Agaricus bisporus

Agaricus bisporus, known as table mushroom, cultivated mushroom or button mushroom, is an edible basidiomycete fungus which naturally occurs in grasslands, fields and meadows across Europe and North America, though has spread much more widely and is one of the most widely cultivated mushrooms in the world. The original wild form bore a brownish cap and dark brown gills but more familiar is the current variant with a white form with white cap, stalk and flesh and brown gills.

Some grocery stores in the Western world sell this mushroom in canned and fresh preparations. An agaric, its gills are often left on in preparations. It can be found cooked on pizzas and casseroles, stuffed mushrooms, raw on salads, and in various forms in a variety of dishes. Some mycologists, including Paul Stamets, have raised concerns that this mushroom contains trace quantities of a chemical agaritine known to have carcinogenic properties, though whether levels are sufficient to cause harm in consumers is debated.

Taxonomy and naming

Agaricus bisporus is known by many names several of which refer to different stages; "button mushroom" when sold, collected or eaten in young, unopened form, "Crimini mushroom" or "baby bella" as an immature portobello, or "Portobello mushroom" as a large brown mature mushroom. It is known as the champignon de Paris in France. It is also often called simply "champignon" (the french word for "fungus") in several languages.

The cultivated mushroom is a member of the large genus Agaricus, which has numerous members which are edible, tasty and collected worldwide. The next best-known is the commonly collected wild mushroom (A. campestris), known in North America as the meadow mushroom or field mushroom in England and Australia. This can be found throughout much of the United States and Europe.

The common mushroom has a complicated taxonomic history. It was first described as a variety (var. hortensis) of A. campestris in 1884, before Danish mycologist Jakob Emanuel Lange reviewed the cultivated form, naming it as a variety Psalliota hortensis var. bispora in 1926, its epithet derived from its two-spored basidia (as distinct from other members of the genus which had four-spored basidia). Mõller and Schäffer raised the mushroom to species status as Psalliota bispora in 1938. It was given its current binomial name of Agaricus bisporus by Emil J. Imbach upon the renaming of Psalliota to Agaricus in 1946.

The earlier Agaricus brunnescens was a name coined by Charles Horton Peck in 1900 and proposed as the correct name for the mushroom, however this description referred to a four-spored collection and cannot be ascribed to A. bisporus.

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 always two-spored.

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 spp.), however 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). This 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 on mossy woods and lives symbiotically with spruce.

A more common and less dangerous mistake is to confuse this with the inedible yellow-staining mushroom (Agaricus xanthodermus), a common mushroom found worldwide in grassy areas which can be distinguished by its chemical smell reminiscent of phenol and its flesh which turns yellow on bruising. This fungus causes gastrointestinal symptoms of nausea and vomiting in some people.

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

Culinary use

There have been few studies on the nutritional value of mushrooms, with what is known derived from chemical analyses of the composition and few animal studies. Thus much of what is said about their nutritiousness is speculative. The water content of fresh Agaricus bisporus has been measured at 89%.

Common mushrooms are fairly rich in vitamins and minerals. The mushroom contains high amounts of vitamin B group, sodium, potassium and phosphorus. Raw mushrooms are naturally cholesterol and fat free. The mushrooms also have very low energy levels — five medium-sized common mushrooms added together only have 20 calories.

Common mushrooms have a unique flavor that can be matched by few other mushrooms. No specific flavor can be defined; most people describe the mushroom as "plain", but other people say that the common mushroom tastes slightly sweet or "meaty".

Like potatoes, apples and avocados, table mushrooms oxidize ("rust") quickly when exposed to air. When sliced and exposed to air for ten minutes or more, the mushrooms quickly soften, turn a brownish color, and lose their original flavor.

History of cultivation

Cultivation of Agaricus bisporus originated in France, when agriculturist Olivier de Serres noted that transplanting mushroom mycelium 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 sterilised, 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 cultivated in at least 70 countries around the world. In most supermarkets, common mushrooms are marketed as "table mushrooms" and are often packed in small quantities. Mushrooms may be sold sliced or whole.

Portobello mushroom

The 'Portobello mushroom' is a large brown strain of the same fungus, left to mature and take on a broader, more open shape before picking. Portobello mushrooms are distinguished by their large size, thick cap and stem, and a distinctive musky smell. Because of their size and the thickness of their fleshy caps, these mushrooms can be cooked in a range of different ways, including grilling and frying.

Crimini mushroom

Although sometimes described a sub-variety of the portobello mushroom, the Crimini or Cremini mushroom is actually an immature portobello. Marketers have begun to refer to Crimini mushrooms as baby Portobellos and Portabellinis or BabyBellas. Left to grow another 48 to 72 hours, a Crimini mushroom will more than quadruple in size, taking on the large-capped Portobello shape. They are more delicate in texture but still have the meaty Portobello flavor.

Agaritine

All mushrooms of the genus Agaricus contain the hydrazine agaritine (and lesser amounts of other hydrazines like gyromitrin), a suspected carcinogen.. Most hydrazines (over 80%) are known carcinogens.

Notes

References

• Benjamin, Denis R. (1995). Mushrooms: poisons and panaceas — a handbook for naturalists, mycologists and physicians. New York: WH Freeman and Company.
• Genders, Roy (1969). Mushroom Growing for Everyone. London: Faber.
• Kuo, M. (2004, January). Agaricus bisporus: The common mushroom Retrieved from the MushroomExpert.Com Web site:

External links

• Complete common mushroom nutrition info
• Mushroom cookbook and information
• An informative video illustrating the cultivation and harvest of Agaricus bisporus