212 BACTERIA, YEASTS, AND MOLDS 



ture can easily be maintained. The lower the temperature 

 the better the results ; and, consequently, the more ice used 

 the better. The refrigerator has become a practical neces- 

 sity for families who try to keep milk for even a few hours 

 in hot weather. If not cooled, milk will sour very rapidly. 

 In a moderately warm room it will keep for a few hours 

 only, and in summer it will sometimes sour almost as soon 

 as delivered to the customer. The housewife should there- 

 fore place the milk in as cold a place as she can find, im- 

 mediately after receiving it from the milkman. If she 

 does not do this, she must not blame the milkman if the 

 milk does not keep. 



One caution must be given in regard to milk preserved 

 at low temperatures. If milk is put in an ice chest with a 

 temperature in the vicinity of 40°, it may keep for many 

 days or even weeks without souring. It is usually assumed 

 that milk is perfectly good and wholesome so long as it is 

 not sour. This is based upon the assumption that the only 

 important change to be feared is souring; so that if it is 

 not sour, it is almost universally regarded as wholesome. 

 Now, although the lactic-acid bacteria do not grow at low 

 temperatures, certain other species do grow readily enough. 

 Milk kept in an ice chest for many days, even though per- 

 fectly sweet and showing no trace of souring or curdling, 

 usually contains great numbers of bacteria. The bacteria 

 that grow under these circumstances may cause the milk 

 to become unwholesome, while the lactic-acid bacteria 

 are not injurious, although they render the milk unpleas- 

 ant. If any unusual smell or taste should appear in milk 

 which has been kept for a day or two in an ice chest, it is 

 undoubtedly wisest to discard the milk. 



