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Alternate Decoction Mash Purpose


David Corr — Centerville, Ohio asks,

I’m a bit confused, and I am hoping you can help. I read an article on how Sam Adams uses decoction mashing to make their light beer. The article stated the idea is that enzymes are destroyed during the decoction process to lower the concentration of fermentable sugars, which in turn lowers both ABV and calories. I also read that decoction mashing helps to destroy the cell walls and makes the starches more accessible to the malt enzymes. To me, more starch would result in more fermentable sugars and thus increase the ABV. This seems to contradict what is said above. What am I missing?


Thanks for the fun question, David! I want to begin with a bit of housekeeping. I was able to find an article on the Food & Wine website from June 22, 2017 ( that has quotes from Jim Koch from Boston Beer explaining the Sam Adams Light as you have described.

The other thing I want to quickly review is the enzyme content of malt and the differences between dextrinizing units (DU) and diastatic power (DP). DU is a measure of alpha amylase activity, the enzyme that liquefies starches and increases wort extract; some fermentable sugars are produced by alpha amylase, but not much. DP represents both alpha and beta amylase activity, but the DP value is heavily influenced by beta amylase (the enzyme in malt that yields most of the fermentable sugars in wort). Finally, beta amylase is denatured at lower temperatures than alpha amylase, and increasing the concentration of starch in wort does not give more fermentables without beta
amylase activity.

Okay, so Jim Koch is stating that Sam Adams Light uses the decoction mash method as a technique to reduce wort fermentability via enzyme denaturation. What he doesn’t explain is what type of decoction mash method is used to accomplish their goal. But knowing their method is not required to explain how this can work. In classic triple decoction mashing, the first mash rest starts at around 95 °F (35 °C). A thick portion of mash is removed, heated to boiling with a brief rest around 158 °F (70 °C) for alpha amylase to chop up starches and reduce mash viscosity, boiled for some time, then pumped back to the rest mash. This results in an increase in the blended mash temperature to about 122 °F (50 °C). The process is repeated (minus the pause at 158 °F/70 °C) two more times for a mash rest at 158 °F (70 °C) and mash-off at 168 °F (76 °C). The norm in decoction mashing is to boil portions of the thick mash because the enzyme concentration is not as high as in the liquid phase above the mash.

If the goal is to really limit wort fermentability, the heating rate up the mash ladder is usually too slow because mash mixers and decoction kettles can only heat so quickly.

The decoction method is pretty darn intense, typically results in high material efficiency, and can also produce highly fermentable wort depending on malt DU and DP and how quickly the mash is pumped from the decoction kettle back to the rest mash (that’s the portion hanging out reading the paper in the mash tun). How is Boston Beer using decoctions to selectively beat up on enzymes? My guess is some variant of a jump mash using boiling water as part of the decoction process.

When any mash is heated from about 140 °F (60 °C) to 154 °F (68 °C) there is interplay between beta and alpha amylases. And with slower heating rates comes higher wort fermentability. This important ramp is part of step mashing, decoction mashing, and double mashing (primarily used for rice and/or maize adjunct brews). The ramp’s heating rate is affected by how quickly the mash can be heated on the way up in temperature for all of these methods and is also affected by how quickly the boiling mash is returned to the rest mash for decoction and double mash methods. If the goal is to really limit wort fermentability, the heating rate up the mash ladder is usually too slow because mash mixers and decoction kettles can only heat so quickly (about 1.8 °F/1 °C per minute is typical). What’s a brewer to do?

One option is to mash-in at a higher temperature to limit beta activity. Another is to reduce enzyme activity through malt choice, dilution with adjuncts, or heat in mashing. Koch states that they are using a combination of malt choice and heat. My bet is on the jump mash. Here’s one approach. Mash-in all grains at 122 °F (50 °C). Pump about half of the thick mash to the decoction kettle, very quickly “drop” in sufficient boiling water to jump the mash temperature from 122 °F (50 °C) to 158 °F (70 °C). This essentially deletes the ramp and allows virtually no beta-amylase activity. It also makes for one big decoction; a surefire way to destroy a big chunk of enzymes. Boil the decoction and then quickly pump back to the rest mash to jump the temperature from 122 °F (50 °C) to 158 °F (70 °C). The combined mash now has about 40% less enzymes than before the decoction. There is sufficient alpha to convert the starches, less beta at a temperature that is going to finish off what was present before the heating step, and not much of a ramp involved.

Why go to the trouble? In the Food & Wine article, Koch states “It [Sam Adams Light] had to be flavorful and not a pale, fizzy, flavorless brew. When it came to brewing a beer with more flavor, we knew we needed to play around with both the recipe and the brewing process.

Response by Ashton Lewis.