Enhanced antioxidant capacity of brown rice vinegar supplemented with soy residue

Background and Objectives: This study aimed to prepare and use soy residue as a supplement to enhance the antioxidant activities of grain vinegar, which has relatively lower antioxidant activities than fruit vinegar. Brown rice vinegar (BRV) was supplemented with 30% (BROV3) and 50% (BROV5) soy residue, respectively. Findings: After 3 days of alcohol fermentation, the BROVs had lower sugar content (6.43–9.4 °Brix [Bx]), alcohol content (4.47%–7.5%), and pH (3.65–3.66) but higher acidity (0.75%) than BRV (12.47 °Bx, alcohol content 11%, pH 3.72, acidity 0.66%). During 10 days of acetic fermentation, the BROVs showed higher pH (3.23–3.38) and lower acidity (4.05%–4.35%) than BRV (pH 2.88, acidity 4.86%). As the soy residue was increased, the contents of essential amino acids increased from 22.03 mg/L (BRV) to 515.27 mg/L (BROV5), but those of desirable volatile compounds decreased, and those of undesirable compounds increased. The polyphenolics, flavonoids, and isoflavone contents and 2,2‵-diphenyl-1-picrylhydrazyl (DPPH) and 2, 2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS)⁺ radical-scavenging activities of the BROVs increased while increasing soy residue. Conclusions: Although soy residue addition decreased certain desirable volatile compounds and increased certain undesirable ones, overall, it improved the antioxidant activity and quality of brown rice vinegar. Significance and Novelty: The potential of soy residue for enhancing vinegar functionality was confirmed.