Milk processing

The perishable nature of milk means that some form of processing is necessary to extend its shelf-life, transform it into different products to expand its market and to generate income by adding value. Ordinary heat treatment or pasteurization, while destroying harmful bacteria, does not make milk absolutely free from spoilage organisms. In a tropical climate, milk becomes unfit for human consumption within a day or two.

Processing milk into dairy products makes it more stable for storage over extended periods of time. In the tropics where ambient temperatures are high and refrigeration is not readily available, milk may be concentrated by boiling or made into butter, ghee or other products which keep better at room temperatures. When there is an abundant local supply, storage and marketing may have a low priority, leading to wastage. Processing helps eliminate wastage and also adds value.

In many tropical areas, there is a cultural reluctance to use fresh milk, which probably derives from safety considerations. In Africa and Asia, lactose intolerance (an inability to digest milk sugar) is common.

The consumption of fermented cheeses and yoghurts, in which the lactose has been converted into lactic acid, has helped reduce the problem. The limited market for liquid milk is a problem which can thus be remedied by creating a demand for processed milk products. Through community milk centres, such processing can create jobs, while at the home level it provides added income for the family.

General methods of milk processing

While there are numerous ways of processing milk that vary in detail, they can be classified into three broad areas. These areas may overlap and a combination of the two main methods may be used to make a better preserved product.

The first general method involves increasing the acidity (or lowering the milk pH). This slows down, or may prevent, the growth of spoilage micro-organisms and the action of enzymes (natural substances that cause changes in flavour, etc.). Increasing the milk's acidity can be achieved by:

* lactic acid fermentation: beneficial micro-organisms ferment the milk sugar, turning lactose to lactic acid.

* the addition of organic acids: for example, vinegar or lime juice.

The second method involves lowering the moisture content to a level which is sufficiently low to control the growth of microorganisms and the action of enzymes, so making the product more stable. Moisture content can be lowered by:

* evaporation of water using heat;

* curdling the milk and removing the whey, or watery part, as in the case of cheese;

* mechanical separation of the fat by churning, in the making of butter;

* adding salt and sugar to bind part of the water as in the salting of cheese or making milk sweets;

* sun or air drying of products such as cheeses or milk casein, the milk protein;

* mechanical drying, for example roller and spray-dried milk powders.

The third method simply involves heating to produce either pasteurized or sterilized milk.

Milk composition

Milk contains the essential nutrients in the right proportions necessary to support a young mammal in the first stages of its life. It is a good source of carbohydrate, fat and protein as well as many vitamins and minerals.

The composition of milk varies from one species to another. In most countries, the principal source of milk is the cow but other animals such as goats, sheep, buffaloes, camels and yaks are also raised for milk production, particularly in the tropics. Table 1 shows the milk composition in various mammals.

Milk composition within a particular species also varies according to such factors as breed, the type of feed and nutritional status of the animal, stage of lactation and milking and the effects of changes in season.

Milk fat

The value of milk is influenced by its fat content and most collective milk purchasing schemes pay farmers based on fat levels. Milk with high fat content is creamy and smooth and yields more butter and cheese. It also contains more fat-soluble vitamins such as A, D, and E and provides a good source of energy.

Milk protein

From a nutritional point of view, milk is a very valuable source of high quality protein. This protein is mainly casein together with minor amounts of other proteins called albumin and globulin which are essential for disease resistance in the young. These last two are lost in the whey during cheese processing, while the casein is coagulated and therefore stays in the milk solids.

Lactose

Lactose is the main sugar present in milk. It gives milk its sweet taste. Some individuals are unable to digest lactose, most commonly in countries without a dairy tradition.

Lactose is also important in the production of yoghurt and cheese since, as it is fermented to lactic acid, the milk becomes sour. The increased acidity then causes coagulation of the casein.

Minerals

Milk contains important minerals such as calcium and phosphorus which are necessary for the growth and repair of teeth and bones. A number of other minerals are also present in trace amounts.

Water

Milk has an average water content of 87 per cent. This is the medium in which water-soluble components of milk are dissolved, including the water-soluble vitamins such as B complex and C. Since the bulk of milk is water, its removal reduces the volume significantly which can help overcome transport and bulk storage problems. The effect on the nutritional quality depends on the method used.

Microbiology of milk

As milk is a highly perishable commodity whose composition is ideal for the growth of spoilage micro-organisms, a basic understanding of milk microbiology is important when considering any improvement to processing.

The high moisture content, abundant supply of nutrients combined with the almost neutral acidity (pH 6.7) and temperature of raw milk make it a very good medium for the growth of microorganisms including those that cause food poisoning and for enzymatic changes such as those that cause rancidity in milk fat.

The types of micro-organisms that occur in milk can be divided into three broad groups:

* those that cause milk spoilage;

* those that cause infection in humans, called pathogens;

* beneficial micro-organisms such as those which bring about the natural fermentation of lactose to lactic acid. These are used by milk processors to prepare products such as cheeses and yoghurts.

Sources of contamination

Micro-organisms are found everywhere on animals and people, in the air, soil, water and milk. Good quality, safe milk is the result of good sanitary practices being observed throughout the entire process from milking through processing to...