FERMENTATION 241 



first be inverted to grape sugar by invertin, 1 and 

 malt sugar by maltase. Lactase is present in some 

 yeasts, enabling them to ferment milk sugar. Diastase 

 is also found. 



The action of these several ferments becomes clear 

 when the chemical nature of the carbohydrates is 

 recalled. 



Chemical Relations of Carbohydrates. Carbohy- 

 drates were formerly defined to be compounds con- 

 taining six, or a multiple of six carbon atoms, together 

 with hydrogen and oxygen atoms in the proportion in 

 which they exist in water. The researches of E. 

 Fischer have shown that all aldehydes (bodies which 

 are the first oxidation products of primary alcohols, and 

 which contain the carbonyl group CO) and all ketones 

 (bodies which are the first oxidation products of second- 

 ary alcohols and which likewise contain the carbonyl 

 group CO) contain carbon, hydrogen, and oxygen, there 

 being two atoms of hydrogen to one atom of oxygen, 

 as in water. 



The carbohydrates, therefore, no longer occupy an 

 isolated position, but are to be classed with the fats, 

 being methane derivatives in which the carbon atoms 

 are arranged in an open chain; thus, grape sugar is an 

 aldehyde alcohol, and fruit sugar a ketone alcohol. 



The carbohydrates are divided, according to the size 

 of their molecule, into monosaccharides, disaccharides, 

 and polysaccharides. The monosaccharides (e. g. grape 



1 For extraction, see Lea : Journal of physiology, 1885, vi, 



p. 142. 



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