2 hours ago, gfweb said:Many advocate not washing them at all.
Isn't that a hangover from the days when people insisted that mushrooms absorb the water they are washed in - a theory that has long since been debunked?
2 hours ago, gfweb said:Baking soda is alkaline, not acid.
True, but baking powder is mildly acidic.
That said, this sounds like a nonsensical old wives' tale. And a complete waste of time, flour and baking ingredients. You can buy special mushroom brushes if you want to waste more money.
Just wash them in water like any other vegetable.