Isolation and identification of the spoilage microorganisms in Sichuan homemade Paocai and their impact on quality and safety

The spoilage microbiology in Paocai (fermented vegetables) affects not only the quality of this popular traditional Chinese food but also its safety. This study aimed to isolate and identify the microorganisms commonly leading to spoilage in homemade Paocai from the Sichuan region and, further, to scientifically assess the impact of these microorganisms on product quality and safety. Seven putrid Paocai samples were collected from 7 families in different Sichuan cities. From these, 45 strains were isolated by means of a nutrient agar medium and rose bengal agar. All of the 22 strains (5 fungi and 17 bacteria) with different colonial morphologies and corruption phenomenon in Paocai were determined 16S rDNA/18S rDNA gene sequences. Bacteria were identified as Bacillus spp. (16 strains) and Staphylococcus saprophyticus (1 strain), while the 5 fungi were identified as Candida spp. (3 strains), Kodamaea ohmeri (1 strain), and Geotrichum candidum (1 strain). Based on the results of identification, 7 representative strains of different species were determined as the spoilage characteristics in paocai. All the representative strains metabolized to produce nitrite. Strain SPF21 (K. ohmeri) has a particularly serious impact on the crunchiness of Paocai; however, strains SPF19 (Bacillus subtilis) and SPF21 have the greatest influence on its color. All five of the fungi were seen to form a film on the surface of the Paocai, with SPF5 (G. candidum) exerting the most extreme influence. The growth characteristics in the broth showed that all the representative strains investigated metabolized most of the carbohydrates and were able to tolerate the salinity and acidity of the ordinary homemade Paocai. Moreover, these strains were found to have obvious impacts on the volatile components of Paocai, especially SPF2 and SPF8, with higher dimethyl disulfide and dimethyl trisulfide, which were found to have a pungent odor when highly concentrated.