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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, 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

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?

Next: The Babcock Milk-tester

Previous: Milk Cream Churning And Butter

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