Decoction technique
http://www.picobrewery.com/askarchive/decoction.htm
http://www.homebrewtalk.com/wiki/index.php/Decoction_mash
Estimate the decoction volume with a formula that gives part of the full mash volume (temp in Fahrenheit)
decoction as portion of wort volume = target wort temp – current wort temp/ 212 – current wort temp
this will yield something like .36 to go from 120 degrees to 153. So you need 36% or your wort volume to decoct. Add a bit more to make sure you hit your target (you can let extra cool) and you see you are boiling almost half your wort.
Hochkurz Double Decoction
All the decoction schedules provided here assume a decoction rise temp of 2-4 *F/min (1-2 *C/min). This is what is generally recommended in the literature for heating the mash.
There is also a saccrification rest at 155 – 162 *F (68 – 72 *C). The purpose of this rest is to utilize the enzymatic power of the decoction before its enzymes are destroyed by further heating. This is particularly important when brewing beers with a large percentage of the enzymatic weaker dark base malts. This rest doesn’t have to be held at the main saccrification temperature. It is sufficient to rest in the alpha amylase range where the conversion is also done much quicker.
The decoction is then boiled for 10 – 40 minutes. Shorter boil times for light colored beers, longer boil times for dark colored beers. If only gentle heat is applied during the boil, stirring should only be necessary occasionally. Similar to wort boiling, excessive thermal loading of the decoction can result in a burnt flavor of the beer. If the decoction is boiled for an extended amount of time evaporation losses can be compensated with the addition of water (which can also be added after the decoction has been pulled, where it helps in thinning it out and makes it more manageable).
