The casein of human milk is always precipitated from solution in finer flakes than are the caseins of the ruminants, and, under conditions similar in other respects, they yield no pseudonuclein on digestion by pepsin.
The lactalbumins agree in all their properties with the serum albu mins of the same species of animals. The lactalbumin of cows, however, is said to have a Higher limit of precipitation with alcohol than the corresponding albumin of the blood. Lactalbumin passes through the clay cell in filtration.
Lactoglobulin is found in abundance only in colostrum and has not yet been clearly defined.
The proteids of milk are specific: that is, their injection into rabbits (but less often with guinea-pigs and never with dogs) occasions in the serum of the blood the formation of a substance which causes a precipi tate with the milk or the proteids of the same, or closely related, species of animals. It has not yet been determined whether this specific action derives its qualities from the group of proteids separated chemically or from sonic substance attached to them.
The nitrogenous extractive substances found in milk are urea, ammonia (probably only in milk already decomposed, or by absorp tion from the atmosphere of the stable), orotic acid; i.e., a ureide with the formula, CO , and traces of creatin, creatinin, NH—Co—CO hypoxanthin, and sulphocyanate. The so-called nucleons are artificial products.
Milk sugar is identical in all kinds of milk. It is as yet uncertain whether some other carbohydrate is also present in small quantities. It is probable that we have here to do with a substance that can be split off from lactomucin by acids.
Finally, milk contains citric acid, substances producing color and odor, perhaps also a trace of substances giving an iodoform reaction (alcohol or aldehyde), ferments, and alexins.
In nutrition great importance has been ascribed to the ferments or enzymes. It is a question as to whether the milk carries the fer ments which are contained in the cells and blood serum, as a cell product or as a transudate. There have been demonstrated, so far: (1) Superox idases, which decompose hydroperoxides by liberating the oxygen. By
centrifugation they pass into the cream, where they adhere physically to the milk globules. Consequently, in fractional precipitation, they are deposited along with the casein.
(2) Re(hictacs.—Milk in contact with sulphur, selenium, arsenic, phosphorus, etc., and with water, produces the corresponding hydrides. It also induces other reductions, for example, that of methylene blue.
It is. however, not yet quite certain that this latter is not caused by bacteria. It is likewise still doubtful whether these reducing agents are identical with the milk ferment called by Smith aldehyde-caw lase, which reduces methylene blue when small quantities of warm formaldehyde are present. Both pass into the cream under centrif ugation.
(3) A Idehydages.—These oxidize aldehydes when air is admitted. Perhaps they are identical with the reductases.
(4) Pcroxidases.—In the presence of the so-called true superoxides BaO,, etc.) or substances closely related to them (as ozonized oil of turpentine and the resinous part of tincture of guaiacum), these hasten the oxidization; c.g., of guaiacol, p-phenylendiamine, iodide of potassium, and many similar substances. It is probable that they hasten also other oxidative syntheses: e.g.. the formation of indo phenol from naphthol and p-p1ienylencliantine. They pass into the skim milk by centrifugation and in fractional precipitation are only precipitated along with the globulins.
(5) Amylolytie Ferments. which transform starch into dextrins and maltose. They appear to pass into the whey when precipitated.
(6) Glycolytic Ferments.—The action -usually attributed to them should perhaps be assigned to bacteria.
(7) Lipase, which decomposes neutral fats into fatty acids and glycerin. It has not been ascertained how these substances, and the two that follow, act in fractional precipitation and in centrifugation.
(S) It has not yet been determined whether the decomposition of salol by milk is produced by a ferment. salolase, or merely by saponifi cation. Salolase does not occur in the alcoholic precipitate.