About 95 percent of all cultivated mushrooms are saprobic and can be cultivated on various dead organic materials such as straw, wood, and wastepaper. Some saprobes are produced in what are little more than cottage industries, while others are grown in huge, highly efficient factories, their production and sale the basis of multimillion-dollar industries. Certain saprobes, called the sugar fungi, can only use sugars available in the growth medium. Others, including many edible varieties, can break down much more complicated molecules, such as cellulose or other highly resistant compounds found in wood.
Although around fifty saprobic mushrooms are cultivated in China, only eight of these make up the lion's share of the market worldwide: the button mushroom (Agaricus bisporus and A. bitorquis), shiitake (Lentinula edodes), oyster mushroom (Pleurotus spp.), wood ear (Auric-ularia spp.), straw mushroom (Volvariella volvacea), enokitake (Flam-mulina velutipes), white jelly fungus (Tremella fuciformis), and nameko (Pholiota nameko). The other cultivated saprobic mushrooms are consumed in relatively small quantities, although locally they can be very important. For example, Hypsizygus marmoreus, Hericium aff. eri-naceus, and Grifola frondosa are important in East Asian countries, and Stropharia rugosoannulata is eaten widely in Germany, Hungary, and Poland. Many other saprobic mushrooms are not cultivated at all, and supplies of many are restricted to what can be collected from nature.
The principles of growing many saprobic mushrooms can be found in scientific literature and other published sources, some of which include descriptions of apparently detailed methods. However, there is a significant difference between producing a few mushrooms in a pilot trial and continuously turning out a consistent, high-quality, high-yielding, disease-free product to be sold to a discerning public. Many important factors are discovered only through experience, and these trade secrets are often jealously guarded. Hygiene and climatic controls are crucial to a successful operation, and failure to prevent and control pests and diseases can jeopardize one or more production cycles. Methods developed in one country are rarely directly applicable to another without first making adjustments. Different growth media, pests, diseases, and ambient climatic conditions will affect the productivity of particular strains.
As previously mentioned, people in Asia are accustomed to eating a wide range of mushrooms, whereas people in Australasia, Europe, and North America have only recently begun to acquire a taste for something more exotic. Therefore, careful market research is required to determine if there is a local demand for a new mushroom and, if so, what quantity the market can absorb—and at what price. If mushrooms are to be exported, it is also necessary to determine whether it is possible to produce mushrooms at a price and quality that will compete with those produced in Asian countries such as China, India, Indonesia, Thailand, and the Philippines, where economies of scale and cheap labor allow production costs to be kept low. It is particu larly important to consider the Chinese mushroom industry, which employs ten million people, generates $1 billion in exports, and which increased its production from 0.59 million tons in 1986, 26.8 percent of the world's production, to 4.35 million tons in 1998, more than half of the world's requirements (Sun and Xu 1999).
Quarantine restrictions should also be considered, as some countries prohibit the importation of mushrooms that might have been in contact with contaminated soil. Species with pathogenic traits may also be prohibited imports. For example, Pleurotus ostreatus, a common species of oyster mushroom cultivated in many parts of the Northern Hemisphere, cannot be imported into New Zealand because it does not occur there naturally and can pose a risk to plant health (Buchanan 1996).
When it comes to mushrooms as familiar as the button mushroom (Agaricus bisporus and A. bitorquis), there is a wealth of information available on cultivation. Such subjects as the buildings and equipment required, how to prepare a growing medium, where to get the best strains of mushroom for the climate and growing medium, and what pests and diseases are likely to be encountered have all been widely studied (Penn State Mushroom Laboratory 2002). There are societies and research organizations, such as the American Mushroom Institute and the Mushroom Council, that coordinate market analysis, advertising, and research, and that hold educational conferences, farm visits, and training courses. The Penn State University mushroom industry short course has been given annually in Pennsylvania since 1959. There are seminars on the cultivation of specialty mushrooms as well, such as those run by Paul Stamets, founder and president of Fungi Perfecti.
One essential component in commercial mushroom production is the use of high-quality spawn. Spawn is a culture of a fungus used to inoculate the medium on which the mushroom will grow and fruit, called the substrate. It may be purchased from a spawn company or produced using starter cultures and appropriate equipment. Unfortunately it is rarely possible to simply produce a starter culture from a fresh mushroom and then expect to obtain quality spawn. Not just any spawn will do. It must be one produced from a superior strain, selected from the dozens available, and capable of producing the best-quality mushrooms at the best price for the substrate and ambient climate, a task that usually requires a lot of research. During this preliminary research it is also necessary to find out how much latitude is allowable in the control of environmental conditions and substrate. The methods adopted or developed for the cultivation of the mushroom must also be efficient in terms of staff time, energy costs, and materials.
Button mushrooms (Agaricus bisporus) growing in plastic bags filled with substrate. Huon Valley Mushrooms, Australia. EDIBLE. (Hall)
A high standard of hygiene is essential when cultures and spawn are being prepared and when the fungus is first introduced into the substrate. Special items of equipment such as a sterilizing autoclave and a lamina flow cabinet or sterile room are needed to do this. Nevertheless, no matter how much care is taken, there still may be a build-up of pests and diseases, and expertise and special methods must then be used to deal with these problems. For some specialty mushrooms there may be only skeletal published information available, with crucial commercial details treated as trade secrets. Finally, before mushrooms are produced in commercial quantities, the grower must have already made contacts in the marketplace. The mushrooms must then be effectively marketed, packaged to enhance shelf life and appeal, and labelled to comply with food regulations as well as to attract, inform, and educate the consumer.
Agaricus bisporus and Agaricus bitorquis (Button Mushroom)
The impetus for developing cultivation techniques may be a mushroom's popularity, but the ease and efficiency with which it can be cultivated, the convenience of the product, and its price are very important. It is for these reasons that more button mushrooms are consumed in the Western world than any other type of mushroom.
Button mushrooms are typically cultivated on a composted medium made up of moistened wheat or barley straw, horse and/or chicken manure, and gypsum (calcium sulfate—plaster of Paris). The raw ingredients are mechanically mixed and brought to a suitable moisture content. Composting material is formed into ricks, also known as windrows, 2-3 m wide and high, or may first be formed into piles before preparation of ricks. Piles and ricks are mechanically turned at regular intervals. Supplements to regulate nitrogen content may be added. On a simple concrete pad this Phase I composting will take more than three weeks, but the speed can be increased, aerobic conditions maintained, and the level of unpleasant odors reduced by introducing air into the composting material through pipes and holes in the concrete pad. During composting, the internal temperature should be 65-80°C.
Traditionally Phase I composting occurs outdoors, but increasingly strict environmental restrictions in many countries are encouraging indoor composting in bunkers. Indoor composting reduces odor emissions and allows environmental conditions to be closely monitored and controlled in order to achieve a more consistent product. Successful production of compost is both an art and a science. Parameters
Mechanical assembly of compost materials for button mushroom (Agaricus bisporus) cultivation. Prewetted bales of straw are opened, and straw mixed with poultry manure and gypsum is added from the hopper on the right. Meadow Mushrooms, New Zealand. (Buchanan)
Completion of mechanical assembly. The compost mixture will be placed in piles and ricks (in background) for Phase I composting. Meadow Mushrooms, New Zealand. (Buchanan)
Turning button mushroom (Agaricus bisporus) compost. Meadow Mushrooms, New Zealand. (Buchanan)
Air pumped through the concrete floor assists in the preparation of the compost. Meadow Mushrooms, New Zealand. (Buchanan)
such as pH, moisture, ammonia, nitrogen, ash, actinomycete concentration, and carbon-nitrogen ratio can be prescribed and measured, but compost quality is often best assessed from experience. This usually involves careful evaluation of odor, appearance, and texture of handfuls extracted from deep within the composted material.
Mushrooms are cultivated on compost in wooden trays, in bags, or on shelves. The compost may be mechanically transferred directly into these containers, followed by pasteurization, or alternatively the compost may be pasteurized and spawned before it is loaded into the growing containers. Pasteurization (also known as Phase II composting) requires temperatures of 50-60°C for five to eight days, during which time an adequate supply of air is essential. In this phase the nutritional status of the substrate is altered to favor growth of the button mushroom and to eliminate potentially contaminating microorganisms and pests.
Spawn is prepared by inoculating sterilized grain with a high-yielding, commercially bred strain of the button mushroom. After cooling the pasteurized compost, spawn is introduced, allowed to colonize for some days, and a layer of casing is applied. A form of spawn known as casing inoculum may be added to the casing layer to improve yield.
In countries where the button mushroom industry is most highly developed, composting, spawn-making, casing, and mushroom production may be performed by separate companies. Compost companies may supply compost ready for pasteurization or compost already spawned. The growing rooms of farms that employ a shelf system, for example, can be rapidly loaded by trucks simultaneously delivering spawned compost and the overlying casing layer. In modern Dutch farms, mechanically harvested mushrooms are grown in large rooms of 1000 sq. m growing surface; smaller rooms with 300 sq. m growing surface are used for hand-harvested mushrooms (J. Janssen, personal communication).
A temperature of 16-20°C with high humidity and low carbon dioxide is required for fruiting, although optimal conditions will vary with each strain used. Mushrooms fruit and are harvested in flushes, six to twelve days apart, with usually three to four flushes per crop. A typical mushroom farm is run on a continuous cycle, with several rooms each at a different stage in the process. Mushrooms are harvested at different stages of maturity to generate products of different appearance (for example, buttons, cups, and flats). Mushrooms to be sold fresh are handpicked, while those to be canned are mechanically
A sophisticated shelving system with automatic watering used for cultivating button mushrooms (Agaricus bisporus). Quality Mushrooms, New Zealand. (Buchanan)
harvested. Selective breeding has produced a range of commercially available strains, including the firmer Swiss brown strain.
Pests and diseases can be a major problem, not just with button mushrooms but with all cultivated species. Bacterial blotch, viruses, and flies can wipe out a producer's commercial venture (Fletcher et al. 1989).
In industrialized countries, most button mushrooms are produced by large, efficient, and highly mechanized companies. This, in addition to the requirement that mushroom growers meet strict environmental pollution standards, means that small new growers may find it difficult to become established. Nevertheless, a number of publications provide a good introduction to the cultivation of button mushrooms. Due to the scale of production in some parts of the world, the disposal of spent mushroom compost presents a major headache for button mushroom growers. In 1994 a conference was held in Philadelphia where the problems were highlighted (Wuest et al. 1994).
Lentinula edodes (Shiitake)
Cultivation of shiitake originated in China about a.d. 1000, but much of its technological development (and the capture of its marketing
Shiitake (Lentinula edodes) sawdust spawn has been placed in holes drilled into bedlogs and then sealed with wax. Tottori Mycological Institute, Japan. (Buchanan)
Bedlogs inoculated with shiitake and stacked under a canopy of bamboo. Tottori Mycological Institute, Japan. (Buchanan)
name) has come from Japan. Although shiitake is primarily consumed as a food, a lot of attention is paid to the medicinal properties of lentinan, a polysaccharide that can be extracted from this fungus (Chang et al. 1993, Mizuno 1995).
The traditional method of growing shiitake—a method that arguably produces the highest-quality product—involves growing it on logs in a shady situation. Just before bud break in spring, logs about 1 m long and 75-150 mm in diameter are cut from trees and carefully dried. Oak (Quercus) is traditionally used, but other hardwoods can be substituted. Examples include birch (Betula), hornbeams (Carpinus), hickory (Carya), chickapin (Castanopsis), beech (Fagus and Nothofagus), tanoak (Lithocarpus), and poplars (Populus) (see also Oei 1996). Once the logs have dried, a netlike pattern of small holes is drilled into the wood. Spawn, prepared by growing the fungus on sawdust or small wooden dowels, is pushed into the holes. These are then sealed over with something like wax to prevent the entry of other fungi and to keep the inoculum from drying out. The inoculated logs are stacked under a forest canopy or in artificial shade, where the temperature is 10-27°C (optimally 20-25°C).
During the six to twelve months following inoculation, the moisture content of the logs is monitored and maintained at 35-55 percent. The fungus grows out from the inoculum and along the log, breaking down the wood and thereby gaining nutrients—a phase called the spawn run. Eventually the fungus forms its fruiting bodies on the sides of the log, but this is usually induced by soaking the logs in cold water. Once the first flush of fruiting bodies has been harvested, the logs are "rested" by maintaining the moisture content at 30-40 percent and the temperature at 15-25°C. When the fungus has accumulated sufficient nutrients, another cycle of fruiting can be induced by again soaking the logs in cold water. Using these techniques, the natural six-month fruiting cycle can be increased in frequency for some strains to four three-month flushes per year. Colonized logs may remain productive for two to five years until decomposition is nearly complete. A maximum yield of about 2 kg of shiitake is possible from a log with an initial fresh weight of about 11 kg (dry weight of 6 kg) (Przybylowicz and Donoghue 1988).
Today most shiitake is produced on sawdust in bags. A typical procedure is to first fill special heat-resistant plastic bags, similar to oven bags, with a mixture of sawdust and supplements (for example, bran). The top of the bag is gathered together and a collar is passed around the outside. The neck is plugged with foam plastic or cotton
Shiitake (Lentinula edodes) fruiting on logs that have been artificially inoculated with the fungus. EDIBLE. (Buchanan)
wool to allow gaseous exchange. The bag and its contents are then heated to 121°C using steam under pressure in an autoclave. When the bag has cooled down, shiitake spawn is inoculated into it through the neck and the plug is replaced, or the spawn is inoculated through holes cut in the sides of the bag, which are later sealed. This has to be done in very clean conditions such as in a lamina flow cabinet. As with logs, it is essential that a strain of shiitake is used that suits both the climatic conditions and the sawdust mix in the bag. The next step is spawn run: the bags are incubated, preferably at about 25°C, on racks for four to fifteen weeks.
During the spawn run the fungus completely colonizes the sawdust mix, which turns white and is referred to as a "block." The collar is removed from the top of the plastic bag, and the bag is either rolled down the sides of the block or removed completely. The exposed surfaces of the block develop a brown outer layer that functions rather like the bark of a log. At this point the temperature of the blocks is reduced either by plunging them into cold water or lowering the air
temperature. Shiitake fruiting bodies then form on the sides of the block. High carbon dioxide levels inhibit fruiting, and so good ventilation is essential. Light is also required to trigger fruiting, which is perhaps a surprising fact since fungi, unlike green plants, do not require light to produce carbohydrates. Once fruiting has been completed, further harvests can be triggered by allowing the blocks to recover for two to four weeks and then again inducing fruiting. Cultivation procedures that use a shelf system, similar to those for growing button mushrooms, have been developed for shiitake, but they are not popular. Year-round fruiting under ambient conditions can be achieved by using strains that have different temperature requirements at different times of the year.
Size, thickness of the cap, taste, texture, moisture content, and the pattern on the cap contribute to the perceived quality of shiitake fruiting bodies. The highest grade, hua gu (Chinese) or donko (Japanese), has a pattern of cracks that resembles a flower on the surface of the cap and can command much higher prices than the lower grade, dong gu
(Chinese) or koshin (Japanese). While it is possible to grow hua gu on sawdust blocks, the more usual method is to grow the fungus on logs, using the traditional techniques. Some growers of shiitake on sawdust-based media will scar the surface of the fruiting bodies when they are at the button stage. When these grow to maturity they have the cracked appearance of hua gu, although the texture of the caps usually belies their origin.
Once the last flush of mushrooms has been picked, the grower is left with the spent remains of what was once sawdust. The spent material, like spent button mushroom compost, can be used as a garden mulch. However, Forest Foods Company in Christchurch, New Zealand, developed a novel way of growing shiitake on pine sawdust, of which there is a surfeit in New Zealand, and then using the nitrogen-rich spent material as a stock food (Reddish 1995).
Over the past decade there have been major advances in China in minimizing the cost of producing specialty mushrooms like shiitake. The techniques used by Luo Qi Gin in Zhang Jia Jie, Hunan, are typical of those that can be seen throughout China. Mr. Luo's substrate is made from 1000 kg of cotton husks mixed with a suspension
of 2 percent calcium hydroxide, 2 percent calcium sulphate, and 2-5 percent ground rock phosphate in water. The mixture is then covered with clear polyethylene and left in the sun until it reaches pasteurization temperature—about five days. The pasteurized materials (2.5 kg) are then placed in polypropylene bags cut from a roll of tubing. The ends of the tubing are passed through 50-mm rings, the bag folded backward over the outside of the ring, and the open neck covered with newspaper. The bags are then inoculated in three positions along the length of each bag, stacked four to five high in rooms, and incubated at approximately 22-27°C for three to four weeks. Remarkably, contamination is not a significant problem. The bags are then removed and the colonized substrate buried horizontally in a field, leaving only the upper 3-5 cm exposed. Fruiting occurs after two weeks and there are a total of five flushes separated by intervals of two weeks. Alternatively the bags may be incubated in rudimentary shelters where, despite a complete lack of hygiene, yields are remarkably high, typically in the order of 1 kg of fresh mushrooms from each kilogram of fresh substrate. A modification of the above technique is to form the sawdust mix into rectangular blocks and incubate these on shelving similar to those used in Agaricus mushroom cultivation.
Yet another method that has gained great favor in China is to raise shiitake in beds of inoculated media between other crops in a field. A sawdust-based mix is first steamed in a steel vessel for five hours and allowed to cool. Channels are then made in the soil between rows of a crop, lined with polyethylene, and filled with the steamed mixture, after which point the spawn inoculum is added. The polyethylene is folded over the bed, and a protective cover of straw supported on wooden hoops is placed over the bed to prevent overheating by the sun. When the mycelium has completely colonized the substrate, the covers and polyethylene are removed and the beds kept moist to ensure fruiting.
Pleurotus Species (Oyster Mushroom)
The common name "oyster mushroom" refers to several species of edible mushrooms all belonging to the genus Pleurotus. These are among the most colorful edible mushrooms, ranging from golden yellow (P cornucopiae var. citrinopileatus) and pink (P. djamor) to gray (P pulmonarius—phoenix mushroom, also widely but incorrectly called P. sajor-caju), bluish gray, cream, and brown. Although many
As the first step in the production of shiitake between rows of maize, steamed sawdust medium is used to fill a polyethylene-lined channel cut in the soil. (Tian)
The fruiting bodies of Pleurotus djamor (pink oyster mushroom) showing the distinctive color of the gills. EDIBLE. (Douglas)
oyster mushrooms are easy to grow, the same factors as those outlined for shiitake must be considered: strain selection, optimal substrate, climatic conditions, and prevention of pests (especially flies) and diseases.
Fruiting bodies of most oyster mushrooms have a characteristic form, with circular to tongue-shaped or shell-like caps developing from a lateral or eccentric stalk and gills that tend to extend partway down the stalk. Like shiitake and most other specialty mushrooms, oyster mushrooms grow naturally as saprobes on dead wood, although a few species, such as Pleurotus eryngii (king oyster mushroom), are parasitic.
Oyster mushrooms can be cultivated on a range of waste materials from forestry and agriculture, including hardwood sawdust, paper, cereal, maize, sugar cane bagasse, and banana leaves. This versatility of substrates and relative ease of cultivation, coupled with the number of oyster mushrooms that are edible, have led to rapid increases in production in recent years. As a result, oyster mushrooms are now second only to button mushrooms in terms of volumes produced.
Although oyster mushrooms may be cultivated on hardwood logs, the most common system involves bags and is similar to that used for shiitake. The bags are filled with moist substrate material and addi-
tives, and a collar is placed around the neck, which is plugged to prevent entry of contaminating organisms while still allowing for gaseous exchange. After being sterilized in an autoclave, typically at 121°C, the bags are cooled and inoculated with spawn. During the spawn run the substrate becomes paler as it is colonized by the fungus. Fruiting occurs through perforations cut in the bag or over the exposed surface if part or all of the bag is removed. High carbon dioxide concentrations reduce the size of the caps; consequently, adequate ventilation is essential. Each bag often produces three to four flushes of mushrooms. The bags are rested between flushes and successive fruitings are induced by soaking the bags in water.
A simpler process popular among farmers in China involves pasteurizing the bagged substrate instead of sterilizing it. This is usually carried out in a steel vessel about 1.5 X 1.5 X 1.5 m fitted with a grid toward the bottom. Below the grid is a space for water. Several large mesh boxes containing twenty to thirty bags are placed in the vessel, and a fire is lit underneath it. The water eventually boils, raising the temperature of the bagged substrate above 70°C. After several hours the substrate is pasteurized. However, there is a greater risk of contamination using this technique, and as a result, poorer yields are
Pleurotuspulmonarius (phoenix mushroom) fruiting through slits made in the side of a large plastic bag filled with substrate. Huon Valley Mushrooms, Australia. EDIBLE. (Hall)
achieved. In another technique, pasteurized or sometimes even raw, untreated, or fermented substrate is formed into columns, horizontal or vertical slabs, or loaded into trays and inoculated with the spawn. When the first flush of mushrooms is harvested, the bags or slabs are rested and successive fruitings are induced by soaking them in water. Typically four flushes are harvested.
In contrast to the labor-intensive systems used in China, Japanese production is largely automated and based on unit trays, each containing sixteen wide-mouthed polypropylene bottles fitted with caps containing an air filter. Following sterilization in an autoclave, each bottle is inoculated with spawn inside a clean room. Sterility of the room is ensured by maintaining absolute cleanliness, pumping in filtered air and using ultraviolet germicidal lamps. The bottles are then taken to climatically controlled rooms where they are incubated at a temperature that suits the particular strain being cultivated. Pleurotus pul-monarius, for example, typically requires 18-25°C, a high relative humidity (85-95 percent), low carbon dioxide concentration, and moderate light levels. In this species, fruiting is initially induced by dropping the temperature of the bottles to 10-20°C. Species such as P. ostreatus and P. eryngii fruit best at lower temperatures, typically 15-20°C. Additional details of the cultivation process for several species of oyster mushrooms can be found in the books Growing Gourmet and Medicinal Mushrooms (Stamets 2000) and Mushroom Cultivation with Special Emphasis on Appropriate Techniques for Developing Countries (Oei 1996), and in a short paper titled "Indoor Cultivation of Paddy Straw Mushroom, Volvariella volvacea, in Crates" (Gutierrez Reyes 2000). An extensive literature on oyster mushroom cultivation can also be found in Edible Mushrooms and Their Cultivation (Chang and Miles 1989), while details of a simple technique for producing small quantities are described in an article titled "Cultivation of the Oyster Mushroom in Traditional Brick Pots" (Abate 1996).
As some workers may develop an allergic reaction to the massive numbers of airborne spores released by even immature oyster mushrooms, respirators may have to be worn during harvest. Sporeless strains of some species have been developed to reduce this problem.
Oyster mushrooms are sold fresh, canned, or in brine. The shelf life of fresh oyster mushrooms is maximized by picking the mushrooms before they are mature. Most are rather delicate and so are usually prepacked at the factory to prevent damage. Of all species, the robust fruiting bodies of Pleurotus eryngii store best. Many believe this species also has the best flavor.
Oyster mushrooms are among the most efficient converters of substrate to fungal fruiting bodies among the edible mushrooms. Fresh-weight yields of mushrooms in excess of the dry weight of the substrate can be achieved (expressed as a biological efficiency rating greater than 100 percent). The spent compost is used as an animal feed, as a soil supplement, and as a substrate for the cultivation of other edible fungi such as Stropharia rugosoannulata.
Auricularia Species (Wood Ear)
Very different in appearance to a conventional mushroom, the wood ear is thin, rubbery, ear-shaped, and lacks gills. In Auricularia poly-tricha the typically convex upper surface may be covered with whitish hairs, while the concave lower surface, from which spores are produced, is purplish brown. This species has a broad geographic and host range and is commonly seen fruiting on dead wood in the forests of many regions. Auricularia polytricha and the related A. auricula are widely cultivated, particularly in Asia, and both species have been used in traditional medicine. An aqueous extract (probably adenosine) obtained from A. polytricha is thought to have the property of reducing atherosclerosis, while A. auricula has been used as a treatment for hemorrhoids.
Spawn for Auricularia polytricha is prepared using cereal grains such as millet, rye, and wheat or aged hardwood sawdust amended with rice bran and other supplements. Alternatively, if the mushrooms are to be grown on logs, spawn can be grown on hardwood dowels. Cultivation follows, using a procedure similar to that described for shiitake, with both log and, more recently, bag techniques being common.
Logs 1 m long and about 10-20 cm in diameter of selected hardwood trees, such as Acacia, beech (Fagus), and mulberry (Morus), are cut between late autumn and early spring and inoculated within one week by pushing spawn or dowels into holes drilled into them. The holes are sealed to prevent contamination and to retain moisture. The inoculated logs are then moved to a laying yard, where shade is provided by a natural forest canopy or by artificial shelter. There must be adequate ventilation and a steady temperature. The logs are turned
and watered for thirty to forty days. The logs are then transferred to a shaded cropping yard, where they are placed in an almost upright position. Fruiting is induced by raising the humidity to about 85 percent, sometimes after soaking. The optimum temperatures for fruiting are 23-28°C. Successive crops of mushrooms can be harvested from the logs while the temperatures are high. During winter the logs are covered with straw, and in spring fruiting is induced by heavily watering or soaking the logs.
The procedure for cultivating wood ear in bags is similar to that used for shiitake. Substrates include hardwood sawdust, cotton seed hull, sugar cane, corn, and rice straw. The optimum temperatures are 22-30°C during the spawn run and 20-24°C during the fruiting phase. The bags are stood on end and fruiting is induced by cutting small holes in the sides of the bags and raising the humidity to 90-100 percent. Alternatively, the tops and bottoms of the bags are opened and the bags stacked on their sides to form walls. Although fruiting is quicker on bags of sawdust than on logs, typically only three to four flushes are obtained.
Most wood ear continues to be marketed today in dried form, equating to about 10-12 percent of fresh weight, with simple rehydra-tion prior to cooking. The mushroom is better known for its soft, crunchy, somewhat cartilaginous texture than for any distinctive flavor. It is often added to vegetable and meat dishes in Chinese cooking.
Volvariella volvacea (Straw Mushroom)
The straw mushroom derives its name from the fact that in China this fungus was traditionally cultivated on rice straw. The mushroom is adapted to subtropical and tropical climates, with optimal temperatures of 32-36°C for vegetative growth of the mycelium and 28-30°C for fruiting (Chang and Miles 1989). The cultures must also be stored at high temperatures and will die if stored in conventional low-temperature systems.
Spawn can be prepared on a range of materials, including cereal grains, short lengths of rice straw, and cotton waste. Rice straw is a suitable substrate, but in recent times it has been largely superseded by cotton waste from the textile industry. The highest yields are produced indoors, where climatic conditions can be managed, using pasteurized composted substrates. To create the compost, a mixture of cotton waste, lime, and water is fermented outdoors for three to seven days. The composting pile is turned once and protected from
rain and cold. The compost is then loaded onto shelves in the growing house and pasteurized with steam for two hours at 62°C, followed by eight to sixteen hours at 50-52°C. This is a very important step because inadequate fermentation of the compost and incomplete pasteurization can result in severe nematode infestations. The pasteurized compost is then cooled to about 36-38°C, spawn is introduced, and the inoculated compost is covered with plastic sheeting. Full colonization occurs in three to four days at 32-34°C. The covers are then removed, and the first primordia appear about the fifth day. The first mushrooms are harvested eight to ten days after spawning. Increased aeration and light levels and a slight reduction in temperature and humidity assist initiation and development of fruiting bodies. Generally there is a large first flush of fruiting bodies followed by a smaller second one.
Straw mushrooms may also be cultivated outdoors when ambient temperatures are high enough to support fruiting. Cultivation beds consist of layers of water-soaked bundles of unfermented rice straw, with each layer spawned as it is laid down. The beds are then covered with a layer of rice straw, followed by a sheet of polyethylene. The sheet
is removed after four days, after which time the stacks are watered regularly until fruiting ceases. Further details of cultivation procedures can be found in Edible Mushrooms and Their Cultivation (Chang and Miles 1989) and Mushroom Cultivation with Special Emphasis on Appropriate Techniques for Developing Countries (Oei 1996).
When mature, the mushrooms have a typical stalked appearance, but they are usually harvested when immature and in what is known as the egg stage, with the cap still inside the volva. When fresh, the mushrooms have a shelf life of only three days, so these mushrooms are often canned. Straw mushrooms are frequently used in Chinese cuisine.
Flammulina velutipes (Enokitake)
Enokitake is a common saprobic wood-decay fungus in the forests of many countries. While the mushroom can be cultivated on a small scale, production in countries such as Japan (where it was first commercialized) and Taiwan is now in the hands of a limited number of very large companies, which have adopted fully automated systems. These farms use sterile circulated air, fully climate-controlled
Flammulina velutipes (enokitake) being harvested and automatically weighed in a factory near Hiroshima. (Hall)
Flammulina velutipes (enokitake) fruiting through the necks of bottles of sawdust and other ingredients. Note the clear plastic collar around the neck of each bottle to maintain high carbon dioxide concentrations. Tan Mushrooms, Tai-chung, Taiwan. EDIBLE. (Hall)
Flammulina velutipes (enokitake) fruiting in the light through holes in the side of a bag of substrate. Note the difference in the form of the fruiting bodies with those grown in bottles of sawdust. EDIBLE. (Cole)
environments, conveyor transportation, and automated manipulation of the growing containers.
The mushroom is cultivated on a mixture of aged sawdusts amended with rice bran. In Japan, Japanese red cedar (Cryptomeria), cypress (Chamaecyparis), and pine (Pinus) sawdusts are commonly used. Spawn is also prepared on a mixture of sawdust and rice bran. Wide-necked polypropylene bottles are then filled with the substrate, capped, placed in trays, and autoclaved. After cooling, spawn is introduced through the neck of the bottle and the cap is replaced.
The bottles are incubated at about 18-20°C for twenty to twenty-five days. Just before the fungus has colonized all the substrate, the caps are removed and the surface of the sawdust is mechanically scraped to give a flat, even surface or cone-shaped depression. Fruiting body primordia are initiated by dropping the temperature to 10-12°C and adjusting the humidity to 80-85 percent. The temperature is then further reduced to 3-5°C for three to five days, during which time the stalks begin to elongate. When the stalks are about 2 cm high, the temperature is raised to 5-8°C and the mouth of each bottle is surrounded by a tall, paper or plastic, cylindrical collar. This supports the elongating stalks and raises the carbon dioxide concentration, which reduces the size of the cap to little more than a slightly swollen end on the top of the stalk. The humidity is also reduced to 70-80 percent, and overwatering is avoided in order to produce dry fruiting bodies. When the stalks are 13-14 cm high, the collars are removed and the harvested mushrooms are either sold in 5-kg bags for the wholesalers or 100-g packets for the retail trade. The mushrooms can be grown in tall narrow bags instead of bottles, but in this case the bag is only partially filled with substrate and the open extended mouth is used to support the elongating stalks.
Consumers prefer mushrooms with white stalks and caps rather than the golden wild type, and this has resulted in breeding programs, particularly in Japan, for white strains. These strains dominate the market. Enokitake is sold fresh or canned and is a frequent ingredient in Asian soups and stir-fried meat and vegetable dishes.
Tremella fuciformis (White Jelly Fungus)
Members of the genus Tremella are common saprobic wood-decay fungi in the forests of many countries, from tropical to temperate regions. Commercially produced fruiting bodies of the white jelly fungus consist of white to pale yellow convoluted folds of gelatinous
Tremella fuciformis (white jelly fungus) drying in the sun in Gutian, Fujian Province, China. EDIBLE. (Buchanan)
translucent fungal material 8-15 cm across that somewhat resemble a chrysanthemum flower. They were first cultivated on hardwood logs by a method very similar to that used for shiitake, but most are now grown on bagged sawdust or cottonseed hulls.
Unlike the cultivation of other mushrooms, acceptable yields of the white jelly fungus can only be achieved if a companion fungus is grown with it. This fungus degrades the substrate in advance of the white jelly fungus. One companion species that can be used is the ascomycete Hypoxylon archeri (Chang and Miles 1989). Spawn production is a specialized procedure. First the slower-growing white jelly fungus is inoculated onto the spawn medium, which consists of sterilized sawdust or bagasse mixed with rice bran, and incubated at 23-25°C. Only when it has become established is the companion fungus introduced. The quality of a spawn is assessed by judging the fruiting bodies that develop on its surface. High-quality spawn is then bulked up for inoculation of the substrate.
Substrates for cultivation include hardwood sawdust (Oei 1996) and cottonseed hulls, along with sources of nitrogen such as corn cobs and wheat bran. These are mixed with gypsum and other ingredients and compressed into long, narrow (perhaps 12 X 55 cm) bags, which are then autoclaved. Spawn is introduced through three to five holes punched in the sides of the bag, which are then resealed with tape. The inoculated bags are incubated at 28-30°C. The temperature is subsequently lowered to 20-25°C and the tape covering the inoculation points lifted to allow gaseous exchange. The first fruiting body primordia appear about sixteen to eighteen days after inoculation. The tape is removed, the inoculation holes enlarged, and the temperature raised to 23-25°C. Once the fruiting bodies have formed, they are covered with newspaper until mature—about thirty-five to forty days after inoculation. In China harvested mushrooms are commonly dried on racks in the sun. Alternatively they can be dried in an oven (first drying the outer surface at about 30°C, followed by higher temperatures to complete the drying process).
Tremella fuciformis has both medicinal and culinary uses. The polysaccharides and steroids it contains reportedly have antitumor and anti-inflammatory properties and also serve as a general tonic. The white jelly fungus can be included in desserts and added to soups and other dishes. Though it has little flavor, the smooth gelatinous texture of this mushroom is appealing to the Asian palate. Other species of Tremella, such as the golden ear (T. aurantia), are also cultivated in China and used in traditional medicines.
Pholiota nameko (Nameko)
The small caps of nameko are very popular in Japan. Although the gelatinous surface of the cap sometimes reduces its immediate appeal to Westerners, the slime is not readily apparent once the fruiting body is cooked. The mushroom makes a delicious addition to Japanese soups. Nameko fruits at low temperatures, like enokitake, and is cultivated by a similar procedure (Chang and Miles 1989, Stamets 2000), although Japanese producers usually employ broader-mouthed bottles than those used for enokitake. A range of sawdust substrates has been used, including both hardwood sawdust, such as oak (Quercus), and softwoods such as pine (Pinus) and Japanese red cedar (Cryptomeria), to which is added cereal bran. The fully colonized substrate is subjected to high humidity, low light, and low temperatures (10-15°C) to initiate fruiting. Adequate ventilation is required to lower carbon dioxide concentrations so that the caps develop normally. The production of orange slime on the surface of the substrate often precedes the development of primordia. During maturation of the mushrooms, the temperature is maintained at 13-18°C. In Japan the short-stalked mushrooms are sometimes harvested using scissors as they mature, so that each bottle has to be harvested on more than one occasion. A
second flush can be induced by scraping the substrate surface to expose growing mycelium (Stamets 2000).
Several other mushroom species are commercially cultivated in relatively small quantities, often for the production of medicinal products. These include the almond portobello (Agaricus blazei), grown as an alternative to button mushrooms (Stamets 2000); several species of Agrocybe, which have only recently gained acceptance in the marketplace; bamboo mushroom (Dictyophora indusiata), a phalloid mushroom that emerges from an egg and is cultivated mainly in China on bamboo and wood chips outdoors, for both taste and medicinal effects; reishi (Ganoderma aff. lucidum), a polypore cultivated for its fruiting bodies and their spores; hen of the woods (Grifola frondosa), cultivated mainly in Japan; lion's mane mushroom (Hericium eri-naceus); shimeji (Hypsizygus marmoreus), which is popular in Japan and increasingly so in China; Kuehneromyces mutabilis, a small, brown, stalked mushroom grown on hardwood pieces in eastern Europe; fat pholiota (Pholiota adiposa), cultivated in China; Pleurotus
The fruiting bodies of Hypsizygus marmoreus (shimeji) growing in wide-necked plastic bottles. EDIBLE. (Buchanan)
Ganoderma aff. lucidum (reishi) cultivation in China on bags of substrate covered with soil. The sheets hanging from the roof are used to collect the more valuable spores. MEDICINAL. (Tian)
The deformed fruiting bodies of this Ganoderma species were produced by growing the mushroom with an elevated level of carbon dioxide. Qi Feng free market, Changsha, Hunan, China. MEDICINAL. (Hall)
The large and distinctive fruiting bodies of Grifola frondosa (hen of the woods) growing in wide-necked plastic bottles. EDIBLE. (Hall)
Trametes versicolor (many-colored polypore) is an extremely variable species. Here it grows on a Nothofagus species. MEDICINAL. (Buchanan)
The fruiting bodies of a white variety of Trametes versicolor, clustered on a decaying trunk. MEDICINAL. (Wang)
tuber-regium, a species cultivated not for its fruiting bodies but for the medicinal properties of the sclerotia it produces (Chen and Huang 2001); and many-colored polypore (Trametes versicolor), another polypore grown for its medicinal properties.
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