The specific gravity of milk gives an easy measure of the solids in solution, but unfortunately it gives no accurate estimate of the amount of fat suspended in the emulsion. Therefore, to test milk adequately two methods must be employed: one to estimate the degree of density of solution, and the other the degree of opacity of the emulsion.
I. To test the density, a specially graduated form of hydrometer is generally used. This is graduated so as to indicate specific gravities from 1014 to 1042. The latter being the maximum density of pure milk (the average being about 1030), and the former being about the density of pure milk when mixed with an equal bulk of water. Every reduction of 3 in the specific gravity may be said to correspond to about 10 per cent, of water.
II. The degree of opacity is estimated by the amount of water required to render a small quantity of milk sufficiently translucent to allow a candle flame to be seen through a layer of the mixture one centimetre thick. One cubic centimetre of the milk (which has been shown by the microscope and the iodine test not to contain chalk or starch) is placed in a test glass with flat parallel sides, just one centimetre apart, and water is cautiously added from a graduated pipette. The more water required the richer the milk is in fat; good fresh milk requires about 70 times its bulk of water to become translucent.
Another method employed for the same purpose consists in the comparison of the color produced by a layer of milk 1 mm. thick in a black cell with a previously prepared standard of grayish colors.
The quantity of fat may also be estimated by placing the milk in a tall graduated vessel for twenty-four hours, at the end of which time it should show at least 10 per cent, of cream.
Butter is made from milk, or better from cream, by breaking, by agitation, the coating of proteid which before churning prevents the oil globules from running together. It is almost completely composed of fat, the larger globules having run together to form the solid butter, which can be removed, leaving some small fat globules with the proteids, milk sugar, lactic acid, and salts in the water forming "buttermilk." *
Fig. 43. Microscopic appearance of milk in the early stage of lactation, showing colostrum cells (a).
Cheese is another form of food made from milk by precipitating the proteid either by lactic fermentation or the addition of rennet - an extract of calves' stomach which, without the presence of any acid, curdles milk - and draining off the solution of milk sugar, and salts (whey). It contains most of the proteid and a great deal of the fat of the milk. During the ripening of the cheese more fat is formed, apparently from the proteid while leucin and tyrosin also appear.