Regulation of the phenolic release and conversion in oats (Avena sativa L.) by co-microbiological fermentation with Monascus anka, Saccharomyces cerevisiae and Bacillus subtilis

Microbial fermentation is an effective method to improve the functional activity of oats (Avena sativa L.), while there are some limitations to the advantages of single microbial fermentation. In this study, a microbial co-culture fermentation system with Monascus anka, Saccharomyces cerevisiae and Bacillus subtilis to release and conversion oat phenolics was established. Results showed that the optimal microbial co-fermentation system was obtained by adding Saccharomyces cerevisiae on the fourth day and Bacillus subtilis on the eighth day during Monascus anka fermentation (MF + 4S + 8B). The phenolic content was reached 26.93 mg GAE/g DW, which increased 41.08 times compared to un-fermented oats (UF). In the process of co-fermentation systems, cellulase and β-glucosidase (r² = 0.97, p < 0.01) had a positive correlation with the release of phenolics. SEM combined with HPLC showed that the complex enzyme system produced by microbial co-fermentation enhanced the disruption of oat cell structure, as well as altered the phenolics fractions and facilitated the conversion of bound phenolics to free phenolics, especially the content of chlorogenic acid and vanillic acid in the free forms was increased 31.42 and 14.15 times, respectively. Additionally, the phenolic contents were increased and the components were changed with the microbial co-fermentation of crude enzyme solution further added, which validated the positive influence of complex enzyme system of MF + 4S + 8B in the phenolic release and transformation of oats. Therefore, this study systematically investigated the phenolic mobilization in oats during the co-fermentation period, which provides a viable option for improving the functional properties of cereal products, as well as the application of microbial cell factories.