208 



PHYSIOLOGY OF NUTRITION 



Two kinds of fermentation are distinguished in the brewing industry: top- 

 fermentation, which occurs at high temperatures, and bottom-fermentation, 

 which occurs at lower ones, these two kinds of fermentation being produced by 

 two different groups of yeast races. Experiments aiming to change bottom into 

 top yeasts, or the reverse, have never been successful. 



Pasteur called attention to the fact that the properties of beer depend upon 

 the character of the yeast employed in its manufacture. Since bacteria cause a 

 deterioration in beer, Pasteur suggested a method for yeast purification, by 

 means of cultures with tartaric acid or phenol. Hansen proved, however (1883), 

 that the most widespread and injurious "diseases" of beer are not caused by 

 bacteria but are due to wild species of yeasts. c The same writer has also shown 

 that treatment of yeast with tartaric acid fails to have any good effect and is 

 positively harmful when wild species are present; such treatment weakens the 

 cultivated yeast and the wild forms become ascendant in the culture. To ob- 

 tain a perfect product pure cultures of yeast must be employed. Comparative 

 studies have shown that different varieties of beer are produced from the same 

 beer- wort by different forms of yeast. Thus, Saccharomyces pastorianus I 

 produces a bitter taste and an unpleasant odor, while the use of S. pastorianus 

 III or S. ellipsoideus II results in cloudy beers. 



In a mixture of yeasts the wild forms may be identified by the time required 

 for ascospore production at a temperature of i5°C, as is brought out by the 

 following scheme. 



If a drop of a yeast culture a day old is thinly spread on a sterilized plaster 

 plate impregnated with beer-wort, and if the preparation is kept at a tempera- 

 ture of 15 °C, no ascospores are found after seventy- two hours unless wild yeasts 

 were present in the original culture; ascospore formation does not occur till 

 later. If spores are found, on the other hand, then wild yeasts are present, and 

 the amount of these may be estimated by the number of ascospores that have 

 been formed. It is possible in this way to detect the presence of wild yeasts in 

 mixtures where they comprise no more than one two-hundredth of the total 

 amount of yeast present. 



Another method of identifying yeasts is based on the forms of their "giant 

 colonies," 1 which are formed from cell masses/ A drop of a young yeast cul- 



1 Lindner, 1909. [See note 1, p. 45-] 



c See Hansen, 1896. [See note 1, p. 45.] — Ed. 



d This paragraph is omitted in the 7th Russian edition. — Ed. 



