340 BIOLOGICAL ACTIVITIES OF YEASTS 



when first mixed is about 5.8 but it falls during the process to 5.5 

 or 5.2. Toward the end of the mashing step, the spent grain and 

 proteins settle out and the clear amber wort is filtered off. The spent 

 grain is extracted or leached a number of times with hot water (usu- 

 ally 75° C.) in a process known as sparging. The sparged grain 

 can be used as stock feed. The wort and spargings are pumped to 

 copper kettles where hops are added and the whole boiled. Tannins, 

 bitter acids, and resins are extracted from the hops in this process. 

 The tannins react with proteins, forming complexes, and aid in clear- 

 ing the wort. A characteristic flavor is imparted to the beer by- 

 substances extracted from the hops. Antiseptic principles are also 

 extracted. The boiling process also destroys enzymes, coagulates 

 proteins, and carmelizes the sugar. The wort is then filtered and 

 cooled. 



The wort is now ready to be pitched. Selected strains of bottom 

 yeasts are commonly used in the manufacture of beer. The terms 

 beer and ale are often used interchangeably. In the United States 

 the industry terms brew made with top yeasts ale and those with 

 bottom yeasts beer. See page 278 for top and bottom yeasts. As 

 soon as the krausen stage is reached, as indicated by the appearance 

 of white foam on the surface of the fermenting mixture, it is often 

 pumped to another fermenter. This affects the removal of more 

 proteins which have settled during the initial phase of fermentation. 

 The temperature is generally kept at about 5° to 14° C. in the man- 

 ufacture of beer, and at 13° to 22° for ale. Cooling coils have to be 

 used in the fermenters because the activities of yeast tend to raise 

 the temperature. After a fermentation period not longer than ten 

 days, a product known as "green" beer is obtained. This is now 

 stored for two weeks to several months. In this maturing process, 

 undesirable substances are eliminated by settling, and mellowness is 

 acquired. 



The finishing process involves the carbonization of the mature 

 beer. This may be accomplished either by carbon dioxide under 

 pressure or by the krausening process. The latter process involves 

 the addition of about 15 per cent of fermenting beer to the stored 

 beer placed in pressure tanks. The gas evolved by the fermenting 

 yeast cells carbonate the entire mixture. This process requires from 

 three to four weeks and the beer is further stored for from three to 

 eight weeks. Bottled beer is generally pasteurized at 63° C. for 

 twenty minutes. For further details on brewing see Tauber ^^ and 

 Prescott and Dunn.^^ 



