Mushrooms are not plants, and require different conditions for optimal growth. Plants develop through photosynthesis, a process that converts atmospheric carbon dioxide into carbohydrates, especially cellulose. While sunlight provides an energy source for plants, mushrooms derive all of their energy and growth materials from their growth medium, through biochemical decomposition processes. This does not mean that light is an irrelevant requirement, since some fungi use light as a signal for fruiting. However, all the materials for growth must already be present in the growth medium. Mushrooms grow well at relative humidity levels of around 95–100%, and substrate moisture levels of 50 to 75%.
Mycelium, or actively growing mushroom culture, is placed on a substrate—usually sterilized grains such as rye or millet—and induced to grow into those grains. This is called inoculation. Inoculated grains are referred to as spawn. Spores are another inoculation option, but are less developed than established mycelium. Since they are also contaminated easily, they are only manipulated in laboratory conditions with a laminar flow cabinet.
All mushroom growing techniques require the correct combination of humidity, temperature, substrate (growth medium) and inoculum (spawn or starter culture). Wild harvests, outdoor log inoculation and indoor trays all provide these elements.
Mushrooms can be grown on logs placed outdoors in stacks or piles, as has been done for hundreds of years. Sterilization is not performed in this method. Since production may be unpredictable and seasonal, less than 5% of commercially sold mushrooms are produced this way. Here, tree logs are inoculated with spawn, then allowed to grow as they would in wild conditions. Fruiting, or pinning, is triggered by seasonal changes, or by briefly soaking the logs in cool water. Shiitake and oyster mushrooms have traditionally been produced using the outdoor log technique, although controlled techniques such as indoor tray growing or artificial logs made of compressed substrate have been substituted.
Shiitake mushrooms grown under a forested canopy are considered non-timber forest products In the Northeast[where?] shiitake mushrooms can be cultivated on a variety of hardwood logs including oak, American beech, sugar maple and hophornbeam. Softwood should not be used to cultivate shiitake mushrooms. The resin of softwoods will oftentimes inhibit the growth of the shiitake mushroom making it impractical as a growing substrate.
In order to produce shiitake mushrooms, 1 metre (3 foot) hardwood logs with a diameter ranging between 10–15 cm (4–6 in) are inoculated with the mycelium of the shiitake fungus. Inoculation is completed by drilling holes in hardwood logs, filling the holes with cultured shiitake mycelium or inoculum, and then sealing the filled holes with hot wax. After inoculation, the logs are placed under the closed canopy of a coniferous stand and are left to incubate for 12 to 15 months. Once incubation is complete, the logs are soaked in water for 24 hours. 7 to 10 days after soaking, shiitake mushrooms will begin to fruit and can be harvested once fully ripe.
Indoor growing provides the ability to tightly regulate light, temperature and humidity while excluding contaminants and pests. This allows consistent production, regulated by spawning cycles. This is typically accomplished in windowless, purpose-built buildings, for large scale commercial production.
Indoor tray growing is the most common commercial technique, followed by containerized growing. The tray technique provides the advantages of scalability and easier harvesting. Unlike wild harvests, indoor techniques provide tight control over growing substrate composition and growing conditions. Indoor harvests are much more predictable.
According to Daniel Royse and Robert Beelman, "[Indoor] Mushroom farming consists of six steps, and although the divisions are somewhat arbitrary, these steps identify what is needed to form a production system. The six steps are phase I composting, phase II fertilizing, spawning, casing, pinning, and cropping."
Six phases of mushroom cultivation
|1. Phase I composting||6–14 days||Regulate water and NH3 content through microbial action. |
Add fertilizer / additives
|2. Phase II composting or pasteurization||7–18 days via composting method, ~2 hours for pasteurization (heat sterilization)||Reduce number of potentially harmful microbes through further composting, or apply heat sterilization. |
Remove unwanted NH3.
|3. Spawning and growth||14–21 days||24 to 27 °C (75 to 80 °F); must be above 23 °C (74 °F); for rapid growth.
Must be below 27 to 29 °C (80 to 85 °F) to avoid damaging mycelia
|Add starter culture. |
Allow mycelium to grow through substrate and form a colony.
Depends on substrate dimensions and composition.
Finished when mycelium has propagated through entire substrate layer
|4. Casing||13–20 days||Promote the formation of primordia, or mushroom pins. |
Add a top covering or dressing to the colonized substrate.
Fertilizing with nitrogen increases yields.
|5. Pinning||18–21 days||Earliest formation of recognizable mushrooms from mycelium. |
Adjusting temperature, humidity and CO2 will also affect the number of pins, and mushroom size
|6. Cropping||Repeated over 7- to 10-day cycles||Harvest|
Complete sterilization is not always required or performed during composting. In some cases, a pasteurization step is not included to allow some beneficial microorganisms to remain in the growth substrate.
Specific time spans and temperatures required during stages 3–6 will vary respective to species and variety. Substrate composition and the geometry of growth substrate will also affect the ideal times and temperatures.
Pinning is the trickiest part for a mushroom grower, since a combination of carbon dioxide (CO2) concentration, temperature, light, and humidity triggers mushrooms towards fruiting. Up until the point when rhizomorphs or mushroom "pins" appear, the mycelium is an amorphous mass spread throughout the growth substrate, unrecognizable as a mushroom.
Carbon dioxide concentration becomes elevated during the vegetative growth phase, when mycelium is sealed in a gas-resistant plastic barrier or bag which traps gases produced by the growing mycelium. To induce pinning, this barrier is opened or ruptured. CO2 concentration then decreases from about 0.08% to 0.04%, the ambient atmospheric level.
- Wood chips or sawdust
- Mulched straw (usually wheat, but also rice and other straws)
- Strawbedded horse or poultry manure
- Waste or recycled paper
- Coffee pulp or grounds
- Nut and seed hulls
- Cottonseed hulls
- Cocoa bean hulls
- Cottonseed meal
- Soybean meal
- Brewer's grain
- Ammonium nitrate
Mushrooms metabolize complex carbohydrates in their substrate into glucose, which is then transported through the mycelium as needed for growth and energy. While it is used as a main energy source, its concentration in the growth medium should not exceed 2%. For ideal fruiting, closer to 1% is ideal.
One of the most sustainable ways of mushroom cultivation is using coffee grounds as a substrate. This process was pioneered by Prof. Chang Shuting  in the early 1990s while he worked at the Chinese University in Hong Kong. Coffee ground are sterile, and rich in fibers. It is more environmentally-friendly; as an estimated millions of kilos of coffee waste disposed in landfill every day, which could be diverted into sustainable food production. The spent substrate, after harvesting mushrooms, is enriched in essential amino-acids, and therefore an ideal feed for animals.
Pests and diseases
Parasitic insects, bacteria and other fungi all pose risks to indoor production. The sciarid fly or phorid fly may lay eggs in the growth medium, which hatch into maggots and damage developing mushrooms during all growth stages. Bacterial blotch caused by Pseudomonas bacteria or patches of Trichoderma green mold also pose a risks during the fruiting stage. Pesticides and sanitizing agents are available to use against these infestations. Biological controls for insect sciarid and phorid flies have also been proposed.
A recent epidemic of Trichoderma green mold has significantly affected mushroom production: "From 1994–96, crop losses in Pennsylvania ranged from 30 to 100%".
Commercially cultivated fungi
- Agaricus bisporus, also known as champignon and the button mushroom. This species also includes the portobello and crimini mushrooms.
- Clitocybe nuda, or blewit, is cultivated in Europe.
- Auricularia polytricha or Auricularia auricula-judae (Tree ear fungus), two closely related species of jelly fungi that are commonly used in Chinese cuisine.
- Flammulina velutipes, the "winter mushroom", also known as enokitake in Japan
- Hypsizygus tessulatus (also Hypsizygus marmoreus), called shimeji in Japanese, it is a common variety of mushroom available in most markets in Japan. Known as "Beech mushroom" in Europe.
- Lentinus edodes, also known as shiitake, oak mushroom. Lentinus edodes is largely produced in Japan, China and South Korea. Lentinus edodes accounts for 10% of world production of cultivated mushrooms. Common in Japan, China, Australia and North America.
- Pleurotus species are the second most important mushrooms in production in the world, accounting for 25% of total world production. Pleurotus mushrooms are cultivated worldwide; China is the major producer. Several species can be grown on carbonaceous matter such as straw or newspaper. In the wild they are usually found growing on wood.
- Rhizopus oligosporus - the fungal starter culture used in the production of tempeh. In tempeh the mycelia of R. oligosporus are consumed.
- Sparassis crispa - recent developments have led to this being cultivated in California. It is cultivated on large scale in Korea and Japan.
- Tremella fuciformis (Snow fungus), another type of jelly fungus that is commonly used in Chinese cuisine.
- Tuber species, (the truffle), Truffles belong to the ascomycete grouping of fungi. The truffle fruitbodies develop underground in mycorrhizal association with certain trees e.g. oak, poplar, beech, and hazel. Being difficult to find, trained pigs or dogs are often used to sniff them out for easy harvesting.
- Ustilago maydis (Corn smut), a fungal pathogen of the maize plants. Also called the Mexican truffle, although not a true truffle.
- Volvariella volvacea (the "Paddy straw mushroom.") Volvariella mushrooms account for 16% of total production of cultivated mushrooms in the world.
- Fusarium venenatum - the source for mycoprotein which is used in Quorn, a meat analogue.
Production regions in North America
The borough of Kennett Square is a historical and present leader in mushroom production. It currently leads production of Agaricus-type mushrooms, followed by California, Florida and Michigan.
Other mushroom-producing states:
- East: Connecticut, Delaware, Florida, Maryland, New York, Pennsylvania, Tennessee, and Vermont
- Central: Illinois, Oklahoma, Texas, and Wisconsin
- West: California, Colorado, Montana, Oregon, Utah and Washington
Vancouver, British Columbia, also has a significant number of producers — about 60 as of 1998 — mostly located in the lower Fraser Valley.
Production in Europe
The oyster mushroom cultivation lately is taking off in Europe. Many entrepreneurs nowadays find it as a quite profitable business, a start-up with a small investment and good profit. Italy with 785,000 tonnes and Netherlands with 307,000 tonnes are between the top ten mushroom producing countries in the world. The world’s biggest producer of mushroom spawn is also situated in France.
According to a research carried out on Production and Marketing of Mushrooms: Global and National Scenario Poland, Netherlands, Belgium, Lithuania are the major exporting mushrooms countries in Europe and countries like UK, Germany, France, Russia are considered to be the major importing countries.
Education and training
Oyster mushroom cultivation is a sustainable business where different natural resources can be used as a substrate. The number of people becoming interested in this field is rapidly increasing. The possibility of creating a viable business in urban environments by using coffee grounds is appealing for many entrepreneurs.
Since mushroom cultivation is not a subject available at school, most urban farmers learned it by doing. The time to master mushroom cultivation is time consuming and costly in missed revenue. For this reason there are numerous companies in Europe specialized in mushroom cultivation that are offering training for entrepreneurs and organizing events to build community and share knowledge. They also show the potential positive impact of this business on the environment.
Courses about mushroom cultivation can be attended in many countries around Europe. There is education available for growing mushrooms on coffee grounds, more advanced training for larger scale farming, spawn production and lab work and growing facilities.
Events are organised with different intervals. The Mushroom Learning Network gathers once a year in Europe. The International Society for Mushroom Science gathers once every five year somewhere in the world.
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