Metabolic and Transcriptomic-Based Characterization of Streptococcus salivarius ssp. thermophilus Snew Fermentation in Milk

Fermented milk has a long history. It is fermented by lactic acid bacteria and is rich in protein, minerals, vitamins, and other nutrients. As people's pursuit of quality of life improves, consumers are paying increasing attention to fermented milk. <i>Streptococcus salivarius</i> ssp. <i>thermophilus</i> is commonly used to make fermented milk. This study investigated the fermentation characteristics and physicochemical properties of <i>Streptococcus salivarius</i> ssp. <i>thermophilus</i> Snew-fermented milk, as well as transcriptomic and metabolomic analyses of different fermentation stages. <i>Streptococcus salivarius</i> ssp. <i>thermophilus</i> Snew can be used as a fermenter strain, as evaluated from the point of view of fermentation time, titratable acidity, post-acidification, viable bacteria count, water holding capacity, and viscosity. The flavor and odor of Snew-fermented milk varied across fermentation stages. The analysis of the detected volatiles revealed that ketones and esters were the main substances responsible for the flavor of Snew-fermented milk. The differentially expressed genes and differential metabolites screened from several categories, such as carbohydrates, proteins, amino acids, fats, and fatty acids, varied at different fermentation stages, while differentially expressed genes and differential metabolites were also threaded together for joint analysis in this study. This study provides theoretical guidance for the practical production application of <i>Streptococcus salivarius</i> ssp. <i>thermophilus</i> in cow's milk fermentation.