Mushrooms and moisture have a tense relationship. Mushrooms seem to love moisture and grab onto it every chance they get, but the relationship is a toxic one, as moisture hastens the shrooms’ slimy demise. That makes it your job, as the mushroom storer, to keep them as dry as possible (until you’re ready to eat them).
The easiest way to do this is to not buy them too far ahead of time — a few days before you plan to eat them is best, but some varieties will stay edible for up to a week. Buy whole, loose mushrooms if possible, so you can inspect each one for damage and make sure it’s firm and free from blotches or soft, mushy wet spots.
When it comes to storing, not everyone agrees which method is the “best.” According to Harold McGee’s “On Food and Cooking,” mushrooms “exhale” moisture as they sit in the fridge, and this moisture can collect on the surface of the shroom, encouraging spoilage. McGee recommends wrapping them loosely in moisture-absorbing packaging (such as a paper bag), while Cook’s Illustrated prefers an open Ziploc-style bag, which they say “maximizes air circulation without drying them out,” and “allows ethylene gas emitted from the mushrooms to be released, prolonging their shelf life.”
Unsatisfied with this conflicting information, I reached out to Farmer Dan, who just happens to be organising the mushroom hunt I am attending in June. Dan was not enthusiastic about the plastic bag method :
I’m team paper bag. They can get slimy really fast in a plastic bag even if it’s open. Also depends on the mushroom and the conditions they were picked in…is the shroom waterlogged or dry? Better to have them dry out a little in a paper bag than to get slimy in a plastic bag.
If you choose the paper bag route, make sure the bag is dry, and change it out if it starts to feel damp.
If you buy packaged mushrooms, leave them in the packaging until you are ready to cook them. That packaging is well ventilated and designed to keep them as fresh as possible for as long as possible until they get picked up by a shopper. If you only use a few shrooms at a time, you can approximate the original packaging by covering the container with new plastic wrap and poking a few holes in it.
Whether in grocery store packaging or a farmer’s market paper bag, keep your mushrooms out of the produce drawer — it’s far too moist of an environment. Store them in the main compartment of the fridge, and toss them if they start to smell sour or look or feel slimy to the touch.
A little shrivelled is ok, however. Cook’s Illustrated actually found that slightly old-looking white button mushrooms “boasted deeper, earthier, more mushroomy flavour than the unblemished samples,” most likely “because some moisture had evaporated and the flavours had concentrated and because some of the proteins had broken down to peptides and amino acids such as glutamic acid that add to the umami taste.”
Washing mushrooms is another topic that has garnered a lot of conflict. Though Cook’s Illustrated found that varieties with exposed gills tend to absorb a good bit of moisture — even during a quick rinse — most mushrooms don’t absorb enough water to affect their moisture content in any meaningful way. You do, however, want to make sure you don’t wash them until right before you cook and eat them, as you don’t want any residual moisture hanging out on their surfaces and speeding up their decay.
A quick rinse to knock off any loose dirt should be plenty — just follow it up with a trip in the salad spinner, or blot them with paper towels before roasting, searing, or sautéing. (The dryer they are, the less likely they are to steam, and steam is the enemy of browning.)