Bidirectional fermentation of Monascus and Mulberry leaves enhances GABA and pigment contents: establishment of strategy, studies of bioactivity and mechanistic

Bidirectional fermentation is a technology that utilizes fungi to ferment medicinal edible substrates, with synergistic and complementary advantages. In this work, a fermentation strategy was established to produce a high yield of γ-aminobutyric acid (GABA) and <i>Monascus</i> pigments (MPs) using <i>Monascus</i> and mulberry leaves (MLs). Firstly, the basic fermentation parameters were determined using single-factor experiments, followed by Plackett-Burman (PB) experimental design to identify MLs, glucose, peptone, and temperature as significant influencing factors. The fermentation parameters were optimized using an artificial neural network (ANN). Finally, the effects of bidirectional fermentation of MLs and <i>Monascus</i> were investigated by bioactivity analysis, microstructure observation, and RT-qPCR. The outcomes showed that the bidirectional fermentation significantly increased the bioactive content and promoted the secondary metabolism of <i>Monascus</i>. The established fermentation conditions were 44.2 g/L of MLs, 57 g/L of glucose, 15 g/L of peptone, 1 g/L of MgSO₄, 2 g/L of KH₂PO₄, 8% (v/v) of inoculum, 180 rpm, initial pH 6, 32 °C and 8 days. The content of GABA reached 13.95 g/L and the color value of MPs reached 408.07 U/mL. This study demonstrated the feasibility of bidirectional fermentation of MLs and <i>Monascus</i>, providing a new idea for the application of MLs and <i>Monascus</i>.