DIVISION V. 



MICROBIOLOGY OF SPECIAL INDUSTRIES. 



CHAPTER I. 

 DESICCATION, EVAPORATION, AND DRYING OF FOODS. 



FACTORS THAT BRING ABOUT CHANGES IN DRIED FOODS. 

 The factors that bring about changes in dried foods may be considered 

 under two general heads, chemical and microbial. Enzymes, although 

 the product of living cells, may represent the chemical changes, and the 

 activity of bacteria, yeasts and molds, the microbial changes. Enzymes 

 are normally present in food stuffs derived from animals or plants which 

 have not been subjected to heating. All living cells contain enzymes, 

 and these may remain active for a considerable time after the death of the 

 cell. Some of these enzymes attack carbohydrates, some fats, some 

 proteins, and some other compounds. Enzymes are responsible for 

 the stiffening of the muscles after death (rigor mortis), others later 

 break down the tissues and bring about a ripening of meat whereby it 

 becomes more tender. Autolytic enzymes may in some instances 

 produce rancidity in food products by a splitting of the fats. Bacterial 

 enzymes are known that duplicate the action of practically all those pro- 

 duced by higher animals and plants. Some of the changes produced are 

 desirable, others undesirable, particularly if action is allowed to continue 

 for too long a time. In foods dried for preservation, it is therefore im- 

 portant that sufficient heat be used to destroy the enzymes or that enough 

 water be removed to inhibit their activity. Ordinarily the activity of 

 enzymes will be inhibited by the removal of water sufficient to prevent 

 the growth of microorganisms. The action of enzymes is characterized as 

 reversible, that is, after a certain concentration of enzymic products has 

 been reached, the transformation ceases until a part of the accumulation 

 has been removed by diffusion or otherwise. Since many of these actions 



* Prepared by R. E. Buchanan. 



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