336 BIOLOGICAL ACTIVITIES OF YEASTS 



deleterious effect on the growth of yeast. They may also be un- 

 desirable because the alcohol produced slows down the fermentation, 

 a longer time then being required for the complete utilization of the 

 residual sugar. On the other hand, concentrations of sugar lower 

 than 10 per cent lead to loss of valuable fermentation space. The 

 nitrogen deficiency is made up by the addition of ammonia, am- 

 monium phosphate, or ammonium sulphate. 



Since it would be costly to sterilize the wort, the manufacturer 

 depends on the use of a large inoculum of yeast cells, ordinarily from 

 4 to 6 per cent of the final volume of mash, and the low pH to pre- 

 vent contaminants from multiplying appreciably. The large in- 

 oculum overwhelms the relatively small numbers of undesirable or- 

 ganisms and prevents them from developing. Sulphuric or phosphoric 

 acid is generally added to adjust the pH to approximately 4.5. The 

 yeasts are able to propagate and dissimilate the sugar at this fairly 

 low pH but undesirable bacteria are retarded in their development. 

 Lactic acid is sometimes added to or produced in the mash by allow- 

 ing a lactic acid fermentation to occur, prior to inoculation with 

 yeasts. This organic acid inhibits the growth of the butyric acid 

 bacteria which may develop as undesirable contaminants. 



After the mash has been pitched, an aerobic condition is developed 

 by aeration. It has been noted previously in the discussion of the 

 Pasteur effect that aerobic metabolism is economically superior to 

 anaerobic breakdown of sugar. Once the number of yeast cells has 

 been brought up to a satisfactory point, aeration is ceased. Dis- 

 similation of sugar then begins, anaerobic conditions are established, 

 and alcohol and carbon dioxide are formed. 



Although the initial temperature of the mash when pitched is 

 usually between 21° and 27° C, the temperature rises because of the 

 activities of the yeast. Therefore, cooling coils inside or sprays on 

 the outside of the fermenting tanks must be used to control the tem- 

 perature. 



After a fermentation period of approximately 48 hours, the fer- 

 mentation is generally complete, with a yield of about 90 per cent 

 of theoretical on the basis of fermentable sugar. 



A continuous process for preparing the starter ^^ as well as a con- 

 tinuous process for the alcoholic fermentation of molasses ^ have 

 been developed by Kolachov and his associates. 



Alcohol is distilled off the "beer" (fermented mash). Fractions 

 containing various concentrations of alcohol are separated during 

 the distillation. Those fractions high in alcohol are called "high 

 wines," those low in alcohol "low wines." The former are rectified 



