Application of Membrane Separation Technology in Dairy Processing

1. Development of dairy industry

The vigorous development of the dairy industry is inseparable from the development of society and the advancement of science and technology. Today, milk has become an essential drink in human’s daily life, but milk is not favored by human preference as soon as it appears, even for a long time, people turn pale at the mention of milk.Before the 19th century, due to the lack of sterilization technology, fresh milk was easy to be contaminated and deteriorated by bacteria at higher temperature, which caused diseases to the drinkers. At that time, the mainstream view was that milk was a high-risk food.Until the 19th century, due to the development of technology, the promotion of pasteurization methods improved the safety of milk, resulting in unprecedented development of dairy products, and milk has become a common drink on residents' tables.According to the 2019 National Economic and Social Development Statistical Bulletin, the milk production in 2019 was 30.75 million tons, an increase of only 1.2% compared with 2018, and the domestic milk supply remained tight.

2. Nutritional value of dairy products

Milk is an important source of protein, calcium, phosphorus, vitamin A and vitamin B2. The protein content of milk is about 3~4%, of which casein is the main ingredient, and the rest is whey protein and lactoglobulin. The fat content of milk is about 2.8~4%. It exists as particulate fat globules, which are easy to digest and absorb.Milk is rich in lactose, which can promote the absorption of calcium, iron, zinc and other minerals, regulate gastric acid, promote gastrointestinal peristalsis, and promote the secretion of digestive juice. It plays an important role in the normal work of our brain and intellectual development.Due to some people's intolerance to lactose, there are lactose free milk on the market. Under the condition of not changing the milk flavor and other nutrients, the lactose is removed by membrane technology.Milk also has the effect of calming nerves. Drinking a cup of warm milk before going to bed in life can help sleep. In addition, milk is rich in animal calcium, which is more easily absorbed by human body. Milk is the best source of calcium in diet.In daily life, adults need to keep intake of about 300 g of dairy products every day. In addition, when brewing milk powder, it is most appropriate to choose the water temperature of about 50 ℃. In this way, the nutrients in milk will not be destroyed, and the intake of human body will not stimulate the intestines and stomach.

3. Application of membrane separation technology in dairy product 

In order to meet the consumers' increasing demand for the quality of dairy products, membrane separation technology is more and more widely used in the production process of dairy products. At present, the application of membrane separation technology in dairy industry mainly includes: sterilization of dairy products, concentration of dairy products, desalination of whey, fractionation of milk protein, etc.

3.1 Application of membrane separation technology in sterilization of dairy products
The traditional sterilization method of dairy products is thermal sterilization, mainly including pasteurization and ultra-high temperature instantaneous sterilization. Membrane technology is developed in recent years to remove bacteria from dairy products. Microfiltration technology can replace thermal sterilization and chemical preservatives to intercept bacteria and spores in dairy products, and the effective ingredients in dairy products can pass through, without affecting the flavor of dairy products, while extending the quality assurance period of dairy products.
Cross-flow filtration technology (membrane pore size 1.2 um) is used to remove bacteria in skimmed and low-fat milk, and its sterilization rate is >99.6%. The shelf life of dairy products produced by membrane separation technology has been extended from 6-8 days to 16-21 days.

3.2 Application of membrane separation technology in milk concentration
Membrane separation in milk concentration mainly includes: whole milk concentration, skim milk concentration, whey protein concentration, yogurt concentration, etc. The natural milk contains about 87% moisture. The transportation cost of milk can be reduced by concentrating the milk before transportation and then adding water to reduce the whole milk when it is put into production.
At present, more than 60% water can be removed by reverse osmosis, while skimmed milk with protein content up to 80% can be obtained by ultrafiltration. Ultrafiltration membrane is widely used in milk protein concentration in dairy processing plants. The protein in raw milk is intercepted by ultrafiltration separation technology to let lactose and ash pass through. By adding washing water repeatedly, lactose and ash can be removed to the greatest extent, so as to produce concentrated milk protein and lactose with high protein content. Traditional concentration methods, such as flash evaporation, require heating, high energy consumption and can cause protein denaturation in milk. The introduction of membrane separation technology can greatly reduce energy consumption.     

3.3 Application of membrane separation technology in whey desalination
The whey produced by cheese production has a high salt content, which must be desalted before recovery, evaporation and drying. Nanofiltration membrane separation technology can intercept divalent ions and components in whey to a certain extent, and allow univalent ions to pass through. For concentrated solution, appropriate amount of washing water can be added to desalting whey to meet the product requirements. The supernatant can be directly discharged. Nanofiltration membrane separation technology can effectively remove miscellaneous and salty taste without damaging the flavor and nutritional value of milk.

3.4 Application of membrane separation technology in milk protein fractionation
Each milk protein has different nutritional functions and specific functional properties. Therefore, in recent years, milk protein fractionation has become the focus of the world dairy industry.
The separation and fractionation of milk protein includes the separation and fractionation of whey protein and the fractionation of casein. Different molecular weight membranes are used to fractionate and purify milk proteins to obtain different proteins, such as whey protein, serum albumin, -lactoglobulin/-lactalbumin, egg albumin, simmond wood protein, etc.

At present, various membrane separation techniques, chromatographic methods, chemical treatment and enzyme treatment are being studied abroad to separate various components of milk protein. 
In developed countries, membrane separation technology has been maturely applied in dairy production, and the scope of application has been continuously expanded. The application of membrane separation technology in dairy products in China is in the preliminary stage.However, with the improvement of people's living standards, the demand for dairy products and the quality of dairy products continue to improve. Modern membrane separation technology, with its unique performance, has a broad prospect in the development of dairy industry.