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Mushrooms are the edible fleshy fruiting bodies of certain fungi, which may be gathered wild or grown under cultivation. The most commonly cultivated mushroom species is Agaricus bisporus, though many other species are now gaining recognition in Australia due to the widespread consumption of Asian cuisine. This page describes the cultivation of Agaricus species.
Cultivated mushrooms are usually grown in the dark in climate-controlled rooms. The fungal innoculum or 'spawn' is added to a pasteurised substrate in growing containers or beds. After the fungal strands (mycelia) have spread through the compost, a layer of peat or soil (the 'casing') is added. The fruiting bodies begin appearing about 6 weeks after spawning and continue appearing in flushes about 7-10 days apart for the next 6-8 weeks. The first three flushes are the most productive. The cap and a small section of connected stem are usually harvested before the caps are fully expanded.
The mushroom farm should be situated within easy access of markets, compost suppliers and labour sources. Mushrooms require a high capital outlay, and production efficiency will become increasingly important as the market becomes even more competitive.
Mushrooms are grown in specially constructed sheds. (Existing farm buildings can be used, but require major modifications and even then still have some limitations.) There is no standard size or design of buildings for mushroom culture. Factors to include when planning include construction costs, machinery space requirements, tray or bed size, and stacking design. Doors must be designed to suit all machinery and equipment that is used. Windows are not required. Although mushrooms do not require complete darkness to grow, do not allow direct sunlight to reach the beds. Any electrical equipment installed must be able to withstand high humidity. Buildings should be rodent-proof.
Cement floors with adequate drainage are required to allow for ease of cleaning and hygiene operations. Flat roofs should have sufficient slope to prevent condensation dripping onto the beds. Insulation (commonly polystyrene panels) prevents temperature fluctuations and increases the energy efficiency of the air conditioning.
Good ventilation to supply a constant flow of fresh air and prevent carbon dioxide build-up is essential. Ventilation units should be fully adjustable in terms of circulation volumes, and include a filter that will prevent entry of insects and airborne spores. The filters should be cleaned regularly. Do not recycle unfiltered air between different growing rooms. Trays or shelving should be arranged to allow ease of air circulation.
Two systems are currently used: the one-zone system where peak heating, spawn run and cropping are done in the same room, and the two zone system where separate rooms are used for some production stages. The second option requires a larger turnover to cover extra capital expense.
Controlled environment rooms (temperature and humidity) are required for efficient production of high-quality mushrooms. Computer monitoring equipment to maintain the temperature and humidity at the required levels during the production cycle is expensive but streamlines production considerably.
Temperature and humidity
Raise the temperature to 60oC for several hours to pasteurise the compost. Both compost and air temperatures should be maintained at 48-52oC for several days. Ventilate the room, but do not allow the surface of the compost to become too dry. The humidity throughout this stage should remain between 90 and 100 per cent.
Spawn run and casing
Maintain a room temperature of 25oC for 10-14 days with high humidity (95-100%). Ventilation is not required at this stage as high carbon dioxide levels encourage mycelium growth. Metabolic heat from the spawn will raise the temperature of the compost and it may require cooling. After casing, keep the surface moist and maintain constant temperature and humidity.
Maintain an air temperature of 14-18oC in order to keep the compost at 16-20oC. In summer, this will require a cooling system. Ventilate well, avoiding fast or dry air currents. Maintain the humidity at 85-90 per cent.
At the end of the cropping season, use live steam to at least 70oC for 10 hours on the compost to prevent the spread of pests and diseases.
The three main production containers for mushrooms are trays, shelves or bags. Trays are the most commonly used, although yields per tonne of compost are comparable for all systems. Plastic bags are disposable and cheap, but require extra handling as the compost must be peak heated in other containers (such as trays).
Trays can be constructed out of timber (if any preservative used is not detrimental to mushroom growth), concrete, or rust-protected metal. Ensure construction is sturdy enough to withstand rough handling. Trays are more expensive to build and maintain than shelves, but allow ease of maintenance and hygiene, and more continuous production.
The growing trays are filled with a fresh compost mix which is then allowed to ferment for several days. The compost is usually made using other industry wastes such as wheaten straw, horse and/or poultry manure, cottonseed meal or cottonseed hulls, combined with gypsum and water. Composting transforms these products into a suitable nutritional substrate for mushrooms (all nutrients must be supplied in the compost as mushrooms contain no chlorophyll). Composting should be done on a large scale with mechanical turners and handling equipment. Hand-composting is very labour intensive. Ready-mixed compost is available commercially. If composting is to be done on site, be aware that various odours will occur and possible objections from neighbours may result.
Compost production involves two phases. Phase 1 (outside) is where the raw materials are mixed and chemical and biological process proceed for 1-12 days. Phase 2 (peak heating) is conducted inside. The compost is filled into trays in special controlled temperature room and pasteurised for 2-3 hours at 60oC, then conditioned for 6-7 days at decreasing temperatures to free the compost of ammonia. Excessive temperatures will damage the compost.
Bulk pasteurisation may be done by placing the compost in a room with a perforated floor. Conditioned air is then forced through the compost. It is possible to grow mushrooms without peak heating by extending the length of phase 1 but is not recommended, as lower, more variable yields are common.
Ideal pH ranges for mushroom production are 6.5-7 for the compost (although up to 8 will be tolerated) and 7-8.5 for the casing. As the mycelium grows, the compost will become increasingly alkaline. Peat moss is acidic, and should be adjusted by mixing with lime when using it as a casing material.
Spawn is the mushroom propagating material, containing mushroom spores with a mixture of other materials to promote spawning. Spawn is commercially available from specialist spawn producers. Once purchased, it should be mixed with the compost as soon as possible, though if required, spawn can be stored at 15-20oC for several days or at 2oC for several weeks. Spawn performance can be adversely affected by excessive temperatures or rapid temperature changes.
Use 5-7 L (3-5kg) of spawn per tonne of compost. Maintain the growing room temperature at 25oC and humidity at 95-100 per cent. When a whitish growth covers the compost surface, the beds should be cased. From this point on, use detachable sideboards to monitor growth rather than digging through the casing layer.
A layer of specially prepared soil or peat moss is spread over the compost to protect it from drying out and allow for formation of the fruiting bodies. The preferred casing material is a mixture of peat moss and lime. Soil mixtures require additional pasteurisation and peat does not adhere to the cap like soil does. Chemicals to assist in the aid of pest and disease control are often added to the casing mix. The soaked peat mixture should be drained until it no longer drips - do not squeeze. Do not allow the casing layer to dry out.
Harvesting usually commences at the first sign of buttons, often on a 7-10 day cycle and may last for 1.5-2 months. Mushrooms may be picked at the button, cup or flat stage depending on market requirements. Timing is important as mushrooms grow quickly, doubling their size within 24 hours. Buttons are small unopened mushrooms, cups are older buttons where the cap has begun to open, and flats are cups that have fully expanded to expose all of the gills.
The fruiting bodies are harvested by hand with a twisting motion. The stems are trimmed and the mushrooms are usually graded straight into boxes for transport and sale. Mushroom deterioration (brownish discolouration, stalk elongation etc) can be reduced by cooling. Mushrooms are highly perishable and should be marketed as soon as possible after harvest. Buttons are volume packed, whereas cups and flats are packed with the gills facing up to prevent spores from dropping onto lower layers.
Yields are influenced by compost depth and quality, length of cropping and grade of mushrooms picked, spawn productivity, moisture and climatic conditions, and disease factors. Yield is usually quoted in kilograms of mushrooms per square metre of compost. An average yield would be about 16 kg/m2 (15 cm deep compost) over a picking time of six weeks. The majority of mushrooms will be picked over the first three flushes (about four weeks).
Marketing costs include packaging, transport, commissions and levies (the mushroom industry collects a voluntary levy to fund research, promotion, education etc.). As mushrooms are grown indoors under controlled environment conditions, they can be available all year round, although there are usually peaks and troughs in the market as some growers cease production during the hotter summer months.
Mushroom cultivation requires high capital input and labour costs, and a strict adherence to hygiene procedures. Operation efficiency is essential to maintain viability. Remember to check the local regulations to ensure your operation complies with all relevant legal requirements.
Spent compost can be marketed as a potting mix or garden soil additive. Do not attempt to re-use it in mushroom cultivation.
See more information on hygiene procedures for mushrooms