96 BACTERIA AND FERMENTATION 



various different subdivisions of the non-flowering portion of the 

 vegetable kingdom. A large part of fermentation is based upon the 

 growth of a class of microscopic plants termed yeasts. These differ 

 from the bacteria in but few particulars, mainly in their method of 

 reproduction by budding (instead of dividing or sporulating, like the 

 bacteria). Their chemical action is closely allied to that of the 

 bacteria. Secondly, there are special fermentations and modifications 

 of yeast fermentation due to bacteria. Thirdly, a group of somewhat 

 more highly specialised vegetable cells, known as moulds, make a 

 perceptible contribution in this direction. According to Hansen, 

 these latter, so far as they are really alcoholic ferments, induce 

 fermentation, that is, inversion of sugar, not only in solutions of 

 dextrose, but also in maltose. Mucor racemosus is the only member 

 that is capable of inverting a cane-sugar solution ; Mucor erectus is 

 the most active fermenter, yielding 8 per cent, by volume of alcohol 

 in ordinary beer wort. Both of these will be referred to as they 

 occur in considering the five important fermentations already 

 mentioned. 



The general microscopic appearance of yeast cells may be shortly 

 stated as follows : They are round or oval cells, and by budding 

 become " daughter " yeast cells. Each consists of a cellulose 

 membrane and clear homogeneous contents. As they perform their 

 function of fermentation, vacuoles, fat-globules, and granules make 

 their appearance in the enclosed plasma. Just as in many vegetable 

 cells a nucleus was detected by Schmitz by means of special methods 

 of staining, so Hansen has found the nucleus in old-yeast cells from 

 " films " without any special staining. 



1. Alcoholic Fermentation 



Cause, yeast ; medium, sugar solutions ; result, alcohol and carbonic acid. 



It was Caignard-Latour who first demonstrated that yeast cells, by 

 their growth and multiplication, set up a chemical change in sugar 

 solutions which resulted in the transference of the oxygen in the 

 sugar compound from the hydrogen to the carbon atoms, that 

 is to say, in the evolution of carbonic acid gas and the production, 

 as a result, of alcohol. Expressed in chemical formula, the change 

 is as follows : — 



CijHjgOg (plus the fermenting agent) = 2 CjH^O -I- 2 COj. 



A natural sugar, like grape-sugar, present in the fruit of the vine, 

 is thus fermented. The alcohol remains in the liquid ; the carbonic 

 acid escapes as bubbles of gas into the surrounding air. If we go 

 a step further back, to cane-sugar (which possesses the same elements 

 as grape-sugar, but in different proportions), dissolve it in water, 



