between hexane and ethanol in terms of selectivity towards lipids. This solvent is completely miscible with water and its volatility is higher than that of hexane due to its lower boiling point, which is not an advantage from a safety point of view. Acetone has a relatively low latent heat of vaporisation (534 kJ/kg) and does not form an azeotrope with water, which allows easy recovery by distillation. As with ethanol and isopropyl alcohol, the oil extracted with acetone can be recovered by simply cooling the miscella. On the other hand, phospholipids are not soluble in acetone, which facilitates refining but does not allow recovery of the lecithin. The presence of water in acetone significantly reduces its ability to extract the oil. In addition, acetone can reduce the amount of the gossypol and aflatoxins in the meal. One of the main problems with acetone extraction is that it usually imparts a very unpleasant odour (cat's urine smell) to the meal. This smell is due to the reaction of mesityl oxide (which is obtained from acetone by condensation reactions) with hydrogen sulphide (coming from the breakdown of one or more of the sulphur-containing amino acids). Further drying of the flakes and solvent before extraction can reduce these unpleasant odours but significantly increases production costs [105].

Ethyl acetate is a colourless ester of acetic acid and ethanol, characterised by low toxicity and a pleasant odour. Ethyl acetate is one of the solvents approved for food production (Directive 2009/32/EU) and is safer and cheaper (33% less) than hexane. It has a similar boiling temperature and latent heat of vaporisation, but the main disadvantages of using ethyl acetate are its higher viscosity, density and dielectric point than hexane [106]. This solvent is partially miscible with water (8.5% at 20 °C) and forms with it an azeotrope with a boiling point lower than that of the pure compound (70 °C). Ethyl acetate seems to have been relatively little studied compared to other solvents. The researchers who have made comparisons seem to prefer isopropanol. However, as the economic environment has changed, the impact of energy costs may tip the balance in favour of a solvent that is easier to distil. Lohani et al. (2015) used an accelerated pressure extractor (at 1500 psi) to extract seed oils (rapeseed, flaxseed) using ethyl acetate and hexane for comparison. Ethyl acetate shows a lower viscosity and polarity under subcritical conditions, affording an oil yield (120 °C, 90 min) close to that of Soxhlet extraction with hexane, reaching 40.38% for rapeseed and 33.33% for flaxseed, compared to 42.96% and 35.62% with hexane, respectively. These results evidenced that the quality (heating value, density, viscosity, fatty acid profile and unsaponifiable matter) of the oils extracted from different oilseeds with ethyl acetate is almost equivalent to that of the oils extracted with hexane [106]. The use of solvent mixtures such as water/ethanol/ethyl acetate was investigated for the extraction
of rapeseed oil, obtaining the maximum rate of oil yield (32.35 mg/mm) with the mixture containing 9.17%, 6.61% and 84.17% of water, ethanol and ethyl acetate, respectively [107].

Dimethyl carbonate (DMC) is a non-polar aprotic solvent with good miscibility with water (139 g/L), and it is readily biodegradable in the atmosphere and non-toxic. Currently, the most common commercial route to produce DMC is the oxidative carbonylation of methanol using O₂; however, new alternative processes have been developed to produce DMC from CO₂. DMC showed a good extraction performance for triglycerides and could also be used as a mild solvent for biocatalyst-assisted biodiesel conversion [108].

Although cyclopentyl methyl ether (CPME) is currently synthesized by petrochemical-based routes (using cyclopentene and methanol to obtain CPME with excellent atom economy), some bio-based alternatives paved the way for a future biogenic source of this solvent. In particular, several biomass-based processes focused on the production of chemical precursors that can be used for CPME production, such as cyclopentanone or cyclopentanol [109]. CPME is endowed with interesting properties, such as low solubility in water (1.1 g CPME/100 g), the formation of a positive azeotrope and low enthalpy of vaporisation, as well as stability in strongly acidic or basic conditions. CPME also shows low toxicity, a negligible peroxide formation rate and a narrow explosion range. However, it has a high flash point (compared to hexane) and a higher boiling temperature (106 °C). The main studies on the extraction of vegetable oils with other organic solvents are listed in Table 8.

<table><caption>**Table 8.** Vegetable oils' extraction with other organic solvents.</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Solvent</th><th>Oil Yield (g/100 g DM Seed)</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="3">Rapeseed</td><td rowspan="3">Coarsely ground</td><td rowspan="3">Soxhlet extraction</td><td>Ethyl acetate</td><td>42.8</td><td rowspan="3">[66]</td></tr><tr><td>CPME</td><td>41.5</td></tr><tr><td>DMC</td><td>42.8</td></tr><tr><td>Rapeseed</td><td rowspan="2">Ground</td><td rowspan="2">Pressurised solvent extraction</td><td rowspan="2">Ethyl acetate</td><td>40.4</td><td rowspan="2">[106]</td></tr><tr><td>Flaxseed</td><td>33.3</td></tr></tbody></table>

Sicaire et al. [66] investigated the use of ethyl acetate, CPME and DMC as alternatives to hexane for the extraction of rapeseed oil by conventional Soxhlet extraction. All these three solvents gave similar oil yields (42.83%, 42.77% and 41.53% for ethyl acetate, DMC and CPME, respectively), but these values were about 10% lower than those of hexane (46.71%). Furthermore, HPTLC analysis showed that the oils extracted with these solvents had different compositions. Only 88% of the extracted components were triglycerides (TAGs), compared to 99% of the oil extracted with hexane.

### 5.7. Enzymatic and Surfactant-Assisted Aqueous Extraction

The aqueous extraction of oil from oilseeds involves the diffusion of lipids in water and the subsequent formation of emulsions. The emulsified oil can be separated by demulsification, e.g., through temperature change or using enzymes. Aqueous oil extraction is an environmentally friendly and safe process that allows the simultaneous production of edible oil and protein isolates or concentrates in the same process. This process also brings economic advantages as it avoids the use of organic solvents, effectively removes antinutritive factors and toxins from the meal, and avoids the need for solvent evaporation and for degumming of crude oils [110]. The main limitations are the lower efficiency of the aqueous oil extraction, the necessary demulsification of the extract to recover the oil, the wastewater treatment, and the lowest protein content and quality of the meal. To improve the yield of the aqueous processes, enzymes have been used to facilitate the oil release. These mainly hydrolyse the structural polysaccharides (cellulose, hemicellulose and pectin) that form the cell wall, or the proteins of the cytoplasm or of the lipid bodies' membrane.

Selected enzymes were tested on different types of oilseeds and achieved much higher extraction yields than the original aqueous process (in some cases over 90%) [111]. Several
of rapeseed oil, obtaining the maximum rate of oil yield (32.35 mg/mm) with the mixture containing 9.17%, 6.61% and 84.17% of water, ethanol and ethyl acetate, respectively [107].

Dimethyl carbonate (DMC) is a non-polar aprotic solvent with good miscibility with water (139 g/L), and it is readily biodegradable in the atmosphere and non-toxic. Currently, the most common commercial route to produce DMC is the oxidative carbonylation of methanol using $O_2$; however, new alternative processes have been developed to produce DMC from $CO_2$. DMC showed a good extraction performance for triglycerides and could also be used as a mild solvent for biocatalyst-assisted biodiesel conversion [108].

Although cyclopentyl methyl ether (CPME) is currently synthesized by petrochemical-based routes (using cyclopentene and methanol to obtain CPME with excellent atom economy), some bio-based alternatives paved the way for a future biogenic source of this solvent. In particular, several biomass-based processes focused on the production of chemical precursors that can be used for CPME production, such as cyclopentanone or cyclopentanol [109]. CPME is endowed with interesting properties, such as low solubility in water (1.1 g CPME/100 g), the formation of a positive azeotrope and low enthalpy of vaporisation, as well as stability in strongly acidic or basic conditions. CPME also shows low toxicity, a negligible peroxide formation rate and a narrow explosion range. However, it has a high flash point (compared to hexane) and a higher boiling temperature (106 °C). The main studies on the extraction of vegetable oils with other organic solvents are listed in Table 8.

<table><caption>**Table 8.** Vegetable oils' extraction with other organic solvents.</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Solvent</th><th>Oil Yield (g/100 g DM Seed)</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="3">Rapeseed</td><td rowspan="3">Coarsely ground</td><td rowspan="3">Soxhlet extraction</td><td>Ethyl acetate</td><td>42.8</td><td rowspan="3">[66]</td></tr><tr><td>CPME</td><td>41.5</td></tr><tr><td>DMC</td><td>42.8</td></tr><tr><td>Rapeseed</td><td rowspan="2">Ground</td><td rowspan="2">Pressurised solvent extraction</td><td rowspan="2">Ethyl acetate</td><td>40.4</td><td rowspan="2">[106]</td></tr><tr><td>Flaxseed</td><td>33.3</td></tr></tbody></table>

Sicaire et al. [66] investigated the use of ethyl acetate, CPME and DMC as alternatives to hexane for the extraction of rapeseed oil by conventional Soxhlet extraction. All these three solvents gave similar oil yields (42.83%, 42.77% and 41.53% for ethyl acetate, DMC and CPME, respectively), but these values were about 10% lower than those of hexane (46.71%). Furthermore, HPTLC analysis showed that the oils extracted with these solvents had different compositions. Only 88% of the extracted components were triglycerides (TAGs), compared to 99% of the oil extracted with hexane.

### 5.7. Enzymatic and Surfactant-Assisted Aqueous Extraction

The aqueous extraction of oil from oilseeds involves the diffusion of lipids in water and the subsequent formation of emulsions. The emulsified oil can be separated by demulsification, e.g., through temperature change or using enzymes. Aqueous oil extraction is an environmentally friendly and safe process that allows the simultaneous production of edible oil and protein isolates or concentrates in the same process. This process also brings economic advantages as it avoids the use of organic solvents, effectively removes antinutritive factors and toxins from the meal, and avoids the need for solvent evaporation and for degumming of crude oils [110]. The main limitations are the lower efficiency of the aqueous oil extraction, the necessary demulsification of the extract to recover the oil, the wastewater treatment, and the lowest protein content and quality of the meal. To improve the yield of the aqueous processes, enzymes have been used to facilitate the oil release. These mainly hydrolyse the structural polysaccharides (cellulose, hemicellulose and pectin) that form the cell wall, or the proteins of the cytoplasm or of the lipid bodies' membrane.

Selected enzymes were tested on different types of oilseeds and achieved much higher extraction yields than the original aqueous process (in some cases over 90%) [111]. Several
factors influence the efficiency of aqueous enzyme extraction. Pre-treatment to reduce the size of oily materials by grinding or flaking increases the extraction yield due to there being better accessibility for the enzymes. The type of enzyme is a key factor in the whole process. The use of enzymes alone or in combination can be determined based on the structure of the oilseed, the types of enzymes and the experimental conditions. Due to the high protein content of soybean, the use of proteases led to more efficient oil extraction. In the extraction of rapeseed, whose cell wall consists mainly of pectin, treatment with pectinase resulted in an 85.9% increase in oil yield. The use of enzyme mixtures in some cases showed better performance than single enzymes, probably due to a synergistic effect. Since for each enzyme there is a certain optimal pH value, special attention must be paid to optimising this parameter. In general, higher extraction efficiency is obtained at pH values far from the isoelectric point. For example, low oil yields were achieved in soybeans, rapeseed and sunflower seed extractions at the isoelectric point due to low protein solubility.

Temperature also plays an important role as enzyme activities are generally effective at 45 °C or less and an increase in temperature can lead to the denaturation of proteins, reducing the release of oil from the oilseed. In general, increasing the concentration of the enzyme (up to about 1% enzyme by weight of raw material) until the active sites of the substrate are saturated results in greater degradation of the desired product and greater oil recovery. Further increases in enzyme concentration are not only not cost-effective, but can also induce off flavours, bitterness and caramelisation of sugars that can interfere with the oil extraction process. Adequate agitation allows partial breakdown of the mechanical barriers (cell wall) and uniform mixing of the entire contents of the reaction mixture. However, too intensive stirring is not only energetically costly but can also form stable emulsions that are difficult to separate. In general, oils obtained by aqueous enzyme extraction contained low levels of phosphatides and peroxides and similar levels of free fatty acids compared to oils extracted by conventional methods [112].

The optimisation of the entire process must not only focus on achieving the highest possible oil yield but must also consider the ease of splitting the resulting emulsion. In particular, some parameters such as oil droplet size and viscosity of the bulk emulsion are very useful in evaluating emulsion stability [111]. In addition, some proteins show an emulsifying capacity that generally increases during enzymatic proteolysis until a certain degree of hydrolysis is reached, and then decreases. Therefore, the extent of the enzymatic proteolytic action must be optimised to obtain both a higher oil yield and a less stable oil in water emulsion. The main studies on the extraction of vegetable oils with aqueous enzymatic extraction are listed in Table 9.

<table><caption>**Table 9.** Vegetable oils' extraction with aqueous enzymatic extraction.</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Enzymes</th><th>Oil Extraction (% of Total Oil)</th><th>Protein Extraction (% of Total Protein)</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="4">Soybean</td><td rowspan="4">Expanded</td><td rowspan="4">Thermo-controlled bath</td><td>Alcalase and Celluclast (adjusted pH)</td><td>26.8 <sup>a</sup></td><td>16.5</td><td rowspan="4">[113]</td></tr><tr><td>Alcalase and Viscozyme L (adjusted pH)</td><td>29.5 <sup>a</sup></td><td>20.2</td></tr><tr><td>Alcalase and Celluclast</td><td>20.6 <sup>a</sup></td><td>7.3</td></tr><tr><td>Alcalase and Viscozyme L</td><td>20.2 <sup>a</sup></td><td>10.7</td></tr><tr><td>Soybean</td><td>Dehulled, flaked and extruded</td><td>Thermo-controlled bath</td><td>Protex 51FP; Protex 6L or 7L</td><td>90–93</td><td>Nd</td><td>[114]</td></tr><tr><td>Soybean</td><td>Dehulled, flaked and extruded</td><td>Two-stage countercurrent extraction</td><td>Protex 6L</td><td>98</td><td>92</td><td>[112]</td></tr><tr><td>Rapeseed</td><td>Dehulled and cold-pressed</td><td>Thermo-controlled bath</td><td>Viscozyme L and Alcalase 2.4L</td><td>71.9</td><td>82.1</td><td>[115]</td></tr></tbody></table>
factors influence the efficiency of aqueous enzyme extraction. Pre-treatment to reduce the size of oily materials by grinding or flaking increases the extraction yield due to there being better accessibility for the enzymes. The type of enzyme is a key factor in the whole process. The use of enzymes alone or in combination can be determined based on the structure of the oilseed, the types of enzymes and the experimental conditions. Due to the high protein content of soybean, the use of proteases led to more efficient oil extraction. In the extraction of rapeseed, whose cell wall consists mainly of pectin, treatment with pectinase resulted in an 85.9% increase in oil yield. The use of enzyme mixtures in some cases showed better performance than single enzymes, probably due to a synergistic effect. Since for each enzyme there is a certain optimal pH value, special attention must be paid to optimising this parameter. In general, higher extraction efficiency is obtained at pH values far from the isoelectric point. For example, low oil yields were achieved in soybeans, rapeseed and sunflower seed extractions at the isoelectric point due to low protein solubility.

Temperature also plays an important role as enzyme activities are generally effective at 45 °C or less and an increase in temperature can lead to the denaturation of proteins, reducing the release of oil from the oilseed. In general, increasing the concentration of the enzyme (up to about 1% enzyme by weight of raw material) until the active sites of the substrate are saturated results in greater degradation of the desired product and greater oil recovery. Further increases in enzyme concentration are not only not cost-effective, but can also induce off flavours, bitterness and caramelisation of sugars that can interfere with the oil extraction process. Adequate agitation allows partial breakdown of the mechanical barriers (cell wall) and uniform mixing of the entire contents of the reaction mixture. However, too intensive stirring is not only energetically costly but can also form stable emulsions that are difficult to separate. In general, oils obtained by aqueous enzyme extraction contained low levels of phosphatides and peroxides and similar levels of free fatty acids compared to oils extracted by conventional methods [112].

The optimisation of the entire process must not only focus on achieving the highest possible oil yield but must also consider the ease of splitting the resulting emulsion. In particular, some parameters such as oil droplet size and viscosity of the bulk emulsion are very useful in evaluating emulsion stability [111]. In addition, some proteins show an emulsifying capacity that generally increases during enzymatic proteolysis until a certain degree of hydrolysis is reached, and then decreases. Therefore, the extent of the enzymatic proteolytic action must be optimised to obtain both a higher oil yield and a less stable oil in water emulsion. The main studies on the extraction of vegetable oils with aqueous enzymatic extraction are listed in Table 9.

<table><caption>**Table 9.** Vegetable oils' extraction with aqueous enzymatic extraction.</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Enzymes</th><th>Oil Extraction (% of Total Oil)</th><th>Protein Extraction (% of Total Protein)</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="4">Soybean</td><td rowspan="4">Expanded</td><td rowspan="4">Thermo-controlled bath</td><td>Alcalase and Celluclast (adjusted pH)</td><td>26.8 <sup>a</sup></td><td>16.5</td><td rowspan="4">[113]</td></tr><tr><td>Alcalase and Viscozyme L (adjusted pH)</td><td>29.5 <sup>a</sup></td><td>20.2</td></tr><tr><td>Alcalase and Celluclast</td><td>20.6 <sup>a</sup></td><td>7.3</td></tr><tr><td>Alcalase and Viscozyme L</td><td>20.2 <sup>a</sup></td><td>10.7</td></tr><tr><td>Soybean</td><td>Dehulled, flaked and extruded</td><td>Thermo-controlled bath</td><td>Protex 51FP; Protex 6L or 7L</td><td>90–93</td><td>Nd</td><td>[114]</td></tr><tr><td>Soybean</td><td>Dehulled, flaked and extruded</td><td>Two-stage countercurrent extraction</td><td>Protex 6L</td><td>98</td><td>92</td><td>[112]</td></tr><tr><td>Rapeseed</td><td>Dehulled and cold-pressed</td><td>Thermo-controlled bath</td><td>Viscozyme L and Alcalase 2.4L</td><td>71.9</td><td>82.1</td><td>[115]</td></tr></tbody></table>
<table><caption>**Table 9. Cont.**</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Enzymes</th><th>Oil Extraction (% of Total Oil)</th><th>Protein Extraction (% of Total Protein)</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="4">Canola</td><td rowspan="4">Ground</td><td rowspan="4">Thermo-controlled bath</td><td>Protex 7L</td><td>23.4<sup>a</sup></td><td>5.9<sup>a</sup></td><td rowspan="4">[116]</td></tr><tr><td>Multifect Pectinase FE</td><td>22.2<sup>a</sup></td><td>4.5<sup>a</sup></td></tr><tr><td>Multifect CX 13L</td><td>26.0<sup>a</sup></td><td>3.5<sup>a</sup></td></tr><tr><td>Natuzyme</td><td>22.7<sup>a</sup></td><td>4.3<sup>a</sup></td></tr><tr><td>Rapeseed</td><td>Dehulled</td><td>Thermo-controlled bath</td><td>Pectinase, Cellulase, and b-Glucanase</td><td>91.6, 73–76<sup>b</sup></td><td>80–83</td><td>[117]</td></tr><tr><td rowspan="4">Sunflower</td><td rowspan="4">Ground</td><td rowspan="4">Thermo-controlled bath</td><td>Protex 7L</td><td>28.3<sup>a</sup></td><td>4.3<sup>a</sup></td><td rowspan="4">[118]</td></tr><tr><td>Kemzyme</td><td>32.2<sup>a</sup></td><td>2.4<sup>a</sup></td></tr><tr><td>Alcalase 2.4L</td><td>26.6<sup>a</sup></td><td>3.1<sup>a</sup></td></tr><tr><td>Viscozyme L</td><td>39.7<sup>a</sup></td><td>3.7<sup>a</sup></td></tr><tr><td rowspan="2">Sunflower</td><td rowspan="2">Ground</td><td rowspan="2">Thermo-controlled bath</td><td>Natuzyme</td><td>35.5<sup>a</sup></td><td>2.7<sup>a</sup></td><td rowspan="2">[119]</td></tr><tr><td>Celluclast 1.5L (Not buffered)</td><td>17.8<sup>c</sup></td><td>Nd</td></tr><tr><td rowspan="2">Sunflower</td><td rowspan="2">Ground</td><td rowspan="2">Thermo-controlled bath</td><td>Celluclast 1.5L (buffered)</td><td>20.3<sup>c</sup></td><td>Nd</td><td rowspan="2">[120]</td></tr><tr><td>Alcalase 2.4L</td><td>16.4<sup>c</sup></td><td>Nd</td></tr></tbody></table>

Cheng et al. [121] assessed the environmental impacts of soybean oil production through an extruding-expelling process, hexane extraction and aqueous extraction. Hexane extraction showed the highest environmental impact due to the use of an organic solvent. Extrusion led to the highest greenhouse gas and criteria pollutant emissions due to the high energy required for heat-pressing processes. Finally, enzyme-assisted aqueous oil extraction had similar environmental impacts as the extrusion process but also reduced greenhouse gas and criteria pollutant emissions.

Enzyme-assisted oil extraction was also studied in combination with other green technologies such as microwaves and ultrasound. Enzyme-assisted extraction combined with microwaves, in addition to the general extractive benefits of microwaves, accelerates the enzyme reactions and delays the denaturalisation of the enzymes compared to conventional heating. Similarly, the use of ultrasound in combination with enzymatic extraction can increase oil yield and improve its quality and stability over time [4].

The necessary drying of the wet meal to be stored and the limited recycling of the enzymes represent additional costs in this process [4]. Immobilisation of the enzyme on a solid support is a possible strategy that allows many cycles of oil extraction and reduces costs [110]. Aqueous enzymatic extraction has good potential for simultaneous oil and protein recovery from oilseeds. However, the application of this technology is still hampered by the high cost for the production of enzymes, the lower extraction yields compared to solvent extraction, the long incubation time and the additional de-emulsification process.

Surfactant aqueous oil extraction (SAOE) is another aqueous extraction technique that seems to be a promising method on a laboratory scale and allows an oil yield of slightly more than 90%. The basic concept of SAEP is to use surfactants to reduce the interfacial tension (IFT) between oil and water (about 19–24 mN/m), making aqueous oil extraction possible [122,123]. Surfactants reduce the IFT due to their amphiphilic structure, which gives them the property to adsorb at the oil-water interface. Compared to conventional surfactants, extended surfactants contain polar intermediate groups, such as polypropylene oxide and/or polyethylene oxide, inserted between the hydrophilic head and the lipophilic tail. This molecular structure gives surfactants the ability to achieve a very low IFT (≤10⁻² mN/m). Lowering the IFT to such low values allows the formation of so-called microemulsions, which are ideal systems for the co-solubilisation of oil and water. In addition, salts (e.g., NaCl, CaCl₂ and KCl) and alcohols (from ethanol to medium-chain alcohols such as octanol and decanol) are often added to surfactant solutions to improve their interfacial properties and thus reduce both the critical micelle concentration and the surface or interfacial tension. SAE0 involves two main steps: extraction and

<footnote><sup>a</sup> Oil or protein yield (g/100 g DM seed); <sup>b</sup> free oil recovered (% of total oil); <sup>c</sup> free oil recovered (g/100 g DM seed).</footnote>
<table><caption>**Table 9. Cont.**</caption><thead><tr><th>Material</th><th>Preparation</th><th>Extraction Technique</th><th>Enzymes</th><th>Oil Extraction (% of Total Oil)</th><th>Protein Extraction (% of Total Protein)</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="4">Canola</td><td rowspan="4">Ground</td><td rowspan="4">Thermo-controlled bath</td><td>Protex 7L</td><td>23.4<sup>a</sup></td><td>5.9<sup>a</sup></td><td rowspan="4">[116]</td></tr><tr><td>Multifect Pectinase FE</td><td>22.2<sup>a</sup></td><td>4.5<sup>a</sup></td></tr><tr><td>Multifect CX 13L</td><td>26.0<sup>a</sup></td><td>3.5<sup>a</sup></td></tr><tr><td>Natuzyme</td><td>22.7<sup>a</sup></td><td>4.3<sup>a</sup></td></tr><tr><td>Rapeseed</td><td>Dehulled</td><td>Thermo-controlled bath</td><td>Pectinase, Cellulase, and b-Glucanase</td><td>91.6, 73–76<sup>b</sup></td><td>80–83</td><td>[117]</td></tr><tr><td rowspan="4">Sunflower</td><td rowspan="4">Ground</td><td rowspan="4">Thermo-controlled bath</td><td>Protex 7L</td><td>28.3<sup>a</sup></td><td>4.3<sup>a</sup></td><td rowspan="4">[118]</td></tr><tr><td>Kemzyme</td><td>32.2<sup>a</sup></td><td>2.4<sup>a</sup></td></tr><tr><td>Alcalase 2.4L</td><td>26.6<sup>a</sup></td><td>3.1<sup>a</sup></td></tr><tr><td>Viscozyme L</td><td>39.7<sup>a</sup></td><td>3.7<sup>a</sup></td></tr><tr><td rowspan="2">Sunflower</td><td rowspan="2">Ground</td><td rowspan="2">Thermo-controlled bath</td><td>Natuzyme</td><td>35.5<sup>a</sup></td><td>2.7<sup>a</sup></td><td rowspan="2">[119]</td></tr><tr><td>Celluclast 1.5L (Not buffered)</td><td>17.8<sup>c</sup></td><td>Nd</td></tr><tr><td rowspan="2">Sunflower</td><td rowspan="2">Ground</td><td rowspan="2">Thermo-controlled bath</td><td>Celluclast 1.5L (buffered)</td><td>20.3<sup>c</sup></td><td>Nd</td><td rowspan="2">[120]</td></tr><tr><td>Alcalase 2.4L</td><td>16.4<sup>c</sup></td><td>Nd</td></tr></tbody></table>

Cheng et al. [121] assessed the environmental impacts of soybean oil production through an extruding-expelling process, hexane extraction and aqueous extraction. Hexane extraction showed the highest environmental impact due to the use of an organic solvent. Extrusion led to the highest greenhouse gas and criteria pollutant emissions due to the high energy required for heat-pressing processes. Finally, enzyme-assisted aqueous oil extraction had similar environmental impacts as the extrusion process but also reduced greenhouse gas and criteria pollutant emissions.

Enzyme-assisted oil extraction was also studied in combination with other green technologies such as microwaves and ultrasound. Enzyme-assisted extraction combined with microwaves, in addition to the general extractive benefits of microwaves, accelerates the enzyme reactions and delays the denaturalisation of the enzymes compared to conventional heating. Similarly, the use of ultrasound in combination with enzymatic extraction can increase oil yield and improve its quality and stability over time [4].

The necessary drying of the wet meal to be stored and the limited recycling of the enzymes represent additional costs in this process [4]. Immobilisation of the enzyme on a solid support is a possible strategy that allows many cycles of oil extraction and reduces costs [110]. Aqueous enzymatic extraction has good potential for simultaneous oil and protein recovery from oilseeds. However, the application of this technology is still hampered by the high cost for the production of enzymes, the lower extraction yields compared to solvent extraction, the long incubation time and the additional de-emulsification process.

Surfactant aqueous oil extraction (SAOE) is another aqueous extraction technique that seems to be a promising method on a laboratory scale and allows an oil yield of slightly more than 90%. The basic concept of SAEP is to use surfactants to reduce the interfacial tension (IFT) between oil and water (about 19–24 mN/m), making aqueous oil extraction possible [122,123]. Surfactants reduce the IFT due to their amphiphilic structure, which gives them the property to adsorb at the oil-water interface. Compared to conventional surfactants, extended surfactants contain polar intermediate groups, such as polypropylene oxide and/or polyethylene oxide, inserted between the hydrophilic head and the lipophilic tail. This molecular structure gives surfactants the ability to achieve a very low IFT (≤10⁻² mN/m). Lowering the IFT to such low values allows the formation of so-called microemulsions, which are ideal systems for the co-solubilisation of oil and water. In addition, salts (e.g., NaCl, CaCl₂ and KCl) and alcohols (from ethanol to medium-chain alcohols such as octanol and decanol) are often added to surfactant solutions to improve their interfacial properties and thus reduce both the critical micelle concentration and the surface or interfacial tension. SAE0 involves two main steps: extraction and

<footnote><sup>a</sup> Oil or protein yield (g/100 g DM seed); <sup>b</sup> free oil recovered (% of total oil); <sup>c</sup> free oil recovered (g/100 g DM seed).</footnote>
separation. In the first step, the oil is extracted by immersing the matrix in the surfactant solution, usually with stirring. The oil and water form a turbid mixture that must be separated and purified to obtain a free oil phase. First, the solid phase is separated from the liquid phase by centrifugation. Then, in the liquid phase of interest, three layers are generally observed: (i) a layer of free oil, (ii) a microemulsion of oil in aqueous solution and (iii) the surfactant solution. In order to recover the oil from this liquid fraction, the system must be destabilised, whereby the oil-water-surfactant system is relatively stable. Gagnon et al. [122] described in detail the properties of extended surfactants, the state of the art of SAOE and the limitations that need to be overcome for the industrial application of this extraction method. The authors concluded that although initial results have shown this technology to be effective on a laboratory scale, it is still far from industrial application. From a technical point of view, the fractionation process of the oil-water-surfactant mixture, the recycling of the surfactants, the removal of salts and dehydration of the residual cake after extraction are the main critical points. In addition, the energy costs of the process should be taken into account and more information on the quality of the oil and solid fractions should be collected and compared with those obtained in the conventional way. Finally, the identification of new extended-like, bio-based and environmentally friendly surfactants would be desirable.

## 5.8. *Extraction Optimization Methods*

Oil extraction represents one of the most critical steps in oilseed processing, as it determines the quality and quantity of the oil obtained. By carefully optimising the extraction conditions for each extraction method, yield and quality can be improved. In addition, a proper optimisation process can save time and energy, thus reducing the cost of the entire oil extraction process. While conventional extraction methods are highly optimised today, especially at the industrial scale, new extraction technologies require extensive optimisation. Nde and Fonchaet [1] gave an overview of techniques for optimising the extraction of oil from plant material.

Most of these methods involve the use of statistical software to optimise control variables (e.g., extraction time, solids/solvent ratio, type of solvent, seed pre-treatment, etc.) to reduce costs and increase performance while avoiding degradation of product quality as much as possible. Two main methods for process optimisation can be identified in the literature: the single parameter optimisation method and the response surface methodology (RSM).

In single parameter optimisation, the best conditions for oil extraction are determined for a single factor at a time, holding the other factors constant. In this case, the ideal level of a factor that gives the best response is defined. However, this laborious procedure does not allow the interactions between factors to be considered. The RSM adopts several mathematical and statistical techniques to jointly model and analyse the effects of two or more process factors that have an impact on the desired response. A major step in optimization is screening the factors studied to have the significant effects of the analytical method. RSM reduces the number of experimental trials but maintains the expected accuracy and determines the responses for the interactive effect of multiple factors. However, RSM is limited by fitting the data to a second order polynomial, but not all systems with curvatures are compatible with this model. RSM is based on the relationship between product properties and regression equations that describe the correlations between input parameters and product properties. In many extraction procedures, the regression coefficient R² and/or the absolute/standard error of deviation are used to validate the models used. A model is considered good if R² > 0.70 and/or error < 10%. Among the many RSM software available, Design experts, Sigma Plot version 12.5, MATLAB version 5.0, IBM SPSS and Minitab version 15.5 have been commonly used to optimise oilseed extraction [1].

# 6. Conclusions and Perspectives

In the oilseed industry, the individual operations of storage, crushing and extraction are considered interdependent, and the impact of each step on the overall process must
separation. In the first step, the oil is extracted by immersing the matrix in the surfactant solution, usually with stirring. The oil and water form a turbid mixture that must be separated and purified to obtain a free oil phase. First, the solid phase is separated from the liquid phase by centrifugation. Then, in the liquid phase of interest, three layers are generally observed: (i) a layer of free oil, (ii) a microemulsion of oil in aqueous solution and (iii) the surfactant solution. In order to recover the oil from this liquid fraction, the system must be destabilised, whereby the oil-water-surfactant system is relatively stable. Gagnon et al. [122] described in detail the properties of extended surfactants, the state of the art of SAOE and the limitations that need to be overcome for the industrial application of this extraction method. The authors concluded that although initial results have shown this technology to be effective on a laboratory scale, it is still far from industrial application. From a technical point of view, the fractionation process of the oil-water-surfactant mixture, the recycling of the surfactants, the removal of salts and dehydration of the residual cake after extraction are the main critical points. In addition, the energy costs of the process should be taken into account and more information on the quality of the oil and solid fractions should be collected and compared with those obtained in the conventional way. Finally, the identification of new extended-like, bio-based and environmentally friendly surfactants would be desirable.

### 5.8. *Extraction Optimization Methods*

Oil extraction represents one of the most critical steps in oilseed processing, as it determines the quality and quantity of the oil obtained. By carefully optimising the extraction conditions for each extraction method, yield and quality can be improved. In addition, a proper optimisation process can save time and energy, thus reducing the cost of the entire oil extraction process. While conventional extraction methods are highly optimised today, especially at the industrial scale, new extraction technologies require extensive optimisation. Nde and Fonchaet [1] gave an overview of techniques for optimising the extraction of oil from plant material.

Most of these methods involve the use of statistical software to optimise control variables (e.g., extraction time, solids/solvent ratio, type of solvent, seed pre-treatment, etc.) to reduce costs and increase performance while avoiding degradation of product quality as much as possible. Two main methods for process optimisation can be identified in the literature: the single parameter optimisation method and the response surface methodology (RSM).

In single parameter optimisation, the best conditions for oil extraction are determined for a single factor at a time, holding the other factors constant. In this case, the ideal level of a factor that gives the best response is defined. However, this laborious procedure does not allow the interactions between factors to be considered. The RSM adopts several mathematical and statistical techniques to jointly model and analyse the effects of two or more process factors that have an impact on the desired response. A major step in optimization is screening the factors studied to have the significant effects of the analytical method. RSM reduces the number of experimental trials but maintains the expected accuracy and determines the responses for the interactive effect of multiple factors. However, RSM is limited by fitting the data to a second order polynomial, but not all systems with curvatures are compatible with this model. RSM is based on the relationship between product properties and regression equations that describe the correlations between input parameters and product properties. In many extraction procedures, the regression coefficient R² and/or the absolute/standard error of deviation are used to validate the models used. A model is considered good if R² > 0.70 and/or error < 10%. Among the many RSM software available, Design experts, Sigma Plot version 12.5, MATLAB version 5.0, IBM SPSS and Minitab version 15.5 have been commonly used to optimise oilseed extraction [1].

## 6. Conclusions and Perspectives

In the oilseed industry, the individual operations of storage, crushing and extraction are considered interdependent, and the impact of each step on the overall process must
be evaluated. From this point of view, the introduction of new technologies can signifi-
cantly improve the outcome of the different stages, and therefore the quality of the final
product. Since extraction remains the most critical step because of its impact on oil quality
and quantity, new methods of gas- and solvent-assisted mechanical extraction have been
proposed. The wide availability and low cost of petroleum-based solvents made them
the best candidate for any industrial application. However, their harmful effects on the
environment and health have recently stimulated research towards more environmentally
friendly, non-toxic, bio-based solvents. The technology readiness level (TRL) of green
solvents varies widely; while some have already been studied at semi-industrial or even
industrial scale (SFE-CO₂, ethanol and 2-MeOx), only laboratory-scale results are available
for others (aqueous enzymatic extraction, DMC, ethyl acetate, CPME). Further studies are
therefore necessary to assess the feasibility of using them on a larger scale. In addition, to
meet the huge demand from the industry, highly efficient and cheaper production methods
of green solvents need to be developed. The properties of the green extracts (composition,
stability, etc.) should also be further investigated to ensure their similarity or improvement
compared to conventional products obtained with petroleum-based solvents. Finally, fea-
tures and advantages of bio-based solvents should be included in educational programs
(students, extraction industry and government agencies) to raise awareness and promote
the transition to a greener economy. The industrial use of bio-based solvents will indeed
reduce human health risks and protect the environment, promoting the valorisation of
agricultural waste, according to a circular economy approach.

**Author Contributions:** Conceptualization, C.C.; validation, C.C., O.C., M.B., A.-S.F.-T. and S.T.; investigation, C.C.; resources, O.C., M.B., A.-S.F.-T. and S.T.; data curation, C.C., O.C., M.B., A.-S.F.-T. and S.T.; writing—original draft preparation, C.C., O.C., M.B., A.-S.F.-T. and S.T.; writing—review and editing, C.C., O.C., M.B., A.-S.F.-T. and S.T.; visualization, C.C.; supervision, O.C., M.B., A.-S.F.-T. and S.T.; project administration, C.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This research work was conducted as part of the EcoXtract® Protein project funded by the Future Investments Program (PIA) operated by the French Agency for Ecological Transition (ADEME).

**Conflicts of Interest:** The authors declare no conflict of interest.

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7. Dimić, E.; Premović, T.; Takači, A. Effects of the Contents of Impurities and Seed Hulls on the Quality of Cold-Pressed Sunflower Oil. *Czech J. Food Sci.* **2012**, *30*, 343–350. [CrossRef]

8. Bockisch, M. The Extraction of Vegetable Oils. In *Fats and Oils Handbook*; Elsevier: Amsterdam, The Netherlands, 1998; pp. 345–445, ISBN 978-0-9818936-0-0.

9. Barbosa-Cánovas, G.V.; Fontana, A.J., Jr.; Schmidt, S.J.; Labuza, T.P. *Water Activity in Foods: Fundamentals and Applications*; John Wiley & Sons: Hoboken, NJ, USA, 2020; ISBN 978-1-118-76831-0.

10. Dunford, N.T. *Food and Industrial Bioproducts and Bioprocessing*; John Wiley & Sons: Hoboken, NJ, USA, 2012; ISBN 978-0-8138-2105-4.

11. Rousseau, D. Vegetable Oils in Food Technology: Composition, Properties and Uses. *Food Res. Int.* **2003**, *36*, 754–755. [CrossRef]
be evaluated. From this point of view, the introduction of new technologies can signifi-
cantly improve the outcome of the different stages, and therefore the quality of the final
product. Since extraction remains the most critical step because of its impact on oil quality
and quantity, new methods of gas- and solvent-assisted mechanical extraction have been
proposed. The wide availability and low cost of petroleum-based solvents made them
the best candidate for any industrial application. However, their harmful effects on the
environment and health have recently stimulated research towards more environmentally
friendly, non-toxic, bio-based solvents. The technology readiness level (TRL) of green
solvents varies widely; while some have already been studied at semi-industrial or even
industrial scale (SFE-CO₂, ethanol and 2-MeOx), only laboratory-scale results are available
for others (aqueous enzymatic extraction, DMC, ethyl acetate, CPME). Further studies are
therefore necessary to assess the feasibility of using them on a larger scale. In addition, to
meet the huge demand from the industry, highly efficient and cheaper production methods
of green solvents need to be developed. The properties of the green extracts (composition,
stability, etc.) should also be further investigated to ensure their similarity or improvement
compared to conventional products obtained with petroleum-based solvents. Finally, fea-
tures and advantages of bio-based solvents should be included in educational programs
(students, extraction industry and government agencies) to raise awareness and promote
the transition to a greener economy. The industrial use of bio-based solvents will indeed
reduce human health risks and protect the environment, promoting the valorisation of
agricultural waste, according to a circular economy approach.

**Author Contributions:** Conceptualization, C.C.; validation, C.C., O.C., M.B., A.-S.F.-T. and S.T.; investigation, C.C.; resources, O.C., M.B., A.-S.F.-T. and S.T.; data curation, C.C., O.C., M.B., A.-S.F.-T. and S.T.; writing—original draft preparation, C.C., O.C., M.B., A.-S.F.-T. and S.T.; writing—review and editing, C.C., O.C., M.B., A.-S.F.-T. and S.T.; visualization, C.C.; supervision, O.C., M.B., A.-S.F.-T. and S.T.; project administration, C.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This research work was conducted as part of the EcoXtract® Protein project funded by the Future Investments Program (PIA) operated by the French Agency for Ecological Transition (ADEME).

**Conflicts of Interest:** The authors declare no conflict of interest.

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**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
121. Cheng, M.-H.; Sekhon, J.J.K.; Rosentrater, K.A.; Wang, T.; Jung, S.; Johnson, L.A. Environmental Impact Assessment of Soybean Oil Production: Extruding-Expelling Process, Hexane Extraction and Aqueous Extraction. *Food Bioprod. Process.* **2018**, *108*, 58–68.
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122. Gagnon, Y.; Mhemdi, H.; Delbecq, F.; Hecke, E.V. Extended Surfactants and Their Tailored Applications for Vegetable Oils Extraction: An Overview. *OCL* **2021**, *28*, 7. [CrossRef]
123. Do, L.D.; Sabatini, D.A. Aqueous Extended-Surfactant Based Method for Vegetable Oil Extraction: Proof of Concept. *J. Am. Oil Chem. Soc.* **2010**, *87*, 1211–1220. [CrossRef]

**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.