FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
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Contents:
Attenuation ("A. J. deLange")
Re: Attenuation (Kai Troester)
RE: Attenuation ("RJ")
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Date: Fri, 11 Sep 2009 08:26:46 -0400
From: "A. J. deLange" <ajdel at cox.net>
Subject: Attenuation
Attenuation is intended to be a rough indicator of the amount of
specific gravity decrease over the course of a nominal fermentation with
the yeast strain in question. Formally it is ADF = 100 X (OE - AE)/OE
where ADF = "apparent degree of fermentation" (attenuation) in percent,
OE = "original extract" and AE = "apparent extract". OE and AE are in
grams of extract per 100 grams of beer i.e. extract concentration w/w
i.e. degrees Plato but in common usage, especially by homebrewers, they
are in specific gravity "points" (e.g. a wort with SG 1.040 has 40
points of OE). More meaningfull is the "real degree of fermentation" RDF
= 100*(OE - TE)/OE where TE is the "true extract" of the beer i.e. the
actual number of grams of extract in 100 grams of beer but measuring
that is appreciably more troublesome that just taking a hydrometer
reading on the fermented beer so few do it but you can estimate it from
RDF = 3.271 + 0.766*ADF.
I expect the answer to the question as to where the numbers on the
yeast specs come from is that the manufacturer does test fermentations
with nominal worts under nominal conditions for the particular strain
(ale yeasts would be operated at higher temperatures than lager yeasts)
and records the range of ADF's found. The ADF is going to depend on
several factors such as the composition of the wort (dextrinous worts
will not ferment to the same extent as highly fermentable ones i.e. ones
mashed to produce lots of fermentable sugars as opposed to the larger,
non fermentable ones), its OE, the presence of sufficient yeast
nutrients and enzyme co factors, the temperature at which the
fermentation is conducted, the pitching rate, the amount of oxygen
supplied and when it is supplied and the, ability of the yeast to
transport and lyse the various sugars available in a wort, the relative
amounts of those sugars, how the yeast behave with respect to
flocculation and their ability continue functioning as alcohol level
increased. There are lots of variables here and that is why the
manufacturer would have to pick some nominal set of conditions so that
the results compare based on the properties of the yeasts themselves,
(flocculation, alcohol tolerance...) and not on the conditions in the
lab. But naturally, for these data to be useful to the brewer, the test
conditions would have to be typical of the conditions found in the
brewer's fermenter.
All in all these numbers should be used only as a guide. If you prepare
a 1.050 wort and ferment it with a strain rated at 75 - 80% attenuation
you would expect that your final SG would fall in the range of 1.0100 -
1.0125. If it ends up at 1.020 then you would look for some cause
(insufficient pitching rate, mash temperature too high, insufficient
oxygen etc.) that might be responsible.
Final note: a saccharification rest at 148 F would be expected to
produce a more fermentable (higher attenuation) wort than one mashed at
say 154 F.
A. J.
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Date: Fri, 11 Sep 2009 09:35:46 -0400
From: Kai Troester <kai at braukaiser.com>
Subject: Re: Attenuation
Kevin,
> I find attenuation ratings on a yeast strain confusing and I'm hoping
> someone can explain it to me. Assuming the wort was mashed for a low
> attenuation (say mashed at 148F) and the starting gravity was 1066. A
> Munich Lager is used with an approx. attenuation rating of 75% (77% max). I
> would end up around 1016 as a FG right (assuming correct aeration, nutrients
> etc.)? But what determines this attenuation range on the package? Wouldn't
> the yeast keep going until the fermentables were gone or the alcohol got too
> high for the yeast? Why is it limited as per the strain rating?
The attenuation numbers listed with the yeast strain are pretty
useless and I wish they would not give them. Not only is there no
standard in determining them, they also heavily depend on wort
composition. I'd like yeast data that shows how good of an attenuator
a given yeast strain is and how close to the attenuation limit (i.e.
fermentability) of the produced wort a particular yeast will get in an
average fermentation.
Most of my thoughts about attenuation have been summarized in this
article which is intended to answer the questions that you have:
http://braukaiser.com/wiki/index.php?title=Understanding_Attenuation
To illustrate the idea of the attenuation limit and that all yeasts
can reach is it I one fermented samples of the same wort with a lager
yeast (WY2206) and an English Ale yeast (WLP002) in fast ferment test
conditions. This means warm, high pitch rate and occasional agitation.
They both finished at the same gravity. This means that even an
English Ale yest can be a good attenuator. But we don't necessarily
want this to happen in our beers since the residual fermentable sugars
that this yeast leaves behind by giving up early contribute to the
character and balance of that style of beer.
Kai
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Date: Fri, 11 Sep 2009 18:08:49 -0400
From: "RJ" <wortsup at metrocast.net>
Subject: RE: Attenuation
Kevin,
Check out this web page at WhiteLabs
http://www.whitelabs.com/beer/homebrew_terms.html
Hope that helps
RJ
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End of HOMEBREW Digest #5606, 09/11/09
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