100 × 7.9∕8.5 = 92.9 per cent. of genuine milk,
_i.e._ 7.1 per cent. of water has been added to it.
Half a pint of milk supplies as much nitrogenous nutriment as two good-sized eggs, and as three and a half ounces of beef. Milk may be deteriorated (1) by skimming or “separating” by machinery, or (2) by the addition of water—the first diminishing the proportion of fats, and the second the total amount of solids.
=Skim Milk= still contains nearly 1 per cent. of fat, but =Separated Milk=, in which the cream has been removed by centrifugal apparatus, contains less than 1∕8 per cent.
Skim or separated milk forms a cheap source of nitrogenous food; but when it is sold mixed with new or alone as new milk, the public is defrauded, and infants fed on it are robbed of the fat which is so essential for their growth.
=Condensed Milk= is milk deprived of a large part of its water. It represents three times its volume of fresh milk. There are in the market (=a=) unsweetened and condensed whole milk, (=b=) sweetened and condensed whole milk, and (=c=) sweetened and condensed skim or separated milk. Unfortunately the latter is most largely sold because cheapest; and infants are thus often robbed of fat, a most important element in their food. Always examine the label of each tin carefully, to ascertain whether the milk has been deprived of its cream. The law requires that this fact should be stated on the label. Tins which have bulged should be rejected. Condensed milk is more easily digested by infants than new cow’s milk, but it lacks the anti-scorbutic properties of new milk (see page 28). Even the condensed whole milk if diluted beyond 1 part of milk to 3 of water is deficient in fat. Sweetened condensed milk has one-third its weight of extraneous sugar added to it, and on this account it tends in children to produce fatness, and a distaste for simple food; in children fed on it alone ossification (formation of bone) is retarded, and resistance to illness is diminished. The only dietetic advantages it possesses over fresh cow’s milk are its freedom from possible disease germs and easier digestibility.
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.