COMPRESSED YEAST MANUFACTURE 321 



Pasteur also made the observation, and it has been amply con- 

 firmed since, that aerobic conditions generally increase the rate and 

 efficiency of the synthetic processes of the organism. Hence it has 

 been suggested that the fermentation intermediates are used in the 

 resynthesis of carbohydrates. 



However, none of the above-proposed mechanisms nor the above- 

 suggested loci of action is more than tentative. Considerable experi- 

 mental work still remains to be done. For further information on 

 this important subject the reader is referred to excellent reviews by 

 Burk ^ and Lipmann.-^ 



MANUFACTURE OF YEAST AND ITS USES 



Compressed yeast is manufactured in tremendous quantities. Most 

 of it is used for the baking, industrial alcohol, and brewing indus- 

 tries. Some of it is also used for vitamin preparations, enzymes, and 

 the like. 



Compressed Yeast Manufacture. The most commonly used method 

 for the manufacture of yeast involves the use of the molasses-am- 

 monia process. A dilute mixture of molasses, mineral salts, and 

 ammonia is pitched, i.e., inoculated, with yeast. Concentrated wort, 

 uninoculated mash, is added from time to time at such a rate that 

 there is sufficient sugar for the growth of yeast but not for the un- 

 limited production of alcohol. Sterile air is passed through the mash 

 during most of the growth process to aerate it. Aerobic conditions 

 inhibit the production of alcohol as noted above, but such conditions 

 are difficult to attain in the presence of high concentrations of sugar 

 and other organic matter. Therefore, small amounts of alcohol are 

 produced. The pH of the mash is controlled by the addition of 

 ammonia and sulfuric acid from time to time, a fairly low pH range 

 of 4.0 to 4.5 being generally maintained. The periodic addition of 

 ammonia is necessary because the yeast utilizes it for its nitrogen 

 source. Temperature is controlled by the use of cooling coils in the 

 tank. At the end of the growth period, the mature yeast is freed of 

 the wort by centrifugation and/or filtration. 



Yeast has also been prepared from sulphite liquor, a waste product 

 in the manufacture of pulp from wood, mixed with small amounts of 

 molasses (see page 339). Y^east for animal feed has also been pre- 

 pared from dilute sugar solution prepared from wood by the Scholler- 

 Tornesch process (see page 339). In both these instances, nitrogen 

 in the form of ammonia and essential salts must be added. 



