Agaricus bisporus, the white button pizza mushroom, portabella, portobello, portabello, portobella, crimini, Tom Volk's Fungus of the Month for April 2001

Tom Volk's Fungus of the Month for April 2001

This month's fungus is Agaricus bisporus, the white button mushroom or pizza mushroom, also known as Portabella and Crimini

by Tom Volk and Kelly Ivors

For the rest of my pages on fungi, please click TomVolkFungi.net

Kelly Ivors with a crop of large portabella This is my first collaborative Fungus of the Month! This month's co-author, shown to the left, is Kelly Ivors, was (at the time this was written) a graduate student at Penn State University. Kelly has finished her Ph.D. project of identifying (through PCR and other methods) the bacterial and fungal associates important in composting, the first step in producing this month's fungus. You'll see the important role of composting in the production of these mushrooms further down the page. Kelly has finished her PhD and is now a postdoc at the University of California, Berkeley.

Agaricus bisporus is the interesting name for this mushroom. Agaricus, cleverly, means gilled mushroom. In the early days of mycology, every gilled mushroom was placed in the genus Agaricus. Now Agaricus is restricted to saprophytic mushrooms with a chocolate brown spore print and usually an annulus (ring) around the stalk. The epithet "bisporus" refers to the two-spored basidia lining the gills. You'll recall that most mushrooms have 4 basidiospores on each basidium, with each spore containing one of the 4 haploid products of meiosis. These spores are shed separately and must find a mate in order to form mushrooms again ( see the page on Schizophylum commune for a good explanation of mating types in these so-called heterothallic fungi.) Agaricus bisporus produces only two basidiospores on each basidium. Each of the two spores contains two of the products of meiosis-- the consequence of this is that each spore already contains the nuclei needed for sexual reproduction and does not need to find a mate. It has, in effect, already mated with itself! Such a system is called secondary homothallism. This ensures that every spore that lands on a suitable substrate is capable of forming mushrooms. However the consequence of this mating system is that it is very difficult and tedious to do breeding work with Agaricus.

An apparently older name for this mushroom is Agaricus brunnescens, referring to the oxidative "browning" reaction when the mushroom is bruised. There is a movement underway to conserve the name Agaricus bisporus, so we will refer to it by that species name here.

portabella, crimini and white button Agaricus bisporus is the most commonly grown mushroom in the United States, accounting for up to 90% of the mushroom production here. However in other parts of the world, Agaricus accounts for very little production, so Agaricus accounts for less than 40% of worldwide production. The mushrooms crop is worth more than $800 million per year in the US. The state of Pennsylvania produces about 50% of the fresh mushrooms sold in the United States, with most of that production occurring in the southeast corner of the state.

Agaricus bisporus has increased in popularity in North America with the introduction of two brown strains, Portabella (sometimes also spelled portobello, portabello, or portobella) and Crimini. The three mushrooms you see to the right are all actually the same species. Portabella is a marketing name the mushroom industry came up with for more flavorful brown strains of Agaricus bisporus that are allowed to open to expose the mature gills with brown spores; crimini is actually the same brown strain that is not allowed to open before it is harvested. Per capita consumption of fresh Agaricus in the United States is about 2.2 pounds (1 kg) per year. Many people love to eat mushrooms in omelets, in stir-fry, on salads, and in soups. Marinated and grilled portabella is a treat not to be missed. Mushrooms are an excellent low-calorie meat substitute with great nutritional value and lots of vitamins..

The methods for growing Agaricus are vastly different from growing most other sorts of mushrooms. Agaricus is a secondary decomposer, which means that bacteria and other fungi have to break down raw materials before Agaricus can grow. This is the process known as composting. Other kinds of cultivated mushrooms such as oyster mushrooms, enoki and shiitake are primary decomposers of wood, and composting is not necessary.

Throughout the world, the button mushroom Agaricus bisporus is commercially grown as an agricultural commodity. In North America, A. bisporus has been the primary mushroom grown as a farm crop since the late 1800s.Because these mushrooms are heterotrophic (not able to make their own food as autotrophic plants can), A. bisporus fruit bodies must obtain their nutrients from compost. The aim of commercial mushroom composting is to produce a substrate that is optimal and selective for mushroom mycelium growth. The compost must not contain foods upon which other competitor fungi may grow. During composting, nutrients favored by competitor microbes progressively decrease, while nutrients selective for the mushroom mycelium accumulate. The basic ingredients of mushroom compost are wheat straw and horse manure, but substitutes (called "synthetics") made from hay, corncobs, cotton seed hulls, and other fibrous materials, are also used as bulk ingredients. Supplements such as poultry manure and dried brewers grain can also be added to increase the nitrogen content of the compost.

Mushroom farming is still an art, but science has quantified and explained a number of phenomena in the process.Mushroom production involves 6 sequential steps, which consist of: Phase I composting, Phase II composting, spawning, casing, pinning, and cropping.

Phase I composting

phase I composting The preparation of mushroom compost is usually done in two stages. The breakdown of raw ingredients begins in Phase I. Phase I is characterized by building the raw ingredients into long rectangular piles approximately 2 m high called "ricks" or "windrows" (figure to the left). These stacks are then periodically turned, watered, and formed. This phase is essentially a microbiological process resulting in release of energy and heat. To favor the development of relatively high temperatures, aerobic conditions are maintained by aerating the compost during repeated mixing or turning. Temperature fluctuations during this phase are paralleled by similar changes in the numbers of thermophilic (heat loving) bacteria. These organisms start to grow rapidly and release energy in the form of heat. Thermogenesis by microorganisms initiates the heating of Phase I and also produces heat in Phase II. The internal temperature of a compost pile can reach up to 80^oC. Traditional Phase I composting lasts from 7 to 14 days depending on the condition of the material at the start and its characteristics at each turn. It is considered complete when the raw ingredients have become pliable and are capable of holding water. The odor of ammonia should be sharp, and the color of the compost is dark-brown in color, indicating caramelization and browning reactions have occurred.

Phase II composting

phase II composting It is primarily the control of the environment that distinguishes Phase II from Phase I. Typically, compost is loaded into wooden trays, which are stacked, and then placed in specially designed rooms where the environmental conditions can be manipulated (figure to the right). Phase II is commonly referred to as peak-heating and may be initiated by steam. Pasteurization is accomplished early in the Phase II operation and is necessary to kill many insects, nematodes, and other pests or pathogens that may be present in the compost. Pasteurization requires air and compost temperatures of 66^oC for a minimum of 2 hours. Once pasteurization is accomplished, cool air is introduced into the Phase II room to assure adequate oxygen, and to help dissipate ammonia. An important function of Phase II microbes that survive the pasteurization process is the conversion of residual ammonia into protein. Because ammonia is lethal to the mushroom mycelium, it must be removed by the end of Phase II. A stage is reached when the available food supplies for organisms inhabiting the compost become quite limiting, hence their activity decreases. The substrate is now set for A. bisporus to take over, and the substrate is said to be 'selective' for the growth of the mushroom. Once the odor of ammonia is no longer present, Phase II is over and the compost temperature can be dropped to 24^oC for the addition of the mushroom mycelium, called "spawning."

Spawning

spawn of Agaricus in a breathable bag Mycelium of A. bisporus propagated vegetatively on sterilized cereal grain is known as "spawn" (figure to the left). Commercial mushroom growers purchase spawn from any of about a dozen spawn companies. Farmers have a choice of growing different strains, ranging from smooth white, off-white, cream, to brown capped mushrooms. These strains vary in flavor, texture, and growing requirements. Spawn is introduced and thoroughly mixed into the compost with a special machine that mixes the compost and spawn with small tines or finger-like devices (figure below and to the right). After spawning, the compost is maintained at approx. 24^oC, and relative humidity and CO₂ levels are kept high to minimize drying of the compost. The spawn will begin to grow and produce a mixing spawn into the compost thread-like network of mycelium throughout the compost. Complete colonization of the compost usually requires 12-20 days, depending on the spawning rate and environmental conditions.

Casing

To promote mushroom formation, casing soil is added as a surface layer (1.5 - 2 inches deep) on the colonized compost. The important transition from the vegetative to the reproductive stage of A. bisporus takes place in the casing layer, which is usually a mixture of peat and limestone. Mushrooms form only after the compost is covered with a layer of casing material. In addition to stimulating fruit body formation, the casing layer provides moisture essential for high yields and anchorage for the developing mushrooms. Casing materials do not provide any nutrients to the mushroom mycelium. Environmental conditions after casing are the same as during spawn growth. The compost temperature is kept around 24^oC for up to 5 days after casing to allow for the spawn to grow through the casing layer. Before the mushroom "pins" (primordia) start to develop, later is applied intermittently to raise the moisture level of the casing layer to field capacity.

Most Agaricus is grown in a place with high relative humidity and not much light. Some fungi use light as their signal to form fruiting bodies, but not Agaricus bisporus; the casing layer provides all the signal that is needed. Some Agaricus facilities are actually in underground caves, but more commonly "mushroom houses" are built. These facilities are somewhat like small barns, with relatively small rooms so that humidity and temperature can be more easily controlled.

Pinning

mushroom pins or primordia Primordia or "pins" are knots of mycelium that eventually develop into mushrooms (figure to the left).Once the mycelium has reached the surface of the casing layer, the mushroom is induced to pin by reducing both the air temperature (to 16-18^oC) and the CO₂ concentration (to 0.08%). Fruiting occurs in well-defined flushes or breaks with the first harvestable mushrooms appearing 18 to 21 days after casing.

Cropping

the Agaricus mushroom crop The mushroom crop grows in repeating 3- to 5-day cycles called "flushes" or "breaks" (Figure to the right). These flushes are followed by a few days when no mushrooms are available to harvest. The individual flushes tend to produce progressively fewer mushrooms. In commercial practice, three to five flushes are picked before the crop is removed to make room for the next. Most mushroom farmers crop their mushrooms for 30 - 40 days. During cropping, the casing layer is watered 2 to 3 times per week and air temperatures are maintained between 15-18^oC. This temperature range favors mushroom growth and lengthens the life cycles of both disease pathogens and pests. Most strains of the button mushroom (except for the portabella strains) are picked before the veil breaks and the stem elongates.

An entire production cycle, from composting to final harvest, can take up to 15 weeks. As described in the paragraphs above, it takes many steps and changes in environmental conditions to grow mushrooms. The next time you purchase button mushrooms at your local grocery store, realize that Agaricus bisporus requires a level of care and attention to detail beyond the extent of ordinary agriculture.

A. bitorquis, A. augustus, A. campestris, and A. sylvicola

There are, of course many other species of Agaricus. In our area Agaricus bitorquis, the "double ring agaricus" (shown in A) is rather common and abundant, and has a much stronger flavor than cultivated A. bisporus. B is "the Prince," Agaricus augustus, a rather large mushroom more common in western North America with a pleasant almond flavor. C is known as the "meadow mushroom," Agaricus campestris; it is the most closely related wild relative of A. bisporus. The "horse mushroom," Agaricus arvensis, is shown in D. . The edible wild Agaricus have mostly strong and delicious flavors, ranging from a stronger version of the white button to some that are even almondy in smell and flavor. They're yummy! However (isn't there always a however?), Agaricus species are notoriously difficult to identify, and some of the species, (such as A. xanthodermus and A. placomyces) are poisonous, causing mild to severe gastrointestinal upset, so you must be *absolutely* sure of your identification, not only to the genus, but to the species, before eating any wild mushrooms.

For more information on mushroom growing you might want to visit the site of the American Mushroom Institute, the trade association of the American mushroom industry.

If you are interested in growing your own mushrooms you can contact Field and Forest Products, producers of specialty mushroom spawn and supplies. Or you can visit their website at this page.

I hope you enjoyed learning something about the pizza mushroom today. You should have gained some appreciation of the art and skill that goes into growing these mushrooms. More importantly, now you can call up your favorite pizza place and say "I'd like a large pizza with Agaricus bisporus." I bet they'll hang up on you....

Thanks again to Kelly Ivors for being this month's coauthor!

If you have anything to add, or if you have corrections or comments, please write to me at volk.thom@uwlax.edu

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