Abstract: In the present study, the microbial communities in packaged and unpackaged north Chinese tofu were analyzed. The dominant spoilage microorganisms were isolated and identified from these samples and their spoilage capacity and distribution during tofu processing were evaluated. Also, the effectiveness of low-water pulping in controlling microbial contamination in soybean pulp was determined. As a result, we obtained five dominant strains and identified them as Lactococcus lactis (BDF-1), Kurthia gibsonii (BDF-2), Enterobacter aerogenes (BDF-3), Empedobacter brevis (BDF-4) and Bacillus subtilis (BDF-5) based on morphological characteristics and 16S rDNA gene sequence analysis. Each strain was re-inoculated onto the surface of tofu to examine their effect on the total viable count, pH, soluble amino nitrogen content and sensory quality of tofu. All five strains could result in quality deterioration in tofu samples (minimum spoilage concentration 107 CFU/g). BDF-2, BDF-3, BDF-4 and BDF-5 altered the color of tofu, softened tofu, and produced spoilage odor, while BDF-1 produced acid to decrease the pH. BDF-1, BDF-3 and BDF-4 were the dominant spoilage bacteria during the industrial processing of packaged north Chinese tofu. Low-water pulping reduced the microbial load from 2.3 × 108 to 7.0 × 104 CFU/g, improving the processing stability and safety of north Chinese tofu. Thus low-water pulping could provide a basis for automatic processing of north Chinese tofu.

Key words: north Chinese tofu, microbial communities, spoilage organisms, low-water pulping technology