Abstract: Totally 10 acid-producing strains were isolated from naturally fermented soy whey (Suanjiangshui) collected from two fermented tofu (sufu) producers in Mouding, Yunnan. The strains were identified by morphological observation, physiological and biochemical identification and 16S rDNA sequence analysis, and their growth characteristics were evaluated. The results showed that Lactobacillus fermentum (YQZ1), Lactococcus raffinolactis (YQZ2), Lactobacillus plantarum (YQZ3), Enterococcus casseliflavus (YQZ4), and Microbacterium oxydans (YQZ5) were the dominant strains in one sample, while Enterococcus faecium (XJZ1), L. plantarum (XJZ2), Lactobacillus paracasei (XJZ3), Enterococcus hirae (XJZ4), and L. paracasei (XJZ5) were the dominant strains in the other. The optimal growth temperature was 32 ℃ for strain XJZ4 and 37 ℃ for the other strains and the natural microflora. Strain XJZ2 had the fastest growth rate, and YQZ1 and XJZ2 had the strongest reproductive ability. YQZ1 and XJZ2 had the strongest acid-producing capacity and the largest accumulation of organic acid, giving acid yield of 43.61 and 50.91 g/L after 48 h, respectively. YQZ3 and YQZ5 had the strongest acid tolerance, followed by YQZ1 and XJZ2. The optical density at 600 nm (OD600 nm) for the cell density of YQZ4 was maintained at 1.53 in the presence of 4 g/100 mL NaCl, whereas the growth of the other strains was inhibited by NaCl concentrations of greater than 4 g/100 mL. It is concluded that the strains YQZ1 and XJZ2 had strong reproductive ability and high acid-producing ability, making their mixed cultures promising candidates to be used in the preparation of Suanjiangshui for shortened fermentation period and improved quality of fermented tofu.

Key words: fermented soy whey; acid-producing bacteria; screening; identification; growth characteristics