Oyster mushroom production is on the increase in the United States. This article details the cultivation of oyster mushrooms from spawn to postharvest handling and marketing.
UPDATED: JUNE 27, 2016
Total mushroom production worldwide has increased more than 18-fold in the last 32 years, from about 350,000 metric tons in 1965 to about 6,160,800 metric tons in 1997 (Table 1). The bulk of this increase has occurred during the last 15 years. A considerable shift has occurred in the composite of genera that constitute the mushroom supply.
|Species||Fresh weight (x 1,000 t)||Increase|
Source: Chang (1999)
During the 1979 production year, the button mushroom, Agaricus bisporus, accounted for over 70 percent of the world’s supply. By 1997, only 32 percent of world production was A. bisporus. The People’s Republic of China is the major producer of edible mushrooms, producing about 3,918,300 tons each year–or about 64 percent of the world’s total. China also produces more than 85 percent of all oyster mushrooms (Pleurotus spp.) grown worldwide (Table 2).
|Country||1,000 m.||1,000 lbs.||%|
|Rest of Asia||88.4||194,887||10.1|
|Rest of Europe||5.8||12,787||0.7|
Source: Chang (1999)
From 2001 to 2002, the United States produced 393,197 metric tons of mushrooms, or about 7 percent of the total world supply. A. bisporus accounted for over 90 percent of total mushroom production value, while Lentinula, Pleurotus, Grifola, Flammulina, Hypsizygus, Hericium, and Morchella were the main specialty genera cultivated. The value of the 2001-2002 specialty mushroom crop in the United States amounted to $37 million, down 12 percent from the 2000-2001 season. The average price per pound for specialty mushrooms received by growers, at $2.77, was down $0.27 from the previous season.
Sales volume of oyster mushrooms, at 4.03 million pounds, was up 11 percent from the 2000-2001 season, with a total of 51 growers producing 4.27 million pounds of the mushrooms in the 2001-2002 season. Total production includes all fresh market and processing sales plus amount harvested but not sold (shrinkage, cullage, dumped, etc.). Average oyster mushroom output per farm in- creased 249 pounds (18.3 percent) per week, from 1,359 pounds in 2001 to 1,608 pounds in 2002 (Table 3). The production of oyster mushrooms (Pleurotus spp.) in the United States has increased at an annual rate of 14 percent during the last 6 years, from 1,941,000 pounds in 1996 to 4,265,000 in 2002. This increase reflects an international trend toward increased production of this crop. Oyster mushrooms accounted for 14.2 percent (875,600 tons) of the total world production (6,161,000 tons) of edible mushrooms in 1997, the most recent year for which statistics were available.
|Year||No. growers||Production (lbs.)|
|Annual (x1000)||Per wk/grower|
Source: USDA (2002)
The increase in United States production is due to increased consumer demand and the relatively high compensation growers receive for the product. According to the USDA, farmers received an average of $2.00 per pound for fresh oyster mushrooms while growers of A. bisporus received an average of $1.07 per pound for fresh product in the 2001-2002 growing season. The higher price received for fresh oyster mushrooms reflects, in part, the less-developed and less-reliable technology available to growers for cultivating these species. Thus, growers need potentially higher incomes to help offset the increased risks associated with producing Pleurotus spp.
Oyster mushrooms are grown from mycelium (threadlike filaments that become interwoven) propagated on a base of steam-sterilized cereal grain (usually rye or millet). This cereal grain/mycelium mixture is called spawn and is used to seed mushroom substrate. Most spawn is made with mycelium from a stored culture, rather than mycelium whose parent was a spore. This is because spores are likely to yield a new strain and performance would be unpredictable. Spawn-making is a rather complex task and not feasible for the common mushroom grower. Spawn of various oyster mushroom species may be purchased from commercial spawn makers who usually provide instructions for its use. Spawn frequently is shipped from the manufacturer to growers in the same aseptic containers used for spawn production. Inoculum for spawn production is frequently produced in polyethylene bags containing a microporous breather strip for gas exchange. Most commercial spawn production companies produce spawn only from inoculum that has met strict quality control standards. These standards include verification of inoculum production performance before it is used to produce spawn and insurance of the spawn’s biological purity and vigor.
Before 1970, cultivars used for commercial spawn production were maintained on various agars or cereal grains with periodic subculturing of growing mycelium to a fresh medium. This method, for the most part, was reliable, although spawn makers and researchers reported cases of culture degeneration periodically. In 1970, researchers successfully preserved and maintained stability of spawn stocks of A. bisporus stored in liquid nitrogen. Several research reports on culture maintenance verified the suitability of cryogenic preservation, fundamentally changing the way spawn makers handled their cultures used for commercial spawn production. In practice, cryogenic preservation is used to ensure use of superior spawn-starter cultures. Many vials (perhaps as many as 200 to 300) containing spawn or mycelium from cultures of promising spawn lines are stored in liquid nitrogen. Following successful testing of the spawn lines at both pilot plant and commercial testing facilities, the spawn maker can easily reproduce the superior lines many times during subsequent years.
Production in Bags
In the United States, the primary ingredients used for Pleurotus spp. production are chopped wheat straw (Triticum aestivum L) or cottonseed hulls (Gossypium hirsutum L) or mixtures of both. For production on wheat straw, the material is milled to a length of about 2 to 6 cm. Production of Pleurotus spp. on cottonseed hulls has some advantages over straw-based production systems in that chopping of the hulls is not required. One of the most common substrates used on modern mushroom farms is a mixture of 75 percent cottonseed hulls, 24 percent wheat straw, and 1 percent ground limestone. This mixture of cottonseed hulls and wheat straw has a higher water holding capacity than cottonseed hulls used alone. At Penn State’s Mushroom Research Center (MRC), a large-capacity, scale-mounted feed mixer (Figure 1) is used to simultaneously grind and mix the material as water is added to increase the moisture content to 67 to 69 percent.
On some commercial mushroom farms, ingredients are fed into revolving mixers, water is added to the desired level, and live steam is injected into the mixer while in operation. At the MRC, moistened, mixed substrate is filled into galvanized metal boxes with perforated floors (Figure 2). The substrate is pasteurized with aerated steam at 65oC for 1 hour by passing the air-steam mixture through the substrate from top to bottom. After pasteurization is complete, filtered air (HEPA filter, 99.9 percent efficiency) is passed through the substrate for cooling (approximately 1.5 hours).
Spawning and spawn rate
Growers have sought, in the past, to optimize the amount of spawn used to inoculate their substrate. Increasing the amount of spawn used (up to 5 percent of the wet weight of the substrate) has resulted in increased yields. Increasing spawn rates from 1.25 percent substrate wet weight to 5 percent may result in yield increases of nearly 50 percent (Figure 3a). Yield increases may be due to several factors. First, the increased level of nutrient available in higher levels of spawn used would provide more energy for mycelial growth and development. Second, more inoculum points, available from increased spawn levels, would provide faster substrate colonization and thus, more rapid completion of the production cycle. Finally, a more rapid spawn run would reduce the time non-colonized substrate is exposed to competitors such as weed molds and bacteria.
For increasing levels of spawn used (up to 5 percent), there is a negative correlation between spawn rate and days to production. As the spawn rate increases, the number of days to production decreases (Figure 3b). By using a spawn rate of 5 percent of the wet substrate weight, it is possible to reduce the time to production by more than 7 days compared to a spawn rate of 1.25 percent. Thus, growers could complete the crop cycle faster, minimizing the exposure of the production substrate to pest infestations, especially sciarid (Lycoriella mali [Fitch]) flies. Research has shown that the sciard fly may complete its life cycle in 25 days at 21°C, while 35 to 38 days are required at 18°C. Timely disposal of spent substrate may help to minimize the buildup of fly populations on a mushroom farm.
Use of delayed-release supplements
At time of spawning, a commercial delayed release supplement consisting of paraffin-coated whole soybean or formaldehyde-denatured soybean and feather meal may be added at rates of 3 to 6 percent of dry substrate weight, to stimulate yield of the mushroom. Yield increases of up to 90 percent have been observed when 6 percent (dry weight) supplement is added to substrate at time of spawning (Figure 4a). Delayed-release nutrient supplements have also been shown to decrease the number of days to harvest (Figure 4b). The addition of 3 percent nutrient at time of spawning may reduce time to production by 2 to 3 days. Thus, growers wishing to hasten the production process may do so by supplementing with only small quantities of supplement. Use of supplements, however, may cause overheating of the substrate if growers are unable to anticipate and control air temperatures to maintain a steady substrate temperature. Additional cooling capacity is required when higher levels of supplement are used.
Filling plastic bags with substrate
The pasteurized, supplemented hull/straw mixture is spawned and filled (25 to 30 pounds) into clear or black perforated polyethylene bags and then incubated at 23° to 25°C (substrate temperature) for 12 to 14 days. Mushrooms then begin to form around the edges of bag perforations and they are harvested from the substrate approximately 3 to 4 weeks after spawning depending on strain, amount of supplement used, and temperature of spawn run (Figure 5).
Figure 5. Yellow oyster mushrooms (Pleurotus cornucopiae) fruiting from pasteurized substrate (75 percent cottonseed hulls, 24 percent chopped wheat straw, and 1 percent ground limestone) contained in black bags suspended from overhead supports (MRC).
Production in Bottles
In Japan, bottle production of oyster mushrooms is most common (Figures 6a-d). Bottle production also is increasing in popularity in the United States. Substrate is filled into bottles contained in trays (usually 16 bottles per tray), sterilized, and inoculated with Pleurotus spp. spawn (Figure 6a-d). Upon completion of spawn run, bottle lids are removed and the surface of the substrate is scratched mechanically (1 to 2 millimeters of substrate surface containing mycelium are removed). Scratching is required to stimulate the mycelium to produce mushroom primordia uniformly on the surface. After the mushrooms are harvested, they are weighed and packaged for shipment to market (Figures 7a-d).