GROWTH 175 



assimilated by the yeast (see p. 211). This product 

 amounts to about 1% of the fermented sugar, or to 

 about 4% of the yeast solids. The small amount, in 

 comparison to the crop, makes it appear possible that 

 it might be a waste product of growth. 



(g) BALANCE SHEETS OF FEEDING EXPERIMENTS 



While an enormous literature on growth of micro- 

 organisms exists, there is very little material available 

 which shows complete balances of all foods used, or of 

 all products, including growth. The most complete 

 data available are those of the bread yeast industry. 

 This is the only large industry which is interested in 

 growth as such, and not in the products of fermentation. 



Even in this industry, the number of complete experi- 

 ments is not very great. It is regrettable that the carbon 

 dioxide and the quantity of the evaporated alcohol 

 are measured only exceptionally. But enough material 

 is available to get a good conception of the balance. 



A first attempt at such an account of all foods used 

 and all products formed is the study of yeast metabolism 

 by Brown and Balls (1925) who determined alcohol, 

 CO2, decrease of sugar and amount of yeast growth in a 

 well aerated culture. About 750 c.c. of the mash or wort 

 (consisting of 48 gm. molasses, 1.2 gm. H2(NH4)P04 and 

 1.6 gm. (NH4)2S04 per liter) were kept at 28°C. and 

 aerated with 5 liters of air per minute, or about seven 

 times the volume of the liquid per minute. The prod- 

 ucts formed were determined after 3, 6, 8, 16.5 and 

 24 hours. 



The results of some experiments are given in Table 38. 

 There is a noticeable amount of sugar not used for the 

 formation of either alcohol or CO2, and this amount, 

 in the aerated cultures, is about one-half of the yeast 



