Dynamics of Kojic Acid and Other Compositions during Solid-State Fermentation of Two White Grape Pomaces by <i>Aspergillus oryzae</i> and <i>tamarii</i>

This study aimed to optimize a solid-state fermentation approach to produce valuable substances from two grape pomace (GP) varieties fermented with two Aspergillus strains (A. oryzae and A. tamarii) with aqueous extract samples collected over 6 days. The aqueous extracts were characterized by °Brix, pH, titratable acidity, total phenolics, 4 individual sugars, 5 organic acids, and 4 phenolics. The results showed °Brix, pH, and titratable acidity changed monotonically with fermentation, corresponding to changes in individual sugars and organic acids. Fermentation nearly depleted fructose (98–99%), glucose (>99%), and tartaric acid (62–99%), lowering °Brix and titratable acidity while raising pH. Fermentation also significantly increased gallic acid (124–159%) and produced kojic acid (0.041 g/g pomace dry mass). Phenolic extraction was maximized with 1–3 days of fermentations. The PCA biplot showed that fermentation duration and GP variety were the primary drivers of variation in chemical composition, while strain type had minimal impact. The increased phenolic extraction and novel production of kojic acid demonstrated GP’s potential as a source of valuable microbial metabolites.