Effect of combination of Hom Nil or riceberry broken rice with white rice on antioxidant properties of Monascus-fermented rice

The present work aimed to investigate the effects of combining by-products, namely Hom Nil broken rice (HNB) and riceberry broken rice (RBB) with white rice (WR), in solid-state fermentation (SSF), by Monascus purpureus TISTR 3629. The HNB:WR and RBB:WR systems were varied in different ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 (w/w). The secondary metabolites of Monascus such as pigments (yellow, orange, and red), monacolin K, and mycotoxin citrinin, as well as other bioactive compounds such as total phenolic content (TPC), total flavonoid content (TFC), total anthocyanin content (TAC), and their antioxidant activities were determined. Across these ratios, from 100:0 to 0:100 in HNB:WR and RBB:WR systems, significant increases were observed in pigments, monacolin K, TPC, TFC, and TAC, together with decreases in citrinin contents. Interestingly, strong DPPH and ABTS+ radical scavenging activities (RSA) occurred in all Monascus-fermented rice samples. For the HNB:WR system, DPPH and ABTS+ RSA were 78.2 - 92.5 and 98.0 - 99.7%, respectively, and for the RBB:WR system, they were 81.8 - 95.9 and 94.8 - 99.2%, respectively. Although HNB:WR or RBB:WR at a 0:100 (fermented WR) produced significant amounts of various bioactive compounds without citrinin, the HNB:WR or RBB:WR ratios of 25:75 yielded economic amounts of pigments, monacolin K, TPC, TFC, and TAC, along with low citrinin concentrations. The present work offered a valuable method by applying the co-substrate technique in SSF, for the conversion of low-cost agricultural by-products, like HNB and RBB, into high value-added products, which were rich in bioactive compounds and antioxidant activity.