As brewers, we refer to the entire process of converting the
starch present in grains and adjuncts into sugar as mashing. We
use the enzymes present in pale malted grains to perform this
conversion. These enzymes are active over a range of
temperatures, with more activity within certain ranges than
outside of these ranges. Decoction mashing is the method of
boiling portions of a mash, which can extract flavors and
fermentable sugars from those ingredients while allowing the
remainder of the mash to “rest” (which is why it’s called a
“rest mash”). When the boiled (decocted) mash is re-introduced
to the rest mash, it raises the temperature of the rest mash.
When sufficient decocted mash is added back to the rest mash,
the entire mash is raised from the range at which one or more
enzymes are active to the range at which other enzymes are
active. Grain and Water Decoction mashing began in the days
before the development of scientific method or even simple
instruments such as thermometers and hydrometers. Through