Obtaining pure culture
To get pure culture, mushrooms are either isolated from nature, purified and characterized in laboratory before their use or procured from national or international mushroom culture centres. When they are isolated from the nature the method of isolation is totally microbiological one for which aseptic conditions are essential.
Table 18.6. Nutrient media used for isolation of mushrooms.
Sterilized and cooled potato dextrose agar (PDA) or malt extract medium (see Table 18.6 ) is poured into sterile Petri dishes and when solidified they are inoculated by a piece of tissue or spore(s) of mushroom. In tissue culture method fresh mushroom is removed from the bed (or stipe is collected after cropping), washed in running water to remove adhering soil particles, dried with blotting paper, gently washed with 70 per cent ethyl alcohol and finally cut from centre into two halves. A small portion of pseudoparenchymtous tissue from the centre of stipe is transferred onto Petri dishes. Petri dishes are incubated at suitable temperature for the growth of hyphae. The spore culture (single or multispore) is described elsewhere in detail (Chang and Li, 1982).
Preparation of spawns
Spawn is a fungal growth medium impregnated with mycelial fragments of mushroom which serves as inoculum for mushroom cultivation. There is a great problem in preparing pure spawn of a particular strain of a mushroom because of fungal, bacterial or viral contamination.
Many substrates are used for spawn making either alone or in combinations, for example, rice straw cuttings, cotton waste, hulls of cotton seed and rice, and grains of sorghum and rye. For the selection of substrates to be used in making spawn, care is taken for cost and availability of raw materials and mycelial growth on it as well. The steps of grain spawn (e.g. rye/sorghum/wheat grains) or straw spawn (paddy/wheat straw) preparations follow, (a) cooking the grains in water until they swell/cutting of straw into 5 cm long pieces and soaking in water for 5-10 minutes, (b) decantation of water, mixing of 2 per cent lime (calcium carbonate), (c) transferring into glass tubes/flasks, (d) plugging with cotton, (e) autoclaving at 121°C for 30 minutes and cooling down to 30-40°C, (f) inoculating the substrate with pure culture of mushroom as described earlier, and (g) incubation at suitable temperature for proper infestation of mycelium for their use as spawn (Chang and Li, 1982).
Formulation and preparation of composts
Methods of preparation of composts for mushroom cultivation is known as composting. Initially, composting was restricted to industrial levels by using horse manure, but now can easily be applied to other substrates as the methods of formulation and preparation of composts are the same. The purpose of compost preparation is to provide medium for the rapid growth of mycelium. Therefore, physical and chemical compositions are developed in such a way that can alter the gross micfobial community and promote maximum growth and yield of mushroom. No such chemicals or conditions should be present in compost that inhibit mycelial growth. Sohi (1980), former Director of National Centre for Mushroom Research and Training, Solan (H.P.) has described many compost formulations used in India and abroad (Table 18.7).
Table 18.7. Compost formulations used in India and abroad.
While formulating the substrate care is taken for ease in microbial degradation. The main constituents of straw or plant waste are cellulose, hemicelulose and lignin. The first two are carbohydrates which upon decomposition result in glucose units.
Substrates are filled in small trays or wooden boxes to make beds. Use of trays of plastic bags makes air spaces. In India, tray culture is being replaced with plastic bags and shelf beds due to high cost of wood and carry over of pests and diseases. The use of plastic bags enables the growers to undertake mushroom growing with a lesser initial capital investment. Moreover, in Sonepat (Haryana) areas most of growers have adapted shelf bed system on a bamboo platform in Kuchcha mud houses covered with saccharam stalks or dried sorghum (Sohi, 1988). Trays or plastic bags are transferred in the room for its partial pasteurization at low temperatures. Maximum care should be taken at this stage. Room temperature is maintained around 50-55°C for about 6 hours by live steam generated from a boiler. Later on, it is cooled down to a preferable temperature for casing the compost. Casing is the covering of compost when spread over the beds with a thin layer of soil or soil like materials. It gives support to mushrooms, maintains temperature and prevents drying the compost. Various casing materials including farm yard manure + soil (1:1 ,w/w), moss + garden soil (2 : 1 w/w) have been tried (Sohi, 1988).
Spawing, spawn running and cropping
Inoculation by spawn of compost in beds is known as spawning. Bed material is inoculated by a small amount of spawn by removing it from container and spreading over bed material. Room temperature and humidity is controlled for maximum mycelial growth and spawn running. Environmental conditions greatly influence when not suitable for spawn running.
Mushroom crop becomes mature at different intervals, producing flushes. After five to six flushes, the culture is renewed. Mature mushrooms are picked up without disturbing the neighbouring ones. During this period also, environmental conditions are left undisturbed. Harvested mushrooms are sent to market or canned for their use as food.
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