How do you measure alcohol content in a fermented drink without relying on measurements of gravity?

Say we have a bottle of water (pasteurized), add sugar (that has been treated to have no living organisms on it), and then a packet of 100% pure yeast. Then we can measure the gravity before and after to calculate how much alcohol must have been produced.

However, consider the thought experiment where you do the same thing as above. But you're unsure what yeast actually does. Your hypothesis is that it produces alcohol, but you're unsure that maybe it produces something else. What instrument can you use to specifically measure alcohol % directly, not indirectly be using knowledge of the metabolism of yeast. This includes capturing the C02 produced and calculating the amount of alcohol.

  • I'm curious why you don't want an indirect measurement, and insist on a direct measurement. If you have access to a chemistry lap, you could, for example, use mass spectrometry. Just about every other measurement will be the result of deducing alcohol content through some physical characteristic of the mixture. – Flydog57 Nov 5 at 17:55
  • It might be a bit of a weird question, but say I'm fermenting something using bacteria or yeast on the skin or in the fruit/root (ginger)/cabbage. I know we're moving beyond brewing with the cabbage, but it's related to my question quite closely. How do you know that the fermentation is indeed producing ethanol and it is not homolactic fermentation? The question came into my head as I was gathering information on ginger ale/ginger beer production. Information was very different from different sources, so I wondered if you could directly measure alc % determine the role of yeast. – pkofod Nov 5 at 18:58
up vote 1 down vote accepted

The most reliable way to determine alcohol in any fermented drink, without the need for several thousand US dollars in equipment and some seriously technical laboratory procedures, is distillation.

The answers already given provide good solutions in certain circumstances, but both will give incorrect readings in solutions with significant amounts of dissolved substances (especially sugar), because these compounds alter the physical characteristics of the liquid which are being used to determine alcohol content. In general, these methods only apply directly to the behavior of pure mixtures of water and ethanol, and when you measure something else (e.g. any fermented drink) other dissolved substances (sugar, protein, organic acids) will skew the results (or, in other words, unless you're measuring a pure mixture of alcohol and water, any reading you get is merely an extrapolation, telling how the liquid you're measuring compares to a pure mixture's behavior).

Distillation, however, ensures that you are only measuring volatile substances and leaving behind non-volatile dissolved constituents. Ethanol and water are the most volatile substances you're likely to encounter in the largest quantities. Acetic acid (vinegar), which may be found in some fermented beverages is also mildly volatile (about half as much as water at distillation temperatures), but its density is very similar to water and can likely be ignored in all but the highest concentrations. Volatile acids (including acetic) may be neutralized with addition of a base (sodium hydroxide), which converts them to a non-volatile form and thus removes the error they may otherwise present.

Other substances are highly volatile but are almost always found in such minute quantities as to make them completely negligible (parts per billion for some).

It's not the most practical measurement to be making for the home fermenter (though you could probably get all the tools you'd need for a lab-grade distillation apparatus for a couple hundred US dollars), but it is the most accurate. Most other methods are still calibrated to distillation.

Here is one outline of a common method, and another. You could also buy one of these if you have a large surplus of money on hand.

For reference, the American Society of Brewing Chemists lists distillation as its primary method for measuring alcohol content.

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There are, of course, many other methods used for high-accuracy alcohol determination. Generally, these methods involve pricy equipment and intricate sample preparation and measurement procedure, and for this reason, to me, lie outside of general home fermentation practice:

  • Refractometry - the refractive index of a liquid is used along with specific gravity to determine alcohol content. A few issues include:
    • Refractometer needs to be read in 'refractive index', not Brix or other commonly found units. Something like this, or any other abbe refractometer.
    • This method generally uses a calibration curve that is based on the distillation method (the ASBC method mentions it may be necessary to build a calibration curve for every single type of beverage to be tested).
    • There are refractometers for measuring alcohol content directly, but which require the sample to be distilled first.
    • Honestly this technique may be well within many home fermenters' limits of budget and patience, but may be inaccurate without a proper calibration curve (see above). Here is a sample procedure.
  • Gas Chromatography - you don't have to do much Googling to understand why this is out of the reach of most people. However if you have access to the equipment and methods, it's definitely a good method, even for discriminating and quantifying different kinds of alcohols (i.e. fusel alcohols)
  • HPLC (high performance liquid chromatography) - same as above.
  • Enzymatic analysis - in which alcohol is reduced by specific enzymes in a controlled environment and the results measured with a spectrophotometer.
  • Proprietary methods - such as the Anton Parr Alcolyzer or similar devices.

Ethanol concentration measurement is actually an active field of research, mostly in the pursuit of methods that can be automated and used in-line in large volume plants.

So, while there are many highly accurate methods out there, I still believe distillation to be the best compromise between quality results and being within reach of the average 'highly-motivated' home fermenter.

  • Thank you, this seems like the answer I was looking for. Unfortunately, it's illegal to distill at home in Denmark, but you can still buy "decorative" distilling equipment in some shops ;) – pkofod Nov 8 at 21:23
  • It's illegal to distill at home here in the US as well, but it's still perfectly legal to buy the lab equipment (i.e. authorities don't care if you want to try to distill and sell spirits 100mL at a time). I don't know much about Danish law but I would be surprised if it didn't allow you to own basic laboratory equipment. – Franklin P Combs Nov 9 at 3:06

If you don't want to do refractometry or hydrometry, your next best generic option would be ebulliometry, which is basically measuring the bubble point of the liquid, where the bubble point depends upon how much alcohol is there.

  • Nitpick: It's the "bubble point", not the "boiling point". An alcohol/water mixture doesn't have a boiling point, it has a "bubble point" (where it begins to boil) and a "dew point" (where it begins to condense). Because these two temperatures are not the same, and that both of them vary with alcohol concentration, it's possible to build multi-stage distillation devices. – Flydog57 Nov 5 at 18:01
  • @Flydog57: thank you for the correction, and the explanation. I edited my answer. – Dave Nov 5 at 18:04

A vinometer can mesure alcohol of a dry wine (all sugar needs to be fermented). You would need to add some dye to see the results if you have a transparent liquid. It can usually measure between 0 and 20%.

IMHO HPLC is the best method probably followed by spectroscopy. measuring ABV by distillation is "indirect" due to the formation of an azeotropic mixture between alcohol and water. They cannot be separated easily by distillation although one can do a "back calculation" to esttimate (quite accurately) the percentage of alcohol in the original mixture.HPLC actually separates the mixrture as does GC. The EtOH peak can be measured against a standard to accurately determine the amount of ethanol present. I accept few people actually have GC or HPLC but if a chemist really wanted to analyse a simple ferment then chromatography (or alternatively spectroscopy) might be a preferred option. I am not sure why the Brewing chemists prefer distillation as a measurement method - maybe because it is simple and "traditional".

  • I'm a bit unclear as to how formation of the azeotropic mixture of water and ethanol detracts from the distillation method. As I understand it the azeotrope prevents you from being able to achieve a distillate purity above 95.6% alcohol. But the analytical method never requires achieving this level of purity, only that complete separation of ethanol from the original liquid occurs. As the alcohol level in the liquid being distilled nears zero, so does the alcohol in the vapor, meaning it should be possible to achieve a near-perfect isolation of ethanol from the bulk liquid. – Franklin P Combs Nov 9 at 23:24
  • I agree, though, that these may be the best methods in absolute terms. I suppose I was just imagining the best method that's remotely accessible to the average home fermenter. – Franklin P Combs Nov 10 at 2:23

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