It has come to my attention that there is a potential problem with the Rule of Thumb that exists for Acid Malt usage. Acid malt is also known as Acidulated Malt or Saurermalt. That ROT is: Each 1% acid malt by weight added to a grist will drop the mash pH by about 0.1 unit.
Since acid malt contains lactic acid, adding acid malt to the grist is just like adding a few drops of lactic acid per pound of malt in the grist. Acid malt is typically used when the brewing water's alkalinity is too high to allow the mash pH to drop into the desired range. Under typical usage, its only added when a check of the mash pH shows that the pH has not dropped enough. Under this usage, the ROT should be fairly effective.
The problem comes when programs like Bru'n Water enable the brewer to predict mash pH prior to actually conducting the mash. The prediction indicates that the mash pH will be high and the brewer plans on an acid malt addition. Part of the problem stems from the variability in acid content of various acid malt products. Apparently Weyermann acid malt is soaked in lactic acid solution for several days while other maltsters just spray a lactic acid solution onto the grain. It appears that the quantity of lactic acid in acid malt products is generally between 2 and 3 % by weight. Bru'n Water assumes that the acid malt used has the higher 3% by weight lactic acid content.
Knowing that a mash pH problem may be looming can be helpful, but brewers need to use caution when adding acid malt or acid directly to the mash based on a program prediction. If the mash water alkalinity and residual alkalinity are low, the ROT for acid malt effect may not hold. In that case, the alkalinity that is moderating the pH drop may have been consumed and minor acid additions can have a larger than expected pH drop.
This pH effect is similar to the fact that adding a drop of acid to a fixed quantity of water will generally produce a somewhat consistent pH reduction with each drop. But as the water's alkalinity is used up, each drop of acid has a progressively increasing pH drop (pH falls off a cliff).