Antioxidant activity increased due to dynamic changes of flavonoids in orange peel during Aspergillus niger fermentation

Rich in flavonoids, orange peel (OP) converted into tangerine peel (TP) after long-term storage and metabolism by surface microorganisms. The antioxidant activity of OP increased during ageing, but underlying mechanisms were still not clear. The current study simulated the ageing process by inoculating Aspergillus niger on OP. Antioxidant activity, total flavonoids, enzyme activities and hesperidin content were analysed at different fermentation times. The results showed that antioxidant activity was constantly increased and relatively consistent with total flavonoids. The maximum growth rate of antioxidant activity and total flavonoids appeared in 24-48 h. Total flavonoids content increased from 4.68 +/- 0.04 to 8.32 +/- 0.15 mg RE/g and positively co-relate with cellulase and beta-glucosidase activities. During fermentation, nine flavonoids content accumulated, except that hesperidin began to convert into hesperetin after 72 h. In summary, these findings suggested that A. niger fermentation, which led to an increase in antioxidant activity, promoted the accumulation of flavonoids and transformation of hesperidin in OP. This study provided a microbial method for the effective utilisation of OP pomace.