Mashing the Perfect Sweet Stout
Creating the perfect water profile and mash for a sweet stout is an exercise in contradiction. You want the beer to be very dark, sweet, malty, and full-bodied. It should have some roasted and toasty notes without astringency. To me, that translates to the following information concerning the mash and water profile: Keep the pH of the mash fairly high, around 5.6 (at room temperature). Keep the Chloride to Sulfate ratio high, around 3 to 1. Ensure Chloride and Sodium are both above 100, but below 150. Keep the temperature of the mash up around 158°F. It also means a thicker mash of around 1.25 quarts of water per pound of grain.
Where does the contradiction come in? Mostly with the grain bill. You’re looking at 10-15% of your grist being dark, roasted malt, which will drive the pH of the mash down significantly. It’s quite difficult to get a pH of more than 5.2-5.3 (at room temperature) without ending up with too much of something in the final beer (like sodium or bicarbonates). The reason I want to keep the pH up in the 5.4-5.6 range is because I use diastatically weak base malts for sweet stouts (like Munich or Maris Otter), and it’s better for enzymatic activity in that range. It’s also theoretically going to favor alpha amylase in this higher range, while beta amylase is favored a bit lower. Favoring alpha amylase is the same reason I keep the mash temperature up high at around 158°F.
The higher the Chloride to Sulfate ratio, the more malty the final beer is going to be. You need about 100ppm of Chloride before it has significant impact, and the same generally goes for Sodium (which rounds out the beer at that level, and I find that desirable in a sweet stout). You also, of course, need at least 50ppm of Calcium. The problem with the Calcium is that it lowers pH, but you really don’t want too much Bicarbonate in beer (some people believe it creates undesirable flavors, even though a lot of it will precipitate out, so I err on the side of caution here).
Okay, I think that’s enough of the reasoning behind the numbers I aim for with a sweet stout. On to the solution(s).
The first solution I came up with was to only worry about pH during the mash, and add all of the other minerals to the water after the mash. So, I’d add Sodium Bicarbonate and/or Calcium Carbonate during the mash to raise pH, then Calcium Chloride and Magnesium (or Calcium) Sulfate after the mash. Truthfully, this seemed to work out pretty well (and A.J. deLange corroborated), but I believe there is a better, and easier, way.
Don’t mash your roasted-and-kilned grains (e.g. chocolate and black malts). Roasted grains will drive the pH down considerably, so it’s difficult to keep the pH high no matter what fancy solution you use during the mash. Roasted grains have the wonderful benefit of not needing to be mashed. So, the best solution, in my opinion, is to mash everything except for your roasted grains in your MLT, and steep your roasted grains in a separate vessel (below 170 F) simultaneously at around 2 quarts per pound. Then, combine the wort created by the roasted grains with the mashed wort in the brew kettle.
If you don’t want to steep the grains, you can essentially brew a coffee with the dark grains with either a more traditional method (heat) or you can cold brew it overnight to really avoid the astringency as much as possible… it’ll just take longer. If you cold brew, you should probably bring the temperature of the concoction to 170 F after removing the grain to pasteurize it.
Then, you can add the coffee-like brew whenever you want (start of the boil, end of the boil, directly in the fermentation vessel, even just before bottling). All will impart different character, so experiment!
Note that you might not get full extraction from all roasted grains when steeping. According to some experiments run by John Palmer, it looks like Black Patent and Roasted Barley are some of the only roasted malts that have the same yield as mashing when steeped. As such, you might only exclude those from your sweet stout mash, or you can increase the amount of other grains accordingly (e.g. multiply the ounces of Chocolate Malt you use by ~1.5-1.6 to make up for the difference).
I would not recommend sparging with the roasted wort, in part because you’re going to impact the sparge pH pretty significantly, and in part because you’re going to leave some of your flavors behind.
Hitting the ideal concentrations of all ions in the brewing water as well as the ideal pH is very easy when you leave out your roasted malts (and any other malts that don’t need to be mashed, such as caramel/crystal). By steeping the roasted malts (and, optionally, your crystal malts) separate from the mash, you might end up with a much better sweet stout in the end. As someone commented, you could also steep the grains in a bag while you transfer to the boil kettle from your MLT, which sounds like a great idea.