Mushrooms have entered the new era of food technology as a common universally accepted nutritive food. Their commercial cultivation involving SSF has rapidly spread globally, due to their innumerable applications. They are a rich source of protein, carbohydrates, vitamins, and minerals (2,19). Folic acid content in mushrooms has been found to be higher than in liver and spinach. In addition to their nutritive value, mushrooms also possess medicinal properties (1,2,19,20,127-129). They demonstrate antibacterial, antifungal, and antiprotozoal activities, due to the presence of polyacetylene compounds. Today more than 2000 species of mushrooms are known, although only about 20 are cultivated commercially. Button mushrooms, Japanese mark forest mushrooms, Chinese mushrooms, oyster mushrooms, and winter mushrooms are some of the various types of popular mushrooms being cultivated world wide. However, button mushrooms alone account for about 60% of total world production.
Mushroom cultivation involves SSF at three different stages, namely composting, spawn manufacture, and the growth of mushroom on the moist substrate. In order to produce a selective growth medium on which the mycelia colonize and produce fruiting bodies, the compost is prepared by piling up the substrate for a long period of time. During this period, many changes take place, and the resulting substrate differs from the original. Various physico-chemical factors play a vital role in the process. A temperature range of 22-27°C, substrate moisture content of 55-70%, and a pH of 6.0-7.5 are generally considered as the most suitable conditions. Animal manure from horses and chickens, and agro-industrial residues such as wheat straw, paddy straw, barley straw, rice bran, saw dust, banana, maize stover, tannery waste, wool waste, and sugarcane bagasse are used as substrates.
Spawn or inoculum production involves the growth of mycelia of mushroom on cereal grains such as rye, wheat, sorghum, and millet. These support faster mycelial development because of sufficient nutrients availability, and allow easy handling and steadiness during sterilization. The process is carried out with a pure culture. However, application of these substrates increases the costs input; for this reason other raw materials such as sawdust, cereal bran, or other agro-industrial residues have been recommended for spawn preparation. A mixture of sawdust and coffee husk was found quite suitable for spawn preparation for Agaricus bisporus, Pleurotus sp., Lentinus edodus, Flammulina velutipes, and Volvariella volvacea (127-129). Depending upon the nature of the substrate, optimum conditions were moisture content at 40-60%, a pH of 6.5-7.0, and an incubation temperature of 25°C. The final product spawn should be stored at 2-5°C.
The development of the fruiting body requires a lower temperature than the optimum for mycelial growth; it also requires proper ventilation, which helps in releasing the accumulated carbon dioxide, which retards the formation of fruiting bodies. The crops are harvested at the second or third stage of sporophore development, namely the button or closed-up stage, respectively. At this stage, substrate moisture should be generally higher than previous stages, (i.e., compost formation and spawn preparation). High relative humidity (80-95%) is also a desirable condition in order to control the heat, mass, and gaseous exchange. After harvesting of the fruiting body, the leftover solid residue can be used as manure or as animal feed, depending upon the raw material used as substrate.
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