Milk varies a good deal in composition; the different breeds of cows give varying qualities. The Short-horn gives large quantities of milk of rather poor analysis, while the Jersey yields smaller proportions of very rich milk. During the period of lactation (the time which has elapsed since the cow gave birth to a calf), care in milking, food, health, etc., all have an effect on the quality of the milk.
The limits of variation may be stated as follows:
| Per cent. | Per cent. | ||
| Water | 87.5 | to | 82.5 |
| Fat | 2.5 | " | 6.0 |
| Casein and albumen | 3.0 | " | 4.5 |
| Milk sugar | 3.5 | " | 6.0 |
| Ash | 0.6 | " | 0.8 |
These figures are extreme, and it is very seldom indeed that either the minimum or maximum is reached. Indeed, by the regulation laid down under Clause 4 of the British Sale of Food and Drugs Act of 1899, when the percentage of solids not fat falls below 8.5 per cent., and fat under 3 per cent., it is assumed that the milk has been adulterated. This regulation is a perfectly just one. While genuine milk may, in rare instances, show figures as low as 7.1 per cent. of solids not fat, or 2.5 per cent. of fat, the right can hardly be claimed of supplying such an abnormal article to the public as milk of proper quality, and the dairyman who understands his business, and wishes to deal fairly with his customers, can, by attention to the conditions enumerated above which influence the composition of milk, entirely avoid the production of such a low-grade article.
In the nutrition of both plants and animals large quantities of water are needed. The solids must be supplied in solution or dissolved in the assimilative processes, and this cannot take place without water, which also conveys the dissolved solids to the various parts of the economy, and in the case of animals removes waste materials. For the most part, water passes through the body unchanged, but a certain proportion unites chemically with the food materials and assists in their digestion. It is therefore not surprising that seven eighths of milk is composed of water. Blood contains a similar proportion, and this agreement emphasises the fact that milk is a perfectly balanced food.
The fat of milk, which yields cream and butter, differs in some important respects from other fats. Like these, it is made up chiefly of stearin, palmitin, and olein, but, in addition, it contains an abnormally large proportion of compounds of certain of the volatile fatty acids. It is these which give to butter its agreeable flavour. By the methods of Duclaux, the following is the approximate composition of butter fat:
| Per cent. | |
| Stearin, palmitin, olein, and traces of myristin and butin | 91.50 |
| Butyrin | 4.20 |
| Capronin | 2.50 |
| Caprylin, caprinin, and traces of laurin | 1.80 |
| —— | |
| 100.00 | |
| ——— |
Myristin occurs in nutmegs; butyrin in another combination flavours pineapples and rum; caprinin is found in cocoanut fat, mutton fat, and in the offensive odour given off by the goat (from which the name is derived); caprylin is a by-product of alcoholic fermentation, and also occurs in cocoa fat; laurin is found in sweet bay; from which it is evident that there are some curious relationships in flavouring materials.
Fats are very concentrated foods, furnishing a large amount of energy to the body. At one time they were classed together with starch, sugar, and other carbohydrates as heat-producers, but the distinction which was drawn between the kinds of food which were thought solely to keep up the temperature of the organism, and those which produced force in work and other forms of bodily energy, has broken down, and by direct experiment has been found not to exist. It is usually calculated that one part of fat is equal in food value to about two and a quarter parts of any of the other carbohydrates. Milk fat or butter is more digestible than almost any other fat, and its importance therefore can readily be realised. All the above constituents of milk fat are composed of different proportions of carbon, hydrogen, and oxygen, but milk also contains minute quantities of lecithin, a fat containing phosphorus in addition. Lecithin is also found in the brain and nerve material of animals, in the yolk of egg, and in several plants.
The nitrogenous constituents of milk—casein and albumen—are usually estimated together, and they are reckoned as of equivalent food value. The name protein is very commonly applied to the total of these bodies in milk, or other animal and vegetable foods. They are composed of different proportions of carbon, hydrogen, oxygen, and nitrogen, with small quantities of sulphur, while casein contains phosphorus in addition. Albumen exists to the extent of about 0.6 per cent. in milk. It is very similar in properties to egg albumen. The coagulum which forms on the surface of milk when boiled is largely composed of albumen. Casein is combined with, and kept in solution by, lime, soda, and calcium phosphate, and its amount averages a little over 3 per cent.