COMPOSITION OF MILK.

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Food which is typically perfect, is presented by nature to the young of various animals. In milk, or in the egg, we should ex- Milk as an artipect to find whatever is necessary for the growth of the tis- cle of food: its composition. sues, and for the performance of the functions. An examination of milk will therefore illustrate the essential characters of the different elements of food.

In this we notice, first, the large proportion of water present, almost nine tenths of the whole amount. The double duty of this The water of water has already been mentioned, to remove from the sys- milk. tem effete substances which are not of a vaporous or gaseous form, and which can not escape through the lungs, and to regulate the temperature by evaporation. We might have added to these that it imparts a due fluidity to the blood. These are conditions as necessary to the infant as to the adult, and it should be remembered that two thirds of the weight of the body are water.

Next follows the nitrogenized principle casein, which is closely related in composition to muscular flesh. It is the tissue-mak- The casein of ing, histogenetic, or nutritive element of the milk, and has been milk. elaborated from the albumenoid substances of the mother's system. It is to be converted into the muscular, gelatinous, and other soft tissues of the infant.

Casein is one of a group designated as the neutral nitrogenized bodies, of which some of the more prominent are albumen, fibrin, Nature of proand globulin. From an opinion that these all contain the tein bodies. same organic radical, they are often termed the protein bodies. They appear to exist in two different physical conditions, soluble and insoluble in water; they all contain sulphur, and exhibit a proneness to pass into the putrefactive fermentation. As this takes place when they have reached a certain stage of decay, they act upon other bodies as ferments. Their constitution is represented in common by the formula

Albumen.

Of the whole group, albumen may be taken as the type and most important member. Indeed, as will be found hereafter, in the process of digestion the others are invariably converted into it. The white of the egg and the serum of the blood are usually referred to as examples of albumen, though they differ in several particulars from one

another. Albumen forms basic, neutral, and acid compounds. It is a basic albuminate of soda which is found in the egg and in serum of blood. In certain diseased conditions the blood contains the neutral albuminate.

Casein.

Casein presents nearly the same constitution as albumen, but differs from it in its physical properties; for, while a solution of albumen is coagulable by heat, one of casein is not, but lactic and acetic acids coagulate it, though they have no such effect on albumen. While, so far as their protein nucleus is concerned, the two substances agree in composition, they differ in this respect, that casein appears to contain a less proportion of sulphur, and no phosphorus. It is interesting to remark that, during incubation, casein arises from albumen in the eggs of birds.

Fibrin.

Closely allied to albumen and casein, and having the same protein nucleus, is fibrin, which likewise exists in two states, soluble and insoluble. Its solidification or coagulation can be produced by the action of sulphuric ether, which does not affect albumen. Moreover, in the coagulated state fibrin decomposes the deutoxide of hydrogen, but albumen does not. The most important difference between them is, that in the act of coagulation albumen shows no disposition to assume a definite structure, but fibrin does-fibrillating, as it is termed. The analogy of constitution and closeness of relation of the two substances is demonstrated by the fact that by nitrate of potash coagulated fibrin may be changed into albumen, and the same conversion is accomplished in the stomach by the digestive juices.

It is generally supposed, however, that fibrin contains a larger proportion of oxygen than albumen, a conclusion which seems to be confirmed by physiological considerations respecting its origin. For this reason, Mulder describes it as a higher oxide of his hypothetical protein. It always is associated with fat, or, perhaps more correctly, with soaps of ammonia and lime.

Fibrin is found in the chyle, lymph, and blood. In the latter fluid its quantity varies in different parts of the circulation. The blood of the portal vein yields it in smaller proportion than that of the jugular. It is also affected very much by diet: thus Lehmann found that under an animal diet there was much more fibrin in his blood than under a vegetable one, a result which has been confirmed by experiments on dogs. It has also been observed that its quantity is increased during starvation. But the blood of herbivorous animals contains more than that of carnivorous ones, and that of birds contains the most of all.

These remarks on the composition and physical properties of casein, albumen, and fibrin, have been introduced for the purpose of illustrating the facility with which these bodies are mutually convertible, and more

OF THE SALTS, BUTTER, AND CURD OF MILK.

31

particularly for showing that there is nothing whatever mysterious in the casein or curd of milk arising from the albuminous serum of the mother's blood, and being transmuted into the fibrin structure of the muscular tissues of the infant.

Returning now to our examination of the composition of milk, as set forth in the preceding table, we find that two respiratory el- The sugar and ements are next upon the list: 1st. Sugar of milk, which is butter of milk. to be converted into lactic acid, partly by the agency of the saliva, and chiefly in intestinal digestion; 2d. Butter, which is the oleaginous or fatty portion, and of which a part is to be deposited in the adipose tissues for a time of need, and a part, along with the lactic acid and excess of sugar, is to be burned at once for the production of heat.

The salts of

earth, and

dium.

The inorganic body, phosphate of lime, is necessary for the earthy portion of the skeleton, and probably the reason of the introduction of casein, to the exclusion of other protein compounds, depends milk, particuon the power it possesses of holding phosphate of lime in solu- larly bonetion, not less than 6 per cent. of its weight of this earthy body chloride of sobeing often obtainable from it. Among the other salts of the milk, chloride of sodium may be pointed out as of special importance. It undergoes decomposition in the system of the infant, its hydrochloric acid giving acidity to the gastric juice, its soda entering into the composition of the bile and various salivary secretions. It also imparts solubility to albumen, and, in some degree, regulates the facility with which that substance coagulates. It impedes the coagulation of fibrin.

Milk is not a chemical compound, but a variable mixture of different ingredients, which, under proper circumstances, may be sepa- Making of rated. When the fluid is allowed to rest for some hours at the butter. ordinary temperature, the fat-globules rise to the surface as cream, which, submitted to a strong agitation with air in the process of churning, forms butter.

The casein of milk can be readily coagulated by rennet (which is the mucous membrane of the stomach of the calf) at a temperature Making of of 120°. If parted from the residual whey, mixed with a little cheese. salt and yellow coloring matter, and subjected to the action of a suitable press, it is formed into cheese. No better examples of the tissue-making and heat-making elements of food can be offered than cheese and butter respectively.

When milk is exposed to the air, its sugar, under the influence of the casein or curd, gradually disappears, turning into lactic acid, Lactic acid in and the milk becomes sour. The composition of sugar and sour milk. lactic acid is such, that we might, without much error, say that an atom of sugar symmetrically bisected will yield two atoms of lactic acid. This effect is produced by the casein commencing to pass into a state of de

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VARIOUS KINDS OF MILK.

cay under the influence of the atmospheric air. It is likewise produced during digestion by the saliva, and also by the pancreatic juice. The turning sour of milk on the stomach is due to the transmutation of its sugar into lactic acid.

An infant finds in its mother's milk whatever it wants for the growth Physiological of its own body. In its system the curd resumes the form uses of milk. of albumen, or passes into the condition of fibrin or syntonin, and in this manner its muscular and gelatinous tissues are made. The butter is deposited in the adipose cells, or burned at once for the production of animal heat, a part of it, however, being incidentally consumed, as will be hereafter explained, in the fabrication of fibrin and for other histogenetic purposes. The phosphate of lime is carried to the osseous system, now in a state of rapid increase, and bone is formed from it.

Various kinds

But though milk is so well adapted to the wants of infantile life, it is unsuited to the adult. Its nitrogenized principle, casein, though in sufficient quantity for the repair of muscular waste and development at the former period, is inadequate to these purposes at the latter, when destruction, arising from the incessant activity of the muscular system, is so greatly increased. It is interesting to remark how the of milk for dif- composition of milk is modified when there is a necessity to meet these indications, its nitrogenized principle being increased in the case of animals such as the cow and horse, the young of which commence locomotion almost at birth, or at a far earlier period than the human infant. This excess of casein is necessary for the repair of the resulting waste.

ferent animals.

This table presents an explanation of the unsuitableness which is sometimes remarked in the milk of the cow when used for the nourishment of children. Milk which is adapted to the wants of the calf is not adapted to the functional wants of the child. Experience has taught the nurse that these difficulties may in part be removed by diluting it with water and sweetening it with sugar, the effect of this being to reduce the percentage of the nitrogenized element, the casein, and to increase that of the respiratory, and so approximate the composition more closely to that of human milk.

Moreover, milk is not suitable as the sole nourishment of adult life, since it does not contain in sufficient quantity those phosphorized compounds which are necessary for the repair of the waste of the cerebral and nervous tissues, which at this period are much more active than in infancy.

on the compo

Variations in the composition of milk from its normal standard are observed to depend upon age and bodily health. Young fe- Influence of males, from fifteen to twenty, yield a milk more rich in sol- age and health ids than that which is given at thirty-five or forty. Gesta- sition of milk. tion at a late period increases the solid portions. The following table of Vernois and Becquerel illustrates the influence of disease:

From this consideration of the nature and properties of the food of infancy, we may pass to the examination of that of the mature period. Experience has shown that, of all articles of food, bread made from wheaten flour meets best the requirements of the adult life of man. It seems to contain all that is necessary for support. A very simple analysis will show how it presents both the respiratory and nutritive elements.

Of bread.

of flour of wheat and of

If such flour be made into a paste with water, and be gradually washed with a larger quantity, an elastic coherent mass is left, and Examination the water assumes a milky turbidity. After a time it becomes clear, through the settling of a white precipitate, which other grains. is starch, the leading member of the respiratory group. The elastic substance is gluten, which is a true vegetable fibrin, mixed with another nitrogenized body, gliadine, which may be removed, along with a certain quantity of oil, by washing with ether and alcohol.

Thus, simply by washing in water, flour may be separated into two physiological elements, respiratory and nutritive, the former being the starch, and the latter the gluten. The relative quantity of these substances differs in different samples of flour, and, other things being equal, the greater the amount of gluten the more valuable the sample, because the more nutritious. It is interesting to remark that the liquid from which the starch has settled, if brought to the boiling point, becomes turbid again, from the coagulation of the vegetable albumen it contains.

Other grains, treated in the same manner, yield similar results. The flour of barley and of the oat, when washed with water, do not, however, yield gluten, but a pure fibrin, with a separation of starch.

The fibrin occurring in these grains is replaced in other nutritious seeds, such as peas and beans, by legumin, which, like the casein of milk, does not coagulate by boiling, but merely forms tenacious skins as it is evaporated. These may be removed by skimming. This substance,

C