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 packaging. The number of bacteria present in the end-product reflects the sanitary conditions under which the milk has been processed. It also indicates the storage life of milk or its products. The main sources of contamination by microorganisms in raw milk are the surfaces, such as udder and utensils, which it contacts. The hands also convey bacteria to the milk during handling. This means that it is very important to clean hands and surfaces carefully with clean water. Improving sanitary practices during traditional milk handling and processing may meet resistance because of cultural beliefs or simply lack of time. Ideally, training of a practical nature is needed to demonstrate the effect of improved sanitation on product quality.

The udder

Milk inside a healthy udder contains relatively few micro-organisms. The external surface of the udder, however, harbours a large number of micro-organisms. Dirt such as dried mud, dung on fodder and hair can pass millions of bacteria into the milk. Good milking practice and cleanliness of the udder is vital. In addition, if the animal is suffering from infections such as mastitis, the milk will contain harmful pathogenic micro-organisms.

The whole area of herd management and milking techniques is beyond the scope of this source book. It is strongly recommended that those advising projects involved in milk processing should seek advice from specialists in herd management, as good quality products can never be made from poor quality raw milk.

Equipment and utensils

All dairy utensils such as buckets, milking cans and niters accumulate spoilage organisms if they are not thoroughly cleaned and disinfected after each use. Equipment that is made of wood or to a design which is not smooth, with, for example, joints and corners, is very difficult to clean and provides possible places for micro-organisms to survive. Cloth filters must be properly washed and dried, in the sun if possible, after each use.

The milkers

As the milkers move from one animal to the next, they can transfer pathogenic micro-organisms to all the animals in the herd so that all the milk may become contaminated. A person who is suffering from an infection could thus infect the milk, making it unsafe for consumption.

The milkers play a vital role in controlling the level of sanitation. They can directly check the cleanliness of the premises and utensils and observe the health and cleanliness of the animals as well as their own personal hygiene.

The environment

The environment in and around the milking premises affects the level of contamination in milk. If milking is done inside the stable, as is usually the case with small farmers, there is a high risk of contamination through the air and insects, particularly flies. It is therefore better to milk in a special parlour but if this is impossible, milking in the pasture is preferable to the stable. Utensils containing milk should be kept covered whenever possible.

Water supply

Using polluted water to wash udders, utensils, etc. will cause contamination. A clean water supply is essential to minimize contamination. Some water-borne bacteria are dangerous. Coliforms, which cause stomach disorders in humans, also cause quality defects in dairy products such as cheese. Cholera, another waterborne disease, can cause death. If a supply of clean drinking water is not locally available, water quality can be substantially improved by adding a small quantity of household bleach to water used in the dairy (approximately five drops per gallon or one drop per litre). Alternatively, the water can be boiled, although this uses a considerable amount of fuel.

Once micro-organisms have found their way into milk, they develop easily and multiply rapidly. Micro-organisms grow best at room temperatures, so keeping milk cool will slow down their growth. Heating the milk, a process known as pasteurization, destroys a large number of the micro-organisms. This is described in Chapter 3. Also, making the milk more acid, either by natural fermentation or the addition of acid, suppresses the growth of harmful organisms.

Classification of milk products

Across the world, a wide variety of food products are derived from milk. While they vary from one place to another as they are adapted to suit local conditions and available resources, they can be classified broadly as cheeses, butters, creams, fermented milks and miscellaneous products.

Cheeses

These can either be fresh or ripened by an ageing process. They are made by coagulating the milk and draining off the liquid whey. Cheese is a concentrated food containing practically all the valuable nutrients originally found in the liquid milk. The coagulation can be carried out in several ways. The most common practice involves adding rennet, a natural enzyme found in the fourth stomach of a ruminant. In some situations the milk is coagulated by adding an acid such as vinegar or vegetable enzyme extracts. The final characteristic of the cheese depends in part upon the particular type of coagulant used.

There are more than a thousand varieties of cheese all over the world and there is no absolute method of classification. Classifications are based on several properties and characteristics, such as fat content, source of milk, method of coagulation, and whether ripened or unripened. A simple and commonly used method is according to the moisture content as summarized in Table 2.

Butters

These are made from the fat component of whole milk which is dispersed as very small invisible globules. In buttermaking, the fat globules are made to join together through mechanical agitation. The fat forms a semi-solid mass with 80–85 per cent fat and up to 16 per cent water. Various types of butter and butter-like products are made from either fresh or soured milk. In some cases salt is added, in others the butter is allowed to ripen. In some countries the moisture content is reduced by heating to give a more stable product, known as clarified butter or ghee. The characteristics of common butters are shown in Table 3.

Fermented milks

These are acidified or soured milks produced by the natural fermentation of lactose to lactic acid or by the addition of a starter culture (a previously prepared or commercially produced bacterial culture of selected micro-organisms). The flavour and texture of the final product is greatly influenced by the particular microorganisms used in the process and the length of time they are fermented. Such products may be liquid or semi-solid and can be flavoured with artificial flavourings, fruit pieces, etc. Common types of fermented milks are described in Table 4.

Miscellaneous milk-based products

A wide variety of other products derived from milk do not fall in any of the previous categories. These include creams, milk-based sweets which are very important in Asia, dried milk protein or casein, alcoholic vodka-like beverages, body lotions and soaps.

Ice-cream

Ice-cream is a major milk-based product but it is considered to be beyond the scope of this source book. While large amounts of ice-cream are produced at small-scale levels around the world, it is very difficult to control its microbiological safety and the product carries a very substantial health risk if not produced and distributed under strict conditions. For these reasons, the small-scale manufacture of ice-cream is not recommended in most situations. Readers considering ice-cream manufacture should contact local specialist advisers.

Traditional methods and products

Milk processing is an important activity in many developing countries for many reasons, particularly at the household level. The products can provide income, be consumed in the home, find use in rituals and form an important dietary source, especially for vegetarians. The traditional methods of processing that have evolved over many years of dairy farming take into account local factors such as climatic conditions, indigenous knowledge, availability of resources and the level of hygiene in the kitchen. The resulting products are, to a very large extent, intrinsically safe for local use. Great care is needed in any attempt to replicate the manufacture of a traditional dairy product in another area, since differences in climate, knowledge and experience may result in a product that is unsafe for consumption.

Traditional technologies serve several major functions:

* conserving milk;

* increasing the market value of milk;

* providing improved food security in the home;

* generating income;

* reducing the volume and so lowering transport costs.

Because of the high temperature and humidity in the tropics, farmers have developed an appropriate technology for keeping milk longer without refrigeration facilities. Milk is allowed to sour naturally, which prevents the growth of harmful micro-organisms. Thus, most traditional dairy products start from soured milk. In some countries, such as India, milk is boiled down to remove most of the water and produce a concentrated and more stable product.

During the time of peak production, surpluses may occur and create problems, especially when the market within a small community is limited. The farmer may also not have income from milk during the lean months. To cope with seasonality, milk products are dried, salted, smoked or soaked in salted whey to preserve them. In many cases however, the unhygienic conditions of processing make prolonged storage difficult and the product spoils rapidly.

In general, sanitation and hygiene in household milk processing is poor. Owing to the scarcity of clean water, washing of udders, milk cans and other utensils is inadequate. In some places in Africa, because of the scarcity of water, containers are smoked prior to milking and this is said to give a certain degree of sterilization aside from imparting a smoky flavour to the milk. Alternatively, containers are exposed to the heat of the sun.

These methods are less effective than boiling water or bleach. The quantities of milk processed by most traditional pastoralists are small, perhaps only a few litres per household. The equipment used is therefore very simple and of the type commonly found in the kitchen. The same containers are often used for milking and for fermentation.

Despite the small quantities involved, however, processing milk increases its value. Selling products like cheese and sweets will give the producer a higher return than liquid milk.