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How Milk Sours


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, a
d 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


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. Place them side by

side. Which keeps longer? Why?