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	FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
		Digest Janitor: pbabcock at hbd.org


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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.


Return to table of contents


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




Return to table of contents


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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