522 MICROBIOLOGY OF FOODS 



and raisins, the concentration of sugar and other solutes is the control- 

 ling factor. Frequently as much as 30 per cent of the dried fruit is 

 water. Fruit drying is often accomplished by heating in the sun's 

 rays, in other cases artificial heat and even hydraulic pressure are 

 used. 



Many manufactured products, particularly bakers' goods such as 

 crackers, biscuits, dried yeast cakes, etc., are preserved by the elimina- 

 tion of water. 



Macaroni and vermicelli are prepared by forcing a thick paste of especially pre- 

 pared flour and water through openings of different sizes. The product is then 

 dried in the air until it is brittle and may then be kept indefinitely. 



Copra, one of the principal exports of certain of the islands of the Pacific and 

 Indian oceans, is prepared by cutting the meat of the cocoanut into pieces and dry- 

 ing in the sun. From this copra much of our desiccated and powdered cocoanut is 

 prepared, and from it is pressed the cocoanut oil which finds so many uses in manu- 

 facture. 



Syrups, molasses, jellies, jams, and many other carbohydrate foods 

 are preserved through the concentration of solutes. Many of these are 

 partially sterilized by the heat used in the process of manufacturing. 

 There is usually plenty of opportunity for subsequent infection. They 

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

 exception may be noted in Leuconostoc mesenterioides, a bacterium which 

 causes considerable trouble by a gelatinous fermentation in syrups from 

 which sugars are being manufactured. 



Fatty foods frequently contain little water. Cottonseed, olive, 

 cocoanut, 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 and considerable traces of nitrogenous materials 

 remain in them. With these foods the water is necessary for the 

 growth of the organism and also for the action of the lipoly tic enzymes 

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

 Butter is an exception to the rule that fatty foods contain little water, as 

 it usually has from 12 per cent to 16 per cent. When it is necessary 

 to keep butter fat for long periods under unfavorable conditions, 

 the water and nitrogenous material are removed and the clear fat 

 preserved. This is the so-called ghee of India. Bacteria, enzymes, and 

 a few molds have been described that attack fat. In the process of 

 preparation or manufacture of any fat foods sufficient heat may be used 

 to .sterilize the material, and infection thereafter penetrates to the 

 interior very slowly. This heat destroys the enzymes as well as the 

 bacteria. 



