05-04-2008, 07:41 AM
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SRM Calculations in ProMash
Cruising ProMash help menus and found this...i hadn't really worried too much about color up to this point...maybe that's the next thing i should work on.
thought others might find this useful:
Morey, Mosher and Daniels - SRM Calculations
Some background data points:
1) Real SRM and EBC ratings are taken through a spectrometer, not calculated. The spectrometer measures color absorption to 99.9% accuracy.
2) The basic element for estimating color in beer is the Malt Color Unit, or MCU.
3) There is NO known hard correlation between MCU's and SRM.
Found on the Brewery Website.
"Beer's Law" - Dan A. Morey
It has been well documented that beer color is not a linear scale. So why try to relate Malt Color Units to an estimated SRM by a linear equation? I propose a power equation would better approximate the color phenomenon.
What I did was: Assume MCU = SRM from 0 to 10.5, Daniel's equation holds in the range of 10.5 < MCU < 37, and Mosher's formula holds for MCU < 37. Then I plotted In (MCU) vs. In (SRM) and performed linear regression. This exercise resulted in a R^2 value of 0.97 (This is better than my linear fit with force zero intercept on the MCU vs. SRM graph - R^2 0.96). My resulting equation is:
SRM = 1.4922 [(MCU) ^ 0.6859] - for values of SRM < 50
Now there is a single equation that can be used to estimate the final color of beer. Also, in the age of calculators and computers, it is no more difficult to calculate than a linear equation.
Mosher, Daniels Formula:
From "Beer Color Demystified" - Part III. Ray Daniels, Brewing Techniques Vol. 3, #6 - November/December 1995 Issue.
1) Calculate MCU, which is basically:
MCU = (Lovibond rating x pounds) / gallons.
Each grain in the grist gets this number calculated, the totals are added, and we get a final MCU number.
2) Randy Mosher compared commercial beers with known rating to his own, and came to the conclusion that SRM could be approximated using the formula:
SRM = (MCU x 0.3) + 4.7
Although not perfect, when graphically plotted his scale runs right through the middle of the data set.
3) In Ray Daniels 'Beer Color Demystified' (see reference in ProMash), Ray does further refinement on Mosher's equation and stated that SRM is approx:
SRM = (MCU * 0.2) + 8.4
and when graphically plotted, looks a tad better than Mosher's equation.
A Technical Note On Beer Color, Color Swatches and Color Comparison in ProMash
ProMash produces color patches based upon a calculated SRM color value which is determined from the color values of the malts which make up the grist (Details of formulas ProMash can use for calculating color).
The calculated SRM values should be viewed as approximations for the following reasons:
1) 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.
2) 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.
3) The effects of fermentation on color change cannot be accurately predicted.
4) 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).
5) The ASBC's Standard Reference Method (SRM) value is, by itself, insufficient to specify beer color.
SRM measurement requires that the beer being measured have "average" spectral absorption characteristics. Given that this requirement has been satisfied, the SRM color is 10 times the absorbance of one-half inch of beer at 430 nm. To calculate color from the SRM value and present the color on the screen of your computer requires several assumptions:
1) The beer does have the average spectral characteristics required for a valid SRM computation.
2) The beer is being viewed by transmitted light which has the color characteristics of "Illuminant C" (which resemble mid day northern light).
3) The thickness of the beer being viewed is 5 cm (the same assumption as is required by the Davidson Guide based on the diameter of the base of the typical cup used in homebrew competitions).
4) The user's computer monitor is "profiled" to accurately present the calculated colors to the viewer.
Since most beers (with the exceptions of fruit beers) have nominally the same spectral characteristics, the eye compensates for differences in illuminant spectral characteristics to some extent, and most computer monitors are at least approximately calibrated, the ProMash color patches can be used to roughly compare the colors of beers provided the 5 cm rule is not violated.
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.
Thus, to compare beers to the ProMash color patches (and especially to each other), it is important to use a container that is close to 5 cm (2 inches) in diameter. If possible, use the light from a north-facing window and try to do the comparisons around noon. Even so, do not be surprised if the colors seen on the screen do not match exactly to the beer in your hand. Concentrate on the lightness or darkness of the beer rather than on its hue.