I've brewed a number of all grain beers and have been reading a bit about decoction mashing. However the explanations I've read have been a bit complicated.
How do I do a decoction mash?
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A decoction mash is removing a portion of the mash from the rest, boiling it, and then adding it back into the mash to raise the temperature. It's something akin to today's step mashing. However, you can get better efficiency doing a decoction mash as boiling the grain's cell walls are destroyed and allows for better access to the grains starches by enzymes. Here's a link for volume calculations for decoction mashing.
A decoction mash will not extract tannins because the pH of the mash is too low. For a single decoction, you boil the grain, leaving as much of the liquid behind as possible. The enzymes remain in the liquid, not the grain, so you will not denature them by boiling. If you do a second or 3rd decoction, you include more liquid. At that point, conversion is pretty much done and denaturing enzymes isn't much of an issue. Contrary to brewing myth, pH is much more of a factor with tannins than temp. the effects of decoction are very debatable. Many test have found that tasters don't really find a difference (or improvement) in flavor from a decoction. Here's one experiment I did. Start on pg. 25...
Here's a page with probably everything you'd want to know about decoction mashing: