Enzymes used in dairy industry mainly include:
1. Rennet: making cheese;
2. Milk peroxidase: milk disinfection;
3. Lysozyme: added in infant milk powder;
4. Lactase: decomposes lactose;
5. Lipase: butter adds flavor.
Among them, cheese production and lactose decomposition are the most important.

The world's cheese production consumes more than 100 million tons of milk, accounting for 1/4 of the total milk production. The first step of cheese production is to ferment milk into yoghurt with lactic acid bacteria, and then add rennet hydrolysis κ- Casein is made from casein, which is solidified by Ca2+ in acidic environment, and then cut into pieces, heated, pressed and matured.

Animal rennet has high activity in hydrolyzing protein and it is the first enzyme in cheese production. However, with the development of the cheese industry, it has been unable to meet the production needs by slaughtering calves. Since the discovery of microbial chymosin by Japanese scientists, about 80% of animal-derived enzymes have been replaced by chymosin produced by genetically engineered recombinant microorganisms, and chymosin has become a major commodity next to amylase.

Lactase can hydrolyze lactose into galactose and glucose. Milk contains 4.5% lactose, which is a disaccharide with low solubility and lack of sweetness.

Lactose intolerance caused by the lack of lactase in human body is a very common phenomenon in China, which is characterized by diarrhea, abdominal pain and other adverse reactions after drinking milk. Hydrolyzing lactose in dairy products with lactase can greatly improve the absorption and utilization of dairy products by human body. In addition, lactose is insoluble in water and often precipitates in condensed milk and ice cream as sand crystals, which affects the flavor. If milk is treated with lactase, the above problems can be solved. A process for producing lactose-free milk by treating milk with immobilized Aspergillus niger lactase has been put into production in Western Europe.

Whey is the by-product of cheese production, with an annual output of more than 90000 tons (Its constituent content is 0.2% fat, 0.7% protein, 4% - 4.5% lactose and 0.45% salt respectively). Because lactose is difficult to digest, it has always been used for wastewater discharge, which seriously pollutes the environment. If treated with lactase, when the hydrolytic rate of lactose reaches 80%, its sweetness can be equivalent to DE40's glucose syrup and can be used as food or feed, if treated with glucose isomerase, The sweetness can be further increased by converting part of glucose into fructose. Corning Glass also uses Aspergillus niger lactase fixed in porous glass to decompose lactose in whey, and uses it to produce baker's yeast with an annual treatment capacity of 5000 tons.

The unique flavor of dairy products is mainly the volatile substances (such as fatty acids, alcohols, aldehydes, ketones, esters, amines,etc) produced during processing. Adding proper amount of lipase during dairy processing can increase the flavor of cheese and butter, and the use of flavoring butter in the manufacture of milk sugar, cakes, and other foods can save consumption. Lipase can also be added to the cream to increase the flavor of the cream. For example, adding Aspergillus oryzae lipase to the cream can produce the characteristic flavor of cheese.

The lactoperoxidase system includes lactoperoxidase, thiocyanate and hydrogen peroxide, which can inactivate some bacterial enzymes, prevent the metabolism and proliferation of bacteria, and it can inhibit or kill bacteria.

Hydrogen peroxide can be used for sterilization of milk preservation in the absence of pasteurization equipment or refrigeration. Its advantage is that it will not greatly damage the enzymes and beneficial bacteria in milk. The excess hydrogen peroxide can also be decomposed by catalase from liver or Aspergillus niger. This system is only applicable to the preservation of fresh milk.

One of the differences between human milk and milk is the content of lysozyme. The lysozyme in human milk is 3000 times higher than that in cow milk. Adding egg white lysozyme in milk powder can make up for the low content of lysozyme in cow milk, promote the proliferation of probiotics in infants' intestines, and prevent infection.