212 OUTLINES OF BACTERIOLOGY 



2. Mashing. The bruised or crushed malt is thrown into the mash 

 tun, and water is added at a temperature of from 158 F. to 172 F. 

 After maceration for three or four hours, assisted during the first half- 

 hour by constant stirring, the liquid portion is strained off through 

 finely perforated plates in the bottom of the mash-tun and pumped into 

 the copper. The mashing has converted all the starch 'in the malt into 

 sugar, through the agency of the ferment diastase. The temperature 

 must not be below 140F., otherwise the diastase will not work, and 

 must not be above 185 F., for this temperature will destroy the 

 diastase altogether. A medium temperature of about 165F. is found 

 to be the most suitable. The sugary liquid that is now obtained is 

 called wort. 



3. Boiling. When the wort is all collected into the copper, hops are 

 added and the whole is boiled for about three hours. The boiling 

 coagulates and precipitates the excess of albumen that is present, and 

 also extracts the aromatic oil and the more pronounced of the bitter 

 substances of the hops. This process serves several purposes. The 

 boiling kills all the minute-organisms that are present, and by removing 

 the albumen prevents putrefactive fermentations setting in later, whilst 

 the hops not only give a characteristic flavour to the beer, but by 

 imparting its bitterness prevent too rapid fermentation when yeast is 

 added. 



4. Cooling. After the boiling is finished, the wort is cooled as 

 quickly as possible by exposing it to a current of air in large shallow 

 vessels, or running it over refrigerating pipes. Wort is capable of 

 serving as a nutrient medium to various kinds of bacteria and moulds 

 which are always in the immediate vicinity ; most of these, if they get 

 a chance to multiply in the wort, would produce acid decomposition- 

 products, and thus would render it unfit for the next stage, because 

 yeast cannot multiply in an acid medium. At a low temperature, how- 

 ever, these organisms, even if they find their way to the wort, cannot 

 multiply. 



5. Fermenting with Yeast. The wort is next run into fermenting 

 vats at a low temperature and yeast added. We have already explained 

 how yeast, by multiplying in the wort, changes the sugar into alcohol. 

 The fermentation is allowed to proceed for about 48 hours, when the 

 yeast is skimmed off if it collects at the top, or is run out through 

 holes in the bottom of the tun if it forms a sediment at the bottom. 

 Yeasts can be roughly divided into two groups the Top- and the 

 Bottom-Fermentation Yeasts, so called because, when cultivated in nutrient 

 media containing a fermentable sugar, the former always form a froth 



