Decoction mashes are a form of step mash. The basic idea behind a decoction mash is that the temperature is raised each step by taking a small amount of the mash (the decoction), boiling it, and returning it to the main mash. Decoction mashing was invented by German brewers at the end of the 18th and the beginning of the 19th centuries and evolved because malts were often of inconsistent quality. Decoction enabled them to extract the maximum amount of sugars from the grain and produce malty, flavorful beers using measurements that did not require precision instruments.
Why the bother?
The obvious question is, why bother decocting if modern malts no longer require it and accurate thermometers are readily available? Of course, some homebrewers have an interest in history and traditional methods and like to recreate them. But the real answer lies in the flavor of the beer that results. Some people claim that it is only possible to achieve the full malty flavor of certain beer styles through decoction.
Primarily these are styles that emphasize malt in the flavor profile, including the majority of German styles, especially malty lagers such as bocks, Munich dunkel and Oktoberfest. Also, many of the wheat beer styles, such as Bavarian hefeweizen, have traditionally been decocted. These beers are not highly bittered, although some of them, such as Pilsners, have noticeable hop flavor and aroma.
Boiling the mash, as well as the wort, results in chemical changes. Many of these are complex and involve darkening and caramelizing of the sugars. They are known as Maillard reactions and are responsible for a variety of generally pleasing flavors in food, including roast meat, bread crust and caramel. Some of them already occur during the malting process, but decoction also seems to bring them out more completely than conventional infusion mashing.
Not everyone agrees. A number of brewing academics and professionals argue that with only minor changes to a recipe, there should be no appreciable differences between a decocted and an infused beer. In the interest of reducing time, labor and energy costs, quite a few German breweries have abandoned decoction mashing without a greatly noticeable lessening of flavor.
However, as homebrewers we are less interested in saving time and making a profit. Moreover, there is something to be said for traditional methods and styles. If you like a rich, full malty character, it’s worth trying a decoction mash and deciding for yourself if it results in better beer.
Although boiling of the wort is almost universally practiced, boiling of the mash may seem highly unconventional and raises technical questions. Extract brewers are warned not to boil any steeped specialty grains and a standard infusion mash, by definition, is never boiled. Excessive temperatures can leach tannins from grain husks, leading to a harsh astringency in the beer. Additionally, the amylase enzymes in the malt that convert starches to sugars are inactivated at temperatures above about 158 °F (70 °C) and can be destroyed by boiling.
Why then doesn’t decoction mashing lead to astringent beer and an enzyme-deficient mash? The answers lie in the chemistry of the mash. Only the thicker portion, containing more of the grain solids, is boiled. Malt is rather acidic and — in the thick, boiled portion — the pH is relatively low (about 5.0). This results in relatively few tannins being leached from the husks. As for enzyme denaturing, this is not a practical problem because most of the malt enzymes are dissolved in the mash liquid when the grain and hot water are mixed. In a decoction, the thin liquid in the unboiled portion contains more than sufficient enzymes to convert the starches, both in itself and in the boiled portion when it is returned to the main mash.
Consider a double decoction as a step mash with a protein rest, saccharification rest and mashout. A single decoction is analogous to a single infusion mash with mashout, and a triple decoction is a three-step mash (acid rest, protein rest, saccharification rest or protein, beta rest, alpha rest) and mashout.
Tools for the job
Decoction mashing requires more time (typically another 45–60 minutes for a double decoction) than a conventional infusion mash, and requires some additional equipment. There is also a certain amount of multi-tasking involved. Decoction is not recommended for beginning all-grain brewers; it’s best to have some experience and be relatively comfortable with the mashing process in general before attempting your first decoction.
The primary additional piece of equipment is a pot in which to boil the thick portion of the mash. It can be quite a bit smaller than the mash tun; for normal gravity beers (O.G. of 1.060 or below), 20 percent of the total batch size is a minimum. For example, for a five-gallon (19-L) recipe, a four-quart (3.75-L) pot would be the smallest you would want to use. A somewhat larger size would allow you to decoct higher gravity recipes. A thick bottom is highly desirable to prevent burning of the mash that is boiled.
You will also need a burner with good heat control. A typical large burner on a kitchen stovetop is adequate for 5–10 gallon (19–38 L) batches. On another burner, heat about 2 gallons (8 L) of water nearly to boiling, or you can add an additional volume of water to the vessel you use to heat sparge water. A large slotted spoon is an excellent tool for removing and returning the decocted portion of the mash.
Doing the deed
For a double decoction, mash in at 122 °F (50 °C). For a triple decoction, mash in as if for an acid rest at 104 °F (40 °C). For a single decoction, mash in at the saccharification temperature called for in the recipe, typically from 149–158 °F (65–70 °C). The thickness of the mash is usually slightly thicker than that of an infusion mash.
I like mashing in with a water to grain ratio of about 1.1 quarts of water per pound of grain (2.3 L per kg) for a double decoction. Calculate the temperature of the strike water based on your mash tun’s thermal characteristics and the temperature of the grain. This is typically 8–12 °F (4–7 °C) higher than the temperature of the rest.
Stir the grain and water well so that there are no dough balls. Allow the temperature to stabilize. If you measure the pH and make brewing salt or acid adjustments to the mash, now is the time to do so. At this point (about 5–10 minutes after mashing in), begin the decoction.
Remove approximately one-third of the total volume of the mash to the pot you will use to boil the decoction. It’s important that this be the thick portion of the mash; it should have the consistency of hot oatmeal you would serve for breakfast. What remains in the mash tun should be quite thin and soupy. Cover the mash tun and let it rest at the mash in temperature.
Bubble, toil and trouble
Place the decoction pot on the burner and apply medium-high heat. The goal is to heat the decoction evenly and at a rate of about 4–5 °F (2.5–3.1 °C) per minute. This will require frequent, but not constant, stirring. The reason a thick bottom is recommended is to reduce the chances of scorching the mash. As the temperature increases and the mash thickens, you may find that sticking is a problem. Add a large spoonful or two of nearly boiling water and continue stirring to thin the consistency as needed. This is a deviation from the traditional method, but is a practical solution to the problem of sticking.
Eventually the decoction will come to a boil. Adjust the heat to maintain a slow boil without burning. Stir frequently and add boiling water as necessary to prevent sticking. Maintain the boil for 25–30 minutes, at which time you can remove the pot from the heat. Uncover the mash tun and spoon the boiled portion back into the unboiled mash; stir until it is mixed well and take a temperature reading.
It is entirely possible that the temperature will not be as high as desired for the saccharification rest (or the protein rest or mashout if you are doing a triple decoction or single decoction). If not, merely add boiling water, stir and take readings until the temperature is correct. In the less likely event that the mash is too hot, add small amounts of cold water. Again, this is not a traditional technique, but it works on a homebrew scale.
Cover the mash tun once again and allow the enzymes to convert the starches in the mash to sugars for about 45 minutes. If you are doing a triple decoction, the rest will be 30 minutes for a protein rest, and about 10 minutes for mashout if a single decoction.
Once more with feeling
If you are double or triple decocting, again remove about one-third of the total volume. It should the thickest portion, although not quite as thick as the first decoction. Once again, bring the decocted portion to a boil. You can heat the decoction somewhat more quickly this time, stirring frequently and adding boiling water as necessary to avoid sticking. Second and third decoctions should be boiled for about 20 minutes.
Return the decoction to the mash tun at the end of the boil. If this is the final decoction, the target temperature is 168–170 °F (75–76 °C) for mashing out, which helps ease sparging and facilitates rinsing of the sugars from the mash. If the mash temperature is too low or too high, again add boiling or cold water as necessary. Normally 10 minutes is sufficient for mashout, at which time you can begin recirculation, runoff, sparging and boiling of the wort as you would with any mash.
The first time you decoct it may seem like a lot of juggling and time pressure, but the process soon becomes very logical and straightforward. The extra investment in time and effort will be very rewarding when you savor the rich, full malty character of your beer and feel a kinship with the brewers who developed this traditional technique nearly 200 years ago.
Doctor Decoctor’s Doppelbock
(5 gallons/19 L, all grain)
OG = 1.078 FG = 1.020
IBU = 28 SRM = 14 ABV = 7.4%
13.2 lbs. (6.0 kg) Munich malt
3.4 lbs. (1.5 kg) Pilsner malt
0.6 lbs. (0.27 kg) Carahell Dark malt
6.75 AAU Northern Brewer hops (0.75 oz./21 g of 9% alpha cids)
3.4 AAU Hallertauer Mittelfrüh hops (0.75 oz./21 g of 4.5% alpha acids)
1.0 tsp. Irish moss (15 min.)
Wyeast 2206 (Bavarian Lager) or White Labs WLP830 (German Lager) yeast (2-qt. (1.9-L) starter, minimum)
2/3 cup corn sugar (for priming)
Step by Step
Mash in grains with 4.7 gallons (18 L) of water to 123 °F (51 °C). After the mash has rested 5 minutes, pull your first, thick decoction and bring it to a boil. Boil the decoction for 30 minutes while the main, thin portion of the mash continues to rest at 123 °F (51 °C). Add the decoction back to the main mash to boost the temperature to 158 °F (70 °C). Let the mash rest for 45 minutes, then pull your second decoction. Boil this decoction for 20 minutes while the main mash continues to rest at 158 °F (70 °C). Add the second decoction back to the main mash for the mashout to 168 °F (76 °C). Boil wort for 115 minutes, adding bittering hops with 60 minutes remaining and flavor hops with 20 minutes remaining in the boil. Primary fermentation should last approximately 18 days at 52 °F (11 °C). Lager for 12 weeks at 38 °F (3.3 °C) Prime with 80 grams (about 2/3 cup) corn sugar for 2.3 volumes CO2.