FERMENTATION BY BACTERIA. 595 



the most favourable temperature, but this optimum 

 varies under the influence of various factors. Further, 

 the influence of the aeration of the fermenting mixture, 

 its impregnation with oxygen, is of importance. Accord- Oxygen. 

 ing to Niigeli, the access of oxygen favours fermenta- 

 tion in all cases ; chiefly when, at the same time, 

 materials which offer good nutriment for the yeast are 

 present in the fermenting mixture, because in that case 

 a much more active multiplication of the cells can occur. 

 In investigations recently made by Hansen,* it was 

 found that the same quantitative results require a longer 

 time when air is absent, and that, at the same time, 

 fewer yeast cells are formed and take part in the fer- 

 mentation ; on the other hand, by continued aeration 

 the same result is reached in a shorter time, and there 

 is a much more extensive multiplication of the cells. 

 Therefore, under the influence of the presence of oxygen 

 the formation of the cells and the occurrence of fermen- 

 tation takes place more quickly, but the action of the 

 individual cells is not, on an average, so great as when 

 air is absent. Of chemical agents which disturb or 

 hinder fermentation may be mentioned free alkalies, also 

 sulphurous acid, sublimate, chloroform ; while sul- 

 phuretted hydrogen, arsenious acid, carbolic acid, sali- 

 cylic acid, strychnine, hydrocyanic acid, only hinder the 

 fermentation of yeast when markedly concentrated, or do 

 not affect it at all. 



As regards the fermentation of bread, kephyr, koumiss, 

 see page 607. 



B. Fermentation by Bacteria. 

 (a). Fermentation of Carbo-Jiydrates. 



The carbo-hydrates furnish, under the influence of Fermentation 

 various bacteria, a lactic fermentation, a mucous dr 

 mannite fermentation, a butyric fermentation, a dextran 

 fermentation, and, finally, some fermentations in which 

 nethylic alcohol and various other products are formed. 



* Hanson, Meddekherfra Carlsbcrg Lab. : vol. i., Part 2. 



