111 methods have in common the use of milk derived from animals. This is a glandular secretion analogous to human 111111: Will ell itap proaches in it, Mil k is most used, and goat's milk is also used very much. With us, the ass's and the mare's milk, which are chemically more similar to human milk. do not play a r(ole in artificial infant feeding.
Comparative investigations of human, cow's, and goat's milk have shown that while these kinds of milk have many properties in common, numerous differences exist, particularly of chemical-biological nature For a more detailed account see page 310. llere only the difference of the proteid content will be pointed out.
Cow's milk NMI:1111S about three ti IICS as much pl'HfridS as does human milk. The proteids of the latter are composed of about equal parts of casein and albumin, while cow's milk contains 7 parts of casein to about 1 part of albumin. Beside, there exist certain qualitative dif ferences between the respective proteids of cow's milk and human milk, and these are perhaps responsible for the difference of certain qualita tive reactions (precipitation of human casein in fine flocculi, of cow's casein in coarse curds; residue of pseudonuclein in digestion experi ments with cow's casein, but not with humiln casein: different behavior with regard to the action of certain acids. etc.). The biochemical inves tigations have for the first time shown absolutely that human, milk can not be replaced by milk of animals. Although these results are of great significance, it is impossible at present to decide their practical impor tance, and it appears premature to use them for the explanation of the less satisfactory results of "unnatural feeding." In contradistinction to human milk, which the infant receives directly and without manipulation, cows milk is exposed to changes at the place where it is obtained and to further changes oil its way to the consumer. These changes are of the greatest importance when the milk is to be used for the infant. Bacteria by their activity and growth affect the constituents of the milk. How these changes occur and how they can be avoided are discussed on page 322. The danger of bacterial con tamination is very small, if the milk conies from healthy animals, prop erly kept and fed, and is and kept with the greatest cleanliness and cooled as rapidly as possible, and delivered in this state. Under these conditions the milk may he used raw, provided it is further kept in such a way that the bacteria cannot grow at all or can grow only very slowly, and that the milk is consumed within one or at the utmost two days after milking.
All these conditions can be fulfiled much more readily when goat's milk is used. Goats are less subject to tuberculosis than are cows, and the danger of contamination of the milk is much diminished on account of the solid feces. Furthermore, the deteriorations caused by the deal ing with and the transportation of milk can be avoided with less diffi culty, as many people can afford to keep a goat.
Such a milk, aseptically obtained and kept, seems to be the most suitable substitute for breast-milk, since it has not been exposed to the possibility of changes and has not lost its natural properties. Within recent years, many authors have expressed their preference for raw milk whenever possible, on account of its biological properties.
Heating is the most suitable method to inhibit the growth of bac teria. The total destruction of all the micro-organisms, i.e., complete sterilization, cannot be accomplished with absolute certainty by sim ply boiling the milk. At first this was expected, but certain spore hearing bacteria (particularly the bacterium described by Mugge), which occur very frequently in the milk, are not destroyed. Their continued growth causes dangerous changes in the milk, particularly by their action on the proteids. These bacteria are absolutely destroyed by a prolonged action of temperatures above the boiling-point. But by this the milk is changed to such a considerable degree. that its use as food for infants is excluded, and a suitable milk, which will keep for a longer time, cannot be prepared in this manner.
Boiling. the milk destroys all the micro-organisms, and particularly the pathogenic bacteria, with the exception of certain spores. Thus a partial sterilization takes place, which meets all the requirements of practice, provided the milk is consumed within twenty-four or at the latest forty-eight hours after milking. During this time it must be kept. in such a manner that the germs which have not been destroyed cannot grow or can grow only very slowly, and that no new germs van enter the milk. The best way to accomplish this is to keep the milk at temperatures not. exceeding 10° C. (50° F.). The most danger ous temperatures range between 20° C. and 60° C. (OS° and 140° F.).