Store your mushrooms correctly and avoid the slime
Mushrooms are delicate vegetables, both in taste and temperament. If you don't pay attention, your mushrooms can go from beautiful to a sludgy, slimy mess in a couple of days. That's why it's so important to learn how to store mushrooms correctly—because proper storage and handling is what prevents these fungi from going totally bad before you have a chance to eat them.
The first step in keeping mushrooms fresh for as long as possible is to make sure you're picking healthy-looking mushrooms at the supermarket. Color's not exactly a reliable indicator of quality because every mushroom looks different. Instead, you want to check the mushroom's structure and texture. The experts at Cook's Illustrated recommend looking for mushrooms "with whole, intact caps," and an even texture with no "discoloration or dry, shriveled patches." If you're able to actually touch the mushrooms because you're buying in bulk or at a farmer's market, check that the mushrooms are damp but not slimy; otherwise, you're fighting a losing battle against the sludge.
The reason mushrooms should be damp is because of their high water content: about 80 to 90 percent, according to Harold McGee in On Food and Cooking: The Science and Lore of the Kitchen. But that high water content is also why it's so hard to keep mushrooms fresh. Mushrooms also may continue to grow even after they've been harvested, and as McGee notes, "During four days’ storage at room temperature, they lose about half of their energy reserves to the formation of cell-wall chitin," the molecular name for what are basically mushroom fibers.
This is why you want to keep mushrooms in the fridge and not at room temperature. According to guidance from Giorgio Fresh Co., a Pennsylvania-based company that grows, harvests, and ships fresh mushrooms across the United States, if you keep mushrooms refrigerated, you can use them for up to a week—though they're best within a few days after purchase.
And how you store the mushrooms plays a critical role in how well they'll keep. The number one rule when it comes to keeping mushrooms fresh is to let them breathe. "Storing in air-tight containers or plastic bags will cause condensation and speed spoilage," the folks from Giorgio say. McGee seconds that point, writing that mushrooms, "should be loosely wrapped in moisture-absorbing packaging to avoid having the moisture they exhale wet their surfaces and encourage spoilage." And adding excess moisture, either by washing your mushrooms before your put them away or wrapping them in a damp paper towel, does you and your fungi no favors.
So what's the best way to store mushrooms? You can use the original packaging from the store, because those cases are generally designed to allow for some air flow. You can also place mushrooms in a paper bag; just be sure to leave the top open. And that's it! Pop the mushrooms in your fridge, unwashed, and be sure use those mushrooms quickly. Fungi don't freeze well, so if you don't use them before they turn, you'll never have a chance at all.