396 MICROBIOLOGY OF SPECIAL INDUSTRIES. 



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 crys- 

 tallize, 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 stuff, 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 decomposition 

 of the particular food, are greatly retarded in their activities and other 

 kinds outstrip them under the new conditions. The changes taking 

 place during chilling are very important in some special instances, and 

 often a very definite procedure must be followed to obtain a satis- 

 factory result. 



CHANGES DURING STORAGE. This is often a relatively long period 

 and causes acting very slowly may ultimately produce marked alteration. 

 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 volatile sub- 

 stances may be absorbed from the air of the storage room. 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 micro- 

 organisms, 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 adaptation 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 bacteria can grow at the temperature of zero, and many kinds 

 multiply at a fraction of a degree above that point. In order definitely 



