RECOVERY OF SOLUBLE AND INSOLUBLE PHENOLIC FRACTIONS FROM WALNUT PROCESSING RESIDUES AND THEIR ANTIOXIDANT ACTIVITY

The walnut processing industry has been growing significantly due to the consumer trend of choosing plant-based foods. As a result, significant amounts of walnut residues such as a press cake have been generated from both the walnut processing of plant-based beverage (WPB) and oil (WO). Therefore, within the framework of the circular economy promotion, the aim of this study was to valorize these residues through their phenolic characterization including the soluble (free, esterified and etherified) and insoluble phenolic fractions and the assessment of their antioxidant activity. Alkaline and acid hydrolysis was applied to recover the esterified and etherified soluble phenolic fractions, respectively. Likewise, an alkaline hydrolysis was carried out to obtain the insoluble fraction. Total phenolic compounds (TPC) and antioxidant activity of the different phenolic fractions were analyzed by Folin-Ciocalteu, FRAP and ORAC methods, respectively. The fractions of soluble phenols (free, esterified and etherified) had a higher total phenolic content with an average of 62.5 % in both WPB and WO residues with respect to the fraction of insoluble phenols (37.5 %). In addition, it turned out that 44 % of the phenols present in the walnut were not extracted during the processing of the plant-based beverage, therefore, remained in the WPB residue. Moreover, free phenolic fraction from WPB and WO residue presented the highest reduction power (887.56 µmol TE/g and 109.64 µmol TE/g respectively). Furthermore, the total soluble phenolic fraction presented a higher ORAC antioxidant activity representing 68.5 and 62 % from WPB and WO residues, respectively, in contrast with the insoluble phenolic fraction. There was a significant correlation between TPC and FRAP and ORAC values (p < 0.05). Hence, WPB and WO residues could be considered a potential source for the development of nutraceutical and/or antioxidant food additives.