BACTERIA IN THE INDUSTRIES 189 



to that efiEect and the amount does not exceed a definite percentage, as 

 provided by law. 



In England, a limited amount of certain preservatives added to milk is 

 permissible, the argument being that it is better to supply preserved milk 

 than milk loaded with germs. This argument has its commendable 

 features. In very large, congested cities like London, New York and 

 Chicago, it is impossible to supply the poor with certified milk or milk 

 which can be kept free from excessive germ development until it is wanted 

 for consumption. 



Boiling the milk for twenty minutes kills the germs, but unfortunately 

 the boiling temperature produces certain changes which greatly reduce 

 the food value of the milk, besides the germicidal properties of the milk 

 are destroyed, so that the bacterial development is afterward even more 

 active than before. Sterilizing at lower temperature (50° to 80° C.), 

 known as pasteurizing, does not interfere with the nutritive qualities of 

 the mUk, but destro)rs the bactericidal properties, as already mentioned. 

 The process is, however, generally recommended by physicians. A simple 

 home method may be carried out as follows (Roger) : 



Milk is inost conveniently pasteurized in the bottles in which it is 

 delivered. To do this use a small pail with a perforated false bottom. An 

 inverted pie tin with a few holes punched in it will answer this purpose. 

 Punch a hole through the cap of one of the bottles and insert a thermometer . 

 Set the bottles of milk on the pie tin in the pail and fill the pail with water 

 nearly to the level of the mUk. Put the pail on the stove or over a gas 

 flame and heat it untU the thermometer in the mUk shows not less than 

 65° C. nor more than 70° C. The bottles should then be removed from the 

 water and allowed to stand from twenty to thirty minutes. The tempera- 

 ture will fall slowly, but may be held more uniformly by covering the 

 bottles with a towel. The punctured cap should be replaced with a new 

 one, or the opening sealed with wax or parafiSn, or the bottle may be 

 covered with an inverted cup. 



After the milk has been held as directed it should be cooled as quickly 

 and as much as possible by setting in water. To avoid danger of breaking 

 the bottle by a too sudden change of temperature, this water should be 

 warm at first. Replace the warm water slowly with cold water. After 

 cooling, milk should in all cases be kept at the lowest available temperature. 



It should be remembered that pasteurization does not destroy all 

 bacteria in milk, and after pasteurization it should be kept cold and used 

 as soon as possible. 



Rosenau sums up the pros and cons of milk pasteurization as 



follows: 



Advantages. — The advantage of pasteurization is that it is a cheap and 



