Volatile profiles and aroma-active components of Northern Thai fermented soybean (thua nao) produced from controlled and uncontrolled fermentations

Thua nao is a fermented soybean product that is widely consumed, especially in the northern part of Thailand. Thua nao can be used as a condiment to enhance the aroma and flavor of foods. In general, commercial thua nao is traditionally made by indigenous microbial, especially Bacillus spp. fermentation under ambient conditions, leading to inconsistencies in product aroma quality. To solve this problem, inoculation of selected microorganisms can help control thua nao consistency. The objective of this research was to characterize and compare volatile aroma-active compounds in three samples, including thua nao made by commercial Bacillus subtilis inoculation fermented under controlled conditions (YSB) and two thua nao products made by indigenous fermentation in commercial production sites in Lumpoon (LPN) and Chiang-Mai (CMI) provinces, Thailand. It was found that 2-methyl butanoic acid, phenylethyl alcohol, benzaldehyde, 2,5-dimethylpyrazine, 2,6-dimethylpyrazine and 2-methoxy phenol were dominant compounds in the YSB samples. These compounds contributed to fermented/sour, floral, almond-like and nutty aromas, which were related to positive attributes perceived by consumers. In contrast, several aldehydes and ketones related to off-aroma as oxidized, fatty, green, and earthy attributes derived from lipid oxidations were dominantly detected in samples from uncontrolled fermentations (LPN and CMI). From the study, it can be implied that inoculation of B. subtilis in thua nao fermentation could enhance aroma compounds with positive attributes and thus improve the organoleptic property of the product. These findings could benefit fermented soybean manufacturers to improve their aroma quality and to control products' consistency by controlling fermentation conditions.