Decoction mashing is a form of temperature-controlled mashing. Unlike the more common form of infusion mashing, portions of the mash are directly heated to boiling and returned to the rest of the mash to achieve the appropriate temperature rests. There are many different variations of decoction mashing that are specifically tailored to achieve different flavor and body characteristics of the finished beer.

• Decoction mashing is the only way to achieve the traditional malt flavor/aroma found in beers such as Bock, Marzen, Weizen, and Pilsner.
• Decoction mashing enhances body and head retention.
• Decoction mashing improves extract efficiency (especially with low modified, or low enzyme strength malts).
• Decoction mashing reduces the likelihood of a set mash (especially important when brewing with wheat or rye).
• Decoction mashing reduces the likelihood of chill haze in the finished beer.
• Boiling part of the mash removes oxygen, thus theoretically reducing the risk of hot side aeration (that is, if you are careful when adding the decoct back to the rest mash).
  

• It makes for a very long brew day.
• You run a higher risk of scorching your mash.
• The intensive decoction process can extract tannins from 6-row barley.
• If handled improperly, you run a higher risk of hot side aeration (splashing when decoct is added back to the rest mash).
• You could potentially release unconverted starch in the final decoct that would carry over to the boiling kettle and reduce the stability of the finished beer.
  

Bock, Pilsner, Vienna, Marzen, Weizen, Roggen (German Rye), Schwarzbier, Dortmunder, Munich lagers, some Alts, some Koelsch, and some light Belgian ales. – All can be made using a 1-3 decoction mash.

The boiling process bursts cell walls and other particles that had previously trapped some starch from enzymes. These starches can be converted the next time they are in the alpha-beta-amylase range by the enzymes in the rest mash, resulting in higher efficiency. Boiling dissolves protein gum (chill-haze former and cause of stuck mash) which can then be broken down into albumins (this is what gives beer body and head retention)!!! The break down of protein gum also means there is less trub precipitated in the boil, resulting in clearer beer. Boiling also produces melanoidins, which serves to darken the beer and give it flavor. Lastly, boiling reduces the amount of oxygen in the liquid, thus reducing the likelihood of hot-side aeration.

I will go over the most complicated decoction, the triple decoction, to explain what happens at each step and why it is done. From this, you can decide if the beer you are brewing requires this step or not, and either perform the step or cut it out.

Traditionally, grain and water are mixed together at a temperature around 58. This lower temperature is used to prevent the starch from balling up. A very thick mash is created in this process with very little standing liquid on top of the grain. Alternatively, you can dough in at a higher temperature, just make sure you evenly saturate the grains.

If the grain was doughed in at a lower temperature, a portion of water is infused to bring the mash up to a temperature around 95F for an acid rest. At this point, your mash should be at a ratio of around 1.3-1.4 quarts/pound. The only reason to do an acid rest is to lower the initial mash pH (if the pH is already in the right range… skip it). The pH of the mash must be below 6.2 for proper enzyme activity during sacchrification (ideally closer to ~ 5.3). The primary enzyme at work during this rest is phytase (active 86F – 128F) which converts phytin to phytic acid. Phytin is not found in abundance in highly modified malt as the phosphate that makes up phytin has been inverted during the malting procedure. If you use only highly modified malt (American, British, or most any ale malt), you may have to skip this step and either rely on your dark malts to correct the pH or add an acid to lower the pH.

First Decoction:

The first decoction, or decoct, is pulled right after the acid rest temperature is reached. This decoction is very thick (just enough liquid to have a thin layer of liquid on top of the grains). The reason for this is that most of the enzymes are washed into solution at dough in. Since you are going to bring this to a boil, all the enzymes in the decoct will be destroyed before they are returned to the rest mash, and will not be able to convert starch at that point. This decoction is brought up to temperatures suited for alpha-amylase (154 – 160). The decoct does not go through a protein rest to keep a higher pH for optimum alpha-amylase behavior (alpha-amylase works better at a higher pH of around 5.7). The beta-amylase optimum temperature is skipped as well because it is much more effective after alpha-amylase has split the large starch chains into smaller pieces. The temp is held for starch conversion and then is raised to boiling. Boiling is held anywhere from 10 to 45 minutes… the later for darker and more full-flavored beers. After boiling, the decoct is [carefully, so as not to splash] added back to the rest mash to reach the next temperature rest.

The protein rest is carried out at temperatures of 122 – 131 (122 for malts with less than 37% soluble nitrogen [and wheat beers], and 131 for malts of 37-40%). This rest serves mainly to break down larger proteins into polypeptides (head + body builders).

Second Decoction:

The second decoction is pulled after adequate mixing with the rest mash, and is then heated. This decoction is also thick so as not to destroy the enzyme population. For beers with more body, such as bocks, the decoct may be heated up past beta-amylase’s optimum temperature again in favor of more production from alpha-amylase. If you want a beer with less body, opt for a temp closer to 148, and then another rest in the alpha-amylase zone to break down the starches that were release in the previous boil. In any case, after conversion… heat to boiling. After boiling, return to the rest mash.

Depending on the body you desire, Sacchrification temp can vary from 148 – 160… lower for less body.

Third Decoction:

Unlike the previous 2 decoctions, this decoction should be very thin. This is for 2 reasons. The first is that if a more thick decoct were taken, you would release more starch during the boil, and this starch would not get converted as the enzymes will be deactivated. This results in a less-stable beer. The second reason is that you want to deactivate all enzyme activity, and the enzymes are in the thin portion of the mash. The third decoction is taken only after complete conversion, and is heated to boiling without any rests. When infused back to the rest mash, a mash-out temp should be achieved (168~170).

Additional Notes:

• I have omitted volumes of mash to take away for decoction. Typically, this is about 1/3 of the total mash volume for thick decoctions, and 40% for thin decoctions. This will vary depending on your target rest temps, the main mash thickness, and the thermal characteristics of your system. ProMash has a function built-in to aid in this. If you are in doubt of how much to take for the decoction, take a little more than you think. You can always mix this slowly back in, and cool off whatever you need to. Take good records of the temperatures you hit so that you can come closer next time.
• A typical mash thickness for a decoction mash is 1.3 – 1.4 quarts/lb. This ratio should be achieved before the first decoction is pulled so that the proper amount of enzymes stay in the rest mash.
• The typical double decoction mash omits the acid rest and first decoction.
• Typical single decoctions omit the first and third decoctions. Instead of the third decoction, an infusion of hot water to reach mash-out can be done, or the mash can be heated directly if using a heated mashtun.
• Pay attention to thoroughly dough in. If not done properly, enzymes will not be properly washed into aqueous solution, and a higher percentage will be destroyed in the first decoction.
• Pay attention to prevent scorching of the mash. Stir the decoct when boiling, and pay special attention if your mashtun is heated as well. (after all, it would be a shame to spend all that time on a decoction mash only to have a ruined beer).
  

Probably the most thorough explanation of the decoction mash for the homebrewer can be found in Gregory Noonan’s New Brewing Lager Beer (Brewers Publications, 1986). Also helpful are some of the books of the Classic Beer Style Series. Of the books in the series I have, Continental Pilsener, Bock, and German Wheat Beer all have descriptions of tailored decoction mashing schemes for their respective styles. Altbier and Belgian Ale do not give any information regarding decoction mashing schemes; though to be fair, the decoction mash seems to be of less importance in these styles. Information can also be found online. Homebrew Digest has a useful article at http://brewery.org/brewery/library/DecoctFAQ.html.