Production of vinegar from purple sweet potato in a liquid fermentation process and investigation of its antioxidant activity.

Acetobacter pasteurianus JST-S was screened from solid fermented grains of vinegar in China, identified by molecular analysis, and used for the production of purple sweet potato vinegar using purple sweet potato as the substrate. By orthogonal experiment, maximum total acid concentration (4.26% [v/v]) was achieved under optimized conditions as follows: fermentation time, 3.5 days; ethanol content, 9% v/v; and inoculum size, 8% v/v. During the production of purple potato vinegar, the anthocyanin concentration decreased from 652.07 to 301.73 μg/mL. The antioxidant activity of products, including diphenyl-picryl hydrazide radical-scavenging capacity (above 60%), reducing power (above 0.47), and hydroxyl radical-scavenging capacity (above 46%), showed positive linear regression (P < 0.01), which could be related with the changes in anthocyanin concentration and antioxidant activities at different stages of vinegar fermentation. The acetic acid and other non-phenolic antioxidants in purple sweet potato vinegar may have contributed to the antioxidant activities. Results of these studies may provide a reference for the industrial production of vinegar by liquid fermentation of purple sweet potato.