THE MICROBIOLOGY OF ALCOHOL PRODUCTS 527 



portions, one of which may be at the beginning and one after boiling. 

 Hops contain an aromatic essential oil, resins and tannin. The essential 

 oil is quickly soluble and volatile. To preserve its aroma in the beer, 

 the hops must not be boiled too long. The resins are antiseptic and 

 help to preserve the beer. They dissolve with more difficulty and 

 require longer boiling. 



Fermentation: Work of Yeast. — Aftfer boiling, the wort is sepa- 

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

 refrigerators 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 18° 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 tumultu- 

 ous 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 maltose and much of the dextrins. The. result is high alcohol and 

 low extract. The Frohberg type is intermediate. These differences 

 are probably due to differences in the amount and perhaps in the 

 kinds of enzymes. 



The yeasts of spontaneously fermenting beers are of various species, 

 S. elUpsoideus, S. pasfeurianus 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 f ermeii- 



