Impact of cooking methods of red-skinned onion on metabolic transformation of phenolic compounds and gut microbiota changes

Abstract

The recipient work investigated on the stability and bioaccessibility of phenolics of red-skinned onion (RSO) differently cooked and in turn on its impact on the gut microbiota and phenolics metabolism. In fact, the different process used to cook vegetables can modify and re-arrange the molecular profiles of bioactives compounds, likely that of phenolics for phenolics-rich vegetables, such as RSO. Fried and grilled RSO were compared to raw RSO and to a blank control and were subject to oro-gastro-intestinal digestion and subsequent colonic fermentation. For upper gut digestion the INFOGEST protocol was used and for the lower gut fermentation a short-term batch model, namely MICODE (multi-unit in vitro colon gut model) was employed. During the process, the phenolic compounds profiles (through high-resolution mass spectrometry) and colon microbiomics (qPCR of 14 core taxa) analyses were performed. From the results obtained, the degradation driven by colon microbiota of RSO flavonols resulted in the accumulation of three main metabolites, 3-(3’-hydroxyphenyl)propanoic acid, 3-(3'-hydroxyphenyl)acetic acid and 3-(3’,4’-dihydroxyphenyl)acetic acid. Also, colonic fermentation of raw onions resulted in a substantial increase in beneficial taxa, larger in respect to the heat-treated onions, particularly on Lactobacillales and beneficial clostridia. Also, a higher level of inhibition of opportunistic bacteria was seen for raw onion samples, namely Clostridium perfringens group and Escherichia coli. Thus, our results showed RSO, and especially the raw one, is an excellent dietary source of flavonols that are strongly metabolized by gut bacteria and able to modulate positively the gut microbiota. Even if additional in vivo studies are necessary, this work is one of the first exploring how RSO processed with different cooking methods can differently impact on the phenolic metabolism and microbiota composition of human large intestine, tuning and fining the food antioxidant nature.