ENERGY SUPPLY OF THE CELL 107 



discharge of fermentation products into the medium (see also Slator 

 and Sand, 1910, on the rate of diffusion into yeast cells). 



We can further compute the rate of fermentation of 

 Lad. acidophilus from the observation of Rahn (1929b) 

 that 0.045% of lactic acid are formed per hour by 420,- 

 000,000 cells (microscopic count) of Lact. acidophilus 

 per c.c. This amounts to 45 mg. produced by 42 X 

 10^ X 102 cells, or almost 10 X lO'^^ mg. per cell per 

 hour. The count probably included many dead cells, 

 which would increase somewhat the fermenting capacity 

 per viable cell. 



Only one datum on another fermentation is known 

 to the author, namely, the rate of urea fermentation by 

 Micr. ureae. Burchard (1899) used a wrong formula 

 to compute fermenting capacities, but his data show 

 66 X 10~^^ mg. of urea per cell and hour, four to five 

 times as much as the fermenting capacity of lactic 

 streptococci. 



SUMMARY 



The rate of fermentation is known for only a few 

 microorganisms. It is enormously high if compared 

 with larger organisms. High-bred commercial yeasts 

 (beer, wine or bread yeast) ferment about 30-40% of 

 their own weight of sugar per hour. Bacteria being still 

 smaller decompose still more food. Lactob. acidophilus 

 uses at least 50% of its body weight of sugar per hour, 

 the lactic streptococci about two to three times their 

 own weight, and Micr. ureae at least five times its own 

 weight of urea per hour. 



(c) THE DECREASE OF THE RATE OF FERMENTATION 



It is a general experience that fermentations come to 

 end, sometimes even before all the fermentable substrate 



