What are the functions of common lactic acid bacteria in fermented milk?

Dairy products almost contain nutrients needed by human body and are one of the best foods to enhance physical fitness and human immunity. Facing the sudden outbreak of COVID-19 in early 2020, the government and trade associations issued the document that dairy products are beneficial to human health for the first time in the same year. The "Guidelines for the Prevention and Treatment of Pneumonia in novel coronavirus" issued by National Health Commission proposed "try to drink 300 grams of dairy products every day"; ? During the epidemic, many anti-epidemic experts and experts in the field of nutrition and health have suggested that drinking milk or increasing the consumption of dairy products is of great benefit to enhancing the body's immunity, and drinking milk has become a "just need" to fight the epidemic. Therefore, it is necessary to publicize the popularization knowledge of dairy products, so that more people can understand and eat dairy products, enhance their physique and improve their own resistance.

According to research, lactic acid bacteria is a kind of probiotics in human body. Lactic acid bacteria can ferment carbohydrates into lactic acid, hence the name. Probiotics can help digestion and contribute to human intestinal health, so they are often regarded as healthy foods and added to yogurt. This is a complex group of bacteria, which can be divided into at least 18 genera and more than 200 species. Except for a few, most of them are indispensable flora with important physiological functions in the human body and widely exist in the human intestine. Biologists at home and abroad have confirmed that intestinal lactic acid bacteria have a very close direct relationship with health and longevity, and lactic acid bacteria and probiotics can really improve human immunity.

What are the functions of common lactic acid bacteria in fermented milk?

Fermented milk (commonly known as yogurt) is a kind of product in dairy products and is very popular with the public. Suitable for people of all ages, especially those who are lactose intolerant. Second, rich nutrition, which has long been recognized by people. It is easy to be absorbed and has the function of improving the balance of gastrointestinal flora. Lactic acid bacteria in fermented milk can activate macrophages and enhance human immunity.

Why does fermented milk have such good functional characteristics? Fermented milk contains a large number of active lactic acid bacteria in addition to the high nutritional components in milk. Lactic acid bacteria are a kind of lactic acid bacteria that can produce a large number of lactic acid bacteria by using carbohydrates. The common lactic acid bacteria in fermented milk are Lactobacillus bulgaricus, Streptococcus thermophilus, Bifidobacterium lactis, Lactobacillus plantarum and Lactobacillus acidophilus. Most of these strains have probiotic functions and belong to the category of probiotics.

First, common lactic acid bacteria 1 in fermented milk. Streptococcus thermophilus

Streptococcus thermophilus is the main fermentation strain in fermented milk. Streptococcus is a facultative anaerobic gram-positive bacterium. The optimum temperature is 37℃~42℃ and the optimum pH is 6.8. It has fast growth and reproduction, high acidification activity and stable fermentation performance, which can shorten the coagulation time of fermented milk and improve the texture of the product. At the same time, Streptococcus thermophilus is a bacterium that produces flavor substances in fermented milk. During the fermentation process, it can produce volatile compounds such as caproic acid, acetoin, diacetyl and acetaldehyde, giving the product excellent flavor. 2. Lactobacillus bulgaricus (Lactobacillus delbrueckii subsp. Bulgaria)

Lactobacillus delbrueckii subsp. Lactobacillus bulgaricus is also an important fermentation strain, belonging to the genus Lactobacillus, facultatively anaerobic, and the optimum growth temperature is 37℃~43℃. This strain is the most acid-producing strain of lactic acid bacteria, which is suitable for growing under acidic conditions (pH 5.5~6.2) and has a fast fermentation speed. Lactobacillus delbrueckii subsp. Lactobacillus bulgaricus has strong acid resistance, which is the main strain leading to over-fermentation and post-acidification of fermented milk. In view of this, Lactobacillus bulgaricus and Streptococcus thermophilus are usually used in most fermented milk, and the ratio of cocci to bacilli is 1: 1 or 2: 1, so as not to let bacilli dominate and prevent the acidity from being too high. 3. Bifidobacterium lactis

Bifidobacterium animalis is a probiotic strain widely used in fermented dairy products. It belongs to the genus Bifidobacterium and is a Gram-positive strict anaerobic bacteria. The optimum growth temperature is 37℃ ~ 41℃. Bifidobacterium lactis grows slowly in milk and has weak acid-producing ability. It is not a typical fermenting bacteria, but it is an important physiological bacteria in human and animal intestines. It participates in a series of physiological processes such as immunity, nutrition, digestion and protection, and has the functions of biological barrier, immune enhancement and improvement of gastrointestinal tract. 4. Lactobacillus embryonus

Lactobacillus plantarum has good fermentation ability and probiotic function, and is often used as an auxiliary starter. Belonging to Lactobacillus, anaerobic or facultative anaerobic, the optimum growth temperature is 30℃ ~35℃, and the optimum pH is about 6.5. Lactobacillus plantarum has strong viability and acid-producing ability, is salt-tolerant, can produce Lactobacillus during fermentation, and has antibacterial effect. Lactobacillus plantarum is a recognized probiotic, which has the functions of immunomodulation, reducing serum cholesterol content, maintaining intestinal flora balance and adsorbing heavy metals.

5. lactobacillus acidophilus

Lactobacillus acidophilus is an important microorganism in human intestine, belonging to Lactobacillus, anaerobic or facultative anaerobic. The optimum temperature is 37℃ ~38℃, and the optimum pH is 5.5 ~ 6.0. Lactobacillus acidophilus has outstanding acid and bile salt tolerance, high survival rate, and can successfully colonize the intestine. It has the functions of lowering blood fat, cholesterol, regulating intestinal flora and improving immunity, and is known as the third generation yogurt starter.

Second, lactic acid bacteria cooperate to complete the fermentation process.

The ideal fermented milk has the characteristics of good texture, soft acidity, attractive flavor, full taste and practical function. A single lactic acid bacteria starter usually has some defects in acid production, flavor production, viscosity production, protein hydrolysis and functionality, which can not meet the needs of consumers. In order to make up for the shortcomings and deficiencies of a single strain, all strains need to cooperate. Usually, Streptococcus thermophilus and Lactobacillus bulgaricus are used as starter, lactose is converted into lactic acid through heterotypic fermentation, which reduces the pH value of milk, denatures casein and forms the unique curd state of fermented milk. At the same time, they are also the main sources of flavor components in fermented milk, and more than 0/00 flavor substances such as aldehydes, ketones, esters, organic acids and aromatic hydrocarbons are produced in the fermentation process, among which acetaldehyde and diacetyl are the most important flavor compounds.

Bifidobacterium lactis, lactobacillus plantarum, lactobacillus acidophilus, etc. Mainly used as probiotics. They have good acid resistance and bile salt resistance, and can usually pass the gastric acid test to reach the intestine, promote the proliferation of beneficial bacteria, maintain the balance of intestinal flora, reduce the serum cholesterol content and enhance the immunity of the body.

At present, most fermented milk is fermented by traditional strains (Streptococcus thermophilus and Lactobacillus bulgaricus) combined with probiotics, but some products are fortified by adding probiotics with regional characteristics, such as Greek yogurt, Russian yogurt and Icelandic yogurt.