BACTERIA 261 



this way will keep fresh for several days, because the 

 bacteria, which turn the milk sour, are kept out. 



Another way to keep milk fresh is to heat it for a 

 time. Put some fresh milk into each of half a dozen 

 bottles, put cotton in the necks, and sterilize for half 

 an hour. Open one of these bottles each week after- 

 ward. How long does it keep sweet? 



Steaming or boiling the milk gives it a taste which 

 many people do not like. If we heat it to 155 F. for 

 twenty minutes, we kill most of the bacteria with- 

 out giving it a bad taste. This is called pasteurizing, 

 and helps to keep the milk sweet for some time, espe- 

 cially if it is well cooled as soon as the heating is 

 finished. 



Most milkmen will not go to the trouble of providing 

 clean milk or even pasteurizing it, but kill the bacteria 

 by putting in formalin or some other preservative. 

 The effect of this on the stomach is very bad, particu- 

 larly in the case of infants, and its use is forbidden by 

 law. 



Formalin in milk. If the milk contains formalin, 

 we can find this out in a very simple way. Pour a 

 little strong commercial sulphuric acid slowly down the 

 side of a tumbler containing milk. (Great care must 

 be used in handling the acid, as it burns the skin or 

 clothing.) The acid is heavy and forms a layer beneath 

 the milk ; if the milk contains formalin, a purple line 

 will form just between the acid and the milk. Before 



