Slideshow: How to Clean and Trim Mushrooms

Start with fresh mushrooms
Start with fresh mushrooms
Select mushrooms that are firm, light, smell fresh and are not bruised, wet, or sticky. The stems of fresh mushrooms should not appear dried out and the gills, if exposed, should be undamaged.
Storage
Storage
Use them as soon as possible—within a few days of buying them. If you need to store them before use, refrigerate them in their original plastic packaging. If they were purchased loose (or foraged by a trusted mushroom expert), put them in a paper bag and they’ll hold well for a few days. After that, they’ll start to dry out.

Wrapping them in a paper towel and then storing them in an open plastic bag (to prevent condensation from building up) might lengthen their shelf-life another day or so but can also darken the surface of the mushrooms.

When to wipe
When to wipe
Regardless of how you prepare them, if the mushrooms are mostly free of dirt and debris, a brief wiping is enough to get them clean. A lot of cultivated mushrooms are so pristine they hardly need more than a dusting. I’ve never needed to wash maitake, portobello, shiitake, or oyster mushrooms, for example.

Especially if you'll be eating the mushrooms raw (and they are not very dirty), it’s best to simply wipe them clean with a slightly damp towel or a soft brush. Rinsing them with water, on the other hand, can leave the surface of the mushroom slightly slimy—not a texture you’d want in a raw mushroom.

Dry mushrooms well and use soon thereafter
Dry mushrooms well and use soon thereafter
Blot washed mushrooms with a dry towel and allow air to circulate around them so they can dry further. A wire rack works well.
Trim the stem
Trim the stem
Shiitake stems are a little too tough for eating, trim them close to the cap.

Most other mushrooms, including white button, crimini, morels, portobellos, can be trimmed close to the base of the stem.

If you are stuffing the mushrooms, and therefore need to remove the stem entirely, reserve the stem for stock or chop it up and add it to the stuffing.

Removing gills and peeling are optional
Removing gills and peeling are optional
It’s not necessary to remove the gills. In the case of portobello mushrooms, the gills can stain other food it comes in contact with. If you don’t want this to happen, scrape the gills out with a spoon.

Mushrooms don’t need to be peeled except for cosmetic purposes.