For most beer styles you add hot water to the crushed grain and let it mash for an hour or so while the starches in the gain are converted into usable brewing sugars. In this day and age most of the grains we use are highly modified, meaning they have enough inherent enzymes to do this conversion on their own. If you are using a large amount of adjunct (corn, rice) or older style European pilsner malts you may need to do some fancier footwork to get all the sugars out of them. This is where the decoction method arose. By boiling part of the grain/water mix you can break down some of those starches mechanically and then add back to the whole mash for easier conversion. The boiling will also encourage the Maillard reaction of browning, resulting in more complex flavors that are difficult to get in other ways. I attempted a Doppelbock recipe a few years ago that skipped this decoction process, but had several judges in competitions pick up on that lack of complexity and mouthfeel that comes from decoction.
For my Skelator Doppelbock I modified Gordon Strong's recipe from his new book Brewing Better Beer.
5.25# Dark Munich
5.25# Light Munich
5.5# Weyerman floor malted pilsner
0.5# Crystal 80 (English)
1.6 oz Crystal Hops 2.8%AA at 60 min
1.25 oz Tettnang Hops 3.7%AA at 30 min
1) Protein rest at 124 degrees for 25 minutes by regular infusion of warm water.
2) Recirculation of wort to get up to 144 degrees for 20 minutes. Might be tough is your system can't recirculate or direct fire your pot.
3) First Decoction! Thick mash: Scooping out about 1/3 of the mash and boiling it for 15 minutes. Stir pretty constantly to prevent scorching. I did do a slightly thinner mash than usual, which can make this step easier.
4) Add the boiled mash back into the mash tun. The goal was to get this to 154 degrees for 45 minutes. I came out a bit low and had to recirculate a bit to get up to the right temp.
5) Second Decoction! Thin mash: Run out about 1/3 of the wort (3 gallons) into the kettle and boil for 15 minutes. Sometimes you will do a triple decoction where you basically repeat the first decoction step. Lighter beers like pilsners and hefe will often just use this type of thin mash decoction.
6) Add this back into the mash tun to get to a mash-out temp of 170.
7) Sparge as normal to rinse all the sugars out of the grain and into your boil kettle (about 30-45 minutes.)
8) Boil for 90 minutes, adding hops per recipe above.
9) OG is 1.094
All in all this added about 1 1/2-2 hours to my brew day, but everything seemed to go well. I hit most of my temps, or was able to fix it easily. The recipe call for this being a 5 gallon batch with 70% efficiency, but I did a 6 gallon and hit all my gravity numbers. My efficiency is around 78% typically and hit that perfectly here. The proof of how usefull all this extra work is will not come until the fermentation is done and the beer is tasted. The suspense is tough! I will post about the next few stages of the process.