Abstract: Mulberry juice processing residues are a potential source of polyphenols to be used as functional ingredients in foods, but their low stability to oxidation and thermal degradation limits their practical application. In this study, a 70% ethanol extract was prepared from mulberry pomace, a by-product of mulberry juice processing, and freeze-dried with and without cryoprotectants (maltodextrin and/or whey protein, and fermented litchi residues) to obtain seven different powders (P1-P7). Results showed that the total polyphenol content and antioxidant activity of the freeze-dried polyphenol extracts with each cryoprotectant except maltodextrin increased compared with the control group without cryoprotectant, while the (apparent) content of monomeric anthocyanins significantly decreased (P < 0.05). The cryoprotectants affected the extraction efficiency of anthocyanins by encapsulating them, and fermented litchi residues (P5) showed the strongest effect, followed by the mixtures of the freeze-dried extract and maltodextrin at ratios 1:2 (P4) and 1:4 (P7). The contents of total phenols and total anthocyanins, and antioxidant activity in the control group decreased by 19.27%, 18.16% and 11.41%, respectively, after being stored at 25 ℃ for one month. Maltodextrin and whey protein improved the storage stability of the extract, increasing the content of bioactive components and enhancing the antioxidant activity. In the control group, the contents of cyanidin-3-glucoside and cyanidin-3-rutinoside were (11.63 ± 0.13) and (4.14 ± 0.05) mg/g, respectively, which decreased by 26.72% and 16.53% compared with those before storage, respectively. However, the contents of the two main monomeric anthocyanins in the mixtures of the freeze-dried extract and maltodextrin at ratios 1:2 (P4) and 1:4 (P7) did not decrease significantly, indicating their good stability. During in vitro digestion, anthocyanin contents in each group decreased, and the retention rate of anthocyanins in the 1:1:1 freeze-dried extract + maltodextrin + whey protein (P2), 1:2 freeze-dried extract + maltodextrin (P3), and 1:4 freeze-dried extract + maltodextrin (P6) groups increased in contrast to the other groups, indicating improved digestive stability of the polyphenol extract from mulberry pomace.

Key words: mulberry residue; encapsul; polyphenols; stability