480 ' MICROBIOLOGY OF SPECIAL INDUSTRIES 



prominent at various times, playing its part in^the chemical changes. 

 Bacteria, yeasts and molds are likely to be introduced into the food by 

 the various manipulations and some of these may find conditions favor- 

 able for their proliferation. In some instances the activity of certain 

 kinds of microbes appears to be essential to the proper curing and 

 subsequent adequate preservation of the food; the preservative, the 

 constituents of the food and the microorganisms mutually reacting to 

 bring about the desired result. It is worth noting that the added 

 chemical preservative is never sufficiently potent to destroy with 

 certainty pathogenic microbes which may be present in the food. 



The Period of Storage. — Unless the food has been sterUized and 

 stored in sealed containers, slow changes in water content, in physical 

 appearance and in chemical composition usually take place during 

 storage. The added preservative may continue to react with the food 

 substance and its decomposition products. During this period there 

 is relatively little intimate manipulation of the food and therefore little 

 opportunity for the penetration of new microbes. Some of those 

 already present may continue their activities at a diminished rate, 

 producing slow chemical changes often of a desirable nature rather than 

 otherwise. Accessory conditions, such as desiccation, cold storage, 

 or sterilization and sealing, may greatly retard or check altogether 

 microbic activity. 



The After-storage Changes. — The immediate preparation of 

 preserved food for consumption is frequently important. The pre- 

 servative may be largely removed mechanically, or extracted with 

 water. During cooking pecuhar chemical reactions may occur, and 

 cooking is also important in the destruction of microorganisms remain- 

 ing alive in the food up to that time. 



The Chemical Preservation of Certain Foods 



Meats and Fish. — ^The preservation of meat and of fish by salting 

 depends Upon the increase of osmotic tension in the food, a physical 

 change sufficient to prevent or greatly delay the growth of micro- 

 organisms. Sodium chloride (NaCl) probably owes its preservative 

 value solely to this physical effect. In practice its action is often sup- 

 plemented by the addition of a small amount of saltpeter (KNOs), 

 and sometimes also cane sugar (C12H22O11). The fluids of the flesh 



