BACTERIA IN RELATION TO MILK 347 



4. To show that heating milk reduces the number of active bacteria. 



Sprinkle drops of milk and water on agar-agar pctri dish, 1 oatu 

 milk, (2) pasteurized, (8) boiled. (I'sr mv tenth milk and nine tenths 

 sterilized water.) 



5. To show that bacteria change the medium in which they grow. 

 Besides the number, form, size, and color of the colonies, note whether 

 any change takes place in the agar-agar. 



6. To show that bacteria grow best in the presence of warmth and 

 moisture, compare those grown under such conditions with th »wn 

 in a dry or a cold place. Note the influence | a | of warmth. (6) of cold, 

 on the rapidity of growth. 



7. To show that bacteria are in the air, expose the surface of the cul- 

 ture plate for a few seconds. 



8. To show that flies distribute bacteria. Let a My walk across the 

 surface of the agar-agar in the petri dish. 



If bacteria have an opportunity, they work od every- 

 thing which is capable of decay, and so we need to know 

 how to prevent their working upon food and other things 

 which we do not wish to ''spoil." Several ways in com- 

 mon use are : (1) cold storage, where there is not warmth 

 sufficient for the growth of bacteria: (2) the use of salt 

 and other chemicals to prevent their getting a start, as in 

 the curing and smoking of meat; (o) drying fruit and 

 meat, thus removing water, a necessary condition for 

 growth; and (4) heating fruit, vegetables, milk, etc., and 

 sealing them in cans or jars while hot, thus killing any bac- 

 teria the substances may contain and keeping all othi 

 out. Anything prepared in this way is preserved by 

 being made sterile or aseptic (Greek, sepein^ to make 

 putrid). 



246. Bacteria in Relation to Milk. — (Sec also Part II.) 

 Milk as it comes from the healthy cow is practically I: 

 from bacteria of any kind. The number of bacteria present. 

 however, is not of so much importance as the kind. But 

 if a large number of bacteria are allowed to get into the 

 milk, some of them are sure to be harmful and may find 



