314 
SMALLEST PLANTS ( BACTERIA ) 
To deliver pure milk to the consumer costs the producer 
time, care, and money, and consumers should be willing to 
pay more for milk which has had proper care. 
Ice prevents harmful bacteria from multiplying sufficiently 
to make milk dangerous, unless 
the milk is kept too long a 
time. Preservatives, soda, borax, 
boric acid, formaldehyde, and the 
like, are sometimes used to pre¬ 
vent the growth of bacteria. In 
some cases no immediate harm 
seems to come to the persons us¬ 
ing milk thus preserved, but some # 
of these substances are poisonous, 
and pure milk, properly cared for, 
does not need them. So the use of 
any milk in which preservatives are found should be avoided. 
A harmless bacterium gets into milk kept too long and 
forms lactic acid, thus giving the milk a sour taste and 
causing it to curdle. Sour milk 
is perfectly wholesome for food, 
but the taste is disagreeable. In 
1857 Pasteur discovered this bac¬ 
terium. He also found that milk 
could be kept for several days 
without becoming sour, after it 
had been heated sufficiently to 
kill this bacterium. 
This process, called after its dis¬ 
coverer 'pasteurization, consists in 
heating milk for twenty minutes 
at a temperature of 60° C., or to a higher degree for a shorter 
time, and then cooling it rapidly. This procedure kills nearly 
all the bacteria in the milk and does not change the taste or 
make it hard to digest. Milk is not rendered absolutely 
Figure 287. — Beef Jelly. 
Exposed in unsanitary dairy. 
