Monascus pigments (MPs) with different color characteristics, produced by submerged fermentation of Monascus purpureus M9, have potential application in food industry. In the present study, the effects and regulatory mechanisms of ammonium nitrate (AN) on the color characteristics of MPs were investigated. The concentration of intracellular pigments was significantly decreased when subjected to AN. The hue and lightness value indicated AN altered the total pigments appearance from original red to orange. The HPLC analysis for six major components of MPs showed that the production of rubropunctatin or monascorubrin, was significantly reduced to the undetectable level, whereas the yields of monascin, ankaflavin, rubropunctamine and monascorubramine, were apparently increased with AN supplement. To be noted, via real-time quantitative PCR strategy, the expression level of mppG, closely relative to orange pigments biosynthesis, was significantly down-regulated. However, the expression of mppE, involved in yellow pigments pathway, was up-regulated. Moreover, the broth pH value was dropped to 2.5–3.5 in the fermentation process resulted from AN treatment, along with the increased extracellular polysaccharide biosynthesis. Taken together, the change of MPs categories and amounts by AN might be the driving force for the color characteristics variation in M. purpureus M9. The present study provided useful data for producing MPs with different compositions and modified color characteristics.

红曲霉色素（MPS）具有不同的颜色特性，通过深层发酵产生的紫色红曲霉M9，在食品工业中有潜在的应用。在本研究中，研究了硝酸铵（AN）对MP的颜色特征的影响和调节机制。当进行AN时，细胞内色素的浓度显着降低。色相和亮度值表明AN改变了颜料的整体外观，从原来的红色变为橙色。HPLC对MP的六种主要成分的分析表明，rubropunctatin或monascorubrin的产量显着降低至无法检测的水平，而补充AN可使monascin，ankaflavin，rubropunctamine和monascorubramine的产量明显增加。值得注意的是，通过实时定量PCR策略，mppG的表达水平与橙色颜料的生物合成密切相关的蛋白被显着下调。然而，参与黄色素途径的mppE的表达上调。此外，在AN处理的发酵过程中，肉汤pH值降至2.5-3.5，同时细胞外多糖的生物合成增加。综上所述，AN对MP类别和数量的改变可能是紫癜M9颜色特征变化的驱动力。本研究为生产具有不同成分和修饰的颜色特征的MP提供了有用的数据。