Not usually thought of as a particularly good source of nutrients, mushrooms actually rank rather high in nutritive value. They contain a substantial amount of B vitamins, selenium, copper, and some other trace minerals, and are very low in calories. Moreover, researchers have discovered they contain antibacterial and other medicinal substances, including anti-tumor compounds called triterpenoids.

The mushroom's distinctiveness derives, in part, from the fact that it is not truly a vegetable but a fungus, a plant that has no roots or leaves, no flowers or seeds, and that does not require light to grow (although some do need light to fruit). Instead, it proliferates in the dark and reproduces by releasing billions of spores. There are about 38,000 varieties of mushrooms, from deliciously edible to highly toxic. Most varieties grow wild; in fact, their earthy flavour has been appreciated for thousands of years. The Egyptian pharaohs decreed mushrooms to be a royal food.

While some mushrooms still refuse to be "domesticated," many can be cultivated in caves or cellars, or grown year round in specially designed structures in which all aspects of the environment, light, temperature, humidity, and ventilation can be controlled. As a result, many varieties of mushrooms are affordable and widely available, no longer exclusively reserved for royalty.

To minimize waste in recipes that call for caps only, choose mushrooms with short stems. Alternatively, reserve the stems for another recipe.

For garnishing, or for cooking whole in a stew or braise, choose small mushrooms; for general cooking purposes, select medium-sized ones. For stuffing, look for mushrooms with caps large enough to hold a generous amount of filling.

When shopping for specialty mushrooms, there is no single rule of thumb for judging freshness since they are all quite different and will not have the clean, uniform appearance of cultivated button mushrooms, particularly if gathered in the wild. They should, however, be firm and meaty, as well as dry to the touch but not withered. Even uncooked, they should have an appealing earthy fragrance.

Storage
It is important to conserve just the right amount of moisture when storing mushrooms. If left completely uncovered, they will dry out; if enclosed in moisture-proof wrapping, they will become soggy and begin to decay. A good compromise is to place mushrooms purchased in bulk in a loosely closed paper bag or in a shallow glass dish covered with a kitchen towel or a lightly moistened paper towel. Leave prepackaged mushrooms in their unopened package. Don't wash or trim mushrooms before storing them.

Keep mushrooms on the refrigerator shelf, not in the refrigerator salad drawer , which tends to be humid, for no more than a few days. Unopened, prepackaged mushrooms will stay for up to a week. If mushrooms begin to darken (and their caps open) with age, they can still be used for cooking and flavouring foods.

Dried mushrooms will keep almost indefinitely if wrapped in plastic or placed in a tightly closed jar and stored in the refrigerator or freezer. They can also be stored in a cool, dark place for up to six months.

Preparation
Since mushrooms are very absorbent, try to minimise their contact with water when cleaning them. Simply wipe them with a dry paper towel or a damp sponge or cloth, or use a soft brush (you'll find special mushroom brushes at cookware shops, or you can use a small soft-bristled paintbrush). If the mushrooms have a lot of soil adhering to their stems, just trim off the stem bottoms. If absolutely necessary, place the mushrooms in a colander and rinse them quickly under cold running water; do not soak them or they will absorb too much water.

Most mushrooms stems are edible. All you need to do is trim off the very ends where the stem might be too spongy (try cutting through the stem with a small knife; if it feels spongy, trim off that section). If the recipe does not call for stems, break them off at the cap and save them (chopped, wrapped, and frozen) for later use in a stock or soup. However, shiitake stems are usually too fibrous to be edible and should be cut off before preparing the caps. (Use the stems to flavour stock.).