Boiled milk has generally been considered less digestible than uncooked milk, but some experiments seem to contradict this. The experiment station bulletin states that when cow's milk has been boiled before it is taken into the stomach it is likely to be precipitated in a more floculent form. Hutchison says that it has been found in the case of infants and calves that sterilized milk which has been kept at or above the boiling point for more than an hour is absorbed, quite as well as milk which has merely been boiled in the usual way, and he concludes that boiling does not appreciably diminish the digestibility of milk.
On the other hand, the government bulletin states, after acknowledging that the results of experiments upon the subject are conflicting, that "the more common experience seems to indicate that cooking or heating the milk makes the proteids somewhat more difficult for most persons to digest, but there are exceptions to this rule, if it be a rule, for there are persons who cannot take fresh milk with comfort but with whom boiled milk agrees very well".
In this case as in many others we must wait for a larger number of experiments to be made before we can make very dogmatic statements.
Buttermilk is considered an especially digestible form of milk, while koumiss or fermented milk is of still greater value in this respect. Skim-milk deserves more general use than it has, since the proteid of the milk nearly all remains in this, and it is for the proteids >that we especially value the milk. Where skim milk is sold at a low price, it is economy to use it freely in cooking, supplying the needed fat in a less expensive form than cream.
The composition of milk has already been stated in a general way. If we examine it more in detail, we find that the proteids of milk consist chiefly of two: casein or, as it is sometimes called, caseinogen. This forms about three per cent of the total of the milk. It is held in solution more or less completely by the salts of lime present in the milk. When acid is added to the milk, or it becomes sour, this casein is precipitated. When rennet is added the casein is coagulated and is changed in chemical composition. The scum that forms upon heated milk is chiefly casein.
The other proteid present in milk is lact-albumen. This coagulates when the milk is heated for a long time. It is present in much smaller amount than the casein, forming only about one-seventh of the total proteid of the milk.
The sugar of the milk, forming between four and five per cent, is called lactose or milk sugar. It has two important characteristics. It lacks the sweetness usually associated with the name of sugar, having only a very slight sweet flavor, and it is considered the most digestible form of sugar, apparently fermenting in the stomach or intestines with much less ease than do other sugars. For both of these reasons it is particularly suitable for the use of infants or invalids. The commercial article is obtained from milk, and is sold in the form of a fine white powder looking not unlike pulverized sugar. Aside from its use as a food it is extensively used in the preparation of pills. The fat of milk is present in the form of an emulsion. If one looks at a drop of milk through the microscope one sees a large number of tiny fat globules. That the fat is so finely divided is a factor in its digestibility, though fat derived from milk, either in the form of cream or butter, is also considered particularly digestible.
Fat Globules of Milk Magnified 200 Times.
a Skim Milk. b Whole Milk. c Cream.