Determination of volatile components in fermented soybean prepared by a co-culture of bacillus subtilis and rhizopus oligosporus

Abstract

Fermented soybeans are traditional foods widely consumed in many countries (i.e., Natto in Japan, Jang in Korea, Kinema in India, Douichi in China, and Thua Nao in Thailand). In this study, an attempt was made with an expectation to improve the fermentation process using a co-culture of Bacillus subtilis and Rhizopus oligosporus. Initially, the raw soybeans were washed, sterilized by autoclaving, and inoculated with two inocula; for this, three different ratios between B. subtilis and R. oligosporus used were 100:0, 50:50, and 0:100. The fermentation was then carried out at 30°C for 3 days. The volatile compounds in the non-fermented and the fermented soybean products were determined by gas chromatography/ mass spectrometry (GC/MS) and extracted using a solid phase microextraction (SPME) technique. In total, 165 compounds were identified in the non-fermented and the fermented soybean products. For the non-fermented products, the predominant volatile compounds were alcohols (25.81%), aldehydes (13.64%), acids and esters (7.57%), furans (6.13%) and ketones (0.88%). In contrast, the major volatiles compounds presented in the fermented soybeans were as follows: i) The treatment of 100:0 consisted of acids and esters (35.89%), alcohols (14.55%), aldehydes (8.72%), ketones (4.97%), pyrazines (4.87%), and furans (4.22%); ii) 50:50 comprised of acids and esters (55.62%), alcohols (16.22%), aldehydes (7.80%), pyrazine (3.65%), ketones (2.55%), furans (1.67%), and aromatic compounds (1.46%); and iii) 0:100 included acids and esters (66.50%), alcohols (15.44%), aldehydes (2.59%), ketones (2.72%), furans (1.89%), aromatic compounds (1.80%), pyrazines (1.35%), and sulphur containing compounds (0.24%) respectively.