Microscopic Appearance of Pure and Impure Milk

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.

In addition to this thorough cleansing with hot water, we should be careful never to stir up the dust of the barn just before milking. Such dusty work as pitching hay or stover or arranging bedding should be done either after or long before milking-time, for more germs fall into the milk if the air be full of dust.

To further avoid germs the milker should wear clean overalls, should have clean hands, and, above all, should never wet his hands with milk. This last habit, in addition to being filthy, lessens the keeping power of the milk. The milker should also moisten the parts of the cow which are nearest him, so that dust from the cow’s sides may not fall into the milker’s pail. For greater cleanliness and safety many milkmen curry their cows.

The first few streams from each teat should be thrown away, because the teat at its mouth is filled with milk which, having been exposed to the air, is full of germs, and will do much toward souring the other milk in the pail. Barely a gill will be lost by throwing the first drawings away, and this of the poorest milk too. The increase in the keeping quality of the milk will much more than repay the small loss. If these precautions are taken, the milk will keep several hours or even several days longer than milk carelessly handled. By taking these steps to prevent germs from falling into the milk, a can of milk was once kept sweet for thirty-one days.

The work of the germ in the dairy is not, however, confined to souring the milk. Certain kinds of germs give to the different sorts of cheeses their marked flavors and to butter its flavor. If the right germ is present, cheese or butter gets a proper flavor. Sometimes undesirable germs gain entrance and give flavors that we do not like. Such germs produce cheese or butter diseases. “Bitter butter” is one of these diseases. To keep out all unpleasant meddlers, thoroughly cleanse and scald every utensil.


What causes milk to sour? Why do unclean utensils affect the milk? How should milk be cared for to prevent its souring? Prepare two samples, one carefully, the other carelessly. Country Living for sure. Place them side by side. Which keeps longer? Why Wyoming? Why not?

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