438 MICROBIOLOGY OF SPECIAL INDUSTRIES. 



FERMENTAI ON: WORK OF YEAST. After boiling, the wort is separated 

 from the hop debris by straining. It is then cooled by means of refrig- 

 erators consisting usually of serpentine tubes through which cold brine 

 or water runs. The hot wort runs or drips over the outside of these tubes 

 in contact with the air. The final temperature of the wort is from 12 

 to 1 8 in top fermentation and 4 to 6 in bottom. 



By this means the wort is thoroughly aerated, which is necessary for 

 the proper work of the yeast. It also effects a partial clarification by 

 oxidation which causes a precipitation of solid matters. 



The fermentation takes place in two stages, the violent or tumultuous 

 fermentation in vats and the secondary or after fermentation in casks. 



During the violent fermentation the temperature is allowed to reach 

 a maximum of 7 to 9 with light beers, 8. 5 to 10.5 with dark and 12 

 to 20 in top fermentations. At the end of the first fermentation the beer 

 is cooled gradually to 3.5 or 5.0 and drawn into fermenting casks 

 where the after-fermentation takes place. 



The yeasts used in brewing vary very much. Besides the division 

 into top and bottom yeasts, various types of each are recognized. One 

 of the chief characteristics used for this division is expressed by the per- 

 centage of the total extract fermented by the yeast. The Saaz type leaves 

 all the dextrins and some of the maltose untouched and produces beers 

 light in alcohol and high in extract. The Logos type destroys all the mal- 

 tose and much of the dextrins. The result is high alcohol and low ex- 

 tract. The Frohberg type is intermediate. These differences are prob- 

 ably due to the differences in the amount and perhaps in the kinds of 

 enzymes. 



The yeasts of spontaneously fermenting beers are of various species, 

 S. ellipsoideus, S'. pasteuriamis and others. 



To produce fermentation, yeast is taken from previous vats so long 

 as the yeast remains sufficiently uncontaminated with foreign organisms. 

 The condition of the yeast is determined by the character of the fermen- 

 tation, the degree of attenuation, and by microscopic examination. In 

 breweries where modern pure culture methods are not used, the yeast 

 present is always of several forms or types. 



In any case after a certain number of transfers, the yeast deteriorates 

 and finally may become thoroughly infected with bacteria. The bacteria 

 are revealed by microscopic examination. Where pure cultures are used, 

 contamination with foreign yeasts is shown by a change in the time of 



