BEER-YEAST 747 



latter in form and physiological action. They do not, however, 

 form endospores. 



Several cultivated varieties of Saccharomyces cerevisice are re- 

 cognised ; all of them when grown in solutions of certain sugars 

 are capable of breaking down these compounds into alcohol and 

 carbon dioxide gas, together with small amounts of glycerin, 

 succinic acid and other substances. Their fermentative power 

 is due to an enzyme, termed zymase, which has been extracted 

 from the yeast-cells by subjecting them to very great pressure. 



In the manufacture of beer two different varieties or races of 

 S. cerevisicR are utilised, namely, (i) top-fermentation yeast and 

 (2) bottomrfermentation yeast. The former, which is practically 

 the only race employed in the breweries and distilleries of this 

 country, carries on a rapid and vigorous fermentation at a 

 temperature of 12-24° C; the 'sprout-cells' remain united for 

 some time in branched chains and are lifted up to the surface of 

 the liquid in which they are growing by the carbon dioxide 

 liberated. 



The latter or bottom-fermentation yeast is employed chiefly on 

 the Continent in the manufacture of lager beer. Fermentation 

 with this race is very slow, and goes on at a low temperature, 

 namely, at 5-10° C; the 'sprout-cells' usually separate from 

 each other immediately after their production, and accumulate on 

 the bottom of the vessel in which the fermentation is going on. 



About forty more or less distinct species of Saccharomyces exist. 



S. ellipsoideus Reess, is a wild species of yeast common on 

 the exterior of various fruits and brings about the production of 

 wine from grape juice. It also sets up fermentation in watery 

 juices of cherries, plums, and other fruits. 



Many of the undesirable fermentations set up in beer, wine 

 and other alcoholic beverages which result in the production of 

 compounds having an unpleasant taste or odour, are brought 

 about by wild species of Saccharomyces. 



Ex. 314, — Obtain a small piece of German yeast from a baker. Break it in 



