How to Calculate High Attenuation? - Home Brew Forums

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08-29-2013, 04:40 AM   #1
Foothiller
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My mead (a peach melomel) is still fermenting, so my SG readings use my refractometer instead of the hydrometer to not use a larger sample than needed until it's done. But unless I get informed otherwise, I think BrewCalc on my iPhone does an adequate job of calculating the SG for a fermentation in progress. After racking from the primary fermenter into a carboy and letting it sit for another month, the SG is 0.998, down from OG = 1.069. When I brew beer, I just calculate the attenuation as 1 - (FG points / OG points), which is probably OK for beer's attenuation, but clearly the attenuation can't be more than 100%. I have had wines in the past (when I wasn't curious about attenuation yet) finish with FG less than 0.995 as measured by a hydrometer, so I trust that my refractometer calculation is less than 1.000. My question is: How would I calculate attenuation when it gets as high as it does with mead? (Thanks in advance!)

08-29-2013, 09:45 AM   #2
WoodlandBrew

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The "apparent attenuation" will actually be more than 100%, but the actual attenuation is lower. Because fermentation converts sugar (which is more dense than water) to alcohol (which is less dense) the final gravity can be lower than that of pure water even when some sugar remains in solution.
My blog has a few posts on this including calculations with a refractometer. My book which will be published next month does a little beter job explaining this subject.
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08-29-2013, 06:28 PM   #3
fatbloke

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Quote:
 Originally Posted by WoodlandBrew The "apparent attenuation" will actually be more than 100%, but the actual attenuation is lower. Because fermentation converts sugar (which is more dense than water) to alcohol (which is less dense) the final gravity can be lower than that of pure water even when some sugar remains in solution. My blog has a few posts on this including calculations with a refractometer. My book which will be published next month does a little beter job explaining this subject.
Well glad you have a handle on what the OP was actually wanting to know.....

I only make meads and country wines, so haven't a clue what was being asked.....

I suspect its something about the similarity between making beers and meads ? but can't be certain as I don't speak beer !

Personally I have no illusions of similarity other than both being fermented products.....

If you're putting a book together, maybe you have a link to such terminology as "attenuation"...... ?
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08-29-2013, 07:27 PM   #4
Foothiller
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Sorry if my question wasn't clear. As WoodlandBrew explains, apparent attenuation can appear to exceed 100%, but actual attenuation will be less than 100% - which I have experienced when making wine. When brewing beer, calculating apparent attenuation has been good enough, but this breaks down for mead's higher attenuation. So, the question is: how does one calculate the actual attenuation when it is high, like for mead or wine? Thanks again for the responses.

08-30-2013, 12:50 AM   #5
WoodlandBrew

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There are a few things at work with the calculations including density of alcohol vs sugar, refraction index and weight vs volume measurements. It's explained better in my book, but I think there is enough information in this post to answer your question:
http://woodlandbrew.blogspot.com/201...equations.html

You might also be interested in these:
http://woodlandbrew.blogspot.com/201...ithout-og.html
http://woodlandbrew.blogspot.com/201...-alchohol.html
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08-30-2013, 01:23 AM   #6
Foothiller
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Thanks very much, WoodlandBrew! Very helpful references.

For others, in case "attenuation" wasn't clear, this is the amount of sugar that the yeast have consumed. A typical percentage for beer is 75%, since some types of sugar are unfermentable by the brewer's yeast. Values for mead and wine are higher, using different yeast and having different sugars available.

08-30-2013, 02:00 AM   #7
WoodlandBrew

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Thanks for bringing this to mind again, it spurred me to edit that section of the book. For a 1.069 starting gravity the conversion from Apparent Attenuation is:

Actual Attenuation = 0.824 (Apparent Attenuation) + 0.96 (use 110 for 110% in these equations)
This equation is accurate within about 1% for only beers that start at 1.069

For beers between 1.020 and 1.100 original gravity there is a simple equation that is accurate within 3% in the book.
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08-30-2013, 02:54 AM   #8
fatbloke

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I'll check out those links when I get home, thanks.....

Yet I still only speak English, not American or geeky beer nerd....

So presumably by "attenuation", you're alluding to gravity drop during the actual ferment ?....
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08-30-2013, 04:05 AM   #9
Tom128
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Quote:
 Originally Posted by fatbloke I'll check out those links when I get home, thanks..... Yet I still only speak English, not American or geeky beer nerd.... So presumably by "attenuation", you're alluding to gravity drop during the actual ferment ?....
Attenuation is a percentage of the sugars that the yeast consume when it's finished, calculated by (OG - FG) / (OG - 1) * 100%. So if the OG of a mead is 1.080 and it finishes at 1.010 the attenuation is (1.080 - 1.010) / 1.080 * 100% = 87.5%.

This math is usually fine for lower ABV beverages but in wine and mead you have higher starting gravities and lower finishing. Take an example with OG of 1.120 and FG of 0.990. In that case the attenuation is (1.120 - 0.990) / (1.120 - 1) = 108.3% which clearly isn't possible. This is a side effect of the fact that hydrometers are calibrated against water which has a gravity of 1.000. However pure alcohol has a much lower gravity, something like 0.800 I believe. When you have a brew with high levels of alcohol the normal equation does not work.

As for the original question, I am not sure myself. A quick search on it seems to suggest you need to take a sample and boil off the alcohol, then replace it with distilled water and compare weight ratios. I've honestly never cared about attenuation with wine or mead like you would in beer because most wine yeasts will take almost any batch to dry even if you don't want it to, so it's just never been something I though of.

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08-30-2013, 03:32 PM   #10
fatbloke

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Cheers Tom, that makes perfect sense.....
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