BACTERIA AND FERMENTATION- 1 25 



employed wort-gelatine (5 per cent, gelatine), and found 

 that at 25° C. in a fortnight the growths which develop 

 show such microscopic differences as to aid materially in 

 diagnosis. Saccharoinyces ellipsoideus I. exhibits a charac- 

 teristic network which readily distinguishes it. 



There is one other point to which reference must be 

 made. The process of fermentation may be set up by a 

 '* high*' or a " low" yeast. These terms apply to the 

 temperature at which the process commences. " High " 

 yeasts rise to the surface as the action proceeds, accomplish 

 their work rapidly, and at a comparatively high temper- 

 ature, say about 16° C. ; *' low" yeasts, on the contrary, 

 sink in the fermenting fluid, act slowly, and only at the low 

 temperature of 4° or 5" C. This is maintainable by floating 

 ice in the fluid. Formerly all beer was made by the 

 ** high " mode, but on the continent of Europe *' low " 

 yeast is mostly used, while the *' high " is in vogue in Eng- 

 land. This latter method is more conducive to the develop- 

 ment of extraneous organisms, and therefore risky in all but 

 well-ordered brewing establishments. Whether high ^nd 

 low yeasts consist of one or several species is not known. 



Before proceeding to mention shortly some of the com- 

 moner forms of yeast we must again emphasise Hansen's 

 method of analysis in separating a species. The shape, 

 size, and appearance of cells are not sufficient for different- 

 iation, because it is found that the same species when ex- 

 posed to different external conditions can occur in very 

 different forms. Hence Hansen established the analytical 

 method of observing (i) the microscopic appearance, (2) the 

 formation of ascospores, and (3) the formation of films. In 

 addition, the temperature limits, cultivation on solid media, 

 and behaviour towards carbohydrates, are characters which 

 aid in the separation of yeasts. By basing differentiation 

 of species upon these features, the following can be dis- 

 tinguished : 



