Valorization of Prickly Pear Peel Residues (Opuntia ficus-indica) Using Solid-State Fermentation

Prickly pear peel (Opuntia ficus-indica) residues can be used as a substrate in solid-state fermentation to obtain bioactive compounds. The kinetic growth of some Aspergillus strains was evaluated. A Box-Hunter and Hunter design to evaluate the independent factors was used. These factors were temperature (°C), inoculum (spores/g), humidity (%), pH, NaNO₃ (g/L), MgSO₄ (g/L), KCl (g/L), and KH₂PO₄ (g/L). The response factors were the amount of hydrolyzable and condensed tannins. The antioxidant and antimicrobial activity of fermentation extracts was evaluated. Aspergillus niger strains GH1 and HT3 were the best for accumulating tannins. The humidity, inoculum, and temperature affect the release of hydrolyzable and condensed tannins. Treatment 13 (low values for temperature, inoculum, NaNO₃, MgSO₄; and high values for humidity, pH, KCl, KH₂PO₄) resulted in 32.9 mg/g of condensed tannins being obtained; while treatment 16 (high values for all the factors evaluated) resulted in 3.5 mg/g of hydrolyzable tannins being obtained. In addition, the fermented extracts showed higher antioxidant activity compared to the unfermented extracts. Treatments 13 and 16 showed low inhibition of E. coli, Alternaria sp., and Botrytis spp. The solid-state fermentation process involving prickly pear peel residues favors the accumulation of condensed and hydrolyzable tannins, with antioxidant and antifungal activity.