Back to Ask the Brewmaster.

This month's question: Last month you said a decoction mash will add rich malty melanoidin flavors to my beer. How do I do a decoction mash?

Answer: A decoction mash, quite simply, is one where you take a portion of the mash and bring it up to a boil. Once the decoction is completed, mix the boiled mash back in with the portion of the mash that was not boiled. The combined temperature then raises the temperature of the whole mash tun up to the next target temperature. Make sure you have an extra boiling pot and some extra time before you start

Historically, decoction mashes were invented as a simple means of achieving temperature control before thermometers were invented. Since boiling always occurs at the same temperature, brewers discovered that all they had to do was to make sure the volume they decocted every time was the same. Using this methodology, they could hold their mash temperature within a few degrees without any temperature measurement at all. Various breweries making various beer styles came up with their own profiles, depending on the style color, residual sweetness, mineral content of the water and degree of modification of the grain they were using. Some of the profiles were pretty elaborate.

When you plan out a decoction profile, the first step is to select the temperature steps you want to hit. Here is a list you might consider. Note that most mashes don't need all of the steps:

  1. Acid Rest, typically at 95°F. Used with undermodified malts and water with low calcium content. Recommended for Bohemian Pilsner.
  2. Protein rest, typically at 120°F. Improves head retention and clarity. Good for most decoction styles.
  3. Low temperature saccharification, typically at 149°F. Makes for highly fermentable wort and a dry finish. Good for German Pils.
  4. High temperature saccharification, typically at 155°F. Makes for less fermentable wort and a sweet finish. Good for Oktoberfest.
  5. Mash-out typically at 170°F. Stops all enzyme action and allows sparging to be more efficient. Good for most decoction styles.

For most styles, hitting 120°F, 153°F and 170°F works well using a double decoction, which I'll describe here. There are other methods, including combined infusion-decoction methods which work well too.

Start the mash by doughing in with hot water, just like a typical infusion mash. For example, if you want to start with a protein rest at 120°F, and you use 1½ quarts per pound of grain, heat the water to 133°F. Once you have doughed in, take careful note of the mash volume. This is easy if you have volume marks on your mash-tun like Gott coolers have. If not, measure the mash depth with a ruler.
Then, you need to figure out how much to decoct to get to the next temperature step. Suppose you want saccharification at 153°F. You need to work out a weighted average calculation, knowing that the boiling decoction will be at 212°F. It goes like this:

Decoction Volume = Sacc. temp - Protein temp = 153 - 120 = 33 = 0.36
                                        212 - Protein temp        212 - 120    92

Note that 0.36 in this case is the volume fraction (or depth fraction) that needs to be heated. For example, if your mash has a volume of 4 gallons, you need to heat 4 x 0.36 = 1.44 gallons. You'll find that the volume usually works out to about 1/3, or just a bit more.

Now comes the actual decoction. Scoop out a portion of the mash (in this case 1.4 gallons) and put in a boiling kettle. Try to get the thicker part, since the enzymes quickly end up in the liquid phase and you don't want to heat those. Heat the mash to 150°F, while maintaining constant stirring. Make sure you get to the bottom of the kettle. Otherwise it will scorch, leaving you with a foul burnt-smelling wort. When you get to150°F, shut off the heat. This will allow the starch enzymes to break down sugars. Hold this temperature for 15 minutes. Then resume heating until the mash comes to a boil. Whenever the heat is on, keep stirring and don't stop. Continue to boil for an additional 15 minutes. Note the rich malty aroma of the steam while your are stirring. When 15 minutes is up, quickly dump the boiled mash back into the unheated mash. Stir it up good and check the temperature. It should be right on target (153°F in this example). If it is a bit low, just scoop out a few quarts and quickly heat it to boiling. Dump it back in and recheck the temperature once again.

For a double-decoction, repeat the above calculation using the mash-out and saccharification temperatures. For example,

Second Decoction = 170-153 = 17 = 0.29
                                212-153     59

Again, scoop out thick mash and heat, this time .29 x 4 = 1.16 gallons. For the final decoction, you can go straight to boiling, since the starches are already converted. Keep stirring! Boil for 15 minutes, and then return it to the main mash. Stir it back in and your mash should magically be at 170°F. After a 10-minute mash-out rest, you can begin run-off and sparging just like any ordinary mash.
Reading through this, it seems like quite a bit of extra work. However, you'll find that adding at least one decoction to your mash process will make your resulting beer tastes a lot more authentic.