Why doesn't beer color provide a better guide for brewing water adjustments?
Many brewers know that darker beers generally need higher Residual Alkalinity and paler beers need less as indicated in Figure 1 below. Although simple concepts like Residual Alkalinity (a function of water hardness and alkalinity) help a brewer grasp the need to adjust their brewing water, the processes that affect mash pH are more complicated than suggested by beer color alone. The malts and grains added to a mash contribute varying degrees of acidity to the mash. Generally, the amount of acidity contributed by a malt or grain increases as its color increases. Darker kilned malts contribute more acidity than paler malts. Beer color, alone, is not an accurate indicator of what Residual Alkalinity the brewer's water should provide to produce a desirable mash pH.
Figure 1 Residual Alkalinity Chart (from work by AJ Delange)
The difficulty in using beer color to define an appropriate Residual Alkalinity for brewing water is largely due to the acidity of Crystal Malts. The higher acidity per color unit of Crystal Malt has the effect of boosting the overall acidity of a grist out of proportion to the beer color produced. Roast Malts such as roast barley, chocolate malt, or black patent provide a relatively consistent acidity contribution that generally remains in proportion to the color they deliver to the beer.
Figure 2 presents examples of how widely the amount of malt acidity can vary while still producing the same beer color. All the results were simulated using Bru'n Water by using identical malt quantities and water to grist ratios. The impact of Crystal Malts on the overall mash acidity should be apparent in the figure below.
To illustrate this variation, Figure 2 presents four differing grist compositions that show how the acidity of a mash can vary even though their beer color is similar. A grist with only base malt and a portion of roast malt has a relatively linear increase in total malt acidity with increasing color (that is shown as the Red line in graph). A grist that includes increasing percentage of dark crystal malt presents rapidly increasing total malt acidity (Blue line in graph). Adding a portion of crystal malt tends to bump the malt acidity up without an increase in color. The figure presents malt acidity results when 10 percent (Purple line) or 20 percent (Green line) Crystal content is added to a grist. Figure 2 below shows that there is a definite increase in total malt acidity of a grist as the crystal percentage increases. Interestingly, the total malt acidity for these Crystal malt percentages tend to parallel the roast malt line as the color exceeds 20 SRM. This is because more roast malt is added to the grist to achieve the darker beer color. Fortunately, most brewers use darker, roasted malt and grain to produce the deeper color they desire in their beer.
Figure 2 - Beer Color vs Malt Acidity
The Blue line extending through the top of the graph presents how the malt acidity rises if a brewer added ONLY 150L Crystal to the base malt to add more color to the wort (you shouldn't do this in practice since the percentage of Crystal malt in a grist should generally be kept under 20 percent). That Blue 150L Crystal line may represent a worst-case example of what total malt acidity could be.
An explanation follows as to how the various results in Figure 2 were calculated. For the Base Malt with Roast and the Base malt with 150L Crystal malt, the color of the grist was increased by increasing the percentage of the colored malt while keeping the overall weight of the grist constant (in this case, 8 lbs total for a 5 gal batch).
In the case of the 10 and 20 percent Crystal malt grists, the weight of crystal malt was held constant at either 10 or 20 percent, respectively. The color rating (L) of the crystal malt was increased to produce the increased beer color. As evidenced by the 150L crystal line, it becomes infeasible to increase the beer color with only crystal malts (line becomes vertical) at higher beer color. Therefore, roast malt (500L) was added to the grist to produce the beer color greater than 20 SRM. That is why the 10 and 20 percent crystal lines become parallel to the roast line when the beer color exceeds 20 SRM.
So as exhibited in Figure 2, the acidity of mashes that produce the same color beer can vary significantly. To neutralize the malt acidity and produce the desired mash pH, the mashing water must have the proper alkalinity. Basing the mashing water alkalinity (or Residual Alkalinity) on only beer color will not always produce a desirable mash pH.
These are reasons why Bru'n Water includes advanced analytical measures to help the brewer get their mash pH right, the first time.
