Introduction
Starter refers to a group of active microorganisms (either bacterial or fungal species or a mixture of both) that can cause desirable changes in milk products through a controlled fermentation process. In fact, these microorganisms are intentionally and selectively added to milk to initiate fermentation, causing certain changes in the product’s appearance, texture, and taste by affecting the milk’s pH.
The natural flora of milk is unpredictable and uncontrollable, which is destroyed during the heating process of milk.
History of starter culture and fermentation products
Fermented foods enrich the human diet and provide a wide range of flavors, aromas, and textures. Fermented dairy products have been used ever since our ancestors did not know about lactic acid bacteria. The milk was fermented spontaneously at room temperature and then used in the same way or after mixing with water. Then, as a fermented and whey-drained mixture, a new product was introduced, which was raw cheese and had a longer shelf life. Inoculation of milk with a small amount of fermented milk has been tested for a long time.
The starters were still on the road to improvement and production. In the late 19th century, companies were growing to specialize in producing the first starters, although starters contained unknown mixtures of different species. Later, with the advancement of science, starters were defined with specific strains and with fewer species
| Function | Result |
| Produced acid | Gel formation / whey withdrawal / milk preservation / flavor development |
| Flavor | Formation of flavoring compounds such as: diacetyl and acetaldehyde |
| maintenance | Production of lactic acid / Production of antibiotics / Production of H2O2/ Production of acetate |
| Gas formation | Eye formation in some cheeses |
| Formation of stabilizers | Creation of polysaccharide materials and development of viscosity |
| Use of lactose | Reduced gas production and neutral flavors suitable for people with lactose intolerance |
| Reduction of oxide and reduction potential | Help develop product flavor and preservation |
| Proteolysis and lipolysis | Help reach the cheese |
| Other compounds | Production of alcohol in kefir and kumis |
diversity were obtained to produce standard products. Fermented dairy products currently rank second in the fermentation industry after alcoholic beverages.
Definition
The most common reason for using starter cultures is the production of lactic acid from milk sugar (lactose), which by lowering the pH, causes coagulation or helps coagulate milk protein. Lactic acid-producing cultures are generally called lactic acid bacteria or LAB. LABs are of particular importance because of their ability to produce flavoring compounds such as diacetyl, as well as their effect on the texture of fermented or cultured products by fermenting proteins and fats and inhibiting spoilage by producing by-products of growth. (Table 1 – Culture starter functions)
Industrial classification of starter cultures
Starter cultures can be divided into different cases:
Industrial classification based on cell morphology
Starters are either rod-shaped or coke-shaped. Both forms are single-celled or paired but are often connected by short chains at the ends.
Industrial classification based on fermentation
Several lactic acid bacteria, while producing lactic acid, are added to milk to produce flavoring compounds, which are called secondary starters.
Heterofermentative starters convert lactose into about 50% lactic acid and 40% or more into products such as ethanol, acetaldehyde, acetic acid, and carbon dioxide. These bacteria are more likely to be considered secondary flora than primary starters. Homonormative starters convert lactose to more than 90% lactic acid and a small amount to acetic acid.
Industrial classification based on optimal growth temperature
The temperature related to the characteristics of each bacterium is considered as a range due to the natural diversity of species and strains. However, the optimum growth temperature is less than the maximum temperature at which bacteria still grow and even much lower than the temperature at which they begin to die.
Mesophilic stars: The suitable temperature for the activity of these starters is 20-32°C, but this group can ferment lactose at a temperature of 10°C and a maximum of 40°C.
Thermophilic starters: The optimum temperature for the activity of this group is 37-45°C and of course, they can ferment lactose at a temperature of 20 and a maximum of 50°C. Many thermophiles can survive even temperatures and times slightly higher than what is used to meet the minimum pasteurization requirements. Bacteria that can survive pasteurization can form a biofilm in the cooling part of the pasteurizer. These biofilms are the main source of bacterial contamination in milk.
The role of thermophilic cultures in fermented dairy products
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 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 depend on the metabolism of the lactic starter. For cheese, the bacterial cells of the starter release the enzymes that 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 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, gruyere, 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 the 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
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 the 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 resistance to biological barriers (gastric juice and bile salts) have also been reported for S. thermophilus. 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 is different.
For more information about starter cultures and prices, contact our experts.
References:
Auclair, J. and Accolas, J.P., 1983. Use of thermophilic lactic starters in the dairy industry. Antonie van Leeuwenhoek, 49, pp.313-326. DOI: 10.1007/BF00399506
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. DOI: 10.1016/j.idairyj.2009.10.005
Oberg, C.J. and Broadbent, J.R., 1993. Thermophilic starter cultures: another set of problems. Journal of Dairy Science, 76(8), pp.2392-2406. DOI: 10.3168/jds.S0022-0302(93)77576-1
