Effect of lactic acid fermentation of onions (Allium cepa) on the composition of flavonol glucosides

Abstract

SummaryFermentation of red onions inoculated with Lactobacillus plantarum S1 (starter treatment) resulted in acid production to pH 4.5, after 48 h of incubation at 19 °C, which has proven to be adequate for proper preservation. In the presence of erythromycin (starter + antibiotic), acidification to pH 5.2 resulted, which was similar to that of the uninoculated control. The bacterial population was about 108 CFU mL−1 of brine after 3 days for the three treatments, suggesting that in the starter + antibiotic treatment, the antibiotic effectively suppressed the activity of the starter but not the native flora. Before lactic fermentation, the red onions contained 297.4 mg of total quercetin (Q) per kg wet weight, which consisted of 58.3%, 41.6% and 0.1% in quercetin diglucoside (Qdg), quercetin monoglucoside (Qmg) and free Q, respectively. For the starter treatment, Qdg decreased to 41.8% and 18.3% at 48 and 72 h, respectively, and a substantial amount of free Q had accumulated. The fermentation substantially increased the proportion of Qmg, which may have a positive effect as fractions containing higher ratios of Qmg to Qdg have been reported to have higher antioxidant activity.