Comparative analysis of different rice substrates for solid-state fermentation by a citrinin-free Monascus purpureus mutant strain with high pigment production

Monascus pigment (MP) is a safe and natural microbial coloring agent. MP and red yeast rice (RYR) play an important role in the food industry. In this study, a citrinin-free Monascus puprureus MS-12 with high MP production was generated using dual mutagenesis of atmospheric and room-temperature plasma and heavy ion beam irradiation. The fermentation characteristics of eight rice substrates for solid-state fermentation (SSF) were compared to produce a premium RYR. Indica rice was the optimal fermentation substrate (MP = 4526 U/g in RYR), followed by japonica rice (MP = 3556 U/g). The MP value was <1000 U/g using other substrates (red, brown and black rice) for fermentation. RT-qPCR results demonstrated that the expression levels of MP synthesis- and the glycolysis-related genes of the mutant strain were upregulated compared with those of the parental strain, with MpigA exhibiting a 1.70-fold increase, while the relative expression levels of citrinin synthesis-related genes were downregulated; citE was downregulated 7.84-fold. In comparison to M. purpureus MS-12 mRNA levels using indica rice as the fermentation substrate during SSF, genes for MP, citrinin biosynthesis, and glycolysis resulted in the downregulation of other substrates. Our results show that colored and glutinous rice are unsuitable for RYR preparation via SSF. The maximum MP value was obtained using indical rice as the fermentation substrate, resulting from the upregulated expression levels of the pigment biosynthesis gene cluster and glycolytic pathway.