Decoction mashing is a traditional and intensive method of mashing. While the method was once used by most breweries, today decoction is a controversial topic among brewers. Many German brewers (among others) claim that it develops malt character, depth, and superior foam. Others feel that it is a waste of energy and time, and is unnecessary now that modern well-modified malts are available to everyone.
The basic principle of decoction is to remove a part of the mash, boil it, and return it to the main mash, which is held at a constant temperature. There are two different aims in boiling one part of the mash:
The boiling of the grains helps to destroy the cell walls and makes the starches more accessible to the malt enzymes. This is particularly important for under modified malts where boiling helps to break down the cell walls.
There are different types of decoction mashing methods. The archetype is the triple decoction. This very intensive method is, from the current point of view, time-killing and no longer very popular, but it is the basis for understanding the principles behind all decoction methods.
The mash-in temperature is 95°F–98.6°F (35°C–37°C) with one part malt and three parts of water.
The triple decoction mash employs three main temperature rests: the acid rest, the protein rest, and the saccharification rest. At each rest the mash is separated, by interrupting stirring and waiting for a few minutes until there is one part “solid” mash for every three parts “liquid” mash. The thick part of the mash with the main part of the grains is pumped in the mash tun kettle and heated for boiling. The heating must be slow (33.8°F/min [1°C/min]). There can be rests at 143.6°F–149°F (62°C–65°C) or 161.6°F (72°C) during heating. The boiling time is 30 to 45 minutes. In the meantime, the liquid mash rests at a constant temperature of 95°F–100.4°F (35°C–38°C). After mixing up both parts of the mash, the temperature of the full mash will rise to the next (protein)-rest temperature at 122°F–131°F (50°C–55°C). To reach the right temperature a correct calculation of the volume proportions is necessary. The rest temperature and time before pulling the next decoction should be based on the malt that is used.
The next step is similar to the first: separating, heating, and boiling of the thick mash while keeping the liquid mash at the optimal temperature for the enzymes. After the second boiling and recombination of the mash, the rest temperature at 143.6°F–149°F (62°C–65°C) for the amylolysis (conversion of starch to sugars) is reached. Then the stirring stops for 10 minutes and the separation starts again. In contrast to the procedure before, the third decoction step in the traditional triple decoction is the boiling of the liquid mash. This procedure is not logical and is against the basic idea of decoction, because most of the amylase is in the liquid mash and becomes inactivated during boiling. Interestingly, the original idea behind it was to get a sweet and “thick” beer for the Lenten season and keep the degree of fermentation at a low level.
The combined mash temperature after the third mash boiling is at 167°F (75°C) for the saccharification rest. This rest takes about 15 minutes. The duration of the rests and the heating rate can be modified with or without special rests. Therefore, the duration of the triple decoction ranges widely. It lasts approximately 5.5 hours when using dark malts and 3.5 to 4 hours with pale malts. Nevertheless the input of energy is high.
For dark and enzyme-weak malts the triple decoction method could be justified, especially for malt flavor development through Maillard reactions.
To avoid the waste of energy and possible loss of beer quality, the shortened double decoction is an alternative method. The classic version of the double decoction is a shortened triple decoction. The first acid rest is omitted and it starts with the protein rest at 113°F–122°F (45°C–50°C). The following procedure is similar to the triple decoction described above. Both decoctions use the thick mash for physical pulping and to destroy the starch cell walls to improve enzymatic activity. The liquid part rests at constant temperature for the best exploitation of the enzymatic power. The variations of double decoction are based on different mashing-in temperatures and rests during heating. The duration is between 175 and 215 minutes. Depending on the exact method used, the final attenuation, the color, and the viscosity of the wort varies. Beers produced with double decoction, if the procedure is well-matched to the malts used, can be particularly full-bodied and tend to show very good foam stability.
Finally, the single decoction is always a combination of infusion and decoction to reach all the required temperature rests. There are a number of methods. It is possible to proceed with a standard temperature-programmed mash, achieving the regular rests up to the amylase rest at 149°F (65°C), before separating the mash. The decoction fraction can already be separated at the mash-in temperature and then taken through the steps until boiling. Beers from single decoction wort are considered to be more gentle and fresh, with brighter color.