Substrate preparation technique for the button mushroom has witnessed evolutionary changes over the years, from the long-method of composting to the current environment-friendly indoor composting. However, the intermediate short-method of composting, is still the most popular method all over the world.
Long method of composting is the oldest method and now exists only in few pockets of the world mainly because of poor productivity, proneness to attack by the competitors, and also due to more time and labor consuming process (Vijay and Gupta 1995). This method is completely an outdoor process and takes about 28 days, though production of long-method compost in lesser duration has also been achieved. But the biomass loss in this process is very high (30-35%) and the quality as well as productivity is poor, besides the environmental problems it creates.
Based upon the observations of Lambert that productive compost came from the regions of the pile having temperature between 50-60°C and adequate supply of oxygen, Sinden and Hauser (1950) developed the so-called short-method of composting mainly because it took lesser time than the long method. The concept and process was indeed a revolution in the cultivation of button mushroom. The short method of composting mainly consists of two phases: outdoor-composting for 10-12 days (Phase-I) followed by pasteurization and conditioning for 6-7 days inside specialized insulated structures, called tunnels.
Based upon the temperature conditions maintained inside the tunnel, Phase-II can be divided into two sub-phases: pasteurization (57-60°C for 6-8h) and conditioning (45-48°C for 5-6 days). Importance of conditioning has been linked with the growth of desirable thermophilic microorganisms; pasteurization and conditioning are essential for achieving the selectivity in the compost. Short method of composting has many advantages over the long method: more compost per unit weight of the ingredients, higher productivity of mushrooms, less chances for pests and diseases, shorter duration and less environment pollution.
The problems of environmental pollution related with production of stinking gases associated with long as well as short methods of composting drew the attention of the researchers to evolve an alternative "clean" process. Because first time the work on such composting system started using completely indoor system, it was termed as indoor composting (Laborde 1992); other terms like environmentally-controlled composting, rapid indoor composting, and aerated rapid composting have also been used for this process. Based on the temperature conditions maintained inside the tunnel, the process could be divided into two categories, i.e., INRA method and Anglo-Dutch method. In the INRA method, which is popular in France, Italy, and Belgium, phase-I is carried out at constant temperature of 80°C for 2-3 days followed by phase-II at 50°C for 5-7 days (Laborde 1991). As very high temperature attained during phase-I of this method kills most of the microbes including the desirable thermophiles, reinoculation with mature compost or thermophilic fungi becomes necessary in this process. However, in the bunker system used nowadays this can be dispensed with. In the Anglo-Dutch method, a weeklong conditioning at 41 °C follows a short pasteurization phase of 4-6 h at 60°C. The method has attained popularity in several European countries and Australia, and compost with high selectivity and substantial savings on raw materials (Miller 1997) is reported to be produced. Indoor-composting has many advantages even over the short-method of composting: takes lesser time, gives higher yield, is environment-friendly, and conforms to civic laws, lesser loss of raw materials and thus increased end-product and above all, very high degree of selectivity (Miller 1997).
Ready compost is seeded with spawn approximately at 0.5% on fresh weight basis after which the seeded substrate is either filled in polybags or in shelves and the temperature and humidity in the growing rooms are maintained at near 25-28°C and 90-95% RH respectively for 12-15 days for mycelial colonization of the substrate called spawn-running. Mushroom mycelium derives nutrition from the substrate by secreting an array of extracellular degradative enzymes capable of degrading cellulose, hemicellulose, lignin-humus complex, and several bacteriolytic and mycolytic enzymes. Once the compost is completely colonized, various types of supplements, often suitably treated proteinaceous materials like soybean meal is thoroughly mixed at 1% in the upper one third area of the substrate which is then layered with 4-5 cm of casing material. Depending upon the availability and suitability, various types of materials are used as casing in different regions of the world; peat is of course the substrate of the choice in the developed countries while farmyard manure (FYM) in various combinations with soil and other materials are used in many countries. Recently, choir peat (specially decomposed and processed coir pith dust) has found acceptability as casing material in many Asian countries where coconut plantations abound. Many authors (Hayes 1974; Kurtzman 1997; Vijay and Gupta 1995) have discussed materials, techniques, and the role of casing in cultivation of the button mushroom. Though there are varied observations and opinions on the importance of some properties of casing materials, e.g., pH, conductivity, bulk density, water holding capacity, and associated microflora, but that the button mushroom requiring a layering (casing) above the colonized compost for fruiting is a fact well-established. After casing, growing rooms are again maintained at high temperature (~ 25°C), humidity (~ 90%), and CO2 (> 5000-10000 ppm) for mycelium to colonize casing layer called case-run. "CaCing" (mixing of small quantity of colonized compost in casing material at the time of casing) and ruffling of partially colonized casing material few days before readying for fruiting are some of the improvisations practiced in high-tech mushroom production. After the complete colonization of the casing layer when white mycelium becomes visible between the clumps all over the casing surface, room temperature is lowered down to 16-18°C, fresh air is introduced/increased with slight decrease in the humidity (85% RH) i.e., the conditions not conducive for mycelial growth to continue. Under these "adverse" circumstances mycelial aggregation takes place to form pinheads or primordia which differentiate and develop into mushrooms (Figure 2). After growth to the desired size, mushrooms are handpicked or mechanically harvested. It is common observation that during the commercial cropping of A. bisporus and also of some other mushrooms, there is heavy and synchronous appearance, called "flushes," of sporophores at appropriate intervals with very little fruiting between the flushes called flush break. Fructification of mushrooms represents an interesting phenomenon to study the differentiation in multicellular eukaryotes (Rai and Saxena 1991).
Varied levels of mechanization in composting and cropping and automation especially in the environment control of the growing rooms have been introduced in many developed countries in case of the button mushroom. However, old manual system of seasonal growing in makeshift cropping rooms is still practiced in many developing countries to feed the domestic market.
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