Big Monk
Trappist Please! 🍷
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Yes this is absolutely true but we are going to boil the decoction after pulling the wort. If we take a two stage enhanced decoction, dough in at acid rest using pre boiled water and underletting etc then when the decoction is pulled, heated, rested it will be boiled. When some of it is returned to the main mash the temperature will rise driving out dissolved oxygen in the main mash while the rest of the decoction boils. This is added again to the main mash to facilitate reaching scarification level again raising the temperature and driving out any dissolved oxygen. When it is pulled again for the last time, it will be boiled and returned to the main mash to reach mash out. Thus even though we are physically removing the decoction (making oxygenation possible thorough surface aeration) We are compensating by raising the temperature at every stage which should drive out dissolved oxygen at least in theory.
Id love to see some data on dissolved oxygen at the end of a decoction mash.
The "damage", i.e. exposure to dissolved O2, is already done by the point you are referencing. The solubility of dissolved oxygen at mashing temperatures is such that it doesn't take very long to get to the point of no return. If you can't preserve the malt flavors you are after by limiting the dissolved O2 below 1 ppm (or less) on the hot side, then it's all for naught.
That's not to say it isn't worth it, but pulling multiple decoctions is not going to help the effort. You aren't compensating for anything at each decoction stage, you are merely exposing the wort to dissolved oxygen, letting it damage the flavors you've been working to keep, then driving it off.
All the original members and purveyors of these methods at the homebrew level (The original GBF members) gave up on decoctions for just these reasons, despite the romantic notions about its usefulness or necessity.
That doesn't mean you shouldn't decoct if that's your desire, just know that it's working against you if you are pursuing Low Oxygen brewing methods.