Currently, there are more than 40 kinds of lactic acid bacteria known, and there are four genera classified under Lactobacillus, namely Lactobacillus, Streptococcus, Streptococcus, Streptococcus, and Streptococcus. Lactic acid fermented food is currently recognized as a functional health food in the world. The role of lactic acid bacteria mainly includes inhibiting the growth of spoilage bacteria in the intestinal tract and attenuating the toxicity of spoilage bacteria in the intestinal tract, as well as preventing constipation, preventing cellular aging, preventing cancer, and regulating the human body’s physiological functions and other health care and medical effects.

Basic Characteristics

Lactobacillus is a group of Gram-positive bacteria with different morphology, metabolic properties and physiological characteristics, whose general characteristics are G+, non-spore-forming, non-respiratory cocci or bacilli, and whose main metabolite is lactic acid in the fermentation of carbohydrates.

Physiological functions

2.1 Produce special enzyme system
Lactobacillus can produce some special enzymes, such as the enzyme system for producing organic acid, the enzyme system for controlling endotoxin, the enzyme system for synthesizing various vitamins and the enzyme system for decomposing bile acids. These enzymes can not only accelerate the growth of lactic acid bacteria and maintain the micro-ecological balance of the intestinal tract, but also improve the flavor of the products and promote the maturation of dairy products, fermented sausages and other food products.

2.2 Adhesion and colonization ability
Adhesion refers to the specific adhesion between bacteria and the intestinal epithelial cells of the organism through biochemistry, and colonization resistance refers to the host’s resistance or resistance to the colonization and reproduction of pathogenic bacteria and potential pathogenic bacteria in the normal microbiota.

2.3 Metabolites producing bacteriostatic activity
The metabolism of lactic acid bacteria can produce organic acids, hydrogen peroxide and bacteriocins and other metabolites, which can not only improve the flavor and organization of products, but also inhibit spoilage bacteria and pathogenic bacteria in food.

The antibacterial mechanism of lactic acid bacteria on some spoilage bacteria and low-temperature bacteria is mainly manifested in the following aspects: one is the production of lactic acid and other organic acids to make the intestinal in an acidic environment; the second is the production of hydrogen peroxide to activate the hydrogen peroxidase – thiocyanic acid system in cow’s milk, to inhibit and kill the Gram-negative bacteria, catalase-positive bacteria, etc.; the third is to produce a similar bacteriocin of small proteins or peptides.

2.4 Lowering cholesterol
Lactobacillus bacterial constituents or extra-bacterial metabolites have anti-cholesterol factors, and the metabolism of lactobacilli can significantly reduce the absorption of cholesterol in the intestinal canal, meanwhile, lactobacilli absorb part of the cholesterol and transform it into bile salts to be excreted from the body.

2.5 Other functions
Research results show that probiotics or their metabolic by-products can inhibit the proliferation of cancer cells by influencing the process of changes in the colon epithelial cells and blocking non-genotypic and genotypic mutations occurring in the colon and other organs.

Application in food processing

3.1 Application in dairy product processing
3.1.1 Yogurt production
Yogurt is a product produced by fermentation of lactic acid bacteria with fresh milk or milk powder as the main raw material, and generally two or more strains of bacteria are used in combination. The mixed strain of Lactobacillus bulgaricus and Streptococcus thermophilus is more commonly used in the production of pure culture fermentation.

3.1.2 Production of Cream
Cream is a dairy product made from thin cream obtained from centrifugal separation of raw milk, sterilized, cooled, ripened, fermented by lactic acid bacteria, stirred and pressed. Lactic acid bacteria used in the production of cream include Streptococcus lactis, Streptococcus paracitricus, Streptococcus butyricus and so on.

3.1.3 Cheese production
Cheese is the raw milk after sterilization, cooling, adding the appropriate amount of lactic acid bacteria fermentation agent fermentation, so that the protein coagulation, exclude the whey, the curd will be pressed into a block and made of products. Commonly used strains are Streptococcus lactis, Streptococcus lactis, and Streptococcus citrophilus, etc., which are usually selected in the production of more than 2 kinds of lactic acid bacteria mixed use.

3.2 Plant protein beverage processing
The application of lactic acid bacteria in the processing of plant protein beverages is mainly the production of yogurt and peanut lactic acid fermented yogurt, lactic acid bacteria used in the production of yogurt and peanut lactic acid fermented yogurt mainly belong to the genus of Lactobacillus, Streptococcus, in addition, there are also the genus of Streptococcus, commonly used in the fermentation of two or more strains of mixed bacteria.

3.3 Application in meat production
The production of traditional fermented sausage does not add any fermentation agent, and the fermentation and maturation is completed entirely by the meat, spices and microorganisms in the environment. The modern production process of fermented sausage is to add microorganisms pure culture of lactic acid bacteria fermentation agent to the raw materials, so as to ensure that the lactic acid bacteria in the whole process of fermentation and maturation of the absolute advantage. The microorganisms commonly used today are Lactobacillus, Lactococcus, Micrococcus, molds and yeasts.

3.4 Application in food preservation and conservation
The antimicrobial mechanism of lactic acid bacteria is mainly manifested in three aspects: the production of lactic acid and other organic acids, so that the growth and reproduction of most acid-intolerant spoilage bacteria and pathogenic bacteria is inhibited; the production of small proteins or peptides similar to bacteriocins; and the production of H2O2 to activate the peroxidase-thiocyanate system in cow’s milk.

Prospects for the application of lactic acid bacteria

Recent studies have shown that lactic acid bacteria have many special health care effects. Domestic and foreign research on lactic acid bacteria is also more and more in-depth, Japan, Sweden and other countries have carried out research on the treatment of gastric ulcer and gastroenteritis with lactic acid bacteria.

Therefore, how to use modern microbial technology and molecular biotechnology to transform lactic acid bacteria, improve the identification of lactic acid bacteria, detection methods to enhance the quality of lactic acid bacteria, to ensure the safety of their food applications will be the direction of future in-depth research.