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Milk

is the most versatile of all foods. Fresh milk and products made from it (cream, butter, buttermilk, whey, all kinds of cheese, and innumerable soured milk products such as yoghurt and sour cream) are widely used in the cuisines of large areas of the world. Milk has long been held in high esteem for its nutritious quality, which even in pre-scientific days was apparent from the fact that it provided complete nourishment for young animals and humans.

The oldest known record of animals being kept in herds and milked is a series of cave paintings in the Libyan Sahara, showing milking and perhaps cheese-making too, and possibly older than 5000 bc. The Sumerians, around 3500 bc, and the Egyptians a few centuries later used milk and have left reliefs and records showing that they prepared curdled milk products.

In contrast, the Chinese have seldom used milk: nor have peoples in a large part of SE Asia nor in any part of America before the Europeans arrived. It was also unknown in some parts of Africa. The inhabitants of these areas are usually unable, not merely disinclined, to take milk once they have grown up. Technically, they suffer from lactose intolerance, which means they lose the ability to digest the lactose sugar in milk.

Before the techniques of refrigeration and pasteurization were introduced, milk would not stay fresh for long. It went sour, became rancid, or curdled. The whole range of milk products arose as a means of controlling this tendency and even turning it to advantage.

In composition, milk is mostly water. In cow's milk, the water content is 87% by weight. The next heaviest constituent, at about 5%, is milk sugar, which is almost entirely lactose. Its molecule is split in fermentation or digestion to yield the simple monosaccharide sugars dextrose and galactose. Fat content averages slightly under 4% but varies widely. The fatty substances include carotenoids (yellow pigments from plants which impart a creamy colour), free fatty acids which contribute to flavour, and the fat-soluble vitamins A, D, E, and K. Milk also contains useful amounts of the water-soluble vitamins (C and the B group). Proteins total about 3.5%, mostly casein, which is what constitutes the solid curd when milk is curdled. Traces of acetaldehyde, acetone, and formaldehyde contribute to the flavour of milk; and very fresh milk contains methyl sulphide, which gives it its ‘cowy’ smell.

Milks from different kinds of animal vary noticeably in composition. Thus human milk is more watery than cow's milk, has more sugar, less fat, and much less protein. This sort of variation can usually be related to the length of time which the young take to mature. Human babies grow slowly, calves quickly. The average figures for some animal milks are given in the table, in percentages by weight.

But these figures are only averages. The composition of milk from the same kind of animal, indeed even from the same animal, is variable. Changes can be remarked according to the breed (for example Jersey and Guernsey cows give high-fat milk, Holstein low); the animal's diet, which itself changes with the seasons; the animal's age; the stage in its lactation period (see beestings); the time of day at which it is milked; and even the stage reached in one milking (the last milk to emerge has more fat than the first). Flavours vary, too; not only from species to species, as is patent from a comparison of cow, sheep, and goat cheeses, but also according to a particular animal's health (see stracchino) and diet. Odd flavours can get into milk from strong-tasting foods eaten by the animal, and even from smelly substances in the air it breathes. And all this is before the milk even emerges. Many other factors can cause changes thereafter.

Raw milk fresh from the cow contains many kinds of enzymes, most of which are destroyed by pasteurization. The important enzyme actions which occur in the making of milk products are largely brought about by enzymes from outside sources. However, a few of the enzymes naturally present in milk survive and may do useful work. One of them, amylase, liberates sugar from the starch in flour, an effect useful in baking.

From the moment it leaves the udder, milk is subject to invasion by bacteria of many kinds. It is as close to being a perfect food for them as it is for human beings and animals. Some of these bacteria are harmful but others are beneficial, notably the lactic acid-producing bacteria. It is these which have a souring effect on milk and are exploited in making fermented milk products. Although most of these bacteria are present naturally, it is usual in fermentation processes to add a starter culture of the preferred ones to give them a head start. Once these are in the majority they remain dominant and thwart the growth of others.

Physically, milk is a fluid, as all can see. It can be more precisely described as ‘an emulsion, colloidal suspension and solution’. Its structure, in short, is quite complex. The casein (which constitutes most protein) and the fats are clustered separately in tiny micelles (groups of molecules—the closest lay term is ‘globules’) floating independently in water in which the sugar, salts, and other proteins are dispersed. This structure can break down when some chemical substance or physical event disturbs its balance. The casein and fat micelles then agglomerate into a curd, leaving the watery solution as whey.

When milk curdles, the long strands of the casein molecules tangle and link together. The extent to which this happens depends on what has caused the curdling. The fairly mild lactic acid produced in normal souring brings about only a light linkage, resulting in slight thickening of the milk without much separation of curds and whey. Enzymes such as rennin cause much greater linking and contraction of the network of strands, forcing out much of the whey so that there is a clearly visible clotting and separation.

Heating also restructures the proteins, and is a potent weapon in the battle to make milk keep longer than it naturally would. See the box on pasteurization, etc.

Milk may be homogenized to stop the cream from rising to the top. Separation occurs because the fat globules are just too large to remain suspended in the emulsion and, being lighter than water, slowly collect at the top. In homogenization, a process invented in France, the milk is forced through tiny holes which break the fat into smaller globules. Although homogenization is a purely mechanical treatment, it does affect the flavour of the milk, making it blander, and also increases its whiteness. Homogenized milk froths and boils over, and also curdles more readily.

Milk may also be skimmed to remove the cream, lowering the fat content of the remaining milk. This was formerly done by letting the cream rise to the top and then skimming it off. Now it is done in a centrifuge, which whirls the milk around rapidly in a circular vessel. The cream, which is lighter, collects at the centre. A little cream can be removed for ‘partly skimmed milk’; or as much as possible may be extracted.

Evaporated and condensed milk are both made in the same way at the start: by heating milk moderately in a strong vacuum which lowers the boiling point so that most of the water evaporates. The resulting thick liquid is then either given a high temperature treatment to sterilize it, making evaporated milk; or sweetened to preserve it, making condensed milk. In both the flavour is much altered.

Powdered milk is made from skimmed milk. Whole milk is hardly ever made into powder because the fat in it becomes oxidized by exposure to the air and tastes unpleasant. Even skimmed milk suffers from this trouble to some extent. The flavour is altered both by oxidation and by the heat used in the process, which is usually spray-drying: the milk is sprayed into a large drum filled with hot air and the drops dry before they reach the bottom.

Powdered, evaporated, condensed, sterilized, and UHT milks all keep for a long time unrefrigerated, though the liquid ones usually begin to go off once they have been opened. Ordinary pasteurized milk lasts only three days in the refrigerator, or a week or so if held just above freezing. When it does finally succumb to bacteria it becomes very unpleasant. Rather than being soured by lactic acid-producing species, which prefer warmer temperatures and anyway have been almost wiped out by pasteurization, it is attacked by proteolytic (protein-destroying) bacteria which can work at low temperatures. These make the milk alkaline and smelly.

Yet another way of keeping milk has been described by Emerson (1908), in a chapter concerned mainly with myths but in terms which suggest that there is some historical record of what he relates:

When the European first visited India he found that the aborigines had for a beverage a drink which they called dhy, and on investigation it was ascertained to be dried milk. The method of drying it was primitive indeed. The milk of an ass, a mare, or a goat was put into a leather bag or skin and, tightly closed, this was then suspended beneath the belly of a horse. It soon became hard, greatly resembling chalk. When it was wanted for use a piece was broken off and dissolved in water, making a pleasant and invigorating drink. Its taste was slightly acid, but its odour was that of very sour milk. When and by whom this practice was inaugurated is not known; it is only one of the thousands of things that are lost in antiquity.

In cooking with milk the chief problems are with curdling of one kind or another. This may be flocculation: separation at relatively low temperatures. At higher temperatures, above 70 °C (158 °F), coagulation occurs: this is the change in the proteins brought about by heat.

One of the commonest and most annoying instances of coagulation in milk is the formation of a skin when it is heated. Little pieces of denatured protein rise to the surface and join up into a layer which becomes increasingly coagulated and dehydrated, and thus tougher. It can be averted by stirring the milk; but as soon as stirring stops, the skin begins to form. It is no use trying to stir it back in to recombine it once it has formed: the only result is a mess of little bits of skin. There is no special type of milk which is immune from this problem, although semi-skimmed and skimmed milk suffer from it less. Skin formation can be reduced by heating milk only as hot as is needed—if possible not over 70 °C (158 °F)—and at the last feasible moment. In the catering trade milk may be heated with a jet of superheated steam from an espresso machine, which makes it froth rather than form a skin.

Milk sticks to pans when heated, for the same reason. A thin pan which does not spread the heat can cause fierce coagulation in the centre. The coagulated milk begins to ‘burn’—that is, the proteins and sugar break down—producing brown specks and an undesirable taste. A thick pan, stirring, and moderate heating for as short a time as possible all help to avoid sticking.

Both flocculation and coagulation in milk are irreversible once they have occurred. There are no restorative measures as there are with mayonnaise or other emulsions.

Since milk and its products are among our most nutritious and delicious foods, it would be wrong to end this entry on a note of irreversible disaster. Rather should one remind oneself that milk is a great drink; it has built up its own ‘culture’ in many parts of the world—for instance, milk bars, the milk shakes of N. America, and the so far indestructible figure of the British milkman (milko in Australia)—and all this without taking into account the fabulously rich gallery of milk products.

The universal consumption of fresh or processed milk as a drink on its own is very much a product of modern refrigeration, other preservation strategies, and the distribution of powdered milk as country-to-country food aid. It might be among the most important dietary changes of the 20th century, with concomitant effects on agricultural production and human health.

See also milk puddings; milk reduction.

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