Agaricus bisporus is usually called the common or commercial mushroom. These fungi are available at many grocery stores, and have a white cap. The species may be sold under many different names, some of which refer to its size. Small white versions of A. bisporus are generally referred to as button mushrooms, while larger versions may be given a more generic name.
True button mushrooms are picked when they are very young and when the caps of the mushroom are still closed. They should be free of any marks or discolorations, and the stems on their bottoms should not have hardened. Generally, when a cook buys fresher mushrooms, they are easy to slice and the whole mushroom, stem included, can be used. Those that have sat for a few days in the grocery store, or that are larger, may feature a hardened bottom stem that needs to be removed prior to slicing or preparing it.
The name "button" tends to refer to a specific size, but they are not the only mushrooms that are A. bisporus. Portobello mushrooms are large, brown versions, for example, as are smaller Italian brown mushrooms. Consumers might also find wild mushrooms in button size, simply meaning that they’re smaller and picked at an earlier stage.
Many people prefer smaller versions of A. bisporus because they tend to be freshest and are most likely to feature the closed cap. A closed cap means that a person can’t see the tiny black fibers inside the mushroom when it's turned upside down. Open cap button mushrooms should be avoided, as they are of inferior quality.
When preparing button mushrooms, most chefs suggest merely wiping off the mushroom to remove any dirt. The cook can wash them too, but they are porous and likely to absorb liquids. Cooks who do plan to wash them, which is not a bad idea since many are grown in manure, should do so right before they plan to serve or cook them. Though sliced mushrooms are available, most chefs suggest that they be sliced just before they are used so that they have the freshest taste. People who are concerned about the dirt in which they’re grown will also find it challenging to wash pre-sliced mushrooms.
Button varieties are also available in the canned food section. Some are canned in water, while other sare pickled or packed in oil and vinegar. They make great toppings for individual pizzas because of their convenient size, and they can be a good addition to stir-fried dishes and salads. With all mushroom sizes or ages, cooks should be careful not to overcook them, or they can get rubbery and slimy. Cooks who are adding mushrooms to a stew, for instance, should wait until the last five to ten minutes to add them.
Mushroom enthusiasts can find button mushrooms in the wild, but they can easily be confused with A. Californicus, a look-alike that is mildly toxic. Because of the similarities, gathering wild mushrooms should only be done with an expert.