Ratio of Histamine-Producing/Non-Histamine-Producing Subgroups of Tetragenococcus halophilus Determines the Histamine Accumulation during Spontaneous Fermentation of Soy Sauce

Characterization of key off-odor compounds in grass carp cube formed during room temperature storage by molecular sensory science approach

Flavor is a significant factor in determining the popularity of freshwater fish. However, freshwater fish can easily spoil during storage, producing an unpleasant odor. Little research has determined the changes in key off-odor compounds (OOCs) in freshwater fish during storage. In this study, quantitation and odor activity value (OAV) calculations revealed that 19 odorants were important volatile odor compounds in fresh, spoilage, and serious spoilage GCC. Recombination and omission experiments verified that (E)-2-hexenal, acetoin, N,N-dimethyl-benzenamine, trimethylamine (TMA), and ammonia were the key OOCs in spoilage GCC. Additional key OOCs in serious spoilage GCC were cyclohexane isothiocyanato, butylated hydroxytoluene, putrescine, cadaverine and histamine compared to those of spoilage GCC. Correlation analysis showed that 12 amino acids and 10 fatty acids played important roles in the formation of key OOCs. This study provides a theoretical basis for a comprehensive understanding of the formation of key OOCs in GCC during room temperature storage.

The evolutionary mechanism and function analysis of two subgroups of histamine-producing and non-histamine-producing Tetragenococcus halophilus

Tetragenococcus halophilus is a halophilic bacterium that existed in the fermentation of soy sauce and miso for flavor production and probiotic benefits. However, it is composed of two subgroups, histamine-producing and non-histamine-producing, with the former causing histamine accumulation and offering risks to food safety. Exploring the evolutionary mechanisms and physiological function of histamine-biosynthesis is of significance for understanding the formative mechanism of T. halophilus's strain-specificity and is helpful for microbial control. Using systematic genomic analysis, we found that plasmid acquisition and loss is the evolutionary form resulting in the two subgroups of T. halophilus. Two plasmids, plasmid α with 30 kb and plasmid β with 4 kb existed in histamine-producing T. halophilus. We investigated the whole genetic information and proposed their genetic function in both two plasmids. The acquisition of histamine-producing plasmid enhanced the acid tolerance of histamine-producing T. halophilus but did not affect salt tolerance. More interestingly, we found that the existence of plasmid will promote the co-culture growth of T. halophilus. This study deepens our understanding of the formative mechanism of microbial species diversity, and provides our knowledge of the physiological function of histamine-producing plasmids.