As it pertains to flour, it should be specified that it applies to white button mushrooms rather than other cultivated/wild varieties.
While I do not know the exact science other than the prevention of oxidation, "cuire dans un blanc" (as opposed to "cuire à blanc" or "blind bake") is a French technique for cooking vegetables such as artichokes (and occasionally offal; sweetbreads I think) in which the liquid is supplemented with lemon juice to lower the pH (make it more acidic, keep the vegetable firm) and a starch (flour). I believe that the addition of flour to the water makes the cooking liquid opaque and thus limits oxidation through light and any addition of fat (oil/butter/animal fat) limits oxidation through contact with the air. But I could be convinced otherwise.
In Japanese cooking, raw suchi ice is often added to the cooking liquid of bamboo shoots to keep them light in color and to remove the bitterness, as with artichokes, though I have yet to supplement flour with rice.
This leaves me wondering if adding baking soda to my mushroom poaching liquid will have any affect...
