9. Milk and milk products

Nutritional significance
Standard testing procedures
Processing
Separation of milk components
Cultured/fermented dairy products
Suitability for small-scale production

Although cow's milk is the most popular in many countries, milk can be obtained from many different sources. For example, milk from goats and sheep makes a substantial contribution to the total milk production in countries of Eastern and Southern Europe, Malawi, and Barbados, whereas the water buffalo is a common source of milk in much of Asia. The table below illustrates some of the differences in composition between these milks.

Woman milking a goat

Milk is a perishable commodity and spoils very easily. Its low acidity and high nutrient content make it the perfect breeding ground for bacteria, including those which cause food poisoning (pathogens).

Bacteria from the animal, utensils, hands, and insects may contaminate the milk, and their destruction is the main reason for processing. This preservation of the milk can be achieved by fermentation, heating, cooling, removal of water, and by concentration or separation of components, to produce foods such as butter or cheese.

The degree to which milk consumption and processing occurs will differ from region to region. It is dependent upon a whole host of factors, including geographic and climatic conditions, availability and cost of milk, food taboos, and religious restrictions. Where processing does exist, many traditional techniques can be found for producing indigenous milk products. These are more stable than raw milk and provide a means of preservation as well as adding variety to the diet. In addition, the introduction of western-style dairy products and the subsequent setting up of small-scale dairies has provided more choice of dairy products to the consumer.

Nutritional significance

Milk is often regarded as being nature's most complete food. It earns this reputation by providing many of the nutrients which are essential for the growth of the human body. Being an excellent source of protein and having an abundance of vitamins and minerals, particularly calcium, milk can make a positive contribution to the health of a nation. The realization of its nutritional attributes is clearly illustrated by the implementation of numerous 'school milk programmes' worldwide.

Fermented-milk products such as yoghurt and soured milk contain bacteria from the Lactobacilli group. These bacteria occur naturally in the digestive tract and have a cleansing and healing effect. Therefore the introduction of fermented products into the diet can help prevent certain yeasts and bacteria which may cause illness.

Many people suffer from a condition known as 'lactose intolerance'. This means that they are unable to digest the milk fat (lactose). Such people can, however, tolerate milk if it is fermented to produce foods such as yoghurt. During fermentation, lactic acid producing bacteria break down lactose, and in doing so eliminate the cause of irritation.

The quality of milk

The type of animal, its quality, and its diet can lead to differences in the colour, flavour, and composition of milk. Infections in the animal which cause illness may be passed directly to the consumer through milk. It is therefore extremely important that quality-control tests are carried out to ensure that the bacterial activity in raw milk is of an acceptable level, and that no harmful bacteria remain in the processed products.

Average composition (%) of milks of various mammals

Species	Water	Fat	Protein	Lactose	Ash
Human	87.43	3.75	1.63	6.98	0.21
Cow	87.2	3.7	3.5	4.9	0.7
Goat	87.00	4.25	3.52	4.27	0.86
Sheep	80.71	7.9	5.23	4.81	0.9
Indian buffalo	82.76	7.38	3.6	5.48	0.78
Camel	87.61	5.38	2.98	3.26	0.7
Horse	89.04	1.59	2.69	6.14	0.51
Llama	86.55	3.15	3.9	5.6	0.8

Schoolchildren receiving milk

FAO photo

Standard testing procedures

Milk fat

The price paid for milk is usually dependent upon the milk-fat content, and this may be determined either at the collection stage or at the dairy using a piece of equipment known as a butyrometer. Additionally the specific gravity can be measured using a hydrometer. This can also be used as an aid to detect adulteration.

Bacterial activity

Routinely it is necessary to check the microbiological quality of raw milk using either methylene blue or resazurin dyes. These tests indicate the activity of bacteria in the milk sample and the results determine whether the milk is accepted or rejected.

Both tests work on the principle of the time taken to change the colour of the dye. The length of time taken is proportional to the number of micro-organisms present (the shorter the time taken, the higher the bacterial activity). It is preferable to use the resazurin test as this is less time-consuming. For these tests, basic laboratory equipment will be needed such as test-tubes, a water bath, accurate measuring equipment, and a supply of dyes.

After collection the milk should ideally be stored at a temperature of 4°C or below. This is necessary to slow the growth of any contaminating bacteria.

Phosphatase test

For pasteurized milk, it is possible to ensure that pasteurization has been adequately achieved by testing for the presence of the enzyme phosphatase. The destruction of phosphatase is regarded as a reliable test to show that the milk has been sufficiently heat-processed, because this enzyme (present in raw milk) is destroyed by pasteurization conditions.

It is stressed that pasteurization is an effective safeguard against spoilage and food poisoning only if the milk is not re-contaminated after pasteurization.

Processing

Liquid milk

Milk can be kept for longer periods of time if it is heated to destroy the bacteria or cooled to slow their growth. Pasteurization and sterilization are the two most commonly-used heat treatments. Technically, it is possible for both to be carried out on a small scale, but they are most usually performed on a larger industrial scale due to the need for qualified, experienced staff and accurate and strictly controlled hygienic processing conditions.

Production stages for pasteurized and sterilized milk

Product	Store raw milk at 4°C	Test for bacterial activity using resazurin/methylene blue	Filter	Homogenize	Pasteurize	Fill into sterilized bottles	Sterilize	Store
Pasteurized milk	*	*	*		*	*		*
Sterilized milk	*	*	*	*		*	*	*

Equipment required

Processing stage	Equipment	Section reference
Store at 4°C	Refrigerated storage	15.0
	Thermometer	63.0
Test for fat content	Butyrometer	64.5
Test specific gravity	Hydrometer	64.4
Test bacterial activity	Supply of dyes	64.6
	Thermometer Basic laboratory equipment is required for most of the tests	63.0
Filter	Filter cloth	0.80
	Filter press	29.2
Homogenization	Homogenizer	37.0
Fill into bottles	Liquid-filling machine	28.1 - refer to Packaging chapter for notes on the preparation of sterilized bottles
	Capping machine	47.2
Pasteurization	Boiling pan	48.0
	or pasteurizer	50.0
	Heat source	36.0
	Thermometer	63.0
Sterilization	Pressure cooker	48.0
	Retort	05.1
	Heat source	36.0
	Thermometer	63.0
Cool	Bottle-cooling system	Refer to the Packaging chapter for details

Homogenization

Homogenization breaks up the oil droplets in milk and prevents the cream from separating out and forming a layer. This is of particular importance for sterilized milk which has a long shelf-life and when the formation of a cream layer is not desired. Additional changes include increased viscosity and a richer taste. Homogenizers are more usually designed for industrial-scale production, but it is possible to purchase smaller versions.

Filling

The most common packaging material for both pasteurized and sterilized milk is glass bottles sealed with either foil or metal caps, although plastic bottles, plastic bags, and cardboard cartons are all used when bottles are not available or too expensive.

Pasteurization

Pasteurization is a relatively mild heat treatment, (usually performed below 100°C) which is used to extend the shelf-life of milk for several days. It preserves the milk by the inactivation of enzymes and destruction of heat-sensitive micro-organisms, but causes minimal changes to the nutritive value or sensory characteristics of a food. Some heat-resistant bacteria survive to spoil the milk after a few days, but these bacteria do not cause food poisoning.

The time and temperature combination needed to destroy 'target' microorganisms will vary according to a number of complex inter-related factors. For milk, the heating time and temperature is either 63°C for 30 minutes or alternatively 72°C for 15 seconds. Only the former combination is possible on a small scale and for this the simplest equipment required is an open boiling pan. Better control is achieved using a steam jacketed pan, and this can be fitted with a stirrer to improve the efficiency of heating. Both of these are batch processes which are suited to small-scale operation. A higher production rate may be possible using a tubular-coil pasteurizer. This equipment has been tested and has been successful for some fruit products but it is presently still at a developmental stage.

Sterilization

Sterilization is a more severe heat treatment designed to destroy all contaminating bacteria. The milk is sterilized at a temperature of 121°C maintained for 15-20 minutes. This can be achieved using a retort or pressure cooker. Unlike pasteurization, this process causes substantial changes to the nutritional and sensory quality of the milk. In some countries, flavoured milk has become a very popular product.

However, sterilization is not recommended for small-scale production for the following reasons:

· The cost of a retort and ancillary equipment is high for the small-scale processor.

· It is essential that the correct heating conditions are carefully established and maintained for every batch of milk that is processed. If the milk is overheated, the quality is reduced, and it may have a rather burnt taste and aroma.

· If the milk is not heated sufficiently, there is a risk that micro-organisms will survive and grow inside the bottle. In low-acid foods such as milk, many types of bacteria including Clostridium botulinum can grow and cause severe food poisoning.

· Due to the potential dangers from food poisoning, the skills of a qualified food technologist/microbiologist are required in order to routinely examine samples of sterilized milk that have been subjected to accelerated storage conditions. This requires a supply of microbiological media and equipment.

In summary, the process of sterilization requires a considerable capital investment, the need for trained and experienced staff, regular maintenance of sophisticated equipment, and a comparatively high operating expenditure.

Cooling

Pasteurization does not destroy all of the micro-organisms, therefore the milk has to be cooled rapidly to prevent the growth of surviving bacteria. Cooling can be achieved on a small scale by using a bottle-cooling system. A system is outlined in the Packaging chapter.

Storage

Pasteurized milk has a shelf-life of 2-3 days if kept at 4°C. Maintaining this low temperature causes a substantial increase to the cost of transportation and distribution and is therefore a major disadvantage to the development of a small-scale pasteurized milk business. If packaged in sealed bottles and stored at room temperature, sterilized milk should have a shelf-life in excess of six months.

Separation of milk components

Cream

When milk is left to stand for some time, fat globules rise to the surface forming a layer of fat (or cream). This can be separated leaving behind skimmed milk as a by-product. There are different types of cream each with different fat concentrations: single (or light) cream contains 18 per cent milk fat whereas double (or heavy) cream normally contains 30 per cent milk fat. Cream is a luxury item and may be used as an accompaniment to coffee, as a filling in cakes, and an ingredient in ice cream.

Separation

Separation can very simply be achieved by removing the cream with a spoon, however this is a slow process during which the cream may spoil. For this reason it is more usual to use a manual or powered centrifuge.

Production stages for cream

Ingredients	Process	Equipment	Section reference
Raw milk tested	Store at 4°C	Milk churns	62.0
		Refrigerated storage	15.0
		Thermometer	63.0
	Separation of milk fat	Ladle Dairy centrifuge	07.1
	Pasteurization	Large boiling pan or steam jacketed pan	48.0
		Pasteurizer	50.0
		Heat source	36.0
		Thermometer	63.0
	Fill bottles/pots	Funnel or liquid-filling machine	28.1
		Capping machine	47.2
		Pot sealer	47.1
	Cool bottles	Bottle-cooler	See Packaging chapter
	Store bottles at 4°C	Refrigerated storage	15.0

Pasteurization

Cream may be pasteurized in a similar way to milk, using a similar time and temperature combination and the same equipment. Cream can also be sterilized but there is a considerable loss of quality.

Packaging and storage

Cream can be packaged in glass jars or plastic pots sealed with foil lids. Pasteurized cream must be stored at a temperature of 4°C to have a shelf-life of several days. Refrigerated storage is necessary because cream is prone to rapid spoilage.

Butter

Butter is a semi-solid mass which contains approximately 80-85 per cent milk-fat and 15-20 per cent water. It is yellow/white in colour, with a bland flavour and a slightly salty taste. It is a valuable product that has a high demand for domestic use in some countries and as an ingredient in other food processing (e.g. for confectionery and bakery uses).

The principles of preservation are:

· to destroy enzymes and micro-organisms by pasteurizing the milk

· to prevent microbial growth during storage by reducing the water content, by storing the product at a low temperature, and optionally by adding a small amount of salt during processing.

Production stages for butter

Ingredients	Process	Equipment	Section reference
Cream or soured cream	Store at 4°C	Milk churns	62.0
		Refrigerated storage	15.0
		Thermometer	63.0
	Churning	Butter churns	13.0
	Draining (pour off buttermilk)
	Washing
	Draining (pour off washwater)
Permitted colours and salt (optional)	Kneading/working	Butter pats	04.0
	Form into blocks	Butter pats	04.0
	Packaging	Paper/plastic/ foil wrapping Wrapping machines	47.3
	Storage at 4°C	Refrigerated storage	15.0

Churning

Churning disrupts the emulsion of fat and water and as a result the milk-fat separates out into granules. This process takes place in a butter churn.

Churning cream

Churning is continued until fat granules are present and at this stage the mixture is drained to remove liquid that has separated from the granules. This liquid is known as buttermilk and can be used as either a beverage or as an ingredient in animal feed.

Washing

Clean water equivalent in weight to the buttermilk is added to the churn in order to wash the butter granules. The wash water is drained away. Churning is continued for a short time to compact the butter, and once this has been achieved it is removed from the churn.

Forming and packaging

Butter is kneaded to achieve a smooth and pliable texture. This can be done using simple hand-tools such as butter pats. Alternatively for higher production rates a specially-designed kneader can be used. Once the butter has a uniform and smooth texture it is formed into blocks with butter pats and packed in either greaseproof paper or foil wrappers.

Working butter with butter pats

Storage

Due to its high fat composition, butter must be stored at temperatures below 10°C otherwise the fat becomes rancid and imparts undesirable 'off' flavours. The water droplets in butter (20 per cent) can also allow bacteria to grow if it is not kept under cool conditions.

Ghee

Ghee is made from butter which has been heated and clarified. At ambient temperatures it is a semi-solid mass with a granular texture, but on melting (40°C+) it turns into a clear, thin liquid. It has a high demand in some countries for domestic use, as an ingredient for local food production (for example bakeries and confectionery manufacturers), and as an export commodity.

Alternatively, cream is boiled gently to evaporate the water. During boiling the product is stirred continuously until the milk proteins start to coagulate, forming particles, and the colour of the cream darkens. Heating is stopped and the product is left to set. The particles settle at the bottom of the vessel and the milk-fat is separated. The principles of preservation are:

· heating to destroy enzymes and contaminating microorganisms

· to reduce the water-content by evaporation, and in doing so prevent the growth of micro-organisms.

Production stages for ghee

Ingredients	Process	Equipment	Section reference
Butter	Heating	Heat source	36.0
		Large boiling pan or steam jacketed pan	48.0
	Cool to room temperature	Thermometer	63.0
	Filter	Filter cloth	08.0
	Fill into jars/pots	Funnel or liquid-filling machine	28.1
		Capping machine	47.2
	Store at ambient temperatures

Packaging and storage

Metal containers are normally used. They should be thoroughly cleaned, especially if they are re-usable, and they should be made airtight. Alternatives to metal cans include coloured glass jars with metal lids, or ceramic pots sealed with cork/plastic stoppers.

Ghee is usually stored at room temperatures as cold storage affects the granular texture. Thus ghee is useful for those consumers with no access to refrigeration.

Cultured/fermented dairy products

The technology of cultured milk products such as yoghurt, curd, and cheese is based upon the microbial conversion of the milk-sugar lactose to lactic acid (lactic acid accounts for the characteristic 'sourness' of such products). In order for the conversion to take place, lactic acid producing bacteria must be present. This may occur by allowing the milk to sour naturally, but it is better to introduce the appropriate bacteria as a starter culture. Starter cultures may be in the form of a small quantity of previously-cultured product or may be purchased as a commercially-prepared culture.

Yoghurt/curd

Yoghurt is a fermented milk product that evolved by allowing naturally-contaminated milk to sour at a warm temperature. Yoghurt can be either unsweetened or sweetened, set, or stirred. Curd is the name given to a yoghurt-type product made from buffalo milk.

The principles of preservation for yoghurt are:

· Pasteurization of the raw milk to destroy contaminating microorganisms and enzymes.

· An increase in acidity due to the production of lactic acid from lactose. This inhibits the growth of food-poisoning bacteria.

· Storage at a low temperature to inhibit the growth of microorganisms.

Production stages for set yoghurt

Ingredients	Process	Equipment	Section reference
Milk and starter culture (2 per cent)	Preheat to 70°C for 15-20 minutes	Heat source	36.0
		Thermometer	63.0
		Boiling pan	48.0
	Cool to 30-40°C	Thermometer	63.0
	Addition of starter culture	Measuring and weighing equipment	64.1 and 64.2
	Pour into bottles/pots	Funnel or Liquid filler	28.1
		Sealing machine	47.1
		or Capping machine	47.2
	Incubate at 43-45°C	Commercial incubator	39.0
		Thermometer	63.0
	Store at 4°C	Refrigerated storage	15.0

Heating

In the manufacture of yoghurt, milk is normally heated to 70°C for 15-20 minutes, using an open boiling pan, or alternatively a steam jacketed pan.

Addition of starter culture

The milk is cooled to between 30 40°C and inoculated with a mixed culture of Lactobacillus bulgaricus and Streptococcus thermophilus (usually in a ratio of 1:1). If a commercial starter-culture is used, the directions for use will be given. However, if a culture from a previous batch is used, then it is usual to add 2-3 tablespoons per litre of prepared milk.

Yoghurt of the stirred variety can be fermented in the mixing container. To make set yoghurt the inoculated milk should be poured into the individual pots before fermentation.

Selling curd from a roadside stall

Incubation

The micro-organisms that produce yoghurt are most active within a temperature range of 32-47°C. Ambient temperatures are therefore not adequate and a heated incubator is needed. Small commercially-available yoghurt-makers consist of an electrically-heated base and a set of plastic or glass containers. Most yoghurt-makers make four or five individual half litre cups at a time. There are other simple and inexpensive ways of incubating yoghurt such as an insulated box, keeping the jars/pots surrounded by warm water, or by using thermos flasks (the latter is only suitable for stirred yoghurt). Incubation takes approximately five hours.

When fermentation is complete, stirred yoghurt is cooled and flavoured or sweetened prior to packaging. In set yoghurt all ingredients are added before fermentation.

Packaging and storage

Yoghurt or curd is commonly packaged in plastic pots fitted with a plastic lid, or heat-sealed with foil, although traditionally, curd is packaged in clay pots. Such pots are made from local materials and can be re-used or later used for cooking. The shelf-life of yoghurt is usually 3-8 days when stored at temperatures below 10°C.

Cheese

Cheese is made from milk by the combined action of lactic acid bacteria and the enzyme rennin (known as rennet). Just as cream is a concentrated form of milk fat, cheese is a concentrated form of milk-protein. The differences in cheeses that are produced in different regions result from variations in the composition and type of milk, variations in the process, and the bacteria used. The different cheese varieties can be classified as either hard or soft.

Collecting milk for cheese-making

Hard cheeses such as Cheddar and Edam have most of the whey drained out and are pressed. Soft cheeses such as paneer contain some of the whey and are not pressed. Many indigenous cheeses are soft types.

The hardness, flavour, and other qualities of a cheese can be varied by changes to the process conditions, to suit local tastes. However the principal steps of a cheese-making process are basically the same.

The principles of preservation are:

· the raw milk is pasteurized to destroy most enzymes and contaminating bacteria

· fermentation by lactic-acid bacteria increases the acidity which inhibits the growth of food-poisoning and spoilage bacteria

· the moisture content is lowered and salt is added to inhibit bacterial and mould growth.

The table, right, outlines the stages of production and the equipment needed to produce Edam cheese.

Pre-heating

The pasteurized milk is heated to a temperature at which the starter-culture can work.

Addition of starter culture

Starter-culture is added to the milk at the rate of approximately 2 per cent of the weight of milk. The vessel used should be either aluminium or stainless steel.

Addition of rennet

The rennet should be 1 per cent of the weight of milk. The rennet alters the milk proteins and allows them to form the characteristic curd.

Incubation

The milk is allowed to stand until it sets to a firm curd.

Treatment of the curd

The curd is cut into cubes which facilitate the elimination of whey from the gel. The curd is then cooked at 40°C for a period of twenty minutes which has the action of firming the curd. After cooling, the whey is drained off. The curd is pressed to ensure that most of the whey has been removed, and is then cut to fit the cheese-moulds, and finally pressed with weights.

Ripening

This is the final stage in the cheese-making process. It is a process which allows the development of gas in some cheeses and the development of flavour. The longer the ripening process the stronger the flavour. Ripening usually takes place in ripening rooms, where the temperature and humidity must be controlled for the optimum development of the cheese.

Production stages for Edam-type cheese

Ingredients	Process	Equipment	Section reference
Pasteurized milk	Preheat to 35-40°C	Cheese vat	10.0
		or boiling pan	48 0
		Thermometer	63.0
		Heat source	36 0
Starter culture	Addition of starter culture	Measuring and weighing equipment	64.1 and 64.2
Rennet	Add rennet at 30°C	Measuring and weighing equipment	64.1 and 64.2
		Thermometer	63.0
	Incubate
	Cut the curd	Curd cutters	16.1
	Heat to 40°C for 20 minutes	Heat source	36.0
		Thermometer	63.0
	Drain	Filter cloth	08.0
	Cut to fit a cheese-mould	Knife	17.1
	Put into a cheese-mould	Cheese-moulds	09.1
	Press with	Cheese-press weights	09.2
	Cool and dry at 10-12°C	Thermometer	63.0
Salt	Salting in 20 per cent salt solution at 12°C for 12-16 hours	Brine meter	64.6
		Thermometer	63.0
	Ripen for 6-8 weeks at 16°C	Thermometer (optional)	63.0.
	Washing
	Drying for 30 minutes
	Wax with paraffin wax store at 9°C	Refrigerated storage	15.0

Packaging and storage

The packaging requirements differ according to the type of cheese produced. Hard cheese, for example, has an outer protective rind which protects the cheese from air, microorganisms, light, moisture-loss or pick-up, and odour pickup. Cheese should be allowed to 'breathe', otherwise it will sweat. Suitable wrapping materials are therefore cheesecloth or grease-proof paper. Cheese should be stored at a relatively low temperature between 4 and 10°C to achieve a shelf-life of several weeks/months. Soft cheeses are often stored in pots or other containers, often in brine, to help increase their shelf-life of several days/weeks.

Selling ice cream from a bicycle

Ice cream

Ice cream is a frozen mixture which contains milk, sugar, fat, and optional thickeners (e.g. pectin or gelatin), colouring, and flavouring. It may be sweetened and flavoured in numerous ways with nuts, fruit pieces, and natural or artificial flavours and colours.

The principles of preservation are:

· pasteurization to destroy most micro-organisms and enzymes
· freezing to inhibit microbial growth.

Pasteurization

Pasteurization is carried out by heating to 65°C for a period of 30 minutes.

Cooling and beating

Ice cream is a complex mix of small ice crystals and air bubbles in a milk-fat/water emulsion. To achieve this, it is necessary to cool the mixture quickly to produce small ice crystals and at the same time incorporate air into the product by beating.

Ingredients	Processing stage	Equipment	Section reference
Milk, sugar, fat, thickener, colours and flavours.	Mix ingredients	Weighing and measuring equipment	64.1 and 64.2
		Liquid mixer	43.1
	Pasteurize	Boiling pan or steam jacketed pan	48.0
		Thermometer	63.0
		Heat source	36.0
	Cool mixture to approximately -5°C and beat simultaneously	Ice cream maker	38.0
	Fill into containers	Filling machine	28.0
	Freeze at -18°C	Freezer	32.0

Ice cream makers are available commercially and work on the following principle. The mixture is placed in a bowl which is kept at a low temperature (either surrounded by ice and salt, or having been chilled in a freezer). It is then agitated by a manually-operated rotor or by a powered stirrer. At the end of this process the ice cream should be at a temperature of approximately -5°C, and be partly frozen.

Packaging and storage

The ice cream is usually packaged in plastic, waxed paper, or cardboard containers, and is stored at below -18°C. The storage temperature is important for two reasons:

· to maintain the texture of the product
· to prevent the growth of micro-organisms.

Ice cream may be transported in an insulated box (e.g. for sale from a bicycle). It is especially important to guard against thawing and re-freezing as this will cause changes in texture and mouthfeel, and there is the increased possibility of food poisoning by contaminating food poisoning microorganisms.

Suitability for small-scale production

In some regions, there is a high demand for dairy products both for traditional and modern items. Much of the technology and machinery necessary for processing is fairly simple, which may at first sight appeal to the would-be processor. Milk, however, is a highly perishable food and there is a high risk of transmitting food-poisoning bacteria to consumers. It is stressed, therefore, that milk processing of any kind must be done under carefully controlled hygienic conditions.

Furthermore, in developing countries, milk processing is often more problematic than in temperate climates, owing to higher temperatures and humidity. Consequently, milk spoils at a faster rate, cheese ripens too quickly, and it is often difficult to ensure adequate cooling conditions.