562 MICROBIOLOGY OF FOODS 



can be accomplished, later operations are simplified, but settling is not essential to 

 success. 



The fermented liquid and yeast from the vats are passed through continuous 

 centrifuges similar in design to milk centrifuges. A creamy suspension of yeast 

 and liquid is obtained from one outlet and a liquid almost yeast-free from the 

 other outlet of the centrifuge. 



The creamy suspension of yeast is chilled by flowing over brine-cooled coils and 

 flows to the filter supply tank. Chilling the yeast checks the growth of slime-form- 

 ing and other harmful organisms. 



The mixture of yeast and liquid is filter-pressed to remove excess of liquid. The 

 pasty mass of yeast forming the filter press-cakes is usually mixed with a small 

 amount of starch which gives a friable texture to the yeast. A small amount of 

 vegetable oil is used in other plants for the same purpose. The yeast is next molded 

 and cut into blocks of the desired shape and size. After wrapping it is held in cold 

 storage until sold. At room temperatures the yeast liquefies through autolysis or 

 becomes slimy from bacterial growth, moldy or weakened. Therefore, keeping the 

 yeast at a low temperature is essential to longevity of the product. 



The waste liquor from the centrifuges and filter press is distilled in a continuous 

 still. The alcoholic distillate is diluted to about 10 per cent, alcohol and passed 

 through charcoal or coke filled generators to produce distilled vinegar used prin- 

 cipally by pickle manufacturers. The vinegar is sometimes used for aging, or, di- 

 luted to 5 per cent acetic acid, is sold for domestic use (See Division VI, Chapter IV). 



Compressed yeast is also mixed with corn meal and compressed into cakes and 

 dried at temperatures low enough not to affect seriously the yeast's vitality. The 

 dried product is known and used in rural communities as "Magic Yeast," etc. 

 This product makes it possible to obtain relatively pure and active yeast in isolated 

 communities. Such yeast gives the best results if made into a thin batter or potato 

 yeast-starter twenty-four hours before it is to be used in bread making. 



The manufacture of compressed yeast is carefully controlled by chemical analysis 

 and frequent microscopical examinations. Good compressed yeast should be slightly 

 moist but not "sloppy;" the color should be creamy white, and it should show a 

 fine fracture when broken. It should melt readily on the tongue. The flavor should 

 be clean and free from any suggestion of butyric acid or putrefaction. It should 

 show only a very few dead cells when the cells are mounted in a dilute solution of 

 methylene blue. It will normally contain a few lactic bacteria but must be free 

 from B. subtilis or putrefactive organisms. In a case that came to the writer's 

 attention the yeast from a certain factory carried rather large numbers of B. subtilis 

 which caused the yeast to become slimy. The infection was traced to the water used 

 in the factory. In another case the yeast was found to be the cause of ropy bread 

 because of its contamination with B. mesentericus mdgatus. The use of a mash 

 highly acidified by growth of B. delbruckii or other lactic organisms will hold in 

 check most "yeast disease" organisms. 



