Thermophilic Starter Cultures

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Thermophilic lactic starters are composed of streptococci and lactobacilli having an optimal growth temperature of around 45 °C.


The role of thermophilic lactic starters is two-fold. Firstly, they transform lactose to lactic acid, thus lowering the pH of the milk or of the cheese curd. This step is essential for yogurt whose final pH is 4, as it prevents the development of spoilage microorganisms and possibly pathogens. In the case of cooked cheeses, acidification, although limited by the buffering capacity of the curd, contributes to syneresis, i.e. dehydration of the curd, in a fixed proportion and at the proper time (this is the draining stage under the press). After pressing the cheese has low water content and a pH sufficiently low to support the long ripening period (several months) successfully.

Secondly, they contribute to the organoleptic qualities of the final product. In the case of yogurt, this role is particularly important, as the consistency and flavor of the product depending on the metabolism of the lactic starter. For cheese, the bacterial cells of the starter release the enzymes which intervene during ripening in conjunction with rennet. Therefore, varying with the type of product, proteolysis determines the rheological properties of the cheese and gives rise to flavor compounds or aroma precursors which, after modification by various microorganisms or by purely chemical reactions, will give the ripened cheese its organoleptic characteristics.


The genus Streptococcus consists of Gram-positive, nonmotile, spherical, or ovoid cells that are typically arranged in pairs or chains when grown in liquid media. All species are facultatively anaerobic, some requiring additional CO2 for growth. They are non-sporing, catalase-negative, homofermentative, and have complex and variable nutritional requirements. They metabolize carbohydrates by fermentation resulting mainly in lactic acid but no gas. Their temperature optima are usually around 37 °C, but maximum and minimum temperatures vary somewhat amongst species.


Streptococcus thermophilus is of major importance for the food industry since it is massively used for the manufacture of dairy products (annual market of around 40 billion USD) and it is considered as the second most important industrial dairy starter after Lactococcus (Lc.) lactis. This bacterium belongs to the group of thermophilic lactic acid bacteria and is traditionally used in combination with Lactobacillus delbrueckii subsp. bulgaricus (Lb. bulgaricus) or Lb. helveticus for the manufacture of yogurt and so-called hard ‘‘cooked’’ cheeses (e.g., emmental, gruye`re, grana), at a relatively high process temperature (45 C). S. thermophilus is always used together with Lb. bulgaricus for yogurt making, which led to the development of a complex symbiotic relationship (‘‘proto-cooperation’’) between these two microorganisms. S. thermophilus is also used alone or in combination with lactobacilli for the production of mozzarella and cheddar cheeses.


One of the main roles of S. thermophilus in milk fermentation is to provide rapid acidification. In addition to lactic acid, it also produces low levels of formate, acetoin, diacetyl, acetaldehyde, and acetate as additional end-products. Thus the role of S. thermophilus in fermentation of milk is not related only to the production of lactic acid; it also has several other important technological properties, such as sugar metabolism, galactose utilization, proteolytic activity, and urease activity. This diverse technological performance represents the degree of phenotypic diversity existing within the species. In addition, research on the physiology of S. thermophilus has revealed important information on the genetic basis for many of these traits.



Sugar metabolism


S. thermophilus has a limited capacity to utilize carbohydrates, and the primary function of S. thermophilus in industrial dairy fermentation is the conversion of lactose to lactate at elevated temperatures. S. thermophilus, unlike many other Gram-positive bacteria, prefers lactose to glucose as its primary carbon and energy source, which has led to the adaptation of the global control mechanism towards the fine-tuning of lactose uptake and subsequent catabolism by glycolysis. S. thermophilus is unable to metabolize galactose (Gal) and thus expels this sugar into the medium during lactose fermentation.

Proteolytic system


LAB are nutritionally fastidious, needing an exogenous supply of amino acids to initiate growth. The proteolytic system of S. thermophilus comprises more than 20 proteolytic enzymes and is composed of (i) an extracellular cell anchored protease capable of casein hydrolysis, (ii) a set of amino acid and peptide transport systems required for import of amino acids, and (iii) a set of intracellular peptidases involved in the hydrolysis of casein-derived peptides essential for various housekeeping processes.


Probiotic attributes of S. thermophilus


Although S. thermophilus is known to be sensitive to gastric acidic conditions, it has also been shown to survive Gastro-Intestinal (GI) transit and moderately adhere to intestinal epithelial cells. Other probiotic characteristics (deconjugation of bile salts, hydrophobicity and b-galactosidase activity) and the resistance to biological barriers (gastric juice and bile salts) have also been reported for S. thermophilus.
S. thermophilus has been shown to have positive effects on diarrhea in young children, enterocolitis in premature neonates, and inflammatory gut disease.


Exocellular polysaccharides


A large group of exocellular polysaccharides is produced by lactic acid bacteria, including many polysaccharides produced by thermophilic organisms.
Exocellular polysaccharides improve consistency and viscosity in fermented dairy products by binding free water and slowing whey separation. These qualities are particularly valuable in stirred yogurt, which suffers a breakdown in viscosity during processing; exocellular polysaccharides minimize this breakdown.
Danisco thermophilic starter cultures are produced and supplied as pure strains or in combination with mesophilic starter cultures, depending on the type of application and the type of starter culture, the price range of these starter cultures is different.
For more information about starter cultures and prices, you can contact our experts.


References:


-    Auclair, J. and Accolas, J.P., 1983. Use of thermophilic lactic starters in the dairy industry. Antonie van Leeuwenhoek, 49(3), pp.313-326.
-    Iyer, R., Tomar, S.K., Maheswari, T.U. and Singh, R., 2010. Streptococcus thermophilus strains: Multifunctional lactic acid bacteria. International Dairy Journal, 20(3), pp.133-141.
-    Oberg, C.J. and Broadbent, J.R., 1993. Thermophilic starter cultures: another set of problems. Journal of Dairy Science, 76(8), pp.2392-2406.

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