348 



THE SMALLEST PLANTS (BACTERIA) 



conditions so favorable for their growth as to make trouble 

 for the person using the milk. 



A high grade of milk will not contain more than 500 to 

 1000 bacteria per cubic centimeter. Such milk has been 

 well cared for and comes from healthy cows. Some cities 

 permit milk to be sold that contains as many as 100,000 

 and some even more bacteria per cubic centimeter. Such 

 milk comes from unhealthy cows or dirty barns, or has 

 been kept too long, or has "changed hands " too many times. 



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 for too long 

 a time. Preservatives, soda, 

 borax, boric acid, formaldehyde, 

 and the like are sometimes used to prevent the growth of 

 bacteria. In some cases no immediate harm seems to 

 come to the persons using 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 dis- 

 covered this bacterium. He also found that milk could be 

 kept for several days without becoming sour, after it had 

 been heated sufficiently to kill this bacterium. 



Figure 369. — Clean Milk. 

 Showing oil globules. 



