at The Left, Pure Milk; at The Right, Milk After Standing in a Warm Room For a Few Hours in a Dirty Dish, Showing, Besides The Fat-globules, Many Forms of Bacteria

at The Left, Pure Milk; at The Right, Milk After Standing in a Warm Room For a Few Hours in a Dirty Dish, Showing, Besides The Fat-globules, Many Forms of Bacteria

On another page you have been told how the yeast plant grows in cider and causes it to sour, and how bacteria sometimes cause disease in animals and plants. Now you must learn what these same living forms have to do with the souring of milk, and maybe you will not forget how you can prevent your milk from souring. In the first place, milk sours because bacteria from the air fall into the milk, begin to grow, and very shortly change the sugar of the milk to an acid. When this acid becomes abundant, the milk begins to curdle. As you know, the bacteria are in air, in water, and in barn dust; they stick on bits of hay and stick to the cow. They are most plentiful, however, in milk that has soured; hence, if we pour a little sour milk into a pail of fresh milk, the fresh milk will sour very quickly, because we have, so to speak, "seeded" or "planted" the fresh milk with the souring germs. No one, of course, ever does this purposely in the dairy, yet people sometimes do what amounts to the same thing—that is, put fresh milk into poorly cleaned pails or pans, the cracks and corners of which are cozy homes for millions of germs left from the last sour milk contained in the vessel. It follows, then, that all utensils used in the dairy should be thoroughly scalded so as to kill all germs present, and particular care should be taken to clean the cracks and crevices, for in them the germs lurk.