Thermal Degradation of Flavonol Glycosides in Noni Leaves During Roasting

Noni leaves have been used for a variety of health benefits for thousands of years. Noni leaf tea, a commercial product made by a roasting process, is attracting more attention due to its potential health benefits. Flavonoids appear to be some of the predominant constituents in noni leaves. As flavonoids exist mostly in the forms of glycosides or polymers, degradation to corresponding metabolites is usually needed for bio-absorption. This study investigates the effects of thermal treatment (non-aqueous roasting) on flavonoids in noni leaves. Rutin and kaempferol glycoside contents decreased dramatically as roasting time and/or temperature increased, while quercetin and kaempferol aglycones were produced. A quantitative comparison demonstrated that quercetin and kaempferol concentrations were 3.74 and 6.28 times greater in noni leaf tea than in raw noni leaves, respectively. These findings indicate that the roasting process for the noni leaf tea could induce the degradation of flavonol glycosides, and produce their aglycone metabolites, which in turn, may lead to more beneficial bioactivities and bioavailability.