# Oil Press-Cakes and Meals Valorization through Circular Economy Approaches: A Review
## 5 Possible Ways to Capitalize the by-Products Obtained from the Oil Industry
### Extraction of Bioactive Compounds
#### 5.1.1 Protein Isolate, Concentrate and Hydrolysate  
Every year, the world population increases creating an increasing demand for food supply and proteins. Oil cakes/meals are a suitable alternative because they are a valuable source of proteins [111]. Plant proteins, recently, became an alternative for replacing those from animal sources because they are versatile, easily digestible, non-toxic, and nutritional sufficient. Moreover, the isolation process is less costly [121]. However, usually they lack sulfur amino acids and can negatively influence the flavor, color, and texture of foods [17]. Of all plants, proteins from rapeseed contain a large amount of S-amino acids (in an amount that exceeds the requirement for adults and children) and in terms of nutritional value can be compared with those from soy [122]. Napin and cruciferin are the dominating proteins groups (together 85-90%) in rapeseed. Proteins recovered from cold-pressed rape oil cake have higher recovery yield and emulsion properties compared with those extracted with hot-press and solvents. In the recovery process, when the heat was applied the recovery-yield improved slightly (\(p<0.05\), however only for solvent extracted rape oil cake. Conversely, the ability to stabilize emulsion decreased [22]. Soybean and groundnut contain proteins that can be easily absorbed, digested and are nutritionally equivalent to animal protein, the amount is 20-26% and covers the recommended daily dose for adults and children. Regarding the amino acid profile, soybean lacks methionine, and groundnut is rich in arginine. The proteins from linseeds are comparable with those from soya and are also rich in glutamic and aspartic acid and arginine. Furthermore, those from flaxseed have antifungal properties [22; 53; 123]. In hemp oil cake they are present two main proteins, histidine and albumin (a high amount of essential amino acids, especially arginine) comparable with those from egg and soy [17]. Press-cakes/meals are used to prepare protein isolates (with a content of proteins \(>90\%\)), concentrates (with 30-80%) and hydrolysates (resulted from the hydrolysis of protein isolates) [22]. By first solubilizing protein with alkali at a high pH, then isoelectric precipitation with acid, washing, and drying the isolates are prepared. The isolates present high water-holding capacity and emulsifying activity and stability. They find application an as emulsifier and functional food (food and texture retainer and food system size expander) [53]. Proteins isolated from canola by the alkali method are not suitable as food ingredients because they have poor technological properties and solubility. Furthermore, during the process an irreversible denaturation takes place. For this reason, they need chemical or enzymatic improvement [110]. Those obtained from flaxseed and sesame are highly soluble in acidic and alkaline pH with good functional properties such as water holding and fat absorption capacities, emulsifying and foaming properties, bulk density, and poor gelling capacity [68]. [PERSON] et al. used sunflower oil cake and different procedures to obtain protein concentrates and isolates with high solubility (75%). These were then subjected to extraction with various solvents (water, ethanol 70%, methanol 80%, acidic butanol, and sodium thiosulfate 0,1%) to remove phenolic compounds and/or isoelectric precipitation. The residual phenolic compounds (remaining due to the association with proteins) confer to the final products antioxidant properties (without affecting water solubility) and a strong dark coloration (that could limit their potential application) [101]. Hydrolysates resulted from the hydrolysis of protein isolates: the protein structure is modified resulting in improvements of their functionality, solubility, surface activity, hydration, and gelling properties.