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.
1. Split the malt bill and adjust the recipe
The first step is to split your malt bill into two parts. The first should include all the diastatic malts that require a full mash to convert starches to sugars. The second should include the dark specialty malts. You could also add other specialty grains such as crystal malt or Victory malt, but those can stay in the first part if you want. Note that, because this malt will be added at the end of the mash, you’ll need to adjust your quantities upward to compensate. Start with double the original weight; you can always go higher next time if you don’t get the color and flavor you expect.
2. Mash the First Part Normally
Set the second part aside and mash the larger first part of the grain bill as usual. It doesn’t matter whether this is a step mash or single-step infusion mash.
3. Prepare the Reserved Portion of Specialty Malts
While the mash is underway, you can mill the specialty malts and steep them in a little warm water. You should aim for a finer crush than normal. As with the recipe adjustment, the finer crush will help you get the color and flavor you want while compensating for the shorter contact time.
4. Cap the Mash
Once you’re ready to vorlauf or begin your sparge, it’s time to add the steeped specialty grains to the top of the mash. If you don’t normally do a vorlauf, I’d recommend it in this case because it will help you get the most from the grain addition. Also, remember to account for the added grain volume when you start your sparge, regardless whether you fly sparge or batch sparge.
5. Adjust for Next Time
Any time you tackle a new process, remember to track what you did and evaluate the results. If your first attempt seemed too mild or diluted, increase the amount of dark grains for the next time. This seems to be the most common issue that brewers have with mash capping. On the other hand, if the result still seems harsher than you want, try a coarser crush or decrease the amount of dark malts in the recipe.
Final Thoughts
Mash capping is good for more than just making a smoother stout. It’s also helpful for parti-gyle brewing. This is when you use the same mash to make multiple beers. If you don’t want the beers to share the exact same malt bill, you can use mash capping to change the character of the second wort. This would make it possible to brew both a Belgian golden strong and a Schwarz bier with the same base mash, by capping the mash with dark malts before collecting the second runnings.
If mash capping doesn’t give you the results you hoped for, there is another promising alternative. In the book Brewing Better Beer, Gordon Strong has expanded on work that Mary Anne Gruber from Briess Malting did with cold-steeping dark malts. The recommendation is that you steep the dark malts in room-temperature water for twenty-four hours, then add the liquid to the boil during the last five minutes or so. This approach favors the more pleasant simple melanoidins while leaving behind most of the harsher flavors.