Almost 40,000 varieties of mushroom exist in the world, but only a fraction of them make it to the table, where they are enjoyed for their rich, earthy flavor. For culinary purposes, mushrooms are divided into two categories: cultivated and wild.
Today, as demand for different varieties increases, mushroom growers are able to cultivate more and more types, and the line between cultivated and wild is blurring. The most flavorful—and most expensive—mushrooms, however, are still gathered by foragers in forests under trees or on old stumps. Highly prized mushrooms, such as the matsutake and the morel, have eluded all attempts at cultivation. Around the world, mushroom hunting after the rains of spring or during the cool mornings of autumn is still a thriving—and ultimately delicious—tradition.
Although mushrooms can be cooked by almost any method, they taste wonderful when simply sautéed in olive oil, with a little garlic, over high heat. Mushrooms are also good tossed with olive oil (be judicious; these little sponges can soak up a lot of oil) and seasonings, then roasted or grilled, gill side up, to retain their juices. To grill small mushrooms, thread them on skewers.
Fresh mushrooms should be firm and have smooth, unblemished caps. Avoid any that are broken, limp, wrinkled, soggy or moldy. Stems with gray, dried ends indicate that the mushrooms have been stored too long. Some mushrooms have closed caps, like the common button mushroom. For these varieties, if the caps are open so that the gills are exposed, the mushrooms are too old. For varieties where the gills are exposed, like portobellos, check that the gills are unbroken.
As mushrooms age, they dry out, so the heaviest mushrooms should be the freshest. If you plan to cook mushrooms whole, select those with caps of the same size for even cooking. Check that packaged, presliced mushrooms are not wrinkled or discolored. Mushrooms are also available whole or sliced in jars, preserved in a brine, or marinated with oil and herbs.
Refrigerate fresh mushrooms for no more than 3 or 4 days, keeping them in a paper bag to absorb excess moisture. Spread delicate varieties in a single layer on a tray and cover them with a damp cloth. If sealed in plastic, mushrooms will become slimy and mold quickly.
Mushrooms absorb water readily, becoming soggy and flavorless if left to soak. While some cooks insist that you should not wash mushrooms at all, a quick rinse and a thorough drying with paper towels immediately before cooking will not hurt them. If you have time or plan to cook only a few mushrooms, wipe them clean with a damp cloth or brush. Special mushroom brushes are available for gentle removal of dirt, or a toothbrush with soft bristles will work. Trim the dried end of tender stems; but if the stems are tough, remove them completely and save them for soup or stock.
Some wild mushrooms are fatally toxic, and they can closely resemble edible varieties. Do not pick or eat wild mushrooms unless a trained expert collector familiar with local varieties identifies them. Supermarket "wild" mushrooms generally are farmed and certainly are not poisonous.
Adapted from Williams-Sonoma Kitchen Companion: The A to Z Guide to Everyday Cooking, Equipment and Ingredients (Time-Life Books, 2000)