548 MICROBIOLOGY OF FOODS 



elude prompt cooling of the milk after it is drawn from the animal 

 and the maintenance of a low temperature until it is delivered to the 

 consumer. At the low temperature bacteria slowly multiply, unless 

 the milk is actually frozen, but at a temperature slightly above the 

 freezing-point very clean milk may be kept in perfect condition for a 

 week, and it may be kept sweet for several weeks. Refrigeration of 

 milk cannot compensate for unhealthy animals producing it, nor for 

 careless and uncleanly methods of handling. The cold does not 

 destroy the microbes in the milk but only retards their multiplication 

 and chemical activity. In practice, especially in the transportation of 

 milk into large cities, it is frequently most economical to freeze the milk 

 and trust to insulation and the latent cold in the ice to maintain a low 

 temperature during transportation. Such milk should arrive at its 

 destination in a partly frozen condition. 



The cold storage of butter is essential even when it is kept for only 

 short periods, and the seasonal variation in price is sufficient to warrant 

 its storage from summer to winter. The keeping qualities of butter 

 depend upon many factors,* and the most efficient cold storage cannot 

 compensate for previous deficiencies. In refrigerated butter there is a 

 gradual diminution in the number of living bacteria, with possibly 

 a multiplication of a few particular kinds. There is a slow increase in 

 acidity. In frozen butter the bacterial content and the chemical com- 

 position remain practically unchanged. 



FRUITS AND VEGETABLES. These foods are for the most part 

 adapted to preservation for short periods at ordinary temperatures, 

 and cold storage at a temperature slightly above zero is very effective 

 in diminishing the rate of change in them. The humidity of the storage 

 chamber should be kept constant at about 60 per cent saturation in 

 order to diminish evaporation as far as possible without favoring the 

 development of molds. These foods generally remain alive during 

 storage and the changes due to intrinsic enzymes are often important. 

 Some fruits need to undergo further -ripening in storage before they are 

 ready for consumption and this change may be accelerated or delayed 

 by changing the temperature of the storage chamber. The develop- 

 ment of bacteria and molds with consequent rotting is best delayed by 

 maintaining dry clean fruits and vegetables in an atmosphere of very 

 constant humidity and very constant temperature slightly above the 

 freezing-point. 



* See chapter on the microbiology of butter. 



