190 BACTERIA, YEASTS, AND MOLDS 



become a practical necessity 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 some- 

 times sour almost as soon as delivered to the customer. 

 The housewife should, therefore, place the milk in as cold 

 a place as she can find immediately after receiving it from 

 the milkman. 



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 whole- 

 some 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. Nothing could be further from 

 the truth, for, although the lactic 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 perfectly sweet and showing no trace of souring 

 or curdling, usually contains great numbers of bacteria. 

 The bacteria that grow under these circumstances are 

 likely to be more injurious to health than the lactic bac- 

 teria. The latter are not injurious, although they render 

 the milk unpleasant ; while the bacteria that grow at low 

 temperatures are, some of them at all events, mischie- 

 vous forms, and the milk may, therefore, be made very 

 unwholesome by them. 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 not fit to drink, for this means 



