QUESTIONS AND ANSWERS 359 



amount of air space per animal, and is clean and free from germs 

 of infectious diseases. 



(e) A sanitary milk-house is so constructed that it can be 

 readily disinfected throughout with boiling water or steam. All 

 apparatus used in it should be of metal as far as practicable so as to 

 permit of thorough sterilization. Good drainage, ventilation, and 

 a supply of pure water are essential. It should be well fitted with 

 all apparatus necessary for the proper handling, cooling, and storing 

 of milk. 



How is a quantitative bacterial analysis of milk made? 



A cubic centimetre of milk is diluted with sterile water and 

 mixed with melted nutrient agar. This mixture is poizred in a 

 Petri dish, allowed to cool, and then placed in an incubator for 24 

 hours. Each bacterium grows and produces one colony. By count- 

 ing the colonies and multiplying the result by the dilution, the num- 

 ber of bacteria per cubic centimetre is determined. 



What is the significance of the presence of streptococci in milk? 



This question is subject to great dispute. Most authorities are 

 agreed that any number of streptococci in excess of the ordinary 

 number found in milk calls for an investigation of the dairy supply- 

 ing the milk, looking particularly for mammitis, and contamination 

 with dirt, especially manure. 



Name the microorganism that causes milk to become sour. 

 B. lactis acidi. 



What is the difference between pasteurized, sterilized and certified 

 milk? 



Pasteurized milk has been heated to a temperature below the 

 boiling-point but sufficiently high to destroy nearly all living micro- 

 organisms, or at least to check their growth. (Ninety-five to ninety- 

 eight per cent, of all bacteria are destroyed by pasteurization.) 



Sterilized milk is free from all bacteria and spores. Sterilization 

 is usually accomplished by heating the milk to the boiling-point. By 

 this process the milk is altered in composition and taste, and there- 

 fore it is objectionable as a commercial product. 



Certified milk is produced under such cleanly methods as to 

 contain a very low number of non-pathogenic bacteria: not over 

 10,000 per cubic centimetre. 



