© Denzil Green
A Mushroom Brush is a inexpensive kitchen tool for cleaning mushrooms and other delicate food items.
The bristles are usually a synthetic material such as nylon, but you can also get natural. The bristles will be very soft, so that they don't scrape off or gouge the skin of the mushroom. A toothbrush would be too strong (aside from the fact that you wouldn't want to use something that was routinely in someone's mouth.)
The brushes can be round or rectangular. The cap of the brush usually doubles as the handle. You may, though, see a few with actual long handles.
The brushes are washable.
Commercially, mushrooms are grown on top of sterilized compost made from manure. Bits of that can get stuck onto the skin of a mushroom. The purpose of a Mushroom Brush with mushrooms was to get dirt such as that growing material off them in a way that didn't involve water, owing to the persistent myth that mushrooms shouldn't be washed with water, as they would absorb water.
The advice these days is to just go ahead and wash mushrooms.
"I was skeptical about the mushrooms-absorb-water idea and so did the soaking experiments with standard white mushrooms for “The Curious Cook” back in 1990. I've since tried a number of others, and if you make sure to shake the water out of the nooks, fresh mushrooms absorb little if any water. I’d also say that since they’re already around 90% water, a little more or less isn’t going to make much of a practical difference in the subsequent cooking. Now losing flavor to the water is a different question, and it’s possible that you might lose savory amino acids (aroma molecules tend not to be soluble in water). Even here, though, the amino acids are inside cells, and most cells are not going to be breached by surface exposure to water. So I wash my mushrooms with a clear conscience." -- McGee, Harold. 
The chef Jacques Pepin and the TV personality Alton Brown also back McGee on this.
If you switch to washing mushrooms and wonder what to do with your mushroom brush now, there are other uses for them: some people purchase them for the specific purpose of brushing dust off indoor plant leaves.
To clean rehydrated mushrooms, rub the pieces with your fingers, then rinse.
Pastry brush. Paper napkin or paper towel. Or, just rinse under water.
 McGee, Harold. Thread: A clean 'shroom is a happy 'shroom. Cheftalk.com Cooking Forums. 14 December 2005. Retrieved August 2010 from http://www.cheftalk.com/forum/thread/15774/a-clean-shroom-is-a-happy-shroom
Cloake, Felicity. Should you wash mushroooms? Manchester: The Guardian. 18 January 2010.
BrushesMushroom Brush; Pastry Brush; Washing Up Brush
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