THE PRESERVATION OF FOOD BY CHEMICALS. 403 



molds are likely to be introduced into the food by the various manipula- 

 tions and some of these may find conditions favorable for their prolifera- 

 tion. 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 micro- 

 organisms 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 sterilized and 

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

 and in physical appearance 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 con- 

 tinue 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 preserva- 

 tive may be largely removed mechanically, or extracted with water. 

 During cooking peculiar chemical reactions may occur, and cooking is 

 also important in the destruction of microorganisms remaining 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 microorganisms. 

 Sodium chloride (NaCl) probably owes its preservative value solely to 

 this physical effect. In practice its action is often supplemented by the 

 addition of a small amount of saltpeter (KNO 3 ), and sometimes also 

 cane sugar (C 12 H 22 O n ). The fluids of the flesh are in part removed 

 by this treatment, carrying away a part of the soluble constituents. The 

 fluids which remain contain the added preservative substance in solution, 



