Έρευνα και εφαρμογή των τεχνολογιών υπερυψηλής πίεσης και παλμικών ηλεκτρικών πεδίων στην παραγωγή υψηλής ποιότητας φυτικών προϊόντων και στην αξιοποίηση των παραπροϊόντων τους
Authors: Ανδρέου Βαρβάρα 
Published: September 2021

The present phD thesis, titled “Research and application of High Pressure (HP) and Pulsed Electric Field technologies (PEF) for the production of high quality plant origin products and the valorization of their by-products”, aims to study the application of these two food processing technologies for the production of plant-based products at a higher yield of the final product and with enhanced quality characteristics, as well as for the recovery of the bioactive ingredients from their by-products. Specifically, this thesis explored the effect of the two processes as pretreatments at several steps of the production line of industrial tomato processing and olive oil production, as well as on the extraction of bioactive compounds from their by-products. On the first part, HP and PEF pretreatments were applied at several steps of the production line of tomato products and their effect on the yield of each process and quality of the final products was studied. On the first section, PEF and HP were applied on tomato juice before the concentration in order to produce concentrated tomato products with higher yield and improved quality characteristics through the selective inactivation of polygalactorunase (PG) and the preservation of pectinmethylesterase (PME). Initially was performed a complete kinetic study of PME and PG inactivation caused by thermal treatment (55-77oC, 0-60min), HP (200-800MPa, 45-75 oC, 0-60min) and PEF (4.0-12.5kV/cm, 0-12ms, 15μs of pulse width and frequency of 300 Hz) treatments. The behavior of each enzyme was described after each treatment with appropriate mathematical models. The inactivation of PME and PG was described with a first order kinetic model for thermal and HP treatment and with a fractional first order model for PEF treatment. PG appeared more heat resistant than PME. PME proved itself to be quite resistant to the application of HP, displaying an antagonistic effect of pressure and temperature on the PME inactivation. On the contrary, HP inactivation of PG revealed the pressure sensitivity of the enzyme, while a synergistic effect of pressure and temperature was observed. Furthermore, PG was more sensitive than PME regarding PEF treatment, since at all studied conditions the characteristic damage time τ (time required for the 50% of the enzyme inactivation) of PG was lower. Afterwards, when optimum treatment conditions of HP (500MPa, 55οC, 10 min) and PEF (8kV/cm, 6ms) were applied on tomato juice was resulted in yield increase of concentrated tomato products in terms of producing products of higher viscosity with less water evaporation, mainly attributed to the action of remaining PME. By using concentration under vacuum (60 οC, 0.1 bar), it was observed that the final concentrated tomato products with the same viscoelastic properties (5000cp) that were produced after applying HP and PEF at optimal conditions, had a lower total soluble solids content compared with the untreated tomato products, while less concentration time was required. The yields obtained from the untreated (conventional concentration), HP and PEF treated samples were 30,4, 41,0, 34,6% respectively. The effect of each pretreatment to the quality degradation of the final products was also positive, since the shelf life of untreated and the HP and PEF treated concentrated tomato products at 25°C was determined to be 266, 393 and 248 d respectively. On the second section, PEF treatment was applied on whole tomatoes in order to assist or replace the conventional peeling process. Initially, whole unpeeled tomatoes were PEF pretreated (0,5-1,5 kV/cm, 0-8000 pulses, 20 Hz and pulse width of 15 μs) to determine the required work for peel detachment from the rest tomato fruit, as well as the firmness and the lycopene content of the final peeled tomatoes. In order to compare PEF treatment with other conventional peeling methods, fresh tomatoes were subjected to blanching for 1 min at 70 °C and for 2 min with steam at 100 °C. It was observed that the work for peel detachment for blanched and steamed tomatoes was 54.7% and 97.4% lower compared to untreated, respectively. The detachment work achieved after PEF processing at 1.5 kV/cm electric field strength for 500 pulses was 72.3% lower than untreated. Furthermore, the three peeling processes showed no significant differences regarding the fruit losses (<1% w/w). PEF processing decreased the firmness of final peeled tomatoes by only 21.6% compared to untreated, while steam peeled samples reduced the firmness of the final peeled tomatoes by 82.7%, compared to untreated. The lycopene content of PEF pretreated peeled tomatoes remained unaffected (5,29 mg/100g tomatoes) in compare to the untreated sample (5,32 mg/100g tomatoes). This could be attributed to the reduced exposure to heat prevents with PEF treatment. Additionally, PEF processing was applied on chopped tomatoes in the first step of juicing. Electric field strengths ranged from 0.5 to 2.5 kV/cm for 0- 5000pulses, a frequency of 20 Hz and pulse width of 15 μs. Juice extraction was performed on a lab-scale paddle type extractor. The effects of PEF on the juice yield were studied, while the juices from the first step were assessed in terms of quality attributes, such as colour, viscosity and consistency and compared with untreated tomato juice. Secondly, the tomato residues were reprocessed in order to improve the overall juice yield. In this case, PEF processing was applied on the tomato residues from the first step (seeds, peels and a fraction of unpressed tomato flesh) and the overall juice yield was studied in terms of quality of the final juices such as colour, viscosity and Bostwick consistency. At the first step of juicing, the highest yield achieved was 89.2% after PEF processing compared to 71.4% for untreated samples. Regardless of the yield increase, for the most intense PEF conditions led to a significant degradation of the final products on viscosity (<400cp) and Bostwick consistency (>20cm). Thus, choosing milder PEF conditions (for values of Z index ranging from 0.5 to 0.7) the final tomato juice had the optimum viscosity, while the juice yield increase was approximately 10–15% higher compared to untreated samples with improved viscosity (550-700cp). At this PEF treatment conditions the lycopene content of tomato juice was 3,82 mg/100g (22% higher in compare to the untreated sample). At the second step of juicing, the overall tomato juice yield (for both steps) was 82.2% for untreated samples. Reprocessing of tomato residues from the first juicing step with the aim of increasing the yield of tomato juice can be detrimental to the final juice quality. The juice that normally results from the second step of tomato juicing has very high viscosity (0.95 Pa s that corresponds to 10.9 cm Bostwick consistency) and when mixed with the juice resulting from the first juicing, leads to consistencies that are unacceptable by industrial standards (0.55–0.70 Pa s). When PEF processing is applied the observed trend is reversed. PEF processing of tomato before the first juicing step dramatically decreases the juice viscosity, leading to unacceptably low consistencies (values under 0.40 Pa s). However, when PEF treatment is applied to the tomato wastes before the second juicing step a yield increase is observed and the viscosity is higher than that of untreated juice. When the juices from the first juicing (untreated) and second juicing (PEF treated) are finally mixed (at a ratio of approximately 6:1), the overall juice yield is higher (approximately 10%) and the viscosity of the final juice falls within the industrially accepted standard (0.45 Pa s that corresponds to 14.2 cm Bostwick consistency). PEF technology could potentially be used as a pretreatment on the tomato waste in order to extract high-added value compounds. Tomato waste was subjected at different PEF conditions. Electric field strengths ranged from 1.0 to 5.0 kV/cm for 0–1500 pulses, a frequency of 20 Hz and pulse width of 15 μs. Afterwards the bioactive compounds were extracted from tomato by-products with a solid-liquid extraction using ethanol-water solvents. The bioactive compounds extracted were total carotenoids, total phenolics total proteins and the antioxidant activity of the extracts of tomato by-products. More intense PEF conditions resulted in improved extractability of carotenoid compounds, while the increase ranged from 35.9% to 56.4% compared to the conventionally processed extract (11,36 mg/100g) leading to 14.31 mg/100g of extracted lycopene compared with the conventionally processed samples that had 9.84 mg/100 g of extracted lycopene. PEF assisted extraction increased significantly the yield of phenolic compounds, that doubled (56.16 mg GAE/kg) for a PEF treatment at 2 kV/cm for 700 pulses, therefore resulting in a significantly higher antioxidant activity. On the second part, PEF and HP pretreatments were applied at several steps of the olive oil production line and at the valorization of olive pomace, which is the solid by-product generated after the separation step. The olives and olive pomace used came from olives Tsounati variety at the second ripening stage. On the first section was studied the effect of the two technologies as pretreatments at the malaxation step to increase the olive oil yield and improve the quality characteristics of the final olive oils. Additionally, the application of HP and PEF as pretreatments on olive paste, combined with milder malaxalation conditions (time and temperature) was studied, without impairing the oil extraction yield, while improving olive oil quality and sensorial characteristics. Olive paste was subjected to different PEF (0,5-2,0 kV/cm) and HP (100-600 MPa) process conditions and then malaxated at different malaxation conditions (15-40°C, 30 min) for oil extraction. The extraction yield and quality characteristics such as acidity, peroxide value, K232 and K270 indices, total phenolic compounds, chlorophylls and antioxidant capacity were determined for each sample. Initially, based on the kinetic study on the conventional olive oil production process at different malaxation conditions (duration and temperature), the optimum malaxation conditions (30 °C and 30 min), were selected based on the olive oil yield while the quality characteristics were not affected. These conditions led to a high olive oil extraction yield (22.5%), while preserving the quality characteristics of the oil. Malaxation at 15 °C, led to significantly lower olive oil yield and phenolic content, compared with malaxation at 30 °C. An increase in olive oil yield was observed after PEF and HP pretreatments at all the conditions studied. The yield increase of PEF pretreated samples was approximately up to 20.1% and the phenolic content increase was approximately up to 23.1% compared to untreated sample depending on the process and malaxation conditions. The yield increase of HP pretreated samples was approximately up to 18.6 % and the phenolic content increase was approximately up to 20.7% compared to untreated sample (olive oil yield: 22.5% and phenolic content: 743 mg GAE/kg) depending on the process and malaxation conditions. As a result, the highest olive oil yield increase was observed, after using both pretreatments combined with 15°C malaxation temperature. Two pretreatments enhanced the mass transfer phenomena during the malaxation step at 15°C leading to significant higher oil yields (almost equal to the ones obtained by the untreated sample, malaxated at 30°C), while increasing also the phenolic content without impairing the quality characteristics of the final oils. Nevertheless, malaxation temperature was a major parameter on the oil enrichment with bioactive compounds. The maximum phenolic content of the final oil (after combined effect of PEF and HP with 15°C malaxation temperature) was 516 and 501 mg GAE/kg respectively, while the phenolic content of the conventionally extracted oil at 30 °C malaxation temperature was 743 mg GAE/kg. The olive oil yield increase and the phenolic content of the oils were correlated mathematically with the cell disintegration index Z with a sigmoidal model for each pretreatment, proving that the yield is strongly depended on the cell disruption, regardless of the way that this disruption was achieved (field intensity, pulses, applied pressure and time). PEF and HP assisted malaxation was optimized through olive oil yield and phenolic content, using Box-Behnken design with three parameters (temperature, malaxation time and intensity of the PEF and HP treatments), to explore their potential use in order to decrease malaxation time and temperature, while at the same time leading to the maximum possible oil yield and preserving the oil’s quality characteristics. PEF pretreatment with Z=0.7, combined with T=26°C, t=37 min of malaxation conditions, led to the highest oil yield (25.02%) with a phenolic content of 832,05 mg GAE/kg oil. HP pretreatment with Z=0.7, combined with T=26°C, t=37 min of malaxation conditions, led to the highest oil yield (25.72%) with a phenolic content of 839,75 mg GAE/kg oil.A comparison of the application of HP (600 MPa, 5 min, Ζ=0,7) and PEF (5 kV/cm, 100 pulses, Ζ=0,7) pretreatments on olive paste at the optimum selected conditions with the conventional malaxation process, was conducted, based on the oil yield, the content of bioactive compounds, the quality and organoleptic characteristics. It was observed that PEF and HP pretreatments at the optimum conditions at 30°C, increased the oil yield by 8.3 and 7.0% respectively compared to untreated (22.5%), while in half of the malaxation time (15min), the oil yield was 23.13 and 22.84% respectively (approximately equal to untreated at 30 °C). Additionally, PEF and HP assisted malaxation at lower temperature (22°C) led to an oil yield of 23.98 and 22.84% respectively (almost equal to the one obtained from conventional malaxation). At 30°C malaxation temperature with PEF and HP pretreatments led to a 9.3 and 4.9% phenolic content increase respectively, compared to untreated (745 mg GAE/kg), while at 15min of malaxation time the phenolic content of the oil became equal to one obtained from the conventional malaxation. Furthermore, PEF and HP pretreatments increased the individual phenolic compounds (hydroxytyrosol: 68 and 44% increase, tyrosol: 63 and 41.6% increase, respectively) and a-tocopherol content of the final oils by 59 and 65%% (66,9 mg/kg και 66,5 mg/kg) respectively. Also, malaxation conditions at 30°C and 15 min combined with HP and PEF led to an increase of all individual phenolic compounds (increase up to 10%), compared to untreated olive oil. Malaxation temperature at 22°C, combined with the two pretreatments, led to almost equal individual phenolic content of the oil as the one obtained from the conventional process. As expected, the induction period of olive oil was strongly correlated to phenolic and a-tocopherol content. Both pretreatments led to oils with increased oxidative stability (induction period:178 and 197 h, for PEF and HP pretreated samples, respectively) compared to untreated (121h). Both pretreatments had no significant effect on the organoleptic characteristics of the final oils. Oils malaxated at 22°C had a slightly more fruity taste but less pungent and bitter taste compared to oils malaxated at 30°C. The stronger fruity taste could be attributed to the lower malaxation temperature used, as well as the lower bitterness related with lower phenolic content of oils malaxated at 22°C. On the second section, PEF and HP assisted extraction of intracellular compounds from olive pomace was studied using ethanol-water solvent, in order to improve the extractability of high-value bioactive compounds with lower energy consumption. Olive pomace, (by-product of two-phase centrifugation system), was dried under vacuum and then sieved (2mm). Experiments were conducted in different extraction temperatures (25, 40 and 60 °C) and ethanol concentrations (0, 25, 50 and 70%) in order to select the mildest extraction conditions with the highest extraction yield of intracellular compounds. The concentration of total polyphenols (mg GAE/L), proteins (mg/L) and antioxidant capacity (mM Trolox equivalent) of the extracts were measured. For each extracted group of compounds, the optimum yield was achieved for a 50% ethanol-water solution, at 25 °C for 60 min. The phenolic and protein content extracted during 60min ranged between 1.43-2.64 g GAE/100g d.w. and 1.25-3.98 g/100g d.w. of olive pomace, respectively. Olive pomace was treated with PEF (0.5-2kV/cm, 50-2000pulses, 20Hz and 15μs pulse width) or HP (200-600MPa, 1-60min) processing. All the extracts were analyzed for total phenolic compounds, proteins content and antioxidant activity and compared with samples from conventional extraction. The phenolic and protein content for PEF pretreated samples ranged between 1.53 and 3.34 g GAE/100g d.w. and 2,06 έως 5,72 g /100g d.w., respectively. The phenolic and protein content for HP pretreated samples ranged between 1.88-2.94 g GAE/100g d.w. and 1,30 έως 4,69 g/100g d.w., respectively. More intense PEF or HP conditions applied, resulted in higher extraction yield of intracellular compounds. For pressures >400 MPa, the extracted protein amount was decreased, due to a possible denaturation induced by the applied pressure. A comparison of the three extraction methods led to the conclusion that PEF assisted extraction was the most effective for phenolic recovery, compared to the HP treatment at low energy inputs (<6,4 kJ/kg), while the opposite was observed after using higher energy input (>400MPa). The total phenolic content of the extracts obtained after PEF and HP treatments was by 32% and 34% higher than the one obtained conventionally. PEF and HP pretreated samples had 21% (3,44 mMTE) and 15% (3,28 mMTE) higher antioxidant capacity, respectively, compared to conventionally treated sample (2,84 mMTE). Regarding protein recovery, HP treatment increased significantly the protein yield (up to 45% increase) for pressures up to 200MPa compared to conventionally treated sample. On the contrary, more intense PEF conditions (increased energy input) increased the protein yield up to 32% compared to conventionally treated sample. Both pretreatments reduced the extraction completion time t98 (describes the required extraction time needed to recover from pretreated samples the 98% of the phenolic or protein content isolated from untreated sample after 60 min of conventional extraction) to 5 and 12 min, respectively. More intense PEF or HP conditions (>24 kJ/kg) reduced the extraction time t98 less than 1 min, meaning that the extracts had the preferred content immediately after the pretreatments, without any further extraction. PEF and HP assisted extraction was optimized through total and individual phenolic content regarding the extraction time and the intensity of the pretreatments conditions. The amount of total and individual phenolic content (hydroxytyrosol, tyrosol, oleuropein, kampferol, luteolin and ferulic acid) obtained from PEF pretreatment showed a maximum value for less than 60min extraction time (30-40min) and for energy inputs 11-14.5 kJ/kg (3-5kV/cm). The maximum value of extracted polyphenols for HP pretreated samples was reached for less than 60min extraction times (30-40min) using energy input 15.0-36,1 kJ/kg (400-600 MPa). Finally, an optimization for the conventional extraction, assisted by PEF (5 kV/cm, 11 kJ/kg) and HP (350 MPa, 10 min, 15 kJ/kg) was performed, through the extraction time and solvent volume. Regarding the conventional extraction, in order to obtain the maximum yield, 2.78 g GAE/100g d.w. of phenolic content, 4,10 g/100 g d.w. of protein content and an antioxidant activity of 3,27 mM TE, 50-60 min extraction time and 40-50% ethanol content is required. Regarding the PEF assisted extraction in order to obtain the maximum yields, 3.13 g GAE/100g d.w. of phenolic content, 5.40 g/100 g d.w. of protein content and an antioxidant activity of 3,83 mM TE, 40-60 min of extraction time and 35-50% ethanol is required. Regarding the HP assisted extraction in order to obtain the maximum yields, 3.18 g GAE/100g d.w. of phenolic content, 5.13 g/100 g d.w. of protein content and an antioxidant activity of 3,37 mM TE, t=45-60 min of extraction time and 40-50% ethanol is required. To conclude, PEF and HP proved to be one of the most promising processes that could be used as pretreatments for the production of plant-based products (tomato products and olive oil) with increased yield and improved quality characteristics, as well as for effective valorazisation of their by-products. Their use at several steps of the production line led to (a) yield increase, (b) products with improved quality characteristics, enriched with bioactive and antioxidant compounds and (c) the reduction of the processing time and energy consumption of the whole production process. Also, their use as pretreatments to assist or replace the conventional extraction of bioactive compounds from by-products is an approach of high interest that offers a potential industrial use.