Lactic acid bacteria are often called dairy starter cultures and are used for the production of various fermented milk products. In some cases, in addition to lactic acid bacteria, few non-lactic acid starters (bacteria, yeast and mold) are used during the production of specific fermented milk products, such as kefir, kumiss and mold-ripened cheeses. There are two major groups of starter cultures used in the preparation of fermented milk products, classified according to their properties. a) Physiological and growth characteristics, such as (i) Mesophilic starter culture. ii) Thermohillic starter culture (b) Biochemical characteristics such as. i) Homofermentative lactic acid bacteria. (ii) Heterofermentative lactic acid bacteria.
The importance of fermented milk derived from mesophilic fermentation is the consistency which is due to the lactic acid coagulation of milk proteins and the aroma and flavor produced by the fermentation of citric acid and lactose. These lactic acid bacteria are characterized by their ability to ferment lactose almost exclusively to lactic acid while pentoses and gluconate are not fermented. These organisms are coccoid, singly or in pairs, short chains, gram positive, microaerophilic, heterofermentative with lactic acid, produce D(–)-lactate, CO2 and aromatic compounds from lactose, mesophilic (optimal growth 20 to 30°C).
Aldolase-containing lactic acid bacteria, i.e., streptococci, lactococci, pediococci, and obligately homofermenting lactobacilli, perform homolactic fermentation with the production of only lactic acid as the final product. The conversion of lactic acid to propionic acid gives the characteristic sweet taste in Swiss cheese, while CO2 helps form the eyes, a typical regular hole in the cheese body, which is essential in Swiss cheese. In addition, acetaldehyde can also be produced by lactic acid bacteria from nucleic acids, lipids and aromatic compounds in milk (Fig. 5).
The overall proteolytic system of lactic acid bacteria is very weak, but it is sufficient to allow exponential growth in milk. Lactic acid bacteria exert an antagonistic effect on some other organisms, due to the production of some antimicrobial substances. However, nisin is the only one, which is fully characterized and used as a food preservative, other bacteriocins produced by lactic acid bacteria are Acidophilin, Lactocidin, Brevicin, Helveticin, etc.
Early in the century, Metchnikoff claimed that lactic acid and other products in sour milk fermented by lactic acid bacteria inhibited the growth and toxicity of anaerobic spore-forming bacteria in the colon. Lactic acid is biologically active and capable of suppressing harmful microorganisms, especially putrefactive ones, and thus has a beneficial effect on human vital activity. Fermented dairy products are usually spoiled by yeasts and molds and lactic acid bacteria, which can cause sour, bitter and cheesy tastes.
Fermented milks (yogurt, kefir, dahi) have higher protein digestibility due to the precipitation in fine particles of curd by lactic acid which contributes to its higher nutritional value and its capacity to regenerate liver tissue. The importance of lactose is due to the lactic acid produced by the hydrolysis of lactose, which leads to a pH range in the gut that inhibits the growth of putrefaction. The production of free fatty acids as a result of lipolytic activity is increased due to lactic acid bacteria compared to milk.
In fact, there are no significant changes in minerals in milk after or during the fermentation process by lactic acid bacteria, and the nutritional values of fermented milk products remain intact.
Fruit and Flavored Yoghurt (b) Types of Yogurt
Dahi or curd is an Indian fermented milk product which is equally known for its deliciousness, refreshing taste and therapeutic importance as claimed in Ayurvedic literature. The starter culture used in making dahi is normally leftover dahi from the previous day. The traditional method of making dahi invariably involves a small scale, either in the consumer's home or in sweet shops in urban areas.
At home, the milk is boiled, cooled to about 37 °C and inoculated with 0.5 - 1 percent starter (dahi or buttermilk from the previous day) and allowed to solidify overnight. A standardized method for preparing dahi is given below in the flow chart (Figure 10). The composition and quality of dahi varies greatly from one place to another, as it is prepared under different domestic conditions, as well as the milk, of variable chemical and bacteriological quality, used for its preparation.
Dahi quality can be improved in relation to increasing riboflavin and folic acid by including propionic acid bacteria such as Propionibacterium shermani along with the dahi starter culture. The milk is then cooled to 22°C and initial cultures of desirable bacteria, such as Lactococcus lactis, Lac.cremoris, Leuconostoc citrovorum and Leu. At the correct acid and flavor stage, the product is gently stirred to break the curd and cooled to 7.2°C (45°F) to stop fermentation.
It must contain at least 76.0 wt.% of milk fat. a) Types of butter and production processes. In this method of preparation, the cream is cooled to 16-21 °C after pasteurization and inoculated with approximately 4 percent of a mixed starter culture containing the acid agent Lactococcus lactis and/or Lac. The ripening process always takes place in two or three stages to facilitate the cooling of the highly viscous ripened cream.
It is immediately packaged, making it a fresh cheese such as Ricotta cheese or cottage cheese, or it is aged with different curing methods. It is a "stretched" cheese which means that during the manufacturing process the curd is pulled, kneaded and shaped while it is still pliable. Mozzarella is also low in fat; therefore it is ideal to use even when dieting.
It tastes best when it is at room temperature and the center softens and it is a mold-ripened cheese. It is prepared by melting one or more pressed, cooked or uncooked cheeses and adding milk, cream, butter and sometimes flavorings.
Processed Cheese
It therefore absorbs the flavors and juices of the ingredients surrounding it and is perfectly shaped for cooking. One or more ripened cheeses are heated and mixed, then pasteurized at a high temperature (130-140 °C) after adding other dairy products such as liquid milk or milk powder, cream, butter, casein, whey and spices ( figure 4). ). Cheesemaking is essentially a dehydration process in which the casein, fat, and colloidal salts of milk are concentrated 6 to 12 times (about 10 times in cheddar cheese) by removing 90 percent of the milk's water and nearly all of the lactose, the whey. proteins and soluble milk salts.
Concentration of the total colloidal phase of milk by ultrafiltration is now used commercially in the manufacture of a few varieties, for example feta and quarg. Rennet cheeses are usually ripened after manufacture, when the typical characteristics of the individual cheese develop. Consequently, a number of rennet substitutes from animal and microbial sources are used successfully in Cheddar cheese in the cheese industry.
Modern cheese production relies increasingly on microbial sources of the most important enzyme, chymosin protease. It is now produced by recombinant microorganisms. They also affect the taste of the final product by producing specific flavor and aroma compounds and perform essential proteolysis and lipolysis in later ripening. Emphasis is now focused on speeding up cheese ripening because of the obvious economic and technological advantages.
Proteolysis occurs in all cheese varieties and is considered a prerequisite for good flavor development. By controlling the production and ripening conditions, excellent quality cheese can be produced. Lactobacilli and bifidobacteria maintain a healthy balance of intestinal flora by producing organic compounds such as lactic acid, hydrogen peroxide and acetic acid, which increase the acidity of the intestines and inhibit the reproduction of many harmful bacteria.
Probiotics can increase the bioavailability of dietary protein and fat by breaking down these nutrients in the digestive tract. Enterococcus faecium SF68 is a probiotic strain that has been used in the management of diarrheal disease. Adherence, colonization and multiplication in the gut are the most important selection criteria for a probiotic culture.
