Abstract:

The effects of adding different kinds of ammonium salts to the culture medium on the biosynthesis and
metabolism of pigments and citrinin in M. purpureus Y20 during liquid-state fermentation were studied by examining
changes in the contents of monascus red pigment, monascus yellow pigment and citrinin. Also, we elaborated the underlying
causes for the changes. The results showed that pH values of the fermentation broth of M. purpureus Y20 without added
ammonium salt remained stable at 4.8 during fermentation, while those obtained with the addition of CH3COONH4,
NH4H2PO4 or C6H5O7(NH4)3 were maintained at greater than 6. The initial pH of the culture medium with the addition of
NH4NO3, (NH4)2SO4 or NH4Cl was below 5.5, and then decreased to around 2.5 during fermentation. The final citrinin
content of the groups with 0.15 mol/L (NH4)2SO4 and 0.1-0.3 mol/L NH4Cl, respectively, during fermentation dropped to
0.05 mg/L, indicating an 88.6% reduction when compared with the control group with no added ammonium salt. No citrinin
was detected in the fermentation broth with 0.3 mol/L NH4NO3 added, and a 31.0% increase of yellow pigment content
and an 11.6% decrease of red pigment content were observed compared with the control group. Similarly, no citrinin was
detected in the fermentation broth with the addition of CH3COONH4, NH4H2PO4, or C6H5O7(NH4)3, but color value and
biomass were lower, suggesting that the addition of ammonium salts could affect the pH environment of the fermentation,
and plays an important role in reducing citrinin and changing the proportions of monascus pigments. The addition of
NH4NO3, NH4Cl and (NH4)2SO4 can contribute to the biosynthesis of monascus yellow pigment and the inhibition of citrinin.

Key words: Monascus purpureus, monascus red pigment, monascus yellow pigment, citrinin, ammonium salts