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If I get a beer labelled 5% ABV, how accurate is that measurement likely to be?

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2 Answers

tl;dr — By regulation, ±0.3-0.5%.

Chemically speaking, I'm not sure—maybe someone else can go into measuring techniques, alterations (continued fermentation?) during distribution, etc.

But countries and regions specify tolerances for error.

In the United States, according to Title 27: Alcohol, Tobacco, and Firearms, Part 7: Labeling and Advertising of Malt Beverages, §7.71:

(c) Tolerances. (1) For malt beverages containing 0.5 percent or more alcohol by volume, a tolerance of 0.3 percent will be permitted, either above or below the stated percentage of alcohol. Any malt beverage which is labeled as containing 0.5 percent or more alcohol by volume may not contain less than 0.5 percent alcohol by volume, regardless of any tolerance.

http://www.ecfr.gov/cgi-bin/text-idx?SID=1affcd509f9614478fbbc0c85551765a&node=27:1.0.1.1.5.8.41.1&rgn=div8

In the EU,

  • 0.5% vol. for beers having an alcoholic strength not exceeding 5.5 % vol. and beverages classified under subheading 22.07 B II of the Common Customs Tariff and made from grapes
  • 1% vol. for beers having an alcoholic strength exceeding 5.5 % vol. and beverages classified under subheading 22.07 B I of the Common Customs Tariff and made from grapes; ciders, berries, fruit wines, and the like; beverages based on fermented honey
  • 1.5 % vol. for beverages containing macerated fruit or parts of plants
  • 0.3 % vol. for other beverages

http://www.icap.org/table/alcoholbeveragelabeling

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So for example, in the US, by regulation, a beer that is listed as 5% ABV may be anywhere from 4.5% to 5.5%. If a beer is listed as 0.6%, however, it must have no less than 0.5%. (This, I assume is due to the limit that defines a Non-Alcoholic beer). Correct? –  Ryan Kinal Feb 4 at 19:35
    
@RyanKinal - Actually, in the US, the tolerance is 0.3%, so a beer listed as 5% ABV would have to be between 4.7% and 5.3%. But you're correct about the second point: if a malt beverage (more general than "beer," but includes beer) is listed as 0.6%, it must be between 0.5% and 0.9%, as the tolerance ends at 0.5%. Yes, this is due to the threshold for non-alcoholic beverages being 0.5%—similarly a non-alcoholic beverage listed as 0.4% cannot exceed 0.5% ABV. –  Andrew Cheong Feb 7 at 20:37
    
Ah, yes. Got my numbers confused. –  Ryan Kinal Feb 7 at 20:53
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There are two ways a brewery can measure the abv:

  1. By wort gravity: by measuring the specific gravity of the wort (the sugar solution that the yeast ferment into beer) both before and after fermentation. The difference is the amount of sugar consumed, which can be used to approximate the amount of alcohol produced.
  2. By distillation: the sample is heated, so the water and ethanol (alcohol) evaporate off and are collected in a condenser. The specific gravity of this is measured, which can then be used to calculate the percentage of alcohol, since the densities of alcohol and water are both known.

Both methods are exact and the accuracy is entirely due to the accuracy of the equipment used to measure.

The least accurate and precise is the case of (say) a microbrewer using a regular glass hydrometer to measure the OG and FG (method 1 above) - these read to 4 significant digits, with an accuracy of +/-1 SG at best. Without getting into the details, the accuracy then is about +/-5% of the final value for a regular strength beer. So for a 5% beer, that would be +/-0.25%.

With lab grade equipment, the accuracy of method 2 can be as good as 0.05%, since the specific gravities are measured with greater precision (and accuracy.) So our 5% beer would be with accuracy 0.0025%.

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The issue is not the accuracy of measurement but the reproducibility of production. A beer that is brewed one day at, say, 4.5% might end up at 4.6% when brewed the next week because of random variations in, for example, the temperature in the brewery or the activity of the yeast used. They're not going to get new bottle labels etc. printed for each separate batch brewed so the question isn't "how accurately can you measure the ABV of beer?" but "how tightly can you control the brewing process to end up with the ABV you were aiming for?" –  David Richerby Feb 1 at 0:22
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Yes, your absolutely right - I didn't see it from that angle initially. That's why breweries blend their beers to help round out variations from batch to batch, and as well as hitting the right color, flavor, aroma etc, this would also help with hitting abv to within a certain tolerance. –  mdma Feb 1 at 2:48
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