130 MANUAL FOE SUGAR GROWERS. 



growth, hence their growth can be stopped by ex- 

 cluding air, or hastened by admitting it. The acetic 

 ferment belongs to this latter class, and thus in the 

 manufacture of vinegar it is necessary to have free 

 access of air to the fermenting liquid. The alcoholic 

 ferment belongs to the former class, and thus can 

 live and grow in vessels from which air is excluded. 

 In the case of alcoholic fermentation of cane 

 sugar the changes take place in two stages : 1. 

 The sugar is rapidly converted into glucose or in- 

 verted. This change is effected by a soluble fer- 

 ment, " invertase," secreted by the yeast-plant, the 

 change only taking place if the solution is slightly 

 acid, and is at once arrested if the Solution is alka- 

 line. This point is one well worth remembering 

 by sugar-makers, as by tempering their juice as 

 quickly as possible, and by keeping any juice which 

 cannot be boiled at once slightly alkaline to the 

 phenol-phthalein test, this inversion can be largely 

 prevented. It is this inversion, of course, which 

 prevents " sour " cane-juice yielding sugar when 

 boiled, although it may have a high density when 

 tested by the hydrometer before boiling. 2. The 

 glucose thus produced is acted on by the yeast- 

 plant, and converted into alcohol and carbonic-acid 

 gas according to the following equation : 

 Under the action of invertase, 



C 12 H 22 On + H a O = 2C 6 H ia O 6 , 



cane-sugar and water become glucose. 

 Under the action of the yeast-plant, 



C 6 H 12 O 6 2CO 2 + 2C 2 H 6 O, 



glucose becomes carbonic-acid gas and alcohol. 



