The remarkable property possessed by rennet, of curdling or coagulating casein, is well known; rennet is an extract from the stomach of the calf, and similar principles are present in the stomachs of man and other animals, so that the coagulation of milk is the first process in its digestion. If milk is gulped down in large quantities it is apt to coagulate in lumps, and digestion is much interfered with, but if it is taken hot and slowly, it coagulates in small pieces which are readily attacked by the gastric juice, and milk is then one of the most assimilable of foods.
Nature provides that the milk for young animals is supplied in finely divided streams, so that coagulation takes place in the best possible way.
The proteids are the most important constituents of food; they are abundant in the blood, and build up the muscles, brain, nerves, and other bodily structures.
Besides these mentioned, milk contains traces of another proteid of similar composition called globulin.
The sugar of milk is not found anywhere else. It is a carbohydrate like cane and grape sugar—that is to say, the hydrogen and oxygen they contain are in the same relative proportions as in water. Milk sugar is not so soluble or so sweet as the other sugars. It does not ferment with ordinary yeast, but certain special yeasts which are made use of in the preparation of keffir, koumiss, etc., have the power of transforming it into alcohol. Its most remarkable property, however, is the facility with which, under the influence of certain bacteria, it is changed into lactic acid.
Every one is familiar with the souring of milk, but perhaps it is not so generally known that there are great differences in the results obtained in accordance with the conditions under which the souring takes place. The skilled butter-maker, by keeping the milk in a cool and cleanly dairy, obtains a sour milk of a characteristic and agreeable aroma and taste, which beneficially affect the flavour of the butter produced. On the other hand, if milk is kept in hot and dirty surroundings, the development of acidity is accompanied by different bad tastes and odours, and it becomes unfit for use as a food. In the first case, the conditions are favourable to the maximum production of the lactic acid bacteria, and these occupy the field, and largely prevent the development of the other bacteria which are present—the survival of the fittest in the struggle for existence. In the second case, the impure surroundings swarm with the germs of many kinds of putrefactive bacteria, and the high temperature assists these to gain the upper hand. Again, the survival of the fittest, in the particular conditions. Even in cool and cleanly surroundings injurious taints may develop, especially if the milk has previously been subjected to a journey by road or rail, as is the case in the modern creamery system, where the farmers deliver their milk to a central creamery, where it is made into butter. In such establishments it is the regular practice to kill the germs, lactic and others, existing in the milk, by heating it to a high temperature. This process is called pasteurising, after the great French chemist and bacteriologist who invented it. Pure lactic cultures are added to the pasteurised milk, and the souring process is under exact control, with the result that butter of uniform flavour and quality is produced. The same method is made use of in making the special sour milk described in this book, with, of course, modifications in the apparatus employed, to suit the smaller scale in which the manufacture is conducted.
The ash is the mineral matter which is left when milk, previously dried, is burnt in a crucible. It is a complex mixture, and, as we have seen, it amounts to about 0.7 per cent. of the milk. The process of burning destroys all the organic matter, and, at the same time, alters somewhat the state of combination of the inorganic or mineral elements. Attempts have been made from the analysis of the ash to reconstitute the composition of the mineral matter as it exists in the milk. The best known is that of Soldner, and the following is his calculation:
The presence of citrates will be noted in this analysis. Citric acid, which gives to lemons their acidity, and is also found in other fruits, has been proved to exist in milk to the extent of about 0.2 per cent. When alkaline or earthy citrates are burnt or oxidised in the blood, the citric acid is destroyed, and corresponding carbonates remain. No doubt the function of citrates in milk is to furnish to the body the earthy and alkaline carbonates which are required in certain of its parts.
The mineral constituents of milk have many important functions to perform in the building up and nutrition of the bodily organism. Phosphate of lime is the principal constituent of the skeleton, and the blood must be richly supplied with the alkalies, earths, and acids which are comprehended in the ash.