DESICCATION, EVAPORATION, AND DRYING OF FOODS. 379 



are partially sterilized by the heat used in the process of manufacture, 

 but there is usually plenty of opportunity for subsequent infection. They 

 are more frequently attacked by molds and yeasts than by bacteria. 

 An exception may be noted in Streptococcus mesenterioides which sometimes 

 causes considerable trouble by a gelatinous fermentation in syrups from 

 which sugars are manufactured commercially. 



Foods with considerable quantities of fat usually contain little water. 

 Cottonseed, olive and other vegetable oils, the plant and animal fats, 

 as lard, tallow, and butter, are quite resistant to change by bacteria 

 unless water is present. With these foods the water is necessary for the 

 growth of the organism and also for the activity of the lipolytic enzymes, 

 which might hydrolyze fats and aid in the development of rancidity. 

 Butter forms an exception to the rule that fat foods contain little water, 

 as it usually has from 12 to 16 per cent. Where it is necessary to keep 

 butter-fat for long periods or under unfavorable conditions, it is melted, 

 the water removed, and the clear fat preserved. Bacteria, enzymes, 

 and a few molds have been described that attack fats. In the process 

 of preparation or manufacture of many fat foods, sufficient heat is used 

 to sterilize the material and infection thereafter spreads to the interior 

 very slowly. The heat destroys the enzymes as well as the bacteria. 



The third class of foods preserved by drying includes those that 

 contain a high percentage of protein, in large part flesh foods and flesh 

 derivatives. 



Desiccation, however, is only one of the agencies acting to preserve 

 the flesh. 



Jerked meat is sometimes prepared in localities with a hot dry climate. Lean 

 meat is cut into thin slices and exposed to the direct rays of the sun until dry. The 

 bactericidal actipn of the sunlight and the rapid extraction of moisture prevents micro- 

 organisms from producing undesirable changes during the curing process. 



Dried beef is lean meat which usually has been treated with certain condiments or 

 smoked and salted and then dried. 



Dried fish such as cod, mackerel, and herring, is prepared by the use of condiments, 

 salt, and smoke in addition to the drying. 



Pemmican is prepared by drying lean meat, grinding it, and mixing it with sugar 

 and fat, dried fruits, spices, etc. It is highly nutritious, not unpalatable, and compact, 

 and will keep for a long period. It is frequently used as a concentrated form of food 

 by Arctic explorers, etc. 



Beef extract is prepared by cooking minced beef and water in a receptacle under 

 a slight steam pressure. The digestion is continued for several hours. The liquid is 

 filtered off and concentrated in a partial vacuum to the desired consistency. 



