Bacillus cereus emetic toxin production in cooked rice

Three mesophilic strains of Bacillus cereus known to produce emetic toxin were used to model germination, growth and emetic toxin production in boiled rice cultures at incubation temperatures ranging from 8°C to 30°C. Minimum temperatures for germination and growth in boiled rice were found to be 15°C for all strains. Toxin production at 15°C was found to be significantly greater (P<0·01; reciprocal toxin titre of 373±124) than at 20°C and 30°C (reciprocal toxin titres 112±37 and 123±41, respectively). Toxin production became detectable after 48 h incubation at 15°C, with a maximum titre reached by 96 h. At 20°C and 30°C, toxin production was detected at 24 h incubation, with a maximum titre reached by 72 h. Toxin production at 15°C was detectable at lower bacterial counts (6·2 log₁₀ cfu g⁻¹), than with incubation at 20°C and 30°C (>7·0 log₁₀ cfu g⁻¹). In this study, the lower temperature limit for germination and growth on solid laboratory medium was found to be 12°C for all strains, i.e. 3°C lower than that observed in boiled rice.     

Assessment of bacterial profiles in aged,home-made Sichuan paocai brine with varying titratable acidity by PacBio SMRT sequencing technology

Sichuan paocai, a traditional Chinese fermented vegetable, is rife with lactic acid bacteria (LAB). However, the precise bacterial profiles of home-made Sichuan paocai brine (HSPB) remain unclear. In this study, the bacterial compositions of 38 aged HSPB samples with varying titratable acidity (TA) were determined by SMRT sequencing of the full-length 16S rRNA gene. The lactic and acetic acids of HSPBs were also measured to determine any relevance with the bacterial profiles. The SMRT sequencing results reveal that the HSPB bacterial communities were comprised of numerous phylogenetic taxa, including 35 phyla, 371 genera, and 593 species; the bacterial diversity decreased as HSPB acidity increased. Lactobacillus acetotolerans, which was positively correlated to HSPB acidity, was the most dominant species followed by Lactobacillus brevis, which was positively related to acetic acid in the samples. A few opportunistic pathogens (e.g. Serratia marcescens and Stenotrophomonas maltophilia) were also detected. Sample groups with lower acidity had higher bacterial diversity and more Lactobacillus species with relative abundance >1% and opportunistics than higher-acidity samples. The results presented here report the comprehensive bacterial profiles of home-made Sichuan paocai for the first time via SMRT sequencing technology and the correlation between TA and bacterial compositions. It is necessary to further investigate the opportunistics detected in this work as they relate to the safety and quality of paocai.     

Survival of Escherichia coli O157:H7, nitrite content and sensory acceptance of low‐salt Chinese paocai fermented by screened lactic acid bacteria

Starter cultures of low‐salt Chinese paocai were screened from 135 lactic acid bacteria (LAB) by evaluating their antimicrobial activity and growth characteristics. Furthermore, the survival of Escherichia coli O157:H7, nitrite contents and sensory acceptability of the starter‐fermented paocai were evaluated. LAB BC92, which was identified as Lactobacillus pentosus, was used as a starter culture to produce paocai. In comparison with naturally fermented paocai, the disappearance time of E. coli O157:H7 in starter‐fermented paocai decreased from 5.29 to 3.82 days, and the maximum nitrite content decreased from 11.73 to 8.64 mg kg^?1, and the nitrite reduction time decreased from 4 to 3 days. In addition, starter fermentations were able to accelerate the flavour formation and shorten the paocai ripening period. The paocai that was fermented naturally and fermented by BC92 all had good sensory acceptance, and no significant difference (P > 0.05) was observed.