Abstract: The purpose of this study was to isolate and identify the bacteria responsible for spoilage of vacuum-packaged nonrehydrated dougan (a Chinese traditional rod-shaped soybean product prepared with protein-lipid film) and to evaluate the spoilage potential of the heat-resistant bacteria. The bacteria were isolated using various selective media. The dominant spoilage bacteria were identified by morphological observation, physiological and biochemical characterization, and 16S rRNA and gyrB gene sequence analysis. The heat-resistant spoilage bacteria with higher protease-producing capacity were inoculated into sterilized dougan to evaluate the spoilage potential according to the sensory quality, textural properties, total volatile basic nitrogen (TVB-N) content, protease activity and pH of dougan. A total of 20 bacterial strains were isolated from spoiled nonrehydrated dougan, and among these isolates, 12 strains with higher protease-producing capacity could survive autoclaving at 110 ℃ for 20 min. The dougan inoculated with strains DY1a, DY1b, DY2a and DY3 showed obvious signs of spoilage after storage at 37 ℃ for 7 days. The TVB-N, protease activity and pH value of the inoculated samples were significantly higher than those of the non-inoculated control. Texture profile analysis indicated that hardness, springiness, gumminess and chewiness of the inoculated sample decreased significantly compared with control samples. All four specific spoilage bacteria belonged to the Bacillus genus. According to the TVB-N yield factors (Y (TVB-N/CFU) values), the spoilage potential of B. amyloliquefaciens DY1a was the strongest among the spoilage bacteria isolated, followed subsequently by B. amyloliquefaciens DY1b, B. subtilis DY2a and B. subtilis DY3.

Key words: nonrehydrated dougan, Bacillus spp., gyrB gene, spoilage capability, textural properties