Valorization of Grape Seed Cake by Subcritical Water Extraction

Agricultural waste originating from the wine industry presents an environmental and economic issue. Grape seeds, a major constituent of grape pomace, are only partially valorized through oil extraction. The cake remaining after oil production is rich in valuable bioactive compounds. In this study, an advanced extraction technique, which utilizes subcritical water, was employed for bioactive compound recovery from defatted grape seed cakes. Extraction was performed in a nitrogen atmosphere (10 bar) at 130 °C and 170 °C. The extracts were characterized in terms of the total polyphenols, flavonoids, proteins and antioxidant activity. Detailed polyphenol profiles were determined using UHPLC Q-ToF MS analysis. Quantification of the individual sugars was performed by HPLC. The amino acid profile was determined using ion chromatography. The yield of phenolic acids was found to be higher at 170 °C (883 vs. 557 mg/100 g at 130 °C), while the flavonoid content was favored at 130 °C (596 vs. 185 mg/100 g at 170 °C). The total protein, essential amino acid and xylo-oligosaccharide content was higher at 170 °C. The obtained results show that the use of water as the extraction solvent in subcritical conditions is a promising technique for the environmentally friendly valorization of grape seed cakes and biowaste in general.