THE PRESERVATION OF FOOD BY COLD . 543 



CHANGES DURING CHILLING. The period of cooling is a relatively 

 short one, varying from a few hours to a few days in length. The chief 

 physical change is the intentional removal of heat by conduction and 

 convection, but there is usually also some loss of water by evaporation. 

 If cooled to a sufficient degree the water content of the food may 

 crystallize, altering to a considerable extent the physical structure of 

 the food substance (frozen food). Most foods are either actually 

 living when chilling begins, or they are only recently dead and various 

 chemical changes due to intrinsic enzymes continue at a diminishing 

 rate as the heat is removed. Decomposition changes, due to microbes, 

 may also be in progress and continue during the process of chilling. 

 At this time the microbes living in the cold-storage chamber gain access 

 to the newly arrived food and others are added in the process of handling. 

 The extent to which these will grow and multiply depends upon their 

 ability to flourish under the storage conditions. In general the bacteria 

 which flourish at ordinary temperatures, producing the familiar decom- 

 position of the particular food, are greatly retarded in their activities 

 and other kinds of microbes outstrip them under the new conditions. 

 The changes taking place during chilling are of great importance 

 in some special instances, and often a very definite procedure must be 

 followed to obtain a satisfactory result. 



CHANGES DURING STORAGE. This is often a very long period so 

 that causes acting very slowly may ultimately produce marked altera- 

 tions. There is ordinarily some loss of water by evaporation, as well 

 as the evaporation or diffusion of other volatile constituents, some of 

 them at times important factors in the flavor of the food. Other vola- 

 tile substances may be absorbed from the air of the storage room 

 introducing undesirable odor or flavor. The chemical changes of the 

 chilling period continue at a greatly diminished rate, or may be entirely 

 inhibited if the food is frozen. The behavior of the microbic content of 

 the food is the most important factor to be considered during this 

 period. Besides those already present, various other microorganisms, 

 bacteria, yeasts or molds, may gain access to the food from time to time, 

 either from the circulating air or by contact with other things. The 

 fate of the implanted microbes will depend upon their nature and adap- 

 tation to the conditions existing in the stored food. Many of them 

 perish, but many also survive the entire period of storage, and some may 

 actively multiply. There can no longer be any doubt that some bac- 



