The digestion of milk is preceded by its clotting in the stomach. The same thing happens when junket is formed by the addition of rennet to milk. This is a different process from the curdling of milk, which occurs when milk turns sour. The latter is caused by the splitting up of milk sugar and the formation of lactic acid by certain micro-organisms in the milk. When milk is heated, a skin is formed, consisting of coagulated albumin, in which is also a little casein, fat, and salts of lime. Boiled milk becomes sterilized. Cow’s milk should always be boiled, unless it is quite certain that the cows from which it is derived are perfectly healthy, and that the milk has not been exposed to infection before reaching the house. The disadvantages of boiling which are outweighed by its advantages, are that the taste of the milk is altered, some nutritive matter is lost by the formation of the “skin,” and the casein is not quite so easily digested. Pasteurization of milk, i.e. keeping it at a temperature of 70° C. (158° F.) for 20 to 30 minutes has been proposed as an alternative to boiling. This appears to destroy the bacilli causing tuberculosis (see page [312]). The typhoid bacilli are killed at 60° C. in five minutes when suspended in emulsion. Pasteurization is not, however, so certainly efficacious for other disease-germs as is boiling, and is not so easily carried out in domestic life as boiling. By boiling milk in a double saucepan, i.e. in a water-bath, very little change occurs in the taste of the milk, especially if it be cooled rapidly and strained.
Cheese is prepared by coagulating milk by “rennet,” the mucous membrane of the fourth stomach of the calf, salted and dried before using. By this means the casein is precipitated, carrying down with it the cream, and a large proportion of the salts of milk. The whey, containing the sugar, soluble albumin, and remaining salts, is separated by straining, while the mixed curd and fat are pressed in moulds. Cheese thus consists of casein, fat in varying proportions, water and salts, especially phosphate of lime. It is coloured with annatto, a vegetable colouring matter. When new, cheese is tough; when old, its oils tend to become rancid; the best age is from nine to twenty months. It is probable that cheese in small amount helps the digestion of other foods, though it is itself a highly concentrated and comparatively indigestible food. When toasted it is proverbially indigestible.
There are many different kinds of cheese. The following classification gives the more important varieties:—
(1) Cream cheese is the new curd only slightly pressed, and is more digestible than ordinary cheese.
(2) Next to these are cheeses made with whole milk rich in cream, such as Stilton, Gorgonzola, Cheshire, and Cheddar.
(3) Cheeses made of poor or partially skimmed milk, such as Shropshire, Single Gloucester, and Gruyère.
(4) Cheeses made of skimmed milk, such as Suffolk, Parmesan, and Dutch.
American cheeses may belong to any of these classes; they are generally pure, but occasionally are made from separated milk, margarine being added to take the place of cream. The sale of such cheeses, except under the name of “margarine cheese,” is now illegal.
Non-Nitrogenous Animal Foods.—These are all fats, and the most important are the various meat fats and butter. They possess a higher food value than carbohydrates in the proportion of 2¼; to 1. The composition of the various fats differs somewhat; they usually contain varying proportions of olein, palmitin, and stearin, which are compounds of glycerine with the radicle of a fatty acid (stearin = C₃H₅ (C18H35O₂)₃). Thus mutton suet consists of stearin, olein, and palmitin, with a preponderance of stearin. Beef suet contains less stearin and more olein than mutton suet. The more olein a fat contains the less solid it is. Olive oil is composed almost entirely of olein. Palmitin, which melts sooner than stearin, is the chief solid constituent of butter, while olein is its chief liquid constituent. Butter is specially distinguished by containing 7 to 8 per cent. of “volatile fatty acids,” such as butyric, caproic, etc., combined with glycerine. The presence and amount of these compounds is an important test for the freedom of butter from adulterating fats.
Cod-liver oil is next to butter the most digestible animal fat known. The best cod-liver oil is frozen at a low temperature, by which means the stearin is frozen out, and nearly pure olein is left. Traces of iodine have been found in it, and more commonly a small amount of bile, which probably increases its digestibility.