If only some of the tangzhong counts towards total hydration, then we should correct the recipe by that amount. So the question is, what %of tangzhong is "free" water?
Converting each recipe to 75% is just wrong. For example, a recipe that is already 75% would need no water added, yet you would end up with a much lower hydration since no all of the slurry is water.
January 29, 2019 at 10:03am