Another mode of expressing average milk constitution would be thus:
| Fat | 4.1 | per | cent. |
| Solids not fat | 8.8 | " | " |
| —— | |||
| 12.9 | " | " |
It is probably too obvious to need remark that milks vary in standard, but the above figures may be taken as authentic averages.
Milk-sugar, or Lactose (C12H24O12). This is an important and constant constituent of milk. It forms the chief substance in solution in whey or serum. Milk-sugar approximates to dextrose in its action on polarised light. By boiling with sulphuric acid it is converted into dextrose and galactose.
Fat occurs in milk as suspended globules, and by churning may be made into butter.
The Proteids include casein, albumen, lactoprotein, and a small quantity of globulin. These are the nitrogenous bodies.
Mineral Matter. The ash of milk, obtained by careful ignition of the solids, contains calcium, magnesium, potassium, sodium, phosphoric acid, sulphuric acid, chlorine, and iron, phosphoric acid and lime being present in the largest amounts.
(1) Lactic Acid Fermentation. If milk is left undisturbed, it is well known that eventually it becomes sour. The casein is coagulated, and falls to the bottom of the vessel; the whey or serum rises to the top. In fact, a coagulation analogous to the clotting of blood has taken place. In addition to this, the whole has acquired an acid taste. Now, this double change is not due to any one of the constituents we have named above. It is, in short, a fermentation set up by a living ferment introduced from without. The constituent most affected by the ferment is the milk-sugar, which is broken down into lactic acid, carbonic acid gas, and other products.
For many years it has been known that sour milk contained bacteria. Pasteur first described the Bacillus acidi lactici, which Lister isolated and obtained in pure culture. Hueppe contributed still further to what was known of this bacillus, and pointed out that there were a large number of varieties, rather than one species, to be included under the term B. acidi lactici. We have already seen that these bacilli do not as a rule liquefy gelatine, form spores, are non-motile, and are easily killed by heat.