How to do a Decoction
Decoctions have achieved this sort of mythical status among homebrewers, likely because the vast majority of commercial operations haven't decocted in years. For all intents and purposes, aside from some extremely small Czech and German breweries, it's a mash profile that has been phased out in favour of processes that are gentler on the mash. Yet despite the fact that other mash profiles are objectively better for the vast majority of malt on the market, decoctions retain their cachet.
Anyone who's followed me for any length of time knows that I do a lot of decocting in my own brewing and aside from yeast, it's the topic I get questioned on the most often. For a mash profile that has such a reputation, there is a surprising lack of information on exactly how to conduct one. Hopefully I can rectify that on my little corner of the internet.
So you want to do a decoction
Before you even begin to do a decoction, there are some prerequisites. As mentioned previously, most malt on the market is crafted to perform well in a single infusion mash tuns which make up the majority of commercial operations. For malt to perform properly in a decoction, you need malt that has been less well modified. Less well modified malt has more starch and less fermentable sugar, as well as high diastatic power and protein compared to well modified malt. I like using Weyermann's Floor Malted Bohemian Pilsner for decoctions as it was designed for this. Using well modified malt such as North American 2 row, pale malts or even most Pilsner malts will result in some issues that will become apparent as we move on.
You'll also need the right equipment. You'll need a separate kettle and a separate heating source capable of getting your mash to boiling temperatures. You'll also need a way to get a portion of mash from one pot to another and back again - and be prepared to make a rather large mess.
The process of decocting
Decoctions come from the era before thermometers which means that the only things you could accurately measure were weights, volumes, time and whether something was boiling or not. Through trial and error, brewers would write out the amount of boiling liquor to add to the mash, how much to remove for how long and for how many times until they got the best product. We have copies of these recipes, though they don't do us much good as there was wide regional variation in weights and measures. Thankfully, we do have thermometers and we understand the enzymes that need to be activated and inactivated and the temperature ranges in which that happens, so let's get to it.
Step 0 - dough-in
Dough-in is simply the addition of your strike water to your mash (I prefer 1:1 qts/lbs, but 1.5/1 is traditional). Often this initiates the acid rest, though most move directly to the protein rest. I recommend skipping the acid rest for most beers as it tends to augment ferrulic acid, which in turn becomes a clove-like phenolic flavour. For Hefeweizens, that may be desirable, but generally not so much in lagers. Unless your water is already very close to pH neutral, an acid rest also won't acidify the mash so it's largely useless otherwise, unless you're making a Bohemian Pilsner with pH 7.0 water.
If you do chose to do an acid rest, aim for 113F and if not, head straight to 122F for the protein rest.
Your protein rest will synthesise enzymes de novo and greatly augment your diastatic power. This is important because you're going to greatly reduce your diastatic power as we move on. Your protein rest should last about 30 minutes. I've seen some profiles that use a 60 minute protein rest, but unless you're using chit malt, 60 minutes is overkill.
At this point, you should prepare yourself for your first decoction. Take a pH reading and make sure the mash is at 5.2-5.4. The closer to 5.2, the better because of the LOX enzyme which binds oxygen to lipids and releases it later, causing cardboard flavours later on and the lower the pH, the less active it is. The process of moving mash from the tun to the decoction kettle is also going to oxidise it, giving further reason to keep that pH in line.
Move about a third of the thickest portion of the mash into the decoction kettle. Raise the temperature in the decoction kettle to 156F and then hold it for 15 minutes. This will give the enzymes in that portion of the mash a chance to liquify the starches and provide you with a boost to your overall efficiency. Once those 15 minutes are up, head straight to the boil.
How long do you boil for? There's no right answer, which often frustrates new brewers looking to decoct. I prefer 10 minutes for something like a Pilsner to give it a nice, golden hue and something like 20-25 minutes for a doppelbock. The choice, is ultimately yours.
Once the boil has finished, move the decocted portion back into the main mash to raise the temperature to 148F, or your saccharification range. Many brewers find that after doing this step, the mash doesn't get to the right temperature - it's either too high or too low. This is unfortunately where experience comes in. There are calculators that exist which tell you exactly how much mash to remove and return to raise the temperatures, but they're based on theoreticals and aren't always accurate. Often, they'll tell you to remove much more than 1/3rd, which I don't recommend as it will destroy too many enzymes. It's also why I prefer the 1:1 qts/lbs ratio as it makes it easier to hit those temperatures, in addition to providing additional room to sparge and increase efficiency.
You're only going to hold your mash here for 15 minutes, which is quite different than any other mash profile. The reason is that the protein rest has created so many enzymes and the decocted portion has gelatinised so much starch that it won't take very long to get to work. If you leave it longer than 15 minutes and you've done everything else right, your beer will end up drier than you expect. Decoct another third, skipping the 15 minute rest at 156, and return it to the mash tun to raise it to 156F.
156F is your liquifaction range. This is where all of the starch is broken up into dextrins adding body and sweetness, and where all of the previous steps will pay off. I routinely find that my mash hits above 30 degrees Plato at this point, which is far beyond what a single infusion will get me (usually around 21, 22). Rest for 15 minutes again and then perform your last decoction as before, raising the temperature to 168 once you return it. Vorlauf and sparge, proceeding as you would with any other brew.
There are a couple of things to remember about decoctions at this stage. First, you've just done a bunch of stuff that most "how to brew" books tell you not to do. You've horribly abused your mash, releasing a bunch of lipids and beta glucans which can lead to a very slow runoff. Lauter with caution. Your runoff will most likely be very cloudy because of this. It's a good idea to let your mash sit for a bit longer at 168 just so that the bed can float properly. Be patient.
If your lauter does stick, stop, gently rake your mash and let it sit before taking a few quarts to vorlauf again.
If you're using less well modified malt, you're likely also going to need to boil for longer - 90 - 120 minutes in order to reduce any DMS. This will take care of a lot of the excess proteins that will likely have made it into your kettle and your wort should be clear by the end.
Take heart if it doesn't come out perfectly the first time. Decocting is not an exact science! In fact, it exists because of a lack of exact science at the time. But if you follow this advice and only decoct with proper malt and equipment, you'll experience a great deal more success than otherwise.