The Lactometer has been used to discover adulterations, depending upon the difference in specific gravity of the various constituents. The specific gravity of whole milk is about 1.032 which means that, if a certain volume of water weighs 1.000 weight units, the same volume of whole milk weighs 1.032, the same volume of butter-fat weighs, say, .900, or of cream about 1.000, and of skim milk 1.036 units. If the Lactometer shows a sample of milk to have a higher specific gravity than 1.032 it may therefore be suspected of having been skimmed. But it will readily be seen that by removing from whole milk some of the cream and adding water, the specific gravity can easily be brought back to normal for whole milk. This test is therefore unreliable and has been discarded with the advent of the Babcock.
The acidemeter
Acidity Test.[[3]]—The acidity, or sourness, of milk or cream, which depends upon the amount of lactic acid developed in it, may be tested by a liquid normal alkali or by the Farrington Alkaline Tablets, a solution of which added to sour milk neutralizes the acid. A few drops of an indicator, Phenolpthalein, added to the milk, turns it pink when all the acid has been neutralized, and the amount of alkali solution used shows the percentage of acid in the milk. This is quite important in preparing “starters” for ripening the cream in butter making or milk in cheese making, and in the manufacture of “Commercial Buttermilk,” etc.
There are other tests used in scientific dairying as the Fermentation Test to ascertain the relative purity of milk, the Casein Test, etc., but the above are those mostly used besides the Bacterial Count which is mentioned under the chapter on “Milk Supply,” and the Rennet Test described under “Cheese Making.”
FERMENTS
Two classes of ferments are of importance in connection with milk: (1) “unorganized” or chemical ferments, the “enzymes,” and (2) “organized” ferments such as bacteria and yeast.
Enzymes
Rennet.—Among the unorganized ferments, Rennet or Rennin is highly important on account of its power of coagulating or curdling milk by precipitation of the casein. Rennet is extracted from the stomach of the suckling or milk-fed calf, where it serves in digesting the calf’s food. It is in the market in the form of a liquid extract as well as a dry powder compressed into tablets (Rennet Tablets and Junket Tablets). The characteristics and use of rennet are described under “Cheese Making” in Chapter [III].
Pepsin is another enzyme the office of which in the process of digestion is to dissolve albuminoids. It is not considered identical with rennet though in an acid solution it will curdle milk. It occurs in the stomachs of grown animals fed on solid food and is usually produced from hogs’ stomachs.