Phenolic compounds and antioxidant activities of moringa leaf processed by solid-state fermentation with A.niger

Moringa leaf (ML) is rich in phenolic acids, flavonoids, alkaloids, isothiocyanates, tannins and other phytochemicals with diverse biological effects. This study aimed to explore the effect of A.niger fermentation on phenolic compounds and antioxidant activities on the ML. The optimized fermentation conditions as follows: moisture content 60%, fermentation temperature 30°C, fermentation time 8 days and inoculation quantity 10⁷ spores/g fermentation sample. Filter paper enzyme (FPAse), carboxymethyl cellulase (CMCase), xylanase and β-glucosidase in ML increased significantly to 23.91 ± 0.85, 27.40 ± 1.12, 47.85 ± 0.91 and 2.45 ± 0.13 U/g DW respectively. Throughout 14 days of fermentation, a notable increase in total phenolic content (TPC), total flavonoid content (TFC) and antioxidant activity in soluble polyphenols (SPs) was observed. Specifically, TPC, TFC, DPPH radical scavenging activity (DRSA), trolox equivalent antioxidant capacity (TEAC), ferric reducing antioxidant power (FRAP) and hydrogen peroxide scavenging activity (HPSA) for ML were 146.74 ± 1.75 mg GAE/g DW, 2.12 ± 0.04 μmol CE/g DW, 32.56 ± 0.52 μmol TE/g D, 754.22 ± 3.24 μmol FE/g DW, 72.35 ± 0.25μmol FE/g DW, 105.79 ± 0.27μmol FE/g DW, respectively. In particular, the (-)-catechin, (-)-epicatechin were strongly positively correlated with the four antioxidant indexes. Moreover, fermentation with A. niger significantly increased the content of flavan-3-ols in ML. Furthermore, the inhibitory effects of α-amylase, α-glucosidase and lipase were significantly increased compared to unfermented moringa leaf (UF-ML) (P < 0.05). In summary, this study demonstrated that SSF was a new method for releasing polyphenols, which could provide a scientific basis for the further study of ML, as well as novel insights into the sustainable utilization of agricultural waste.