150 

THE “ORIGINAL” SPECIFIC GRAVITY 
OF BEER. 
a ete December (1914) issue of the Journal of 
the Institute of Brewing is an important 
number of this publication, and of more than 
merely technical interest. It contains a series 
of reports and memoranda upon the subject of 
the determination of the “original” specific 
gravity of beer, and the results of the work de- 
scribed have now, by the Finance Act of 1914, 
been legalised as the basis for checking or deter- 
mining the charge of duty upon this beverage. 
The main reports are contributed jointly by Sir 
Edward Thorpe and Dr. H. T. Brown, and the 
latter author supplements them by interesting 
studies of both the historical and the scientific 
aspect of the question. 
Beer is taxed as being an alcoholic beverage, 
and it is popularly supposed to pay duty accord- 
ing to the amount of alcohol it contains. This, 
however, is not strictly the case. The basis of 
taxation is not the actual quantity of alcohol in 
the beer, but the specific gravity of the unfer- 
mented wort, which may be regarded as a 
measure of the quantity of alcohol potentially pro- 
ducible. Essentially, the wort of beer is a solu- 
tion of various sugars—maltose, dextrose, cane 
sugar, and invert sugar—either arising from the 
malt and grain used, or directly added as such. 
The proportion of sugars varies considerably, ac- 
cording to the kind of product desired. In wort 
for producing strong ale, for instance, there may 
be twice as much sugar as in that destined for 
ordinary table beer. But, in the main, the pro- 
portion of sugar determines the specific gravity 
of the unfermented wort, and the specific gravity 
is therefore made the basis of the charge of duty 
levied on the beer. A brewer is thus left at 
liberty to ferment his wort as much or as little 
as he likes, and to produce a stronger or a weaker, | 
a heavier or a lighter beverage, as his fancy dic- 
tates or his customers prefer. 
As soon as fermentation commences, the specific 
gravity of the wort begins to alter, and in general 
to decrease. This is so for two reasons. First, 
the destruction of sugar by fermentation removes 
solid matter from solution; and secondly, the 
alcohol produced is specifically lighter than the 
wort. Hence, unless the specific gravity is deter- 
mined before fermentation has commenced it is 
useless as the basis for the correct assessment of 
duty. It is impracticable, however, to have a 
revenue officer in attendance in all cases to take 
the specific gravity of the wort before fermenta- 
tion begins; moreover, it often happens that the 
“collection” of the wort extends over several 
hours, during which time the first portions may 
have been in contact with yeast, and fermented 
to an appreciable degree, before the last portions 
have been added. Even were this not so, other 
difficulties arise. A large quantity of beer is ex- 
ported, and the exporter is entitled to a “‘draw- 
back” or rebate equivalent to the duty originally 
charged upon the unfermented wort from which 
the beer was made. Hence it is necessary to 
NO. 2371, VOL. 95| 
NATURE 

[Aprit 8, 1915 

devise a method whereby the original specific 
gravity of the unfermented wort can be ascer- 
tained by the analysis either of the partly-fer- 
mented wort or of the finished beer, as the case 
may be. 
As a first approximation, such a method may 
be based upon the well-known equations of Gay- 
Lussae denoting the conversion of sugar into 
alcohol. With maltose, for example, it is readily 
calculable from the equation :— 
CyyH,01, + HzO =4C,H,0 +4C Oz, 
Maltose Alcohol 
that 100 parts of this sugar yield theoretically 
53'8 parts of alcohol. Hence by determining the 
quantity of alcohol in a given specimen of wort 
or beer, we find the equivalent amount of sugar 
destroyed; and from the known density of solu- 
tions of maltose we can thus deduce the corre- 
sponding loss of specific gravity. Adding the 
number denoting this loss to the specific gravity 
of the residual wort or beer freed from alcohol, 
we can, theoretically, obtain the “original” speci- 
fic gravity of the wort, i.e., the gravity before 
fermentation had commenced. 
In practice, however, this calculation is not 
sufficiently accurate. As Pasteur long ago showed 
in his studies of fermentation, a part of the sugar 
destroyed is not converted into alcohol. More- 
over, different classes of sugars yield different per- 
centages of alcohol. Again, the introduction of 
yeast complicates the matter, since part of the 
solid matter of the wort is used up in sustaining 
the growth of the yeast, and a sensible quantity 
of alcohol may be added as such with the yeast 
itself. These and other disturbing factors, small 
individually, together affect the result sufficiently 
to render the figures given by the simple theoreti- 
cal method, as outlined above, only an approxi- 
mation to what obtains in actual brewery opera- 
tions. 
We have, in fact, to fall back upon an empirical 
basis for the calculation of results sufficiently 
accurate to be used in practice. Starting with 
worts of known specific gravity, fermentations 
may be carried out, and from the examination of 
samples drawn as the process continues, it can 
be ascertained experimentally what is the actual 
loss of specific gravity consequent upon the pro- 
duction of known quantities of alcohol. Such ex- 
periments, in fact, have been made, and the results 
have been embodied in statutory tables for use in 
assessing the duty on beer. 
Manifestly it is of much importance that these 
tables should be: true and just. If inaccurate in 
one direction they are unfair to the public by 
reason of the loss of revenue entailed; if errone- 
ous in the opposite sense they are unfair to the 
brewer. 
The first original gravity tables were con- 
structed in the year 1847 by Messrs. Dobson and 
Phillips, of the Inland Revenue Department. They 
were drawn up with considerable care; but some 
discussion having arisen, the whole matter was 
referred a few years later to Profs. Graham, 
Hofmann, and Redwood, whose report was pre- 
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