Abstract
Streptococcus thermophilus is one of the most valuable homo-fermentative lactic acid bacteria, which, for a long time, has been widely used as a starter for the production of fermented dairy products. The key production characteristics of S. thermophilus, for example the production of extracellular polysaccharide, proteolytic enzymes and flavor substances as well as acidifying capacity etc., have an important effect on the quality of dairy products. The acidification capacity of the strains determines the manufacturing time and quality of dairy products. It depends on the sugar utilization ability of strains. The production of extracellular polysaccharide is beneficial for improving the texture of dairy products. Flavor substances increase the acceptability of dairy products. The proteolytic activity of the strain influences not only the absorption of the nitrogen source, but also the formation of flavor substances. Different strains have obvious differences in production characteristics via long-time evolution and adaptation to environment. Gaining new strains with novel and desirable characteristics is an important long-term goal for researchers and the fermenting industry. The understanding of the potential molecular mechanisms behind important characteristics of different strains will promote the screening and breeding of excellent strains. In this paper, key technological and functional properties of different S. thermophilus strains are discussed, including sugar metabolism, proteolytic system and amino acid metabolism, and polysaccharide and flavor substance biosynthesis. At the same time, diversity of genomes and plasmids of S. thermophilus are presented. Advances in research on key production characteristics and molecular levels of S. thermophilus will increase understanding of molecular mechanisms of different strains with different important characteristics, and improve the industrialization control level for fermented foods.
Highlights
Streptococcus thermophilus is the only streptococcal species widely used in food fermentations, especially for yogurt manufacturing
Some research indicates that the proteolytic system, nitrogen metabolism, sugar utilization and transporter systems of S. thermophilus are very important during the adaptation to milk environments [3,6,15,25,28]
Galactose accumulation leads to the growth of undesirable lactic acid bacteria (LAB) in milk products, and cheese browning during baking
Summary
Streptococcus thermophilus is the only streptococcal species widely used in food fermentations, especially for yogurt manufacturing. The acidifying capacity, proteolytic activity, EPS and flavor production abilities are key production characteristics of strains. Strains show different fermentation characteristics, including the ability to produce acid, adaptability to environmental stress, protein decomposition ability, sugar utilization ability, EPS and bacteriocin production abilities etc. Some research indicates that the proteolytic system, nitrogen metabolism, sugar utilization and transporter systems of S. thermophilus are very important during the adaptation to milk environments [3,6,15,25,28]. As a self-replication DNA molecule, plasmids do not have the necessary genetic material for the survival of bacteria, but they often carry some special genes which confer important traits for strains, for example, casein utilization, EPS biosynthesis, potassium transport, bacteriophage resistance, and bacteriocin production related genes etc. Plasmids pSMQ-316 and pSMQ-312b are less than 10 kb, they replicate via θ-replicating mode [39]
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