Part 87

The mean content of citric acid in milk is about one-tenth of one per cent.

Citric acid is not found in human milk, and probably exists only in the mammary secretions of herbivores.

Among the mineral acids of milk, phosphoric is the most important, but a part of the phosphorus found as phosphoric acid in the ash of milk may come from pre-existing organic phosphorus (lecithin, nuclein).

The sulfuric acid, which is found in the ash of milk, is derived from the sulfur of the proteid matter during ignition.

Lactic acid is developed from lactose during the souring of milk as the result of bacterial activity.

Gases are also found in solutions of milk, notably carbon dioxid, which gives to freshly drawn milk its brothy appearance.

The ash of milk has the following composition expressed as grams per liter of the original milk:

Grams Probable form Grams Component. per liter. of combination. per liter.

{ sodium chlorid 0.962 Chlorin 0.90 { potassium chlorid 0.830

{ KH₂PO₄ 1.156 { K₂HPO₄ 0.853 Phosphoric acid 2.42 { MgHPO₄ 0.336 { CaHPO₄ 0.671 { Ca₃(PO₄)₂ 0.806

Potassium 1.80 (as shown above) and as potassium citrate 0.495

Sodium 0.49 sodium chlorid 0.962

Lime 1.90 (as shown above) and as calcium citrate 2.133

Magnesia 0.20 MgHPO₄ 0.336

The percentage composition of the ash of milk, according to Fleischmann and Schrott, is expressed as follows:

Per cent. Potassium oxid, K₂O 25.42 Sodium oxid, Na₂O 10.94 Calcium oxid, CaO 21.45 Magnesium oxid, MgO 2.54 Iron oxid, Fe₂O₃ 0.11 Sulfuric acid, SO₃ 4.11 Phosphoric acid, P₂O₅ 24.11 Chlorin, Cl 14.60 ------ 103.28 Less Cl as O 3.28 ------ 100.00

=428. Alterability of Milk.=—The natural souring and coagulation of milk is attributed by most authorities to bacterial action produced by infection from the air or containing vessels. Pasteur, however, shows that fresh milk sterilized at a temperature of 110° may be exposed to the air without danger of souring. After about three days, however, a fermentation is set up which is totally different from that produced by the microzymes naturally present in the milk. This point has been further investigated by Béchamp, who finds that the natural souring of milk is accomplished without the evolution of any gas, while the fermentation produced in sterilized milk by the microbes of the air, is uniformly attended by a gaseous development. As a result of his investigations, he concludes that the souring of milk takes place spontaneously by reason of milk being an organic matter, in the physiological sense of the term, and that this alteration is produced solely by the natural microzymes of the milk.

According to Béchamp, the milk derived from healthy animals is capable of spontaneous alteration, which consists in the development of lactic acid and alcohol, and of curd in those milks which contain caseinates produced by the precipitating action of the acids formed. Oxygen and the germs which are present in the air, according to him, have nothing to do with this alteration in the properties of milk. Milk belongs to that class of organic bodies like blood, which are called organic from a physiological point of view, on account of containing automatic forces which produce rapid changes therein when they are withdrawn from the living organisms.

After milk has become sour by the spontaneous action of the microzymes which it contains, there are developed micro-organisms, such as vibriones and bacteria from a natural evolution from the microzymes.

Milk which is sterilized at a high temperature, viz., that of boiling water or above, is no longer milk in the true physiological sense of that term. The globules of the milk undergo changes and the microzymes a modification of their functions, so that in milk thus altered by heat, they are able to produce a coagulation without development of acidity. The microzymes thus modified, however, retain to a large extent their ability to become active. Human milk differs from cow milk in containing neither caseinates nor casein, but special proteid bodies, and also a galactozyme or galactozymase functionally very different from that which exists in cow milk. The extractive matter is also a special kind, consisting of milk globules and microzymes belonging particularly to it and containing three times less phosphate and mineral salts than cow milk. Boiling the milk of the cow or other animals does not render it similar to that of woman. There is no treatment, therefore, of any milk which renders it entirely suited to the nourishment of infants. The composition of the milk of the cow may be represented by three groups:

1. Organic elements in suspension; consisting chiefly of the globules of the milk, which are mostly composed of the fat, of an epidermoid membrane containing mineral matter of special soluble albumins and of microzymes containing also mineral matter.

2. Dissolved constituents; consisting of caseinates, lactalbuminates, galactozymase, holding phosphates in combination, lactose, extractive matter, organic phosphates of lime, acetates, urea and alcohol.

3. Mineral matters in solution; consisting of sodium and calcium chlorids, carbon dioxid and oxygen.

It will be noticed from the above classification that Béchamp fails to mention citrate of lime. It is scarcely necessary to add to this brief résumé of the theories of Béchamp that they are entirely at variance with the opinions held by nearly all his contemporaries.

=429. Effects of Boiling on Milk.=—On boiling, the albumin in milk is coagulated and on separating the proteid bodies by saturation with magnesium sulfate no albumin is found in the filtrate. The total casein precipitated from boiled is therefore greater than from unboiled milk. Jager has shown that the casein can be precipitated from boiled milk by rennet, but with greater difficulty than from unboiled. According to this author in 3.75 per cent of proteid in milk there are found 3.15 per cent of casein, 0.35 of albumin and 0.25 of globulin.

=430. Appearance of the Milk.=—The color, taste, odor and other sensible characters of the milk are to be observed and noted at the time the sample is secured. Any variation from the faint yellow color of the milk is due to some abnormal state. A reddish tint indicates the admixture of blood, while a blue color is characteristic of the presence of unusual micro-organisms. Odor and taste will reveal often the character of the food which the animals have eaten. Any marked departure of the sample from the properties of normal milk should at once lead to its condemnation for culinary or dietetic purposes.

=431. Micro-Organisms of the Milk.=—Milk is a natural culture solution for the growth of micro-organisms, and they multiply therein with almost incredible rapidity. Some of these are useful, as, for instance, those which are active in the ripening of cream, and others are of an injurious nature, producing fermentations which destroy the sugars or proteids of the milk and develop acid, alcohol, mucous or ptomaine products. It is not possible here to even enumerate the kinds of micro-organisms which abound in milk and the reader is referred to the standard works on that subject.

For analytical purposes it is important that the sample be kept as free as possible of all micro-organisms, good or bad, which may be accomplished by some of the methods given below.