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HOMEBREW Digest #5052
HOMEBREW Digest #5052 Wed 06 September 2006
FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
Digest Janitor: pbabcock at hbd.org
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Contents:
re: Beer and Beer's Law (was: Color Extraction) ("steve.alexander")
lactose in sweet stout (Robin Griller)
Beers Law & color ("A.J deLange")
Re: Beer and Beer's Law (was: Color Extraction) (J A Stephen Viggiano)
Re: Beer's law and linearity (J A Stephen Viggiano)
Beer-Lambert (mabrooks)
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Date: Wed, 06 Sep 2006 01:05:25 -0400
From: "steve.alexander" <-s at adelphia.net>
Subject: re: Beer and Beer's Law (was: Color Extraction)
J A Stephen Viggiano agrees with AJ's recent note that beer may indeed
follow beers law *BUT* let me note this may be a result based on too
small a sample space. AJ used Guinness as I recall and JohnV notes
that he uses ... "Beamish Stout at five different concentrations",
but dark stouts owe their color to roast barley or malt roast to a
black char color probably with large amount of color from carbon, and
this is not typical of other beer styles.
Based on what I read of Debye-Huckel (and I only learned Deybe-
Sommerfeld at school - rats) and several other effects mentioned by
AJ, it seems likely to me that the color of the phenolic compounds in
beer and possibly of some Maillard products would be most likely to
demonstrate the non-linearity of the "beer does not follow beers law"
aphorism.
It's easy enough to find test beers reasonably dark from caramel and
maillard products, but it's a bit more difficult to think of beers
with significant phenolic color. I suppose that wider cuvettes of a
pils might do but that's a guess.
May I challenge you to fire up your spectrometers on samples of a
beers with more typical coloring agents than stouts ?
tnx,
-S
------------------------------
Date: Wed, 6 Sep 2006 08:13:58 -0400
From: Robin Griller <rgriller at chass.utoronto.ca>
Subject: lactose in sweet stout
I've brewed sweet stouts on several occasions and have always used a
little over 1 lb./5.25 gallon recipe. I wouldn't say that lactose makes
the beer that sweet in any case -- it more gives it a fullness and keeps
it from being dry.
Robin
------------------------------
Date: Wed, 06 Sep 2006 13:00:32 +0000
From: "A.J deLange" <ajdel at cox.net>
Subject: Beers Law & color
When this thread originally kicked off I did an initial check with the
beer that was most handy which was an Oktoberfest of 17.6 SRM. The
linearity was good which caused me to do the Guiness based on the
thought that perhaps the coloring material in the O'fest was too dilute
to show deviation from "ideally dilute" behavior.
I also thought about the spectrophotometer I use. It is programmed with
hundreds of "methods" the very great majority of which are based on
Beer's law i.e. the instrument stores an absorbtion coefficient which is
multiplied by the measured absorbtion to give a reading directly in
concentration. Those methods that tabulate polynomials or piecewize
linear functions of for concentration vs. absorbtion are typically those
where it is not depth of color that is being measured but rather the
scattering of light in a precipitate (for example, to measure sulfate,
barium chloride is added to the sample and the amount of light which
passes through the resulting cloud of barium sulfate is precipitate
measured). Thus it appears that Beers law is generally followed with
rare exception. Why would we suppose beer to be an exception? Up to this
point it's because of the lore. Looking at the collection of articles in
the link Peter Ensminger posted a week or so back I realized that not
everyone accepted that beer was an exception even then. It appears
at this point that I was taken in by a ****** [an apparently offensive
word, as my post was rejected because of it, removed - wondered why I
hadn't seen it recently]. OTOH no
less an authority than Jean DeClerk wrote "This law, however, is not
universally applicable. It is only true with low concentrations
(less than 0.6M)..." (Vol II p 253). And John didn't prove that beer
follows it. He proved that Beamish stout follows it. Nor did I prove
it with Guiness or the O'Fest. These are, at this point, anecdotal
results.The jury is still out.
The broader question, posed by Nathaniel, is what this has to do with
beer color. Determination of beer color has an interesting history. The
motivation behind specifying beer color was to have something which
could be measured in the laboratory. Before the days of
spectrophotometers that was a tall order. The original attempts involved
comparison with iodine solutions of various strengths and I think the
Lovibond series arose from that general concept. With the advent of
decent photometers after WWII the ASBC moved to an instrumental method
which would not depend on subjective judgement of match using north
light in the early afternoon. The SRM method was the result and
conceptually it is seriously flawed (it is the SRM measurement we are
talking about here). It was based on the average spectral
characteristics of a couple of dozzen pale American beers and the words
in the MOA say it applies only to beers of "average spectral
characteristics". Thus, technically, Guiness does not have an SRM
rating. Further the SRM is a single number and as we all know it takes
at least 3 numbers to specify a color. For me to tell you what color a
beer will appear to you as you hold the glass up to the light you must
tell me 3 absorbtions, the thickness of the light path through the beer
and the color quality of the light. It's best if you tell me the
spectral absorbtion of the beer every 5 nm over the visible range. From
that, given the path and the illuminant, I can calculate a tristimulus -
i.e. the color you see.
Now beer is very interesting. It's absorbtion spectra are all very
similar. Most (99%) of the variation in spectral properties over beers
as diverse as Bud light to Watney's stout and including Lindeman's Kriek
can be modeled by a single number (principal component) and 99.9% by
four. Interestingly enough the first principal component correlates with
the SRM value for the beer very, very well (Pearsons r = 0.99934). Thus
if you tell me the SRM number I can calculate the first principal
component and thus the spectal absorbtion curve for the beer. I can
adjust this for any path and any illuminant and tell you what color you
will see in the glass. If you give me the other 3 components I can tell
you the color to an accuracy of 3 units in L*ab space. Thus the SRM
number actually turns out to be more valuable than it might first appear.
------------------------------
Date: Wed, 06 Sep 2006 10:57:46 -0400
From: J A Stephen Viggiano <jasv at acolyte-color.com>
Subject: Re: Beer and Beer's Law (was: Color Extraction)
"steve.alexander" wrote:
> Based on what I read of Debye-Huckel (and I only learned Deybe-
> Sommerfeld at school - rats) and several other effects mentioned by
> AJ, it seems likely to me that the color of the phenolic compounds in
> beer and possibly of some Maillard products would be most likely to
> demonstrate the non-linearity of the "beer does not follow beers law"
> aphorism.
Beer does not obey the Bouguer-Lambert-Beer law if it is visibly
turbid. Kubelka-Munk would probably deal with that.
ASBC Method of Analysis Beer-10 (Color) stresses the importance of
degassing the sample prior to measurement. Don't forget about the
bubbles.
> May I challenge you to fire up your spectrometers on samples of a
> beers with more typical coloring agents than stouts ?
I also have a nice plot of Beck's Oktoberfest at a number of
concentrations. As with my Beamish plot, it's not just at 430
nanometers, but at 400, 410, ..., 700 nm. It was produced using a
different spectrophotometer, a Macbeth Color Eye 7000, which is a
single-beam instrument that is limited to about 2 Absorbance.
==John
------------------------------
Date: Wed, 06 Sep 2006 11:25:26 -0400
From: J A Stephen Viggiano <jasv at acolyte-color.com>
Subject: Re: Beer's law and linearity
Nathaniel Lansing wrote:
> Does a beer that measures 10 SRM appear twice
> as dark as the beer that measures 5 SRM?
Just about, but a 30 SRM does not appear twice as dark as a 15.
As you point out, SRM is logarithmic. The Lovibond Tintometer scale
is logarithmic, as well. This was so that one could add, for example,
use a combination of 2.0 units of Yellow and 3.0 units of Yellow and
obtain the same result as 5.0 units of Yellow. The Series 52 scale is
a bit different, as a decreasing proportion of "Red" is used as the
amount of Yellow increases -- corresponding to a negative correlation
between SRM and Linner Hue Index, but my impression is that the Yellow
scale was linear in Absorbance.
Also, as you point out, the response of the human visual system is
not logarithmic. The 0.4 power relationship you mention is quite close
to the 0.43 used by Ebner and Fairchild (Proceedings 6th Color Imaging
Conference, 1998). The relationship which is most widely accepted today
is Pauli's extension of the Reilly cube root function, upon which the
correlate of Lightness in the CIELUV and CIELAB uniform color spaces
are based:
L* = 116 * (Y/Yn) ** (1/3) - 16, Y/Yn > (6/29) ** 3
L* = (29/3) ** 3 * (Y/Yn), Y/Yn <= (6/29) ** 3
where Y/Yn is the luminous reflectance or transmittance, and L* is the
Lightness.
This implies SRMs are visually closer for darker beers than for lighter,
which is how they actually appear.
I had produced a series of color standards for training beer judges.
They were at 5 SRM, 10 SRM, 15 SRM, and 20 SRM. I verified the SRM
rating of each using a spectrophotometer, and each measured within
1/10 SRM unit (1/20 unit for the 5 SRM standard). We poured small
samples of each into identical sample jars, lined them up, and viewed
them. The difference between the 5 and 10 SRM samples was perceptually
large; between the 15 and 20 samples, much smaller.
SRM was intended to be only roughly visually uniform; cross-functionality
with the Lovibond Series 52 scale was also an issue.
==John
------------------------------
Date: Wed, 6 Sep 2006 10:09:30 -0700 (PDT)
From: mabrooks <mabrooks12 at yahoo.com>
Subject: Beer-Lambert
In a recent posting:
>Date: Mon, 4 Sep 2006 15:27:09 -0400 (EDT)
From: J A Stephen Viggiano <jav9729 at cis.rit.edu>
Subject: Beer and Beer's Law (was: Color Extraction)
In archive 5046, on 19 August, A.J deLange asserted
that "beer doesn't follow Beer's law". Or, more
precisely, I suppose he repeated this assertion. So
it's not my intention to single out A J; many people
have repeated what experimental evidence has shown to
be an urban legend.
I had recently looked into this very question, and
have determined that if the beer is free of visible
turbidity, it does indeed obey the
Bouguer-Lambert-Beer law (to give it its full name).
- -------------------------------------------------------
I did not take the time to research AJ's post,
however, I am in agreement in the statement that beer
color does not follow the Beer -Lambert and cannot due
to the following:
The Beer-Lambert Law states that:
A =abc where:
A= Absorbance
a= species apparent absorption capabilities
b= pathlength
c= species concentration
Can anyone tell me what the concentration of a "color"
is in a beer? No, not hardly(unless they used a
precisely measured amount of pigment dye to color
it?)...why, because there does not exist any standards
by which to measure a color concentration. Color
absorption yes, color concentration, NO! The Beer
-Lambert law is specifically for determining
concentration of a species in solution, by combining
two laws. Separate the two laws and what you find is:
Lambert's law explains that, in solutions, each layer
of equal thickness absorbs an equal fraction of light
which transverses it, thus, when a ray of light passes
through an absorptive medium (beer), its intensity
decreases exponentially as the length of the medium
decreases. Hence when dealing with dilute "color"
concentrations in solution, one must be careful to
select a "path length" that will allow proper
determination of the solutes "color". High
concentrations = smaller path lengths whereas very low
concentrations will require longer path lengths.
Beers law states that: the intensity of a ray of
monochromatic light decreases as the "concentration"
of the absorbing medium increases. It is important to
note that not all colored solutions obey Beers law
(Test question: can anyone tell me when/why it
wouldn't) so new colorimetric methods may need to be
developed to measure these.
So what you have with the Beer-Lambert is the
combining of two laws for the determination of
concentrations of solute in solution.
Beer color does not follow the Beer-Lambert law, as
you are only measuring color, not concentration!
Color determination will follow the Lambert law to
some extent, as that is what is is mostly used for.
There is much, much, more detail to present to here,
but prob not needed/wanted by the masses. It is
imoportant to note that a Spectrophotometer must be
set up properly so it is measuring the proper
absorptive wavelength. To do this and one must know
what this wavelength is before beginning the analysis,
hence the original solution (beer) needs to be diluted
down, and a full spectrum analysis performed to
determine the wavelength of maximum absorbance
etc..... unfortunately one can, at best, only come up
with a comparison chart to use against other beers
whose colors "wavelength of maximum absorbance" falls
within the exact same wavelength as the benchmark
beer(chances are slim to none this will occur), so I
dont see any real benefit from going through all the
trouble?
Matt B.
Northern VA.
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End of HOMEBREW Digest #5052, 09/06/06
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