Enhancement of phenolic antioxidants in industrial apple waste by fermentation with Aspergillus spp.

Apple peel is a waste from industrial juice processing that contains phenolic antioxidants. This study aimed to evaluate valorization of apple peel by fermentation with four different locally isolated Aspergillus spp. Fermentation with Aspergillus spp. augmented phenolic content and antioxidant activity of apple peel. A. niger ZDM2 and A. tubingensis ZDM1 were found to yield the highest level of total phenolic (1440 ± 37 and 1202 ± 88 mg GAE/100 g dm, respectively) and flavonoid contents (382 ± 47 and 495 ± 19 mg CE/100 g dm, respectively). Fermentation of apple peel with these species resulted in 3 and 5 times higher antioxidant activity measured by CUPRAC and DPPH methods compared to those of the unfermented sample, respectively. Quercetin and its glycosides were dominant in unfermented apple peel and their concentrations were reduced by mold fermentation. After fermentation, an isomer of taxifolin was produced in apple peel fermented by A. aculeatus ZGM6 and A. japonicus ZGM4, whereas isomers of eriodictyol and catechin were identified in apple peel fermented by A. niger ZDM2 and A. tubingensis ZDM1.