ALCOHOLIC FERMENTATION 97 



and mix it with yeast, we get exactly the same result, except that 

 the first stage of the fermentation would be the changing of the cane- 

 siigar into grape-sugar, which is accomplished by a soluble ferment 

 secreted by the yeast cells themselves. If now we go yet one step 

 further back, to starch, the same sort of action occurs. When starch 

 is boiled with a dilute acid it is changed into a gum-like substance, 

 dextrin, and subsequently into maltose, which latter, when mixed 

 with these living yeast cells, is fermented, and results in the evolution 

 of carbonic acid gas and the production of alcohol. In the manu- 

 facture of fermented drinks from cereal grains containing starch there 

 is, therefore, a double chemical process : first, the change of starch 

 into sugar by means of conversion, a chemical change obtained by 

 the action of sulphuric or some other acid, or by the influence of 

 diastase; and secondly, the change of the sugar into alcohol and 

 carbonic acid gas by the process of fermentation, an organic change 

 brought about by the living yeast cells. 



In all these three forms of alcoholic fermentation the principal 

 features are the same, viz., the sugar disappears ; the carbonic acid 

 gas escapes into the air ; the alcohol remains behind. Though it is 

 true that the sugar disappears, it would be truer still to say that it 

 reappears as alcohol. Sugar and alcohol are built up of precisely the 

 same elements: carbon, hydrogen, and oxygen. They differ from 

 each other in the proportion of these elements. It is olavious, there- 

 fore, that fermentation is really only a change of position, a breaking 

 down of one compound into two simpler compounds. And this 

 redistribution of the molecules of the compound results in the 

 production of some heat. Hence, we must add heat to the results 

 of the work of the yeasts.* 



It will be necessary subsequently to consider a remarkable faculty 

 which bacteria possess of producing products inimical to their own 

 growth. In some degree this is true of the yeasts, for when they 

 have set up fermentation in a saccharine fluid there comes a time 

 when the presence of the resulting alcohol is injurious to further 

 action on their part. It has become indeed a poison, and, as we 

 have already mentioned, a necessary condition for the action of a 

 ferment is the absence of poisonous substances. This limit of 

 fermentation is reached when the fermenting fluid contains 13 or 

 14 per cent, of alcohol. 



The Biology of Yeast. — Having briefly discussed the "medium" 

 and the results, we may now turn to the other side of the matter, 

 and enumerate some of the chief forms of the yeast plant. Jorgensen 



* When alcohol is pure and contains no water it is termed absolute alcohol. If, 

 however, it is mixed with 16 per cent, of water, it is called rectified spirit, and when 

 mixed with more than half ils volume of water (56-8 per cent.) it is known as proof 

 spirit. 



G 



