Solid-State Fermentation-Assisted Extraction of Flavonoids from Grape Pomace Using Co-Cultures

Eighty percent of grape production is destined for the wine industry, which generates various types of waste, of which grape pomace is the main one, accounting for 50–60% of waste created during processing. This waste could be a promising source of bioactive compounds (e.g., flavonoids and tannin), which are known for their antioxidant properties. Although these byproducts pose disposal challenges, they can be utilized as a substrate for solid-state fermentation bioprocess using co-cultures, where different microorganisms can interact and complement each other, improving the efficiency of metabolite production or substrate degradation. This study investigates the extraction of phenolic compounds and the antioxidant activity of the compounds from grape pomace in the solid-state fermentation bioprocess, comparing fungal and yeast monocultures, and then exploring the use of two co-cultures (P. stipites/A. niger GH1 and S. cerevisiae/A. niger) on the flavonoid extractive process. Fermentation kinetics were evaluated over 120 h, with sampling done every 12 h. Initially, yeasts were used to reduce the content of simple sugars in the medium, and fungus was added at 24 h into the process due to its ability to produce a broad spectrum of extracellular enzymes, allowing a higher efficiency in substrate degradation. Competition or antagonism during co-culture leads to significantly higher production of compounds, which are recovered using different solvents. The evaluation included phenolic compounds (total polyphenols, condensed tannins, and total flavonoids), antioxidant activity (DPPH●/FRAP), molecular characterization (HPLC-MS), and structural microscopy during the bioprocess. The highest titers obtained were 62.46 g/L for total flavonoids and 32.04 g/L for condensed tannins, using acetone as the solvent in co-culture with P. stipitis after 120 h of fermentation. Characterization identified 38 compounds, highlighting families of flavonols, hydroxybenzoic acids, and hydroxycinnamic acids. The co-culture of P. stipitis and A. niger GH1 significantly improved the extraction yield of bioactive compounds through solid-state fermentation.