Fermentation type

1. Homolactic fermentation

Homolactic fermentation refers to the process of lactic acid fermentation in which more than 90% of the final product of fermentation is lactic acid. During lactic acid fermentation of streptococcus and most lactobacilli, lactose is decomposed into glucose and galactose by β-galactosidase. Then glucose is converted into 2 molecules of pyruvate according to the glycolysis pathway, and then catalyzed by lactate dehydrogenase to reduce to acid, galactose is converted into galactose-1-phosphate, glucose-1-phosphate, glucose-6-phosphate, and finally enters the glycolysis pathway (EMP pathway), is converted into 2 molecules of pyruvate, and then restored to lactolactic acid.

2. Heterolactic fermentation

Heterolactic fermentation refers to the lactic acid fermentation process in which in addition to lactic acid, there are also ethanol, carbon dioxide and other components in the final product of fermentation. Lactobacillus of the genus leuconostoc and certain lactobacilli, such as enteromembrane leuconostoc, lactobacillus brevis, lactobacillus mannitol and rhizopus in fungi. Lactose is decomposed into glucose and galactose. Galactose is converted to glucose-6-phosphate through a series of biochemical reactions, and together with glucose-6-phosphate, which is directly produced from glucose, it is produced by the hexose phosphate pathway (HMP pathway) to acetyl phosphate and glyceraldehyde 3-phosphate. The former is then reduced to ethanol, the latter enters the glycolysis pathway to convert to pyruvate, which is further restored to lactic acid. Carbon dioxide is produced during the metabolism of the HMP pathway.

Application of lactic acid bacteria in food industry

1. Application in dairy products

The most common use of lactic acid bacteria in dairy products is to make yogurt. The activated lactic acid bacteria were used as the starter culture to ferment the pasteurized raw milk, and yogurt products could be obtained after fermentation. Yogurt not only has the nutritional value of raw milk itself, but also adds many microelements in the fermentation process, which helps the human body to absorb and utilize protein, and it is a dairy product with special flavor, which is suitable for all ages. The starter of yogurt is generally composed of 2 or more than 2 kinds of lactic acid bacteria.

Dairy products fermented by lactic acid bacteria have become an important part of human food and a world-recognized health drink. In addition to improving the nutritional value of dairy products, lactic acid bacteria also have the following effects:
①Produce lactic acid, relieve lactose intolerance;
②Improve the utilization efficiency of protein;
③Form the substances with special flavor;
④Regulate the immune system and improve the body's resistance to disease;
⑤Improve the absorption of mineral elements;
⑥Improve the balance of intestinal microbiota;
⑦Reduce the  cholesterol content of serum;
⑧Produce bacteriostatic substances, act as the role of sterilization.

1.1. Yoghurt

Yogurt is rich in protein, calcium and life support, it is sweet and sour, which is suitable for all ages, and it has the history of 5 000 years. The process of yogurt is as follows: ① add raw milk; ② Pasteurization; ③ add lactic acid bacteria as starter culture; ④ add white sugar and essence after fermentation; ⑤ Cooling and filling.

For people who are lactose intolerant, yogurt can be used instead of milk. Fruits and vegetables, grains, etc.,added into yogurt,then it can be fermented into products with different types, taste and mouthfeel. At the same time, improved peanut yogurt, mung bean yogurt , banana yogurt etc.,have expanded the dairy market.

1.2. Cheese

Cheese is a natural food. As the concentrated dairy product, cheese is rich in nutrition. Protein, fat, sugar, organic acids, minerals, vitamins and other substances are many times higher than raw milk, and it is favored by consumers. Milk, light cream, partially skimmed milk, buttermilk or the mixture of these products can be made into fresh or mature cheese by the action of lactic acid bacteria starter culture and rennet preparation. Cheese contains various nutrients and bioactive substances, but also contains  lactic acid bacteria for healthcare.

1.3. Fermented cream

Fermented cream is usually made from pasteurized light cream. It is prepared from fresh milk after centrifugation, sterilization, cooling and ripening, by using advanced enzymatic hydrolysis and long-term biological fermentation technology. At the beginning of production, lactic acid bacteria are added to the milk fat and stirred to ferment it. The flavor substances locked in the milk fat are released, and the fermented cream is finally refined to have the special flavor. Fermented cream is often used in the processing and making of cream cakes and breads.

1.4. Lactic acid bacteria drink

The protein content of lactic acid bacteria beverage is not less than 7g/L. Lactic acid bacteria beverage is a beverage made by adding other substances (such as sugar or other sweeteners, juice, tea, plant extract, etc.) according to the proportion of lactic acid bacteria after fermentation of milk or dairy products. According to whether the lactic acid bacteria in the lactic acid bacteria beverage survive, the beverage can be divided into two kinds: active lactic acid bacteria beverage and inactive lactic acid bacteria drink;The former is required by the number of live lactic acid bacteria per milliliter of active milk is more than or equal to 1 million /mL of active milk; The latter is generally inactive.

2. Application in fermented fruit and vegetable products and grain products

2.1. Application of lactic acid bacteria in the fermentation of vegetable ,fruit and juice

Under the action of lactic acid bacteria, the sugars in fruits and vegetables are transformed into lactic acid, and the produced fruit and vegetable drinks are rich in vitamins and minerals, their flavor is unique , they are easy to be digested and absorbed, which make them to have health care effects and their shelf life of food can be extended. Common fermented vegetable juices include cucumber lactic acid bacteria fermented beverage, tomato lactic acid bacteria fermented beverage, carrot fermented beverage, sweet potato fermented beverage, potato fermented beverage,apple fermented beverage and complex vegetable fruit fermented beverage. In addition, eggplant, Chinese yam, mulberry, celery, pineapple, lily, orange, lotus root and other new lactic acid fermented drinks have gradually entered people's lives.

2.2. Application of lactic acid bacteria in the fermentation of vegetables

Fermented by lactic acid bacteria, pickles and sauerkraut have the long shelf life, their taste are unique , and the are added some different tastes between the fat and thick taste, which can be eaten by people in areas which is lack of vegetables, and they are loved by the public. At present, fermented vegetables such as Chinese cabbage, carrot, cabbage, white radish, cucumber, onion, garlic,etc have also begun to be eaten by people.

2.3. Application of lactic acid bacteria in fermented cereal products

Lactic acid bacteria after fermentation of various grains (rice, soybeans, wheat, millet, corn, etc.). The contents of various nutrients are increased, such as small molecular peptides, amino acids, thiamine, riboflavin, and microelements (such as calcium and iron). The cereal products fermented by lactic acid bacteria are more nutritious, which can promote the absorption of the body, increase the intake of nutrients, and regulate the physiological function of the human body. Such cereal products include sourdough bread, fermented soy beans, fermented lactic acid pastries, and fermented cereal drinks.

3. Application in meat production

It is mainly used in the production of fermented sausages. The key of traditional fermented sausage production is to make lactic acid bacteria become the dominant bacteria, if there is an accident in the certain production link, and make other bacteria become the dominant bacteria, it will inevitably lead to the decline of product quality or complete corruption. The modern production process of fermented sausage is to add microbial pure culture of lactic acid bacteria starter to the raw material, so as to ensure that lactic acid bacteria has an absolute advantage in the whole fermentation and maturation process, coupled with reasonable process conditions, so that the safety and quality of products are guaranteed.

4. Application in alcoholic products

Lactic acid bacteria have great influence on the taste and quality of liquor. In the brewing process of solid liquor, there will be a small part of milk in the koji. In the brewing process of solid liquor, there will be a small part of lactococcus or Lactobacillus in the koji. Lactic acid bacteria use the sugars in koji to produce lactic acid, which is partially dehydrated by heat in the distillation process to produce lactyl-lactic acid. Under the action of related enzymes, ethyl lactate is synthesized with ethanol. Ethyl lactate plays an important role in enriching the flavor of liquor, and is known as one of the three esters of liquor. The proper addition of lactic acid bacteria in the brewing process of liquid liquor is also beneficial to the improvement of liquor quality.

For beer production, lactic acid bacteria can produce a variety of bacteriostatic substances to maintain the freshness of beer and improve its taste. Avoid the odor of aging and sterilization caused by pasteurization, lactic acid produced by lactic acid bacteria can soften malt and acidify wort, make beer with lighter color, more nitrogenous substances and suitable polyphenols, and improve the biological stability of beer.

For wine production, lactic acid bacteria are mainly involved in malic acid-lactic acid fermentation. Lactic acid bacteria can decompose malic acid in wine into lactic acid, which greatly reduces the sour and rough feeling of wine. Other metabolites of lactic acid bacteria can increase the special flavor of wine, give wine the unique fruit and wine aroma, and make its taste more coordinated, mellow and soft.

Epilogue

Nowadays, people not only require good sensory quality of food, but also require health and nutrition, and the addition of lactic acid bacteria starter just meets people's needs, and it make food more attractive. Lactic acid bacteria can not only give food special flavor, but also increase the nutritional content of food, and it play a very good health care role in human life and health. With the continuous progress of science and technology, lactic acid bacteria will occupy an increasingly important position in food safety research.