22 BULLETIN 983, It. S. DEPARTMENT OF AGPJCULTURE. 



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. As 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 (cZ-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 cZ-glucose are not the same as 

 the power of (Z-glucose. 



Stone 37 and Browne 3S differ as to the relative reducing powers 

 of arabinose and xylose as compared with <7-glueose, but both writers 



u Stone, W. E., Berichte 23, 3796. 



's Browne, C A., Jour, of the Am. Chem. Soc. 2S, 439. 



