The relationship between pH, pigments production, and citrinin synthesis by Monascus purpureus during red fermented rice fermentation

Red fermented rice (RFR) is consumed as a traditional medicine in many countries. Consumption of food contaminated with citrinin (CIT) has adverse effects on the liver cells and the kidneys. This study aimed to understand the relationship between pH, pigments, and CIT levels during the fermentation of RFR. The rice underwent soaking, steaming, and autoclaving before being inoculated with Monascus purpureus isolates (MF1 or MS1) and fermented at 30 °C for 30 days. The difference between these isolates was that MF1 and MS1 required 4 days and 7 days, respectively, to produce light blue fluorescence on Coconut Cream Agar (CCA), an indicator of the presence of CIT. The pH, pigments, and CIT levels were recorded during fermentation using a pH meter, spectrophotometer, and Ultra-High-Performance Liquid Chromatography with a Fluorescence Detector (UHPLC-FLD), respectively. The pH dropped from 6.8 to 5.3 after 9 days of fermentation. The CIT levels increased from day 4 until day 5 and then decreased during the fermentation period. The pigments increased after 5 days of fermentation, suggesting a relationship between pigments production and CIT reduction. Mixing the CIT standards and pigments extracted from MF1 and MS1 (a mixture of yellow, orange, and red pigments) showed that there was a reduction in CIT of 26–68 % and 16−45 %, respectively. It is suggested that future work should determine which pigments are responsible for CIT reduction. The optimization of pigments production with the control of pH at 5.5 may help to control CIT levels during the fermentation of RFR.