When I released my own brew software, I really struggled with the equation to calculate beer colour. I spent hours pouring over the research and constructing known equations in an excel environment, and as it turns out, estimating beer colour is not so easy. Even with the given equation provided by Dan Morey, I could never come up with a SRM rating that matched the colour that I was actually getting, and if you’ve noticed, many other brewing software don’t get it right either.
Let’s take for example an IPA recipe that I brew. It has the following fermentables:
5.2 kg (11.46 lbs) of 2-Row @ 2 ºL
0.4 kg (0.88 lbs) of Light Munich @ º5L
0.4 kg (0.88 lbs) of Crystal 110 @ º42L
0.4 kg (0.88 lbs) of CaraRed @ º20L
Batch Volume = 5.55 gal
First, I calculate the Malt Colour Units (MCU):
2-Row MCUs = 2 × 11.46 ÷ 5.55 = 4.1
Light Munich = 5 × 0.88 ÷ 5.55 = 0.8
Crystal 110 = 42 × 0.88 ÷ 5.55 = 6.7
CaraRed = 20 × 0.88 ÷ 5.55 = 3.2
Total MCUs = 14.8
If we use the Morey equation, which seems to be the go-to equation, it goes as follows:Therefore:
I verified my calculations on the Brewer’s Friend SRM Calculator, and the result of my recipe resulted in close to the same number: 9.32 SRM.
But, the resulting calculated colour is a deep yellow-orange type of colour —not even close to the actual results. The colour of my beer resulted in something that was closer to 30ºL according to this colour chart below.
Comparing Other Equations
Other equation are slightly different, but the results are not much different.
Beer Colour — Looking at the Raw Data
As I was getting wildly poor results from the above equations when comparing them with colour charts, I was compelled to dig much deeper than gathering existing equations and methods of determining beer colour. I was happy to learn that the Standard Reference Method does employ some solid methods by measuring the amount of light of a specific wave length that passes through 1 cubic centimetre of beer. So, I’ve concluded that I can rely on the SRM.
But what exactly does 20 SRM value actually look like? Well, this is where some of the problems start. Beer charts interpret SRM colours very differently, and any of the tools or charts that you may have seen may not actually be very accurate to the science. After all my research, all that I was really able to determine is that the values given by various SRM calculators are meaningless unless they follow some kind of standard method of interpreting colours.
On top of that, grain lovibonds are still conducted using a visual comparison of a sample with tinted glass discs. As you already know, colour can look very different to different people and depending on what kind of light available at the time. Thus, the calculation of MCUs are skewed right from the start because malters use lovibonds to describe the colour of the grain. This makes it virtually impossible to predict the colour of your beer using the data provided with any degree of accuracy.
So, the conclusion is that there is no reliable way to calculate the real colour of beer using lovibonds as the basis of calculations. The only thing that I’m absolutely sure of is that the beer used in this example is definitely not yellow, and not even orange, but red in colour, and perhaps I will find away to determine real beer colour using some empirically gathered data.
Update — 11 April 2015
I continued my research and I found the following statements on Homebrewtalk.com that generally round out what my observations have been about estimating beer colour:
- The color values of the various malts are determined with a special laboratory mash procedure which is quite different from typical mashing procedures used in actual brewing.
- There is no accurate way to account for the color changing effects of inordinately short or long boils, abnormal water loss (or gain from dilution) in the copper.
- The effects of fermentation on color change cannot be accurately predicted.
- Wort and beer do not follow Beer’s law (which says that total light absorption at a particular wavelength is proportional to the concentration of dissolved material).
- The ASBC’s Standard Reference Method (SRM) value is, by itself, insufficient to specify beer color.
- There is no known hard correlation between MCUs and SRM.
Also, SRM is conducted on a 1 cm path sample, which means 1 cm beer thickness. Obviously, most of use will use much ticker glasses, the width of beer in that light must pass through will darken the perceived colour of the beer.
Beer thickness has a profound effect on the perceived color of beer. One can easily see this by looking at a Pilsner served in the traditional conical glass. At the top, where the light path through the beer is greatest, the beer will appear much redder than near the stem where the path is shortest. For example, consider a beer 5 cm of which passes 10% of blue light and 90% of red light so the ratio of red to blue light exiting will be 9:1. Doubling the path to 10 cm results in 10% of 10% of the blue light or 1% passing though and 90% of 90%, or 81% of the red light, so now the red-blue ratio is 81:1 and the beer appears much redder.
Update 1 May 2015
When all is said and done, I want to know the colour of my beer once it’s in my glass. I really don’t care what it looks like in a 1 cm vessel. My glass is at about 10 cm in diameter, so this certainly plays an important role why the colour of my beer is much darker than 9 SRM. Any calculation will have to take under consideration the diameter of the glass in which the beer is being served. The attenuation of light passing through a glass of beer is easy to calculate as you simply multiply the path factor (the depth) of the beer to the calculations. Thus the following beer colour calculation can be used to get a better approximation of what you’re beer will look like in your glass:
Given that a glass that is 5 cm in diameter.
The 47.5 SRM is a much closer colour match (a deep, dark, red colour) than 9.5 SRM was. So, if you really want to know what you’re been will look like in your glass, add a depth dimension to your calculations. You’ll also need to get a standardized colour chart that will match your findings. Again, these things are really all over the map, so you’ll have to find one that suits your needs. In the end, the only thing that matters is if your forecasts match your results.
Accurate Colour Reference
Based on the work by A.J. Delange, who is a contributor on Homebrewtalk.com, he generously shared a spreadsheet that transforms SRM values into an RGB colour. This give us the ability to actually see what a given SRM actually looks like. Using his spreadsheet, I was able to reconstruct a table similar to what he created that represents the SRM spectra of beer in an RGB colour space. Finally!… a chart that is reasonably accurate and based on some kind of scientific methods! The calculations discussed on Homebrewtalk.com are way above my skill and knowledge, but the Delange’s spreadsheet works great.
The following SRM colour chart is based on average standard daylight (illuminant C).
You can clearly see that 9 SRM is still a yellow or light orange in colour, but because the diameter of my glass is 5 cm, the colour actually darkened. At 10 cm diameter, the colour of my beer matches that in the picture at the beginning of this article. So there you go. Beer colour demystified! If you want to know the colour of your beer, multiply by the depth of the beer (the diameter of your serving glass) to get a better idea of the beer colour.