Production of gallic acid and relevant enzymes by Aspergillus niger and Aspergillus oryzae in solid-state fermentation of soybean hull and grape pomace

Gallic acid is a phenolic organic acid found in plants that has a wide range of applications, as a health-promoting agent and as a food preservative, due to its high antioxidant activity. Gallic acid can be obtained by acid hydrolysis of tannic acid or by the enzymatic action of tannase on tannic acid. A biorefinery approach to produce gallic acid was developed. A combination of grape pomace and soybean hull as support substrate was used for solid-state fermentation by Aspergillus niger and Aspergillus oryzae. The best conditions for the bioconversion of tannic acid to gallic acid were tested. Tannase and other relevant enzymes were produced in the same process. The gallic acid production was monitored and quantified by high-performance thin-layer chromatography (HPTLC). HPTLC was used to separate the phenolic extracts prepared, and bioautography by gel overlay was used to identify antioxidants and tyrosinase inhibitors directly on the plate. The best gallic acid producer was A. oryzae, and the best fermentation condition comprised soybean hull and grape pomace as support substrate, producing 0.36 g of gallic acid/g of tannic acid and 7.2 g/L of fermentation medium in 72 h of incubation. The value addition of these agricultural wastes through a green process to produce gallic acid was demonstrated, creating opportunities for solid waste usage.