Production of citrinin-free Monascus pigments by submerged culture at low pH

• Submerged culture of Monascus sp. in various nitrogen source media was carried out with different initial pH.
• Different final pH values in the fermentation broth were achieved.
• The final pH but nitrogen source strongly affected the secondary metabolite citrinin production.
• Citrinin-free Monascus pigments were produced at extremely low final pH.

Microbial fermentation of citrinin-free Monascus pigments is of great interest to meet the demand of food safety. In the present work, the effect of various nitrogen sources, such as monosodium glutamate (MSG), cornmeal, (NH4)2SO4, and NaNO3, on Monascus fermentation was examined under different initial pH conditions. The composition of Monascus pigments and the final pH of fermentation broth after Monascus fermentation were determined. It was found that nitrogen source was directly related to the final pH and the final pH regulated the composition of Monascus pigments and the biosynthesis of citrinin. Thus, an ideal nitrogen source can be selected to control the final pH and then the citrinin biosynthesis. Citrinin-free orange pigments were produced at extremely low initial pH in the medium with (NH4)2SO4 or MSG as nitrogen source. No citrinin biosynthesis at extremely low pH was further confirmed by extractive fermentation of intracellular pigments in the nonionic surfactant Triton X-100 micelle aqueous solution. This is the first report about the production of citrinin-free Monascus pigments at extremely low pH.