Have you ever tasted a stout that had the harsh edge of truck-stop coffee? Stout, porter, Schwarz bier, black IPA…these styles rely on roast, black, and chocolate malts for color and character, but you can get too much of a good thing. Or rather, you can get some bad with the good. If your water chemistry is off, or your crush is too fine, or if your sparge goes on a bit long, it’s easy to extract a lot of tannins and acrid flavors along with the pleasant qualities of those malts.
One way to tone down the harshness is mash capping. The basic idea is that you leave the dark malts out of your mash until you’re ready to vorlauf or start your sparge. This reduces the amount of time for leaching out tannins.
Mash Capping 101
Mash capping is fairly straightforward. You divide the grain, mash the bulk of the grain as usual, then grind the specialty malts and soak them shortly before adding them to the rest of the mash. Then you can recirculate (vorlauf) or begin your sparge.