Milk

Where does milk come from?

For young mammals and human infants, milk is the first food ingested. In most cases, it continues to be the sole constituent of the diet for a considerable period of time.

Milk is a complex biological fluid, the composition and physical characteristics of which vary from species to species, reflecting the dietary needs of the young mammal. The major constituent of milk is water, but depending on the species, milk contains varying quantities of lipids, proteins and carbohydrates that are synthesised within the mammary gland. Also present are smaller quantities of minerals and other fat-soluble and water/soluble components derived directly from blood plasma, specific blood proteins and intermediates of mammary synthesis. The domestication of animals such as the cow, and the availability of milk surplus to the requirement of the young mammal, has meant that animal milk has also become part of the adult human diet.

Lactating animals

Many animals are kept to produce milk for human consumption. The most important are cows, buffaloes, sheep (ewes), goats, horses (mares), donkey and camels. These animals form the basis of commercial milk production in various parts of the world.

Lactating animals.

The various species produce significantly different quantities of milk. Even within the same species there are wide variations in production, largely depending on:

Domestic purpose Breed and genetic quality Environmental conditions Physiological conditions Level of management

Factors affecting intra-species variation (adapted from van den Berg 1988, p3).

In general, the dominant milk-producing animals in a region reflect the geographic and climatic conditions. Goats, for example, can be successfully farmed in mountainous regions with poor grazing areas, which would be quite unsuitable for other animals.

Milk production is not always the main reason for keeping these animals. Mares, asses and camels are principally kept as draught, pack or riding animals, while milk production is a secondary concern. In many parts of the world the cow is of overwhelming importance in milk production, and in some countries milk from species other than the cow is not legally defined as milk.

Because of different circumstances for the young mammals, milk can be of different consistency. For example, reindeers that live in very cold areas need to have a thick adipose tissue under the skin. The young must consume milk witha high fat content that allows them to quickly develop this protective tissue. The pups of rats are born naked and therefore they need milk that contains the protein necessary to develop a fur coat. (for more information about milk from different lactating animals, see Efficient Milking Alfa Laval Agri AB 1995, Chapter II).

Species	Water	Fat	Casein	Whey protein	Lactose	Ash
Human	87.1	4.6	0.4	0.7	6.8	0.2
Cow	87.3	4.4	2.8	0.6	4.6	0.7
Buffalo	82.2	7.8	3.2	0.6	4.9	0.8
Goat	86.7	4.5	2.6	0.6	4.4	0.8
Sheep	82.0	7.6	3.9	0.7	4.8	0.9
Horse	88.8	1.6	1.3	1.2	6.2	0.4
Rat	79.0	10.3	6.4	2.0	2.6	1.3
Ass	88.3	1.5	1.0	1.0	7.4	0.5
Reindeer	66.7	18.0	8.6	1.5	2.8	1.5
Camel	86.5	4.0	2.7	0.9	5.4	0.7

Composition of milk (g/100g) of different species. 

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What is in milk?

As J.C.T van den Berg explains: “Milk is the first food the newly born human being or mammal receives. To serve its purpose, it is a food that contains all the nutrients the newborn requires. Even beyond the suckling period, it is still the most complete food for human beings and mammals.

Some of the essential minerals and vitamins such as iron and vitamin D are, however, not present in sufficient amounts, or in optimum proportions, to fulfill the requirements for complete nutrition. During the first period of its life, the young animal therefore makes up for the shortage of certain nutrients in milk by exploiting the reserves it receives from its mother at birth, which are normally sufficient until its diet includes other foods. To make the nutrients easily consumable and digestible, they are available in a liquid state, partly as a solution, partly as dispersion or suspension. There is a wide variation in the balance of components in milk from various mammals, although the components themselves are basically the same” (van den Berg 1988, p.5)

Composition of raw milk.

Quantities of the various main constituents of raw milk from cows can vary considerably; between cows of different breeds and between individual cows of the same breed. The numbers in Figure II.4 give examples of the composition of milk. Water is the principal constituent and it is the carrier of all other components. Cows' milk consists of about 87 % water and 13 % dry substance that is suspended or dissolved in the water. Beside total solids, the term solidsnon-fat (SNF) is used in discussing milk composition.

Fat .

Fat weighs less than water and exists as small globules or droplets dispersed in the milk serum (see Figure II.5). The diameter of these globules ranges from 0.1 to 20 µm (1 µm = 0.001 mm), and their average size is 3 – 4 µm. There are some 15 billion globules per ml milk. The emulsion is stabilised by a thin membrane, only 5 – 10 nm thick ( 1 nm = 10 -9 m), which surrounds the globules and has a complicated composition.

Because of its lower weight, fat rises up and floats on the surface of milk, causing a cream layer. The taste of this fat (butter) is creamy and somewhat sweet, and it has a light yellow color.

Protein .

Proteins are the most important nutrient in milk and an essential part of our diet. They are present as a solution in milk, and the proteins we consume are broken down into simpler compounds in the digestive system and the liver.

These compounds are then conveyed to the cells of the body, where they are used as construction material for building the body's own protein. The great majority of the chemical reactions that occur in an organism are controlled by certain active proteins, the enzymes. Proteins are giant molecules built up of smaller units called amino acids, and a protein molecule consists of one or more interlinked chain(s) of amino acids.

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Casein .

The proteins in milk consist to 80 % of casein, which in turn is made up of a number of components that together form complex particles or micelles.

Whey protein

Proteins are built up completely differently and therefore also have totally different characteristics. In general, whey proteins have very high nutritional values and they are widely used in the food industry. Whey protein is also called serum protein.

Non-protein nitrogenous compounds (NPN)

The presence of nitrogen is one of the main characteristics of proteins, but traces of non-protein nitrogenous products are also found in milk.

Percentage of different compunds in milk.

Minerals and salt

Milk contains a number of minerals, with a total concentration of < 1 %. The most important salts are calcium, sodium, potassium and magnesium. These occur as phosphates, chlorides, citrates and caseinates.

Vitamins .

Vitamins are organic substances which occur in very small concentrations in both plants and animals. Vitamins give milk its taste and are essential for normal life processes. Their chemical composition is usually extremely complex, and the various vitamins are designated by capital letters, sometimes followed by numerical subscripts, e.g. A, B1, B2. Milk contains many vitamins and among the best known are A, B1, B2, C and D. Vitamins A and D are soluble in fat, or fat solvents, while the others are soluble in water.

Enzymes .

Enzymes (catalysts) are a group of proteins produced by living organisms. They have the ability to trigger chemical reactions and to affect the course and speed of such reactions, and are able to do so without being consumed. The action of enzymes is specific: each type of enzyme catalyses only one type of reaction. Two factors that strongly influence enzymatic action, are temperature and pH. Several of the enzymes in milk are utilised for quality testing and control.

LIPASE splits fat into glycerol and free fatty acids. When milk has been damaged, lipase causes differences in taste. For example, excess free acids in milk and milk products result in a rancid taste. Many micro-organisms produce lipase.

PEROXIDASE is activated if the milk is heated to 80 °C for a few seconds. This can be used to prove the presence or absence of peroxidase in milk and thereby check whether or not a pasteurisation temperature above 80 °C has been reached.

CATALASE splits hydrogen peroxide into water and free oxygen. Milk from diseased udders has a high catalase content, while fresh milk from a healthy udder contains only an insignificant amount.

PHOSPHATASE is able to split certain phosphoric-acid esters into phosphoric acid and alcohol. Phosphatase is destroyed by ordinary pasteurisation (72 °C for 15 seconds). The phosphatase test can be used to determine whether the pasteurisation temperature has been attained.

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