# Valorization Potential of Oilseed Cakes by Subcritical Water Extraction
## 1 Introduction  
Smart waste management is one of the most important priorities and is a focus of European policy. New solutions that could offer more economical, environmentally-friendly and sustainable technologies are, therefore, a focus of research and are oriented towards the development of sustainable technologies for valorization of agri-food and other biowaste, such as marine biowaste. By applying adequate technology, biowaste value can be increased by several fold by developing new utilization pathways and products that can be brought to the market. Vegetable oil has one of the highest trade shares (42%) of production of all agricultural commodities [1]. Considering its favourable nutritional value and chemical composition, it can be said that oil seed cakes remain underutilized after oil extraction. Due to the high production of oils worldwide from different oilseeds, the quantities of generated biomass are significant, requiring technology development for their valorisation. This biomass is rich in proteins,non-starch polysaccharides, antioxidants, fibres, vitamins and minerals, thus its value can be increased by applying an adequate valorisation approach and technology. In practice, most of the oilseed cakes are used as feed supplements, composts or fertilizers. Alternatively, this biomass can be used for energy production or for the production of protein isolates. Depending on the source, oilseed cakes might contain as high as 40 to 50% of crude proteins, 20 to 60% of dietary fibres and, independent of the source, all oilseed cakes are good sources of minerals (up to 900 mg/100 g), in particular Ca, K, Na, Fe and Zn [2]. Different methods have been investigated for the production of protein isolates (>90% protein) and concentrates (>65% protein) from oilseed cakes, such as enzymatic hydrolysis [3] or extraction [4]. \"Green\" extraction approaches for protein extraction from oilseed cakes have included the use of glycerol-choline chloride deep eutectic solvents [1]. Protein-rich precipitates obtained by adding water as an antisolvent contained up to 9% more proteins than starting raw material. There are also several reports on the use of oilseed cakes as solid-state fermentation substrates for the production of enzymes, antibiotics, antimicrobials and bioethanol [5; 6]. In addition to the valuable nutrients present in oilseed cakes, antimutrients such as glucoisonilates, protease and trypsin inhibitors, cyanogenic glycosides, phytates and lectins, may compromise the use of this by-product for human and animal nutrition. Most of these antimutrients are degraded by thermal treatment [7]. New industrial approaches tend to implement environmentally-friendly, safe, sustainable and economically viable technologies. In this respect, the treatment with subcritical water offers unique features owing to the safety of the used solvent, its availability and low price. In addition, excellent solvating properties and the high reactivity of hot compressed water may offer the unique possibility to simultaneously extract target compounds, while degrading unwanted ones. The high reactivity of hot compressed water is due to an increase in the water ionisation constant with heating, becoming a medium rich in H\({}^{+}\) and OH\({}^{+}\) ions and acting as acid/base catalyst [8]. This offers great possibilities to carry out various catalytic and hydrolytical reactions. The reactivity of water increases with its heating and in addition to hydrolysis many other decomposition reactions take place, such as oxidation, rearrangement, hydratation/dehydratation, elimination, cleavage and hydrogenation/dehydrogenation [9] offering the possibility to degrade or partially degrade raw material components. The oilseed cake extracts that were studied previously used different extraction techniques, such as enzymatic hydrolysis [10], deep eutectic solvents [1] or subcritical water/ethanol mixtures [11].