Fungal Solid-State Fermentation as a Strategy to Release Polyphenols from Orange Peel Waste

Orange peel is an abundant by-product of the citrus industry and a significant source of phenolic compounds with potential applications in the food, nutraceutical, and pharmaceutical industries. However, many of these compounds are bound or glycosylated, with low bioavailability. The objective of this study was to release and biotransform polyphenols from orange peels through solid-state fermentation using Aspergillus niger. A Box–Hunter and Hunter experimental design was employed in which the inoculum size (1 × 106–1 × 108 spores/g) and the concentrations of KCl and MgSO4 (0.76–1.56 g/L) were evaluated as independent factors to assess their effects on the release of hydrolyzable and condensed tannins. After 12 days of fermentation at 28 °C, the resulting extracts were analyzed using colorimetric methods and HPLC-ESI-MS analysis. The results showed significant increases in tannin release, reaching up to 220.63 mg CE/g of condensed tannins, and compounds such as ferulic acid, epicatechin, and quercetin derivatives were identified in the extracts. In conclusion, solid-state fermentation is a strategy for valorizing citrus waste and generating polyphenolic extracts with potential functional and industrial value.