ALCOHOLIC FERMENTATION 101 



A fourth point helpful in diagnosis is the temperature which 

 proves to be the thermal death-point. SaccJiaromyces cerevisice is 

 killed by an exposure to 54 C. for five minutes, and 62 C. kills the 

 spores. As a rule, yeasts can resist a considerably higher tempera- 

 ture when in a dry state than in the presence of moisture. 



It should be noted that yeasts may be cultivated on solid 

 media. Hansen 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. 

 Saccliaromyces ellipsoideus I. exhibits a characteristic network which 

 readily distinguishes it. 



There is one other feature 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 

 temperature, 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 con- 

 tinent of Europe " low " yeast is mostly used, whilst the ' high " is in 

 vogue in England. This latter method is more conducive to the 

 development of extraneous organisms, and therefore risky in all but 

 well-ordered brewing establishments. 



Before proceeding to mention shortly some of the commoner 

 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 differentiation, because it is found that the same 

 species, when exposed to different external conditions, can occur in 

 very different forms. Hence Hansen established the analytical 

 method of observing (1) the microscopic appearance, (2) the forma- 

 tion of ascospores, and (3) the production of films. In addition, the 

 temperature limits, cultivation on solid media, and behaviour towards 

 carbohydrates, are characters which aid in the separation of yeasts. 

 In well-grown cultures on wort-gelatine, a broad division can be 

 made of yeasts according as they produce (a) a dry, hard, cohesive 

 growth ; (b) a soft, moist growth with liquefaction of gelatine ; and 

 (c) those producing pigment. By basing differentiation of species 

 upon these features, the following can be distinguished : 



Saccliaromyces Cerevisice. Oval or ellipsoidal cells; reproduction by budding; 

 ascospores, rapidly at 30 C., slowly at 12 C., not formed at all at lower tempera- 

 tures ; film formation, seven to ten days at 22 C. ; an active alcoholic ferment, 

 producing in a fortnight in beer wort from 4 to 6 per cent, by volume of alcohol 

 (Jorgensen). This species is a typical English high yeast, possessing the power of 

 " inverting " cane sugar previous to producing alcohol and carbonic acid. It is said 

 to have no action on milk-sugar. It is the " true brewing yeast " (Plate 8). 



