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Old 03-24-2010, 02:02 PM   #1
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Default Measuring SRM

So I realize not many people have access to the necessary equipment to directly measure the color of a beer, but fortunately, I work in a research lab and have all this stuff sitting around, waiting to be used by curious brewers such as myself. For those that do have access to a UV/Vis spectrophotometer, centrifuge, and syringe filters, I'll note my procedure below.

Wikipedia (which was the only source I could which gave the spectroscopic determination of beer color) defines the SRM of a beer as the absorbance at 430 nanometers times a constant and a dilution factor.

SRM = 12.7 * D * A, where D is the dilution factor (for an undiluted sample, D = 1, for a sample diluted 1:1 with deionized water, D = 2, etc.) and A is the absorbance at 430 nm.

When I ran my ESB through, I got an absorbance of 1.36, which resulted in an SRM value of 17.3 -- what? An SRM of 17.3 is something expected from a dark lager, not a golden ESB. However, an EBC value of 17.3 looks right on, and conversion to SRM using the formula SRM = EBC * 0.508 results in an SRM of 8.8, which is close to the color calculated by my brewing software, 9.5.

Is Wikipedia wrong, or are my measurements incorrect? Does anyone have a reference for these measurements?

For those interested, here is my procedure:

1. Dispensed 35mL of beer into a centrifuge tube
2. Centrifuged at 10,000 RPM for 10 minutes
3. Passed supernate through a 0.3 micron syringe filter
4. Decanted filtrate to a small vial
5. Pipetted 3 mL into a plastic cuvette
6. Measured the absorbance at 700 nm (This measurement will tell you if your sample is clear of turbidity - a value under 0.039 times the absorbance at 430 nm means your sample is acceptably turbidity free)
7. Measured the absorbance at 430 nm

Results:
A(700) = 0.005784
A(430) = 1.36414


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Old 04-04-2014, 03:51 PM   #2
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I recently corresponded with a guy in the lab at Alaskan Brewing because their listed SRMs are all about twice what their beers look like. Basically, he convinced me they really are measuring and reporting their SRMs correctly using equipment to measure light transmittance. I don't know what it means when their pale ale looks normal and is a 14, their red looks red and is 35, and their 1.065 stout is a seemingly impossible 119. Most breweries don't list their SRMs so I don't know how often this happens, but it certainly doesn't line up with style guidelines and color guides on paper as to what SRM looks like.


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Old 04-05-2014, 01:06 PM   #3
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Quote:
Originally Posted by swankyswede View Post
SRM = 12.7 * D * A, where D is the dilution factor (for an undiluted sample, D = 1, for a sample diluted 1:1 with deionized water, D = 2, etc.) and A is the absorbance at 430 nm.
Yes, that is the correct procedure (for EBC multiply by 25)

Quote:
Originally Posted by swankyswede View Post
When I ran my ESB through, I got an absorbance of 1.36, which resulted in an SRM value of 17.3 -- what? An SRM of 17.3 is something expected from a dark lager, not a golden ESB. However, an EBC value of 17.3 looks right on, and conversion to SRM using the formula SRM = EBC * 0.508 results in an SRM of 8.8, which is close to the color calculated by my brewing software, 9.5.
That's not atypical for an ESB. Here's a list of a few beers of about the same SRM. Note that Fullers is at 17.4 (the less significant figures are useless)

Mad Fox Molotov Hoptail 14.1732
Bitter 14.4145
Samuel Smith Winter Welcome 14.7828
Vienna 15.1638
O'Fest 15.1977
Lindemanns Kriek 15.2654
Anchor Steam 16.002
FdlrsElbow 16.3068
Boom Framb 16.637
Boom Kriek 17.399
Fuller's ESB 17.4498
Oktoberfest 18.0721
Speckled Hen 18.2118
Hercules Double IPA 18.3896
Negro Modello 18.796
Lindeman's Framboise 18.796


Quote:
Originally Posted by swankyswede View Post
Is Wikipedia wrong,
Don't think so, but then again I wrote that article.

Quote:
Originally Posted by swankyswede View Post
...or are my measurements incorrect?
Don't think so again. They seem reasonable to me.


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Originally Posted by swankyswede View Post
Does anyone have a reference for these measurements?
Yes. I have several. 1 (ASBC) and 3 (MEBAK) in the Wikipedia article are the accepted standard practices.


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Originally Posted by swankyswede View Post
For those interested, here is my procedure...
This all looks good to me assuming, of course, that the instrument has been checked for wavelength accuracy, linearity, stray light... Spectral bandwidth doesn't really have much to do with it, despite what the ASBC MOA says, because of the low pass nature of beer spectra. But out of curiosity, what is your instrument's bandwidth?


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Originally Posted by Orthobrewsky View Post
I recently corresponded with a guy in the lab at Alaskan Brewing because their listed SRMs are all about twice what their beers look like.
I am most intrigued by the comments made by both swankyswede and Orthobrewsky to the effect that they know what a given SRM looks like. I too felt that I could guess the SRM based on appearance but thought this strange as what you actually see when you look at a beer depends greatly on the path. A 5 SRM beer in a 10 cm wide glass has the same color (approximately) as a 10 SRM beer in a 5 cm wide glass. I found myself way off when I tried to guess. I had attributed the ability to tell to some sort of adaptation (for example the way we adapt to the different white points associated with different light sources). I think my opinion on this is changing.


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Originally Posted by Orthobrewsky View Post
Basically, he convinced me they really are measuring and reporting their SRMs correctly using equipment to measure light transmittance.
If they are following ASBC procedure then they are doing it correctly.


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Originally Posted by Orthobrewsky View Post
I don't know what it means when their pale ale looks normal and is a 14, their red looks red and is 35, and their 1.065 stout is a seemingly impossible 119.
You could hardly be expected to. Most people are not aware that the SRM actually conveys a lot of color information.

SRM 14 means that, in a 5 cm wide glass viewed under illuminant D65 the beer would have a luminance of about 37, a hue of about 59 ° and chroma ~ 74

35 means that, in a 5 cm wide glass viewed under illuminant D65 the beer would have a luma of about 9, a hue of about 26 ° and chroma ~ 35

Thus in going from 14 to 35 the beer gets darker, and redder.

119 implies, under the same conditions, that a luminance of 0.04, a hue of 10 ° (almost on the red axis) and chroma of 0.24 IOW, black. But if you put a very bright light behind it it would look pure red.


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Originally Posted by Orthobrewsky View Post
Most breweries don't list their SRMs so I don't know how often this happens, but it certainly doesn't line up with style guidelines and color guides on paper as to what SRM looks like.
The color guides on paper or film are an impossibility. The first problem,of course, is that color depends on SRM depth i.e. SRM*thickness_of_glass. The second problem is that you don't need many SRM-cm before you run out of gamut for computer displays, film dyes and CMYK inks. for example, even your SRM 14 beer in a 5 cm glass (a Nonix is 8 cm at the top), has a color which cannot be displayed on your computer monitor or printed on paper.

For example the color chart in the Wikipedia article at http://en.wikipedia.org/wiki/Standard_Reference_Method is way off. No mention is made of illuminant, observer, or path, all of which have an effect on color and with path it is a profound one.

For a beer at 2 SRM to have the brightness (luminance) of the 2 SRM patch in that article it would have to be viewed in a 1.8 cm wide glass (D65) and would have a* of -2 and b* of 20. The patch in the article has a* of -10 and b* of 78.

The SRM 6 patch in the chart has L* = 76.4, a*=1.7 and b* = 76
An average SRM 6 beer would have to be viewed (D65) in a 2.8 cm wide glass to have L* = 76 and would exhibit a* = 11 and b* = 63.

I have no idea where the colors in that chart came from but they aren't beer colors. Do any of us drink beer in 2.8 cm wide glasses (a Kölsch stange is 5 cm)?. I didn't take it out when I re-wrote the article because I knew they'd just put it back in. It is therefore little wonder, I suppose, that most people don't have a feel for the relationship between SRM and color.
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Old 04-05-2014, 01:09 PM   #4
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They sell key cards

http://www.beercolor.com/products.htm
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Old 04-05-2014, 01:36 PM   #5
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Yes, they do, but it appears that these may be leading people astray, for the reasons given in #3, more than they help. Other than a photometer (and I still convinced that I ought to be able to build one for under $100), however, there isn't really another option. I suppose home brewers could go back to the iodine scale. It should be more accurate than the cards/transparencies.
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Old 04-06-2014, 03:27 AM   #6
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I'm glad to see someone knowledgeable about this issue stepped in here because I'm very curious about this. Before I talked to the guy at Alaskan, I knew that SRM was measured with actual instruments and there would be some standard depth through which a standard intensity light passes. Of course it is true that beers look darker in thick containers (like a carboy).

However, for example, I've poured a SN Torpedo into a variety of glasses at various levels of daylight and I never think it looks terribly different. There are naturally color expectations in different styles and BJCP stipulates this in terms of SRM. How did BJCP set this up? Didn't they at some point take some beers typical of the expected color range and measure their SRM?

35 SRM is toward the upper end for an Irish stout and it is what BeerSmith computes for the standard Guinness clone recipe. Alaskan Brewery has a red ale and it looks normal in their picture, but has an SRM of 35. I don't know how they lit that pint glass, but it's hard for me to believe it really looks like Guinness. If Guinness really has an SRM of 70, why do both BJCP and BeerSmith think it has 35 SRM? Why are almost all of Alaskan's beers outside style guidelines on measured SRM? Their beers look normal, not just in their own website's pictures, but also in a variety of consumer photos on a Google image search. Also, they've won lots of awards.

Is it the case that two beers with the same SRM can look vastly different in pint glasses sitting on the same table? If so, I wonder what it is about those beers which cause them to look so different. From what I've read, the SRM machines use a very specific wavelength of light and my eyes are using whitish light. Is this the issue?
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Old 04-06-2014, 01:59 PM   #7
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Quote:
Originally Posted by Orthobrewsky View Post
However, for example, I've poured a SN Torpedo into a variety of glasses at various levels of daylight and I never think it looks terribly different.
That's because your eye adapts not only to the level of the light but it's color quality. Our color vision evolved such that we could tell a red (ripe) apple from a green (unripe) over wide extremes of illumination (fire-light, noon day sun, sunrise and sunset) as long as there is enough illumination for photopic (cone) vision. L*a*b* colors, such as the ones I mentioned in the previous post are calculated normalized to the 'color' of an object that reflects or transmits all wavelengths equally i.e. something white. If your eye can't find something white it averages over the whole scene and calls that neutral. You can find lots of examples of how this works is various places. Typically you will be shown a bunch of small squares of different colors inside larger squares of different colors and then, when you read the material, be told that all the small squares are actually the same color but appear different because the surrounds are different colors.

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Originally Posted by Orthobrewsky View Post
There are naturally color expectations in different styles and BJCP stipulates this in terms of SRM. How did BJCP set this up? Didn't they at some point take some beers typical of the expected color range and measure their SRM?
I really don't know how BJCP came up with their numbers. There are various places one can find lists of beers and their SRM ratings.


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Originally Posted by Orthobrewsky View Post
35 SRM is toward the upper end for an Irish stout and it is what BeerSmith computes for the standard Guinness clone recipe.
That seems quite low for an Irish stout. Most of mine come in at about 80 SRM. I have measured various Guinness products from 50 to 71.

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Originally Posted by Orthobrewsky View Post
Alaskan Brewery has a red ale and it looks normal in their picture, but has an SRM of 35. I don't know how they lit that pint glass, but it's hard for me to believe it really looks like Guinness.
A beer of 35 SRM doesn't transmit much light. In a 5 cm glass the luminance is going to be around 9-10 and in an 8 cm glass around 2 (you can think of those numbers as percentages - white object has L* = 100). As I mentioned in the previous post, the color of an SRM 35 beer is too pure to be represented photographically or on a TV or computer screen but not by much. There are various ways to 'clip' colors such as printing the highest chroma color available from the primaries (display) or inks (printed) that has the same hue. The fact that the beer looks normal in the photo is a testament to the photographers lighting and Photoshop skills.

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Originally Posted by Orthobrewsky View Post
If Guinness really has an SRM of 70, why do both BJCP and BeerSmith think it has 35 SRM? Why are almost all of Alaskan's beers outside style guidelines on measured SRM? Their beers look normal, not just in their own website's pictures, but also in a variety of consumer photos on a Google image search. Also, they've won lots of awards.
It's a simple matter for me to go to their website and measure the color depicted. It is miles away from what a 35 SRM beer is going to look like in a 8 cm (the width of a Nonix at the top) glass. The fact that I can see it on my computer screen says it is within the gamut of the Rec. 709 (ITU) primaries and it is - well within at x = 0.53, y = 0.34 implying that z = .13 which in turn says that this picture contains blue. It also contains green. The color of a 35 SRM beer in an 8 cm path is pure red of wavelength 625 nm with x = 0.74, y = 0.26, z = 0. That picture does not show the color of a beer. But you see the picture as normal. I can only think that this is because by experience you know what beer looks like. Put some of this beer in a glass and hold it next to the monitor and see if you think it looks normal then.

Another interesting aspect of this is that if I ask someone looking at a glass of beer like the one in the picture whether all the beer in it is the same color they say "Of course!" when inspection of the photo with your computers pixel color meter shows that the color is appreciably different between the top and bottom of the glass. Again I think it is the mind playing a trick on you. You know full well that it is the same beer throughout the glass and it must, therefore, be the same color. Now when you tell a person to take a careful look and then tell me whether the color in the thick and thinner parts of the glass are the same, they clearly see the difference.

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Originally Posted by Orthobrewsky View Post
Is it the case that two beers with the same SRM can look vastly different in pint glasses sitting on the same table? If so, I wonder what it is about those beers which cause them to look so different. From what I've read, the SRM machines use a very specific wavelength of light and my eyes are using whitish light. Is this the issue?
In a sense, yes. SRM is based on a single measurement of absorption at 430 nm. The thing that causes beers with the same SRM to look different is that the color you see depends not only on the beer's absorption at 430 nm but on the absorption at all the other wavelengths between 380 and 780 nm. To compute visible color a complete set of spectrum data is needed. Two beers with the same absorption at 430 nm but different spectrum shapes will appear to be different colors. An extreme example of this would be a lambic and an all malt beer. For this reason I don't record the colors of my beers by just the SRM but rather by what I call the 'Augmented SRM' which I write as, for example, SRM 12.38 [ -0.516, 0.104,-0.008]. The three numbers in brackets are measures of the extent to which the spectral shape for the beer deviates from the spectral shape for an average beer and with them I can reconstruct the spectrum of the beer and from that compute actual color.
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Old 04-07-2014, 07:38 PM   #8
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Quote:
Originally Posted by Orthobrewsky View Post
35 SRM is toward the upper end for an Irish stout and it is what BeerSmith computes for the standard Guinness clone recipe. Alaskan Brewery has a red ale and it looks normal in their picture, but has an SRM of 35. I don't know how they lit that pint glass, but it's hard for me to believe it really looks like Guinness. If Guinness really has an SRM of 70, why do both BJCP and BeerSmith think it has 35 SRM? Why are almost all of Alaskan's beers outside style guidelines on measured SRM? Their beers look normal, not just in their own website's pictures, but also in a variety of consumer photos on a Google image search. Also, they've won lots of awards.
Beer recipe software will generally derive its SRM values from the summation of dilution equations specifying Malt Color Units (MCU) and converted to SRM using some non-linear relationship equation such as Morey, Mosher or Daniels. What you have when you do this, is Morey SRM, Mosher SRM or Daniels SRM but you don't have an instrument measured SRM nor do you have a value that can be labeled Lovibond.

Contrast that to a measured value of SRM using the absorbance at 430nm with a certain path, which has a relationship to Lovibond through EBC. ( I believe the previous poster discusses some of that approach. )

The loose use of color units and their context leaves something to be desired. Compound this with the fact that most software is not using ASTM E-308 much less computing the transmission sprectrum, but rather using a simple SRM->RGB lookup table.
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Old 04-08-2014, 03:26 AM   #9
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Beer recipe software will generally derive its SRM values from the summation of dilution equations specifying Malt Color Units (MCU) and converted to SRM using some non-linear relationship equation such as Morey, Mosher or Daniels. What you have when you do this, is Morey SRM, Mosher SRM or Daniels SRM but you don't have an instrument measured SRM nor do you have a value that can be labeled Lovibond.
SRM is linearly dependent on the concentration of coloring materials in the brew because it is proportional to absorption. Beer-Lambert says

SRM = 12.7*A(430) = 12.7*1*sum_over_i(Ci*ei(430))

where Ci is the molar concentration of colorant i and ei(430) the molar extinction of i at 430 nm. The problem with programs that attempt to model color is they don't know Ci and ei. Ci depends not only on how the malt has been treated but on how the beer is brewed. Heat results in the production of colorants, yeast absorb some coloring material etc. Color estimation schemes can at best come up with a WAG.

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Contrast that to a measured value of SRM using the absorbance at 430nm with a certain path, which has a relationship to Lovibond through EBC. ( I believe the previous poster discusses some of that approach. )
SRM = 12.7*A(430); EBC = 25*A(430) with A(430) measured in 1 cm. The 12.7 factor was tweaked so that SRM numbers would agree with Lovibond numbers in the low part of the range.

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The loose use of color units and their context leaves something to be desired. Compound this with the fact that most software is not using ASTM E-308 much less computing the transmission sprectrum, but rather using a simple SRM->RGB lookup table.
E-308 is irrelevant to discussion of SRM and EBC color as they derive from one measurement at a single wavelength (430 nm). E-308 is the prescription for the computation of tristimulus (visible) colors in the CIE system and requires absorption data measured over the entire range of visible wavelengths (380 - 780 nm). ASBC does have an MOA (Beer-10c). Some breweries are using it but it is impractical if your spectrometer is not capable of automatic collection of the data set and easy transmission of it to a computer (as, of course, most modern instruments are). Some instruments will do the computation of at least X,Y and Z internally. These can be plugged into the formulas for L*, a* and b* (or L*, u' and v' or whatever you want).

SRM to RGB lookup is impossible without specifying path, illuminant and observer and, as I mentioned in a previous post, anything over about 40 SRM-cm (8 SRM in a 5 cm glass etc.) is too saturated to be displayed by the R, G and B primaries used in computer and TV displays.
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Old 04-08-2014, 02:13 PM   #10
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E-308 is irrelevant to discussion of SRM and EBC color as they derive from one measurement at a single wavelength (430 nm). E-308 is the prescription for the computation of tristimulus (visible) colors in the CIE system and requires absorption data measured over the entire range of visible wavelengths (380 - 780 nm). ASBC does have an MOA (Beer-10c). Some breweries are using it but it is impractical if your spectrometer is not capable of automatic collection of the data set and easy transmission of it to a computer (as, of course, most modern instruments are). Some instruments will do the computation of at least X,Y and Z internally. These can be plugged into the formulas for L*, a* and b* (or L*, u' and v' or whatever you want).

SRM to RGB lookup is impossible without specifying path, illuminant and observer and, as I mentioned in a previous post, anything over about 40 SRM-cm (8 SRM in a 5 cm glass etc.) is too saturated to be displayed by the R, G and B primaries used in computer and TV displays.
Right. I would guess most software isn't generating the transmission spectrum -> tristimulus values -> whatever color space.

You have to think non-scientific when it comes to most programs. A simple lookup table could be made by taking a chart from the internet, reading the RGB values with a paint program and associating them with the SRM value, perhaps interpolating between values.

1 -> 255, 255, 255
2 -> 255, 250, 230
etc...

The programmer just has to use the MCU -> SRM calculations and voila they have a crude "lookup" table.


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