Fermentation of onion (Allium cepa L.) peel by lactic acid bacteria for production of functional food

Onion (Allium cepa L.) peel is rich in flavonoids (e.g. quercetin), which have attracted interest for their beneficial health effects, including antioxidative and anti-inflammatory activities, but it is usually discarded. This study tested a potential way to utilize onion peel, by producing a functional food by fermentation with lactic acid bacteria (LAB). Of 13 Lactobacillus strains tested, five (E19, E31, J11, J41, and 361) reached viable cell counts > 108 CFU/mL. LAB fermentation did not affect the quercetin content of onion peel. Five strains inhibited the activity of angiotensin I-converting enzyme, which converts angiotensin I into angiotensin II, a potent vasoconstrictor that regulates blood pressure. We selected Lactobacillus plantarum E31 for further study, and onion peel fermented by using this strain inhibited the enzyme activity. Analysis with a taste-sensing system showed that fermentation with strain E31 produced onion peel that was more acidic and had less umami flavor and aftertaste of anionic bitterness than unfermented samples. This study proposes a new approach to utilize a food by-product by adding value via production of a new functional food—fermented onion peel—which contains both flavonoids and probiotics.