THE CHEMICAL COMPOSITION OF FOODS 13 



on being oxidised {i.e., combining with the oxygen taken 

 in by the lungs and circulated all over the system by the 

 blood), is broken down into carbon dioxide and water, 

 and in the process gives up to the body a considerable 

 amount of heat. This is available for keeping the body 

 warm, and also for any internal or external work the 

 animal has to do. If given in excess of the requirements 

 for work, the carbo-hydrates are retained in the body 

 and deposited in the form of fat, this store of energy 

 being available in case of a subsequent shortage of food. 



There are many members of the soluble carbo-hydrate 

 class in vegetable foods, and these differ considerably in 

 their feeding value. Much the most important is starch, 

 occurring largely in the cereal grains, and in roots and 

 tubers like potatoes. Starch is present, not in solu- 

 tion in the sap, but in the form of small grains in the 

 vegetable cells, and the starch grains of different plants 

 show definite characters as to size and shape, so that by 

 this means adulteration of a flour can be detected. 



Starch itself is insoluble in cold water, but when 

 boiled the starch grains burst, and a viscid, mucilaginous 

 mass is formed, not really a solution. Thus, starch as 

 such, being a member of the class of substances known 

 as colloids, is incapable of being assimilated, and it is 

 only when the digestive processes have acted on it and 

 converted it into a soluble substance that it can be 

 absorbed and used in the body. By the action of 

 a dilute acid, or such a ferment as the j^ty^^lin of the 

 saliva, starch is first made soluble and then converted 

 into the gum-like substance dextrin ; whilst, if the process 

 is carried further, a soluble sugar — maltose — is the result. 

 Thus there is an intimate relation between starch and 

 sugar, and it is in the form of sugar that the store of 



