The present study aims to investigate antioxidant activity and phenolic contents of Thai thua nao. Methanolic
extracts of cooked non-fermented soybeans (CNF) and thua nao—fermented soybeans prepared by naturally
occurring bacteria (TNMX) and Bacillus subtilis TN51 (TNB51) were prepared and used to determine antioxidant
activities by β-carotene-linoleic acid system and DPPH-radical scavenging assay. Total phenolic contents of thua
nao samples (both TNMX and TNB51) were significantly higher than those of the CNF samples (with an increase
around 21 and 35%, respectively). A correlation between larger amount of total phenolics and higher antioxidant
activity as determined by the β-carotene-linoleic acid model was observed in these extracts. All these results
indicate that production process and bacterial fermentation are responsible for improvement of total phenolics and
antioxidant activity. The potential use of B. subtilis TN51 appears to be a suitable fermenting strain to promote
antioxidant property in cooked soybean.