386 Effective Farming 



be sufficient to pasteurize it. Bacteria that cause tuberculosis 

 and other harmful bodies are killed by pasteurization. Not all 

 of the lactic-acid bacteria are killed, however, and the milk 

 will sour, but not so quickly as dirty raw milk. Pasteurized 

 milk should be consumed within about twenty-four hours 

 after it has been heated, since a long time before it sours it is 

 likely to become dangerous as human food. This is because 

 in raw milk, certain putrefactive bacteria are kept from grow- 

 ing by the acidity of the milk, but in pasteurized milk, since 

 it does not sour quickly, these bacteria are likely to become 

 active and, if the milk is old, cause it to become impure or 

 even dangerous for use. 



QUESTIONS 



1. Why should the producer of milk know the percentage of fat 

 it contains? 



2. How is milk tested for fat ? 



3. State the advantages of the centrifugal cream separator. 



4. What causes milk to ferment ? 



5. Why should milk be cooled soon after milking? 



6. State some beneficial effects of bacteria in milk ; some harmful 

 ones. 



7. What precautions should be taken to exclude disease-producing 

 bacteria from milk? 



8. Describe briefly how sanitary milk can be produced. 



9. Why should flies be kept out of the dairy? 



10. How is milk pasteurized? Why should pasteurized milk be 

 used within about twenty-four hours after pasteurization? 



EXERCISES 



1. Separating cream from milk. Secure a centrifugal separator 

 and about five gallons of fresh whole milk. Take the separator apart 

 and put it together again, studying its construction. Notice the cream 

 screw. When a rich cream is desired the screw is turned toward the 

 center of the bowl, when a thin cream is wanted it is turned in the oppo- 

 site direction. Read carefully the directions furnished by the manu- 

 facturer about operating the cream screw. Warm the milk until it is 

 about 85 F. and run it through the separator. Milk separates best 



