22 BULLETIN 083, U. S. DEPARTMENT OF AGRICTJLTtTRE. 
tation, expressed in percentage, is multiplied by the percentage of 
sugar fermentable, and by the constant 0.5111 (which is the amount 
of alcohol theoretically possible from 1 unit of dextrose), the result 
is the amount of alcohol theoretically obtainable from the sugar 
present in the neutral juice. The actual percentage by weight of 
alcohol in the beer divided by this theoretical alcohol figure will give 
the fermentation efficiency. 
The first and second causes for the high fermentation efficiencies 
may be grouped together, xls shows on the fermentation record 
sheets, the starting yeast will average 6 per cent of alcohol on the 
fourth day, which is equivalent to 4.5 c. c. of absolute alcohol. A 
12.5° Brix neutral juice will give a beer averaging 2.4 per cent of 
alcohol, the 2 gallons being equivalent to 181.7 c. c. of absolute 
alcohol. The alcohol from the yeast is, therefore, 2.48 per cent of the 
total alcohol and causes the fermentation efficiency to be high by 
approximately this amount. In beers having less than 2.4 per cent 
of alcohol the error will be greater, and in beers of greater alcohol 
content, the error will be correspondingly less. The figure obtained 
for fermentation efficiency is very important for comparative pur- 
poses; and, as the error is nearly a constant one, no corrections were 
made in calculating these values in order to eliminate the alcohol 
derived from the yeast. In addition, it permits of the expression 
of yields in the way they would be obtained commercially, for, in 
either case, commercial or experimental, about 1 per cent by volume 
of the total mash consists of the starting yeast solution. 
The magnitude of the errors involved in the sugar determination 
is more difficult to determine. In addition to sugars, there are other 
reducing substances present. These are principally aldehydes and 
formates, for it is known that considerable quantities of formic 
acid are present before neutralization. Further, although the sugar 
is determined and calculated as dextrose (^-glucose), the sugar is 
actually a mixture of this hexose, sometimes with pentoses and some- 
times with other hexoses. Moreover, the ratio of these sugars to 
each other varies in the different samples taken, and, although after 
fermentation the sugars consist entirely of pentoses and nonfermenta- 
ble hexoses, they are determined and calculated as dextrose. This 
opens two possibilities for error: First, varying quantities of pentoses 
affect the accuracy of the actual dextrose determination ; second, the 
reducing powers of sugars other than (^-glucose are not the same as 
the power of cZ-glucose. 
Stone 37 and Browne 38 differ as to the relative reducing powers 
of arabinose and xylose as compared with J-glucose, but both writers 
*■ Stone, W. E., Berichte 23, 3796. 
as Browne, C A., Jour, of the Am. Chem. Soc. 28, 439. 
