FOOD PEE8EBVATI0N 179 



of the spiees contain essential oils in sufficient quantities 

 to have a decided preservative action. It is a well-known fact 

 in the household that certain acid fruits and vegetables, 

 such as stalks of rhubarb and the fruit of cranberries and 

 gooseberries, may be preserved for considerable lengths of 

 time in water without any sterilization. 



Another class of foods is preserved as a result of chemicals 

 produced during the process of fermentation. Without 

 exception these are foods relatively high in sugar. Sauer- 

 kraut, for example, is preserved as the result of develop- 

 ment of lactic and some acetic acid by bacterial growth. As 

 long as it is kept from the air, deterioration will not take 

 place after the acid has been developed. Dill pickles are 

 prepared in a somewhat similar fashion. The process of 

 preparation of silage from com constitutes another ex- 

 ample. 



In some cases chemicals are added directly to foods in 

 order to preserve them. The preservative may be some 

 organic food acid such as acetic. Vinegar is used for 

 pickling meats, fruits, and vegetables. Sodium benzoate in 

 small quantities, although not advocated, is nevertheless 

 permitted by our pure food laws, providing the quantities 

 added are plainly indicated. Still other chemicals are used, 

 such as spices and the essential oils. Certain preservatives 

 are in general forbidden by our pure food laws. Such, for 

 example, are salicylic acid and the salicylates and formalde- 

 hyde. 



Preservation of Foods by Drying. — The amount of 

 water which must be removed from any food material in 

 order to prevent the growth of microorganisms will depend 

 upon several factors, most important being the amount of 

 soluble materials present in the food, the kinds of solutes 

 present, the distribution of the water, the kinds of organ- 

 isms or enzymes present, and the method of desiccation. 



The amount of material in solution in the water in a food 



