Optimization of Monascus purpureus Culture Conditions in Rice Bran for Enhanced Monascus Pigment Biosynthesis

Monascus pigments (MPs) are the most valuable secondary metabolites of Monascus. To improve the production of MPs is of great importance to food processing. Currently, studies using rice bran as the substrate to produce MPs are rare. In this study, rice bran with different carbon sources and cellulase hydrolysis conditions were explored in Monascus purpureus M9 in this study. Through single-factor experiments and Box–Behnken response surface optimization, we demonstrated that mannitol supplementation combined with cellulase treatment of substrate significantly enhanced the yields of MPs. The optimal conditions (4.00% mannitol, cellulase hydrolysis at 60 °C for 2 h) achieved a maximum color value of 3538 U/g. Furthermore, comparative evaluation under different culture conditions, including only rice bran (RB), cellulase hydrolysis of rice bran (Cel), rice bran supplemented with mannitol (Man), mannitol supplementation combined with cellulase pretreatment of substrate (Opti), and only rice (Rice), confirmed the effectiveness of the optimized treatment. The color value of the Opti group was 27.95 times more than that of the RB group and reached 80.96% of the counterpart of the Rice group. The Opti group also significantly enhanced the yields of two orange pigments (Monascorubrin and Rubropunctatin), induced more sexual spore formation, and exhibited the maximum biomass and colony diameter among different groups. The hyphae of the Man and Opti groups were full, intact, and tubular. The citrinin content in the Opti group was under the limit standard of China. The data provides a theoretical basis reference for improving the yields of MPs with RB as the substrate.