1896 THE MICROSCOPE. 25 



There are three periods in the study of fermentation. 

 The first was based upon tlie question as to the coming 

 into existence of micro-organisms by spontaneous gener- 

 ation. From this followed the method of sterilization. 

 The next was the Pasteur period, in which Pasteur proved 

 that bacteria exert a great influence in fermentation, 

 usually a bad one. The third period may be called the 

 Hansen period, which is certainly as epoch-making as 

 that of Pasteur, for Hansen has proved that the diseases 

 of beer are caused mainly, not by bacteria, but by yeast 

 itself. He it was who devised a method for cultivating 

 pure yeast, obtaining his cultures from a single cell. 

 This makes it possible to separate from one another the 

 different species of yeast. This has been of immense 

 value in the brewing industries, for varieties have been 

 studied, and those possessing certain desirable properties 

 selected for use; besides, a brewing started, absolute cer- 

 tainty could be felt as to the result, and that result dupli- 

 cated at any time. Yet, in bread-making, similar problems 

 present themselves to those of brewing: — To determine 

 which form of yeast will give desirable results as regards 

 taste, lightness, duration of fermentation, and keeping 

 properties. At the present time, one gets yeasts in the 

 market under various names, without knowing just ex- 

 actly what properties will be given by the yeast selected, 

 and whether the results can be duplicated. Then one 

 gets sour bread occasionally without any apparent reason 

 following from the making. 



Of course, the souring is due to the fact that the yeast 

 was not pure, it containing in many cases different kinds 

 of yeast as well as moulds and bacteria. The form most 

 probably causing the souring is B. aceti, a bacterium that 

 attacks acid solutions, containing a small amount of alco- 

 hol. The dough furnishes a good nidus for its growth 

 after the alcoholic fermentation has taken place. There 



