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I experimented with turning a regular bock into an Eisbock. My bock had an OG of 1.088 and a FG of 1.014 (for an abv of ~9.5%).

I froze that bock to varying degrees, and pulled the unfrozen liquid out, letting the ice thaw and refreeze. My "best" result had a gravity of 1.032, the "worst" result had only 1.006 (end of the experiment).

How do I calculate the alcohol by volume in those freeze-distilled beers?

  • Can you re-melt the freeze, take it's volume & gravity then subtract this concentration from the Eisbock? – Kingsley Dec 5 '19 at 3:16
  • I already posted my answer and described how you can do it using pycnometer. There is another way of doing it and it uses hydrometer and refractometer. You can refer to section "Measurement of ABV" here: byo.com/article/refractometers Basically, hydrometer is going to give you accurate SG reading and refractometer is going to give you tilted reading because of the presence of alcohol. Using the error of refractometer we can actually calculate ABV. – MaliMish Dec 5 '19 at 15:33
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You can't determine the alcohol content simply by measuring gravity. We need more information.

Let's try to determine the alcohol by volume in your "best" result beer. For starters, it has gravity of 1.032 which translates into 83 grams of sugar per one liter of beer.

Tools we need to calculate the alcohol content:

  • pycnometer

  • sensitive scale

Steps:

1) Fill the pycnometer to appropriate level and determine the weight of the beer

2) Subtract the appropriate amount of sugar from that result

3) Use ethanol-water density chart to find the weight of ethanol

4) Use formula to convert alcohol by weight into alcohol by volume

Detailed explanations:

1) You can find a lot of information on the internet about pycnometers and how to use them. Basically, they are used to determine the density of the solution. Let's say you have 20mL pycnometer. You have to fill it up to a level marked on the pycnometer and find the mass of the beer. Suppose you got the result of 20.02 grams.

2) Since we are dealing with very small amounts of sugar, we can assume that the volume of beer would stay the same if we removed it(*). In 20mL of liquid there would be 0.02 * 83 = 1.66 grams of sugar. We are going to subtract that from result obtained in first step. To find the amount of sugar in 1L of solution I used this calculator: http://www.vinolab.hr/calculator/gravity-density-sugar-conversions-en19

3) Subtract 1.66 from 20.02 grams. This gives you 18.36 grams. So, the density of water and ethanol mixture (since we removed sugars) is 18.36 / 20 = 0.18 g/mL (notice that we are using milliliters and grams which is OK). Suppose that you did your measurements in the ambient temperature of 20C, using ethanol-water-mixture density chart we can see that the alcohol by weight is 42% (**). Use this chart: https://www.engineeringtoolbox.com/ethanol-water-mixture-density-d_2162.html

4) Using ABW to ABV calculator we can see that alcohol by volume is 52.5%.

(*) You can dig deeper to see how to adjust for the error when subtracting the mass of sugar from the mass of solution but as I said since the percentage of sugar is very small it is not going to make a big difference in calculation (because volume stays almost the same).

(**) Don't be alarmed by this number. It is possible that the very first results you got were this high in alcohol. Since I assumed there was 1 liter of your first results this is what you get. In reality it is more likely that there was like 100mL of this strong stuff.

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  • "For starters, it has gravity of 1.032 which translates into 32 grams of sugar per one liter of beer." - I'm not sure this is actually true - 1.032 is ~8 plato, i.e. 8% extract by weight. 8% of 1L is definitely not 32g. Can you explain what's happening here please? – Frazbro Dec 9 '19 at 5:00
  • You are totally right, it should be 8% by weight. I used this calculator and was looking at the wrong entry: vinolab.hr/calculator/gravity-density-sugar-conversions-en19 – MaliMish Dec 9 '19 at 8:11
  • Ahh, an easy mistake to make. – Frazbro Dec 9 '19 at 21:27

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