Mushrooms

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Key Factspending-external-review

  • Mushrooms are a good source of B vitamins and they are the only source of vitamin D in the produce section and usually the highest source of selenium.
  • Of 300 edible mushroom species, 30 have been domesticated and 10 are grown commercially.
  • Agaricus spp. are the leading mushroom crop worldwide, and accounted for approximately 98% of the total U.S. mushroom production from 2012-2013.
  • Oyster (Pleurotus spp. ) and shiitake mushrooms rank second and third respectively in worldwide production.
  • Pennsylvania produces about half of the U.S. mushroom crop; California and Florida rank second and third respectively in mushroom production.
  • Production systems for domesticated varieties vary by type of mushroom, including indoor and outdoor systems. Either natural or synthetic substrates may be used.
  • Wild harvest remains the largest source of commercially important mycorrhizal species (such as truffles, chanterelles, and morels), despite scientific advances in domestication.
  • Food safety issues associated with edible mushrooms have most frequently been attributed to improper food preservation and handling.

Introduction

Mushrooms are unique in the produce section because they are fungi and not vegetables. What we typically think of as a mushroom is the fleshy, spore-bearing fruiting body of a fungus. The mushrooms we eat are generally composed of a stipe (stem), a pileus (cap), and lamellae (gills). There are, however, many morphological varieties of mushrooms and not all varieties have these features. There are approximately 14,000 different species of mushroom, many of which are inedible. Mushrooms form from a small structure called a primordium which grows on some type of substrate. The primordium enlarges into an egg shaped structure composed of hyphae, called a “button.” Mycelium, called the universal veil, surrounds the button initially. As the button grows, the veil breaks. Remnants of the veil on mature mushrooms often appear as warts or may be found hanging from the cap. The most popular species of edible mushroom is actually sold in three different forms. The white button, cremini, and Portabella mushrooms are all the same species — Agaricus bisporus.  Portobello mushrooms are the mature form of the species and cremini mushrooms are simply a different pigmented variety from the white button. Other commercially available species include oyster, shiitake, chanterelle, enoki, porcini, lion’s mane, and morel. In 2012 -2013, there were 298 mushroom growers in the U.S. who produced nearly 896 million pounds of mushrooms worth an estimated $1.11 billion. Pennsylvania (546 million pounds) and California (118 million pounds) are, by far, the leading producers of mushrooms but it is becoming more common for small-scale producers to sell to restaurants and at farmers’ markets.

Common Edible Varieties

mushrooms Images from The Mushroom Council (http://www.mushroominfo.com/varieties/).

Foodborne Outbreaks

There are few documented cases of foodborne illness due to pathogens associated with mushrooms in the U.S.  A multistate outbreak of Staphylococcal food poisoning was associated with canned mushrooms imported from the People’s Republic of China.  In 1989, a reported 102 people in Mississippi, New York and Pennsylvania became ill shortly after eating products made using the canned mushrooms. Staphylococcal enterotoxin was isolated from unopened cans in several establishments.  Spoilage prior to processing likely created an anaerobic environment which inhibited the normal microbiota present on mushrooms allowing Staphylococcus aureus to grow and produce the enterotoxin. The toxin was heat stable and thus survived the canning process which kills bacteria. A restaurant-associated outbreak of Salmonella Heidelberg was linked to improperly handled mushrooms, the previously canned mushrooms may have come in contact with raw meat and poultry products.

To contribute to the Mushroom-associated Foodborne Outbreak section, please follow this link: http://fsi.colostate.edu/suggest-a-topic/

Production

Mushroom farm
Photo by Orebokech.

The general process for farming mushrooms involves six sequential steps, outlined below for Agaricus species. The process is similar for other species, with the following slight process differences by species. Oyster mushrooms are farmed by a similar process, but require more humidity and fresh air and grow in tubular plastic bags rather than horizontal racks. Shiitake mushrooms are farmed using either natural oak logs or synthetic “logs” made from oak sawdust packed in poly bags. Enoki mushrooms are farmed in plastic bottles at colder temperatures (45°F) and only produce one set of fruiting bodies per crop. Beech mushrooms are farmed similarly to Enoki mushrooms, but at higher temperatures (60-64°F). Maitake mushrooms are farmed on synthetic “logs” similar to shiitake mushrooms, and only produce one set of fruiting bodies per crop.

  1. Phase I composting– Making the compost:
    • The first phase of composting begins by mixing and wetting the bulk compost ingredients on a large concrete slab, called a wharf. Bulk compost ingredients may be natural (manure) or synthetic. Compost can include straw, peat moss, soybean or canola meal, chicken manure, horse bedding straw, grape must from wineries, potash, urea, ammonium nitrate and/or lime. Nitrogen and gypsum supplements are added by topdressing the compost pile. A compost turner is utilized to mix and aerate the compost, and water is added as the bulk ingredients are mixed. The mixture is then stacked in piles with tight sides and loose centers, or ricks. Resulting conditions favor the growth of naturally occurring aerobic microorganisms, which produce heat through aerobic fermentation. Turning the hot (145°F – 175°F) compost pile regularly prevents conditions favoring the growth of anaerobes. Phase I composting usually occurs outdoors, and takes 7-14 days to complete.
  2. Phase II composting – Finishing the compost:
    • During this phase, which lasts 10-14 days, the compost is pasteurized to kill any pests present in the compost and remove ammonia formed during Phase I composting. High-temperature and low-temperature protocols are available. In the high-temperature protocol, the compost is heated to >145°F for 6 hours either through the naturally occurring heat produced by microorganisms or though added steam. After 6 hours, the temperature is lowered to 140°F, then the compost is allowed to cool at a rate of 2-3°F per day until the ammonia is dissipated. During the low temperature protocol, the compost is heated to approximately 126°F using the same methods as in the high temperature protocol, then the temperature is lowered by about 2°F a day for 4-5 days until the ammonia is dissipated. Pasteurization is typically computer controlled, closely monitored, and well documented.
  3. Spawning:
    • During this phase, the compost is inoculated with commercially produced mycelium (mushroom spawn) and held under controlled conditions to allow the spawn to colonize the substrate. This period, called the spawn run, generally takes 14-21 days.
  4. Casing:
    • In this step, casing (a top-dressing of clay-loam field soil mixed with peat moss, ground limestone and occasionally spent, reclaimed mushroom substrate) is applied to the spawn-run compost and the crop is heavily watered. The casing acts as a water reservoir, and is where thicker mycelia (called rhizomorphs) form. After casing, the compost is kept at approximately 75°F for 5 days with high relative humidity, then the temperature is lowered by 2°F per day until young mushrooms (pins) form. Although casing may be pasteurized either on farm or by commercial suppliers prior to receipt, recent research suggests that pasteurization of casing may not be beneficial as it may destroy microorganisms that competitively inhibit the growth of Listeria monocytogenes.
  5. Pinning:
    • When the rhizomorphs change from the vegetative growth stage to the fruiting growth stage, young mushrooms, called pins, push up through the casing layer. Pins develop in response to the introduction of fresh air into the growing room, lowering the ambient concentration of carbon dioxide. The timing of fresh air introduction affects both yield and quality of the harvest, and is best performed when the mycelium shows on the surface of the casing.
  6. Cropping:
    • During this phase, mature mushrooms are harvested in 3-5 day periods called “breaks.” A complete harvest usually takes 30-42 days, but can go on for up to 150 days.

Food Safety

There are several pathogens that are of concern in mushroom production. Campylobacter, a leading cause of bacterial enteritis was isolated from mushrooms sampled in retail markets. Clostridium botulinum is of particular concern in mushroom production and improper home preservation has been linked multiple times to illness and death. The spore forming bacterium can thrive in improperly packaged mushrooms. Mushrooms’ high rate of respiration can rapidly create an anaerobic environment within sealed plastic packages which favors growth of C. botulinum and ultimately production of botulinal toxin. The general recommendation for consumers is to store mushrooms in the refrigerator, unwashed in a paper bag, to prevent trapping moisture which will promote spoilage.

Basket of wild edible mushrooms
Wild edible mushrooms. Photo by Carl F. Bagge.

Wild Mushrooms The vast majority of food illness associated with mushrooms is from the consumption of wild picked mushrooms. Wild mushrooms may contain several toxins, such as muscimol and muscarine, which can cause vomiting, diarrhea, confusion, visual disturbances, salivation, and hallucinations. Some mushrooms even contain toxins which can cause hepatic and renal failure leading to death. Domestic, farm raised mushrooms are an incredibly safe and nutritious food; wild mushrooms should only be consumed under the guidance of a trained mycologist, or mushroom expert who meets criteria required for wild mushroom identification by state or local health departments. Cooking or drying can not reduce the toxicity of poisonous mushrooms.

Nutrition

Nutritionally, mushrooms are often grouped with produce but they add some nutrients not found, or found in limited amounts, in other produce.  Five medium raw mushrooms have only 20 calories, no fat, and provide 3 grams of protein, 3 grams of carbohydrate, and 1 gram of dietary fiber. They are often marketed as a “meat replacer” due to their protein content and fleshy texture.  Mushrooms are a good source of the B vitamins riboflavin, pantothenic acid, and niacin. They also provide several important minerals such as selenium, ergothioneine, copper, and potassium.

Consumption

In the United States, the most popular varieties of mushrooms include the “whites” (agaricus), criminis, portabellas, and shiitakes. In 2009, Americans consumed an average of 3 pounds of mushrooms per person. Of the total amount, 2.2 pounds were fresh mushrooms and 0.8 pounds were processed mushrooms. In the 2006-2007 Population Atlas Survey of Exposures, 33.5% of the survey cohort reported eating fresh mushrooms within the past 7 days.

References

Authors

Daren Harrison

Daren Harrison

Marisa Bunning

Marisa Bunning

Lauren Pecher

Lauren Pecher