There is some dispute as to the reasoning behind this advice. According to DCP:
The Mushroom Council recommends brushing off any dirt with a damp paper towel or fingers, rinsing only briefly under running water and pat dry with a paper towel. Never soak them, as they absorb moisture.
My strong suspicion is that the dispute is due in large part to the differing behaviors of various types of mushrooms. Button mushrooms and shiitakes are quite different, for example. Last night I began some testing that I hope can become a collaborative eGullet effort to document the various "water absorption" properties of differing mushrooms over differing lengths of time.
According to tests run by Alton Brown of Good Eats, the bit about soaking appears to be a culinary myth. Per his tests, 4 ounces of button mushrooms soaked in 1 liter of water for 10, 20, and 30 minutes gained 0.2, 0.25, and 0.15 ounces, respectively (about a teaspoon, max). This works out to 3.75% to 6.25% gain. The kicker? Another 4 ounces of button mushrooms, subjected to a brief blast of cold water, gained 0.2 oz - same as when soaked for 10 minutes.
Mushroom Type Dry Weight Wet Weight Soak Time % change ------------- ---------- ---------- --------- -------- Shiitake 76.0 g 104.6 g 60 s +37.6%I cannot state that this dramatic increase in weight had any particular impact on the finished dish, but for now that's not the point. Anyone else care to play?
ETA: My method here was to soak the mushrooms in cold water, swirling them around in it, for 60 seconds. I then took them out and laid them on towels, patting them dry. So the weight increase is not due to visible water on the surface of the mushrooms or in the gills (though the gills are challenging to dry completely).
Edited by Chris Hennes, 14 March 2008 - 07:45 AM.