CHAPTER VII 

 CARBOHYDRATES 



We come now to the consideration of the third essential 

 organic constituent of animal foods, the carbohydrates. The 

 other two, viz. fats and proteins, have already been discussed. 

 We shall find that the group of compounds known as carbohy- 

 drates contains some of the most important substances that have 

 to do with plant and animal life and also some which have a 

 very great value industrially in other ways than as food. We 

 shall confine ourselves now to the study of these compounds 

 as to their chemical nature, their classification and relationship 

 to each other and their general relation to plants and animals 

 and to manufactured products. Their physiological relation to 

 plants and animals, their special occurrence in plants and their 

 specific industrial uses will be considered later. 



Composition. — The name carbohydrates was originally given 

 because it was supposed that they were carbon and water 

 compounds. It is now known that they bear no direct relation 

 to these two substances other than the fact that they contain 

 carbon plus hydrogen and oxygen with these last two elements 

 in the same proportion in which they are present in water, 

 i.e. H2 : or two atoms of hydrogen (two parts by mass) to 

 one atom of oxygen (sixteen parts by mass). A fact which led 

 to the belief that the carbohydrates contain carbon and water, 

 is that when they are thoroughly dried and freed from all 

 hygroscopic water and also from all water of crystallization and 

 then heated out of contact with oxygen they are decomposed 

 and water is driven off while carbon remains, 



Carbohydrates + heat — > carbon + water 

 This same decomposition occurs when some carbohydrates, 

 e.g. cane sugar, are treated with concentrated sulphuric acid 



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