YEASTS 35 



52. Fermentation. — What makes the yeast of so great 

 practical importance to us is its power of carrying on alcoholic 

 fermentation. This consists in breaking down a sugar into 

 carbon dioxid and alcohol. The carbon dioxid is the gas, 

 bubbles of which are so plentiful in a vigorous yeast culture. 

 This fermentation has nothing to do with the use of sugar by 

 the yeast as food. Neither the carbon dioxid nor the alcohol 

 produced by fermentation is of any use to the yeast ; in fact, 

 alcohol in any considerable quantity is a poison to it, and so 

 when the alcohol accumulates in the solution to a strength of 

 about 12 per cent the activity of the yeast is checked ; 14 per 

 cent of alcohol entirely stops its action, and more than 14 per 

 cent kills the yeast. The sugar that is used by the yeast as 

 food is taken directly into the cell, and is not fermented. 



The usefulness of fermentation seems to be that it sets 

 free large amounts of energy' which the yeast can use in 

 somewhat the same way that most plants and animals use 

 the energy set free in the process of respiration. The ability 

 to obtain energy through fermentation probably explains why 

 the yeast can get along for a time without oxygen from the 

 air — that is, fermentation in part replaces respiration in the 

 life of the yeast. This notion is supported by the fact that 

 fermentation is most rapid when the supply of oxygen is small 

 and when, therefore, respiration is slow. Yeast is used in 

 bread-making because by fermentation it produces carbon 

 dioxid ; this gas makes its way throughout the dough, 

 forming rninute cavities and, as we say, making the bread 

 light. The alcohol which is also produced is driven off by 

 the heat used in baking the bread, and the size of the cavi- 

 ties in the dough is increased because the gas expands when it 

 is heated. In the manufacture of fermented beverages, such 

 as beers and wines, the alcohol is the important product of 

 fermentation and the carbon dioxid is of less consequence. 



53. Enzyms. — The yeasts, like bacteria and all other 

 organisms, produce enzyms which bring about chemical 



