Metabolism of Quercetin and Naringenin by Food-Grade Fungal Inoculum, Rhizopus azygosporus Yuan et Jong (ATCC 48108)

Rhizopus azygosporus Yuan et Jong (ATCC 48108), a starter culture for fermented soybean tempeh, produces figlucosidases that cleave flavonoid glycosides into aglycones during fermentation. However, recent data suggest that fermentation of a flavonoid glycoside-rich extract with this strain did not result in the production of aglycones. Thus, in this paper, flavonoid metabolism of this strain was investigated. Incubation of flavonoid aglycones, naringenin and quercetin, with R azygosporus resulted in the production of flavonoid glucosyl-, hydroxyl-, and sulfo-conjugated derivatives. Naringenin was completely metabolized within 96 h into eriodictyol sulfate and eriodictyol glucoside, whereas quercetin was partially metabolized into quercetin glucoside, diglucoside, sulfate, and glucosyl-sulfate. Most of these metabolites were found to be excreted by the fungi into the culture medium. Toxicity analysis revealed that incubation with both quercetin and naringenin did not exert inhibitory, effects on fungal growth. This study presents an interesting mechanism of fungal detoxification of flavonoids in foods.