The microbial degradation of onion flavonol glucosides and their roasting products by the human gut bacteria Eubacterium ramulus and Flavonifractor plautii

Flavonoids are important constituents of the human diet. One source for flavonols, a major subclass of the flavonoids, is onion. It contains high amounts of quercetin glycosides, primarily quercetin-3,4′-di-O-glucoside (QDG) and quercetin-4′-O-monoglucoside (Q-4′-MG). Due to their high reactivity flavonols are susceptible to thermal degradation as used in food processing. Especially boiling and roasting influence the flavonoid content of food products. Quercetin and several of its glycosides may serve as substrates for human gut bacteria. For example, Eubacterium ramulus and Flavonifractor plautii are capable of cleaving the aglycone quercetin to form 3,4-dihydroxyphenylacetic acid (DHPAA) and phloroglucinol which to some extent can be degraded further. The aim of this study was to find out whether E. ramulus and F. plautii are also capable of degrading Q-4′-MG and QDG by and to investigate the influence of a thermal treatment (roasting) of the onion glucosides on the subsequent microbial degradation. In this study, E. ramulus was capable of degrading Q-4′-MG and QDG, while F. plautii was not. Roasting of QDG at 180 °C for 5 min led to the formation of quercetin with Q-4′-MG and quercetin-3-O-monoglucoside (Q-3-MG) as intermediates. Roasting accelerated the microbial degradation of Q-4′-MG and QDG. In the case of F. plautii, microbial degradation was induced by quercetin which was formed during roasting and is a preferred substrate of this organism.