Brewers today have access to highly-modified malts. We also have modern tools like thermometers which were prohibitively expensive luxuries not available to brewers in the 18th Century. This means the process of extracting sugars from the grains through mashing were far less efficient in 1770 than they are today. Decoction is a method to overcome these problems.
Boiling the grain breaks down the starch and dextrin in grain and allows these sugars to be fully gelatinized. This unlocked the valuable malt sugar held within the cell walls of the grain and ensured more consistency in the mashing process. Since liquids boil at a consistent and constant temperature, brewer could achieve consistent mash without needing to actually measure mash temperatures. Furthermore, since most mash tuns (or kieves) were simply wooden barrels and not well insulated, decoction allows the brewer a means of keeping the mash temperature up for longer times to better facilitate conversion.
The process of conducting a decoction is quite simple. After initial dough-in (mixing the grain with warmed spike water), a portion of the mash is removed, then heated in a separate vessel to a full boil (with constant stirring) and returned to the main mash to raise its temperature. Frequently, this process is repeated several times during the mash cycle with the previous decoction being fully stirred in before the next portion is taken.
Several factors influence the impact performing decoction will have on your beer. First theres the volume of the decoction which influences the amount of heat that you reinject back into the mash tun with each decoction cycle. Next there is the time you allow the decoction to boil which impact the amount of caramelization you are adding back into the mash. Finally, there is the actual heat of the fire (or in the case of the 18th C brewer the distance of your decoction vessel from the fire). Next there is the “thickness” of the decoction or the ratio of water to grains in the portion placed on the boil. Since the enzymes needed to convert starches to sugars are in the cell walls of the grains, and these enzymes are destroyed when the grain actually boils, we must be sure not to make the decoction too thick. This last factor has the biggest impact on sugar conversion (and hence ABV), the slower the decoction comes to a boil the higher the final gravity of the wort will be. As 18th Century brewers, we adjust the number of decoctions, the time for each decoction, the thickness of these decoctions and speed to which the vessel comes to boil to achieve different flavor profiles in our final beers and we do all this without the benefit of thermometers, hydrometers, or even clocks. It’s a true art.
What are those effects? The short answer – melanoidins. Melanoidins were unknown to early brewers, but through trial and error, many master brewers (Brewmeisters!) learned that the decoction process creates some tasty benefits. Melanoidin compounds are formed at high temperatures via a process known as the Maillard reaction. This is the same process that makes your toast brown and makes a charbroiled steak better than one cooked in the microwave. Melanoidin compounds are a family of chemicals that are responsible for the flavors we describe as cookie, biscuit, toasty, bready, and toffee and many of these flavors are highly desirable in many European styles such as Czech Pilsner or German Märzen. The decoction mashing technique can therefore be used to increase the desired malty-sweet flavor profile of the wort without cloying flavors that can result from the use of crystal malts.
Another benefit, and perhaps the one of most interest to brewers in the 1700’s is the fact that decoction allows the brewer to achieve better mash efficiency. This make the beer is stronger without requiring more grain. In short, decoction is a way to keep the cost of production down. Exposing the grain to higher temperatures in the decoction process will guarantee complete gelatinization and liquefaction of the starch. The more starch that is “unlocked” from the grain the more starch that is available to be converted to sugars.
While the decoction has some great benefits, there is a downside. First of all, the process is difficult and requires significantly more time, labor and energy compared to methods such as single infusion mashing. Second, decoctions add lots of color to the beer. Finally, decoction frees up a lot of racemic sugars which are just as sweet to the taste but not easily fermented. This makes a beer demands more hops and botanicals to achieve a certain level of perceived bitterness.
In an era where brewing is done by sight, smell, and taste rather than “by the numbers” as we do today, decoction is a complex tool. It does great things but learning to master it can be difficult.
Michael Carver -- Historical Interpreter/Reenactor 