Biorefining process of agricultural onions to functional vinegar

Biorefinery of onion vinegar (OV) is attractive as a method for producing functional foods from onions or onion by-products. In this study, a two-stage fermentation of OV using Saccharomyces cerevisiae ATCC9763 and Acetobacter pasteurianus CICC20001 was carried out at 28 °C, the titratable acidity reached 4.01%, and the Y_A/E was 69.64% at 72 h. Based on this, semi-continuous fermentation was performed, proceeded to charge-discharge consisting of three cycles, and the yield, productivity, and specific production rate were 76.71%, 17.73 g/(L·d), and 20.51 h^-1, respectively, which was higher than fed-batch fermentation. The in vivo antioxidant experiments showed that OV significantly increased GSH-Px, SOD, and CAT enzyme activities of Caenorhabditis elegans at 271.57, 129.26, and 314.68%, respectively. Nutritional analysis revealed that the total flavonoids and polyphenols were 3.01 mg/mL and 976.76 µg/mL, respectively. It was also shown that the acetic acid to total organic acid (A/T) ratio of OV was 79.02%, and the total free amino acid content was 262.30 mg/100 mL, 1.78-7.44 times higher than other fruit vinegar. The OV prepared in this study showed higher quality than the commercial vinegar.