Biotransformation-Driven Enhancement of Tannins and Bioactivity in Opuntia ficus-indica Peel via Solid-State Fermentation

Agro-industrial residues such as prickly pear peel represent an underutilized source of bioactive compounds. However, comparative evidence on green extraction versus biotransformation strategies remains limited. This study evaluated solid-state fermentation (SSF), ultrasound-assisted extraction (UAE), and microwave-assisted extraction (MAE) for tannin recovery from Opuntia ficus-indica peel. SSF using Aspergillus niger significantly enhanced condensed tannins (>50 mg/g) and hydrolyzable tannins (~7 mg/g), outperforming UAE and MAE. This improvement was associated with fungal-mediated cell wall degradation and metabolic transformation. SSF extracts also showed superior antioxidant activity (DPPH, ABTS, and FRAP) and exclusive antimicrobial activity against Escherichia coli (4.5 mm inhibition zone). HPLC analysis revealed increased phenolic diversity, with rhamnetin as the predominant metabolite. These findings demonstrate that SSF is not only an extraction method but also a biotransformation strategy that enhances both the yield and functionality of phenolic compounds. This approach supports the sustainable valorization of agro-industrial residues within a circular bioeconomy framework.