Analysis of metabolites of coix seed fermented by Monascus purpureus

In this study, an untargeted metabolomics method based on LC-MS was used to investigate the differential metabolites of coix seed after fermentation by Monascus purpureus. The compounds with highly correlated variable importance in projection (VIP) values of orthogonal partial least squares discriminant analysis (OPLS-DA) were selected as the important metabolites of coix seed samples. Coix seed was fermented by M. purpureus for 9 d. The color value units (CVU) of pigment in fermented coix seed reached 157.47 CVU/g, and the contents of total phenols and total flavonoids were 0.32 mg/g and 1.2 mg/g, respectively. A total of 270 differential metabolites were detected, including amino acids, peptides, fatty acids, phenols, steroids, and their derivatives, of which the relative contents of 187 metabolites showed a significant upward trend (p < 0.05). Amino acids, peptides and analogues accounted for the largest proportion at 26%, among which most metabolites were enriched. For example, N-alpha-acetyllysine, l-tryptophan and l-arginine exhibited a fold change (FC) of 6200.87, 330.29, and 35.59, respectively. In particular, the detected steroid metabolites accounted for 3% of the total metabolites, and they all participated in the same pathway, in which androsterone, 2-methoxyestrone and etiocholanolone were upregulated. The synthesis of these steroids was related to the precursors and enzymes in the pathway, such as dehydroepiandrosterone, androstenedione, and 3beta-hydroxy-delta5-steroid dehydrogenase (HSD3B). The results of this study reveal M. purpureus fermentation can successfully transform coix seed, improving the functional activities of metabolites. Thus, this process has high development value with important applications in the food industry.