Production of grape pomace extracts with enhanced antioxidant and prebiotic activities through solid-state fermentation by Aspergillus niger and Aspergillus oryzae

Grape pomace is a winery by-product that accumulates in high amounts in winemaking industry, which usually found difficulties regarding its disposal. Nonetheless, it has a great proportion of polyphenols which can be recovered to valorise this biomass. Filamentous fungi produce hydrolytic enzymes, which can assist in the liberation of polyphenols. Grape pomace was fermented in solid-state by Aspergillus niger and Aspergillus oryzae with the aim of facilitating the aqueous extraction of polyphenols with antioxidant activity by on-site enzyme production. Both fungi increased the antioxidant activity of the extracts, reaching maximum values of 73.7 ± 0.8 (A. niger) and 109.2 ± 0.5 (A. oryzae) mmol of Trolox equivalents/100 g of grape pomace. During fermentations, relevant enzymes were produced at high yields, A. niger produced a balanced profile of enzymes (cellulase, tannase, and pectinase), while A. oryzae switches to cellulase or tannase selective induction according to the fermentation conditions. Positive correlations were found between enzyme production, polyphenols recovery, and antioxidant activity. The extracts obtained after fermentation promoted the growth of Lactobacillus casei cultures. Therefore, the solid-state fermentation was effective for the simultaneous production of relevant industrial enzymes and grape pomace extracts with antioxidant and prebiotic properties, which have potential as functional food additives.