Agaricus bisporus

Agaricus bisporus—known variously as the common mushroom, button mushroom, white mushroom, table mushroom, champignon mushroom, crimini mushroom, Swiss brown mushroom, Roman brown mushroom, Italian brown, Italian mushroom, cultivated mushroom, or when mature, the Portobello mushroom—is an edible basidiomycete mushroom native to grasslands in Europe and North America. Agaricus bisporus is cultivated in more than 70 countries and is one of the most commonly and widely consumed mushrooms in the world.

Taxonomy and naming
The common mushroom has a complicated taxonomic history. It was first described by English botanist Mordecai Cubitt Cooke in his 1871 Handbook of British Fungi, as a variety (var. hortensis) of Agaricus campestris. Danish mycologist Jakob Emanuel Lange later reviewed a cultivar specimen, and dubbed it Psalliota hortensis var. bispora in 1926. In 1938, it was promoted to species status and renamed Psalliota bispora. Emil Imbach imparted the species' current scientific name, Agaricus bisporus, after the genus Psalliota was renamed to Agaricus in 1946. The specific epithet bispora distinguishes the two-spored basidia from four-spored varieties.

Among English speakers, Agaricus bisporus is known by many names. A young specimen with a closed cap and either pale white or light brown flesh is known as a button mushroom or white mushroom. In strains with darker flesh, the immature mushroom is variously marketed as a crimini mushroom, baby portobello, baby bella, mini bella, portabellini, Roman mushroom, Italian mushroom, or brown mushroom. At this stage of maturation, the cap may also begin to open slightly. In maturity, it is called a portobello. The French name is champignon de Paris ("Paris mushroom").

The spellings "portobello", "portabella", and "portabello" are all used, but the first of these spellings is the most common.

Description
The pileus or cap of the original wild species is a pale grey-brown in color, with broad, flat scales on a paler background and fading toward the margins. It is first hemispherical in shape before flattening out with maturity, and 5–10 cm (2–4 in) in diameter. The narrow, crowded gills are free and initially pink, then red-brown and finally a dark brown with a whitish edge from the cheilocystidia. The cylindrical stipe is up to 6 cm (2⅓ in) tall by 1–2 cm wide and bears a thick and narrow ring, which may be streaked on the upperside. The firm flesh is white though stains a pale pinkish-red on bruising. The spore print is dark brown. The spores are oval to round and measure around 4.5–5.5 x 5–7.5 μm, and the basidia usually two-spored, although two tetrasporic varieties have been described from the Mojave desert and the Mediterranean with predominantly heterothallic and homothallic lifestyles, respectively

Commonly found in fields and grassy areas after rain from late spring through to autumn worldwide, especially in association with manure. It is widely collected and eaten, even by those who would not normally experiment with mushrooming.

Similar species
The common mushroom could be confused with young specimens of the deadly poisonous destroying angel (Amanita sp.), but the latter can be distinguished by their volva or cup at the base of the mushroom and pure white gills (as opposed to pinkish or brown of Agaricus bisporus). Thus it is important to always clear away debris and examine the base of a mushroom, as well as cutting open young specimens to check the gills. Furthermore, the destroying angel grows in mossy woods and lives symbiotically with spruce.

A more common and less dangerous mistake is to confuse Agaricus bisporus with Agaricus xanthodermus, an inedible mushroom found worldwide in grassy areas. Agaricus xanthodermus has an odor reminiscent of phenol; its flesh turns yellow when bruised. This fungus causes nausea and vomiting in some people.

The poisonous European species Entoloma sinuatum has a passing resemblance but has yellowish gills turning pink and lacks a ring.

Cultivation
The earliest description of the commercial cultivation of Agaricus bisporus was made by French botanist Joseph Pitton de Tournefort in 1707. French agriculturist Olivier de Serres noted that transplanting mushroom mycelia would lead to more mushrooms. Originally, cultivation was unreliable as mushroom growers would watch for good flushes of mushrooms in fields before digging up the mycelium and replanting in beds of composted manure or inoculating 'bricks' of compressed litter, loam and manure. Spawn collected this way contained pathogens and crops would be commonly infected or not grow at all.

In 1893, sterilized, or pure culture, spawn was discovered and produced by the Pasteur Institute in Paris. Today's commercial variety of the common mushroom was originally a light brown color. In 1926, a Pennsylvanian mushroom farmer found a clump of common mushrooms with white caps in his mushroom bed. Like white bread it was seen as a more attractive food item and was very popular. As was done with the navel orange and Red Delicious apple, cultures were grown from the mutant individuals, and most of the cream-colored store mushrooms we see today are products of this chance natural mutation.

Agaricus bisporus is now cultivated in at least 70 countries around the world. Global production in the early 1990s was reported to be more than 1.5 billion kg, worth more than US$ 2 billion.

Vitamin D
While Agaricus bisporus only contains 40 IU of vitamin D as ergocalciferol (vitamin D2), since they also contain high amounts of ergosterol, by brief exposure to UV light the ergocalciferol contents rise immensely.

Potential medicinal value
Agaricus bisporus also contains sodium, potassium, and phosphorus, conjugated linoleic acid and antioxidants. Protocatechuic acid and pyrocatechol are found in A. bisporus

A clinical trial is scheduled to take place at the City of Hope National Medical Center in Duarte, California to research whether the common mushroom can inhibit aromatase, and therefore lower estrogen levels in the human body, which might reduce breast cancer susceptibility. A 2009 case control study of 2,018 women correlated a large decrease of breast cancer incidence in women who consumed mushrooms. Women in the study who consumed fresh mushrooms daily were 64% less likely to develop breast cancer, while those that combined a mushroom diet with regular green tea consumption reduced their risk of breast cancer by nearly 90%.

The table mushroom has also been shown to possess possible immune system enhancing properties. An in vitro study demonstrated the mushroom enhanced dendritic cell function.

Health risks
Some studies have revealed that raw A. bisporus - along with some other edible mushrooms - contain small amounts of carcinogenic hydrazine derivatives, including agaritine and gyromitrin. However, this research also noted when cooked, these compounds were reduced significantly.