Cider Digest #0259

Published in

· 8 months ago

Subject: Re: potential alcohol 
Date: Tue, 23 Feb 1993 18:19:11 EST 
From: Jay Hersh <hersh@expo.lcs.mit.edu> 

>>                    Be aware that a dry cider will often have a 
>> finishing gravity under 1.000, that is completely dry with no residual 
>> sugar 
> 
>This is not strictly accurate. Pure ethanol has a gravity somewhere in the 
>.800's, so a strong dry cider could read under 1.000 and still have some 
>residual sugar (cf long meads, which can taste sweet even after finishing 
>at .990 or below). 

Taking this rain of thought a step further, this is why there are 2 
ways to measure attenuation, real attenuation and apparent attenuation. 

Apparent attenuation is simply the differenc between original and final  
gravity readings. This is of course, as pointed out above, skewed since 
alcohol is less dense than water and the conversion of sugar to alcohol 
and CO2 causes the alcohol to mix with water in the beverage thus  
lowering the gravity.  

This is of course important in the case of high alcohol drinks like cider 
or mead. I should have thought to point this out earlier in my post 
on potential alcohol readings. This is why it is called potential 
alcohol, since it is not definitive (BTW my numbers were wrong, double 
checking shows the figure to be about 7.5 points of gravity per percent 
alcohol, and I believe that is w/w not v/v) 

So to calculate real attenuation brewers heat the post ferment brew 
at the boiling temp of alcohol to drive off all the alcohol. This leaves 
a solution with only water and sugar. Now the difference between gravity 
of this mixture and the original mixture tells the actual amount of 
sugars fermented, and can thus be used to calculate the exact alcohol 
yield (ie the final reading is no longer offset by the density dilution 
of the alcohol). 

This technique is probably much better suited to mead and cider makers 
since they will tend to have more alcohol and so the potential alcohol 
readings off your normal hydrometer are likely to be less acurate. 
I should have recalled this earlier, but recent study for the Beer 
Judge Certification Program refreshed my memory on this topic... 

        JaH 

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