Residues like peelings from cassava, straw and stover from wild grasses, rice, maize, millet, sawdust, by-products of cotton, oil palm by-products have all been utilized as potential substrates for mushroom cultivation. The application of appropriate bioconversion technology such as slow release of nutrients for mushroom cultivation would reduce the waste profitably.
Moreover, environmental awareness has grown to such a proportion that enforcement of pollution control laws has become more effective. Waste recycling and supplementation techniques in the production of mushrooms, especially Pleurotus species that survive on a wide range of substrates, would be beneficial to ensuring pollution control.
It is estimated that the weight of by-products from twelve major crops grown in Ghana including cocoa, oil palm, cassava and maize amount to more than 9 million metric tons annually. When only one-fourth of this
amount is utilized in growing mushrooms, about 1.2 million metric tons of fresh mushrooms can be harvested within two months assuming a biological efficiency of 50 percent. This is enough to provide 18 million people each with over 1.1 kg of mushrooms daily.
Mushroom is an important food in the diet of Ghanaians.
Depending on the variety, they contain high quality protein with levels ranging from 21 – 40% dry weight. They also contain vitamins B1, B2, B6, B12, C, D and rich in minerals essential for human health.
Dry mushrooms can be powdered and used in infant food preparations for increased nutritional value. Protein energy malnutrition has been identified as one of the biggest nutritional problems of the vulnerable group. Diseases such as Kwashiorkor, marasmus and anemia are becoming widespread because protein is lacking in the daily dietary intake of the average Ghanaian. To combat the crisis, the Food and Agricultural Organization has recommended the use of mushrooms as a potential food source especially since mushrooms have the capacity to convert agricultural wastes into high protein food.
Nutritional analysis showed that mushrooms are a more valuable source of protein than either cattle or fish on dry weight basis, and are good sources of almost all the essential amino acids when compared with most vegetables and fruits.
The cultivation of mushrooms in Ghana is basically the Plastic Bag Method, with the use of decomposed sawdust from cereals (rice or millet) to produce Pleurotus species of mushrooms. It involves an initial composting of the substrate, bagging, sterilization, inoculation with mushroom spawn, incubation, cropping. It is estimated that the weight of by-products from twelve major crops grown in Ghana including cocoa, oil palm, cassava and maize amount to more than 9 million metric tons annually. When only one-fourth of this amount is utilized in growing mushrooms, about 1.2 million metric tons of fresh mushrooms can be harvested within two months assuming a biological efficiency of 50 percent. This is enough to provide 18 million people each with over 1.1 kg of mushrooms daily.
Producing a substrate for the mushroom crop is the first step in mushroom cultivation for the white and brown commercial mushrooms. Poorly prepared substrate will result in lower yields and poor-quality fresh product that will negatively impact the profitability of the farm.
Materials used in this substrate-preparation process are high in soluble nutrients that need to be kept separated from the natural water and land resources.
A substrate is defined as a surface on which an organism grows or is attached. Compost is a mixture of decaying organic matter used to improve soil structure and provide nutrients. Composting is the process by which organic matter converts to compost for mushroom cultivation. Farmers use a composted substrate, defined as organic matter decomposed into a media for organisms (in this case, mushrooms) to grow on. A chain of chemical reactions and microbial decomposers, of which the mushroom is an organism in the chain, complete the composting for mushroom substrate preparation.
Mushroom substrate describes the composted material used to grow a crop of mushrooms, whereas mushroom compost (spent mushroom substrate /compost) describes the material left after a crop of mushrooms is finished. Mushroom substrate preparation can be separated into various stages, often described as pre-wet, preconditioning, and Phase 1; however, these terms are not universally applicable.
Mushroom substrate may be simply defined as a kind of lignocellulosic material which supports the growth, development, and fruiting of mushroom mycelium. The process of preparation of substrate is broadly termed “composting”. The final product of “composting” is called the “compost” or prepared substrate. The process for preparation of substrates has been the subject of much scientific and practical interest over the past two decades. It should be noted that different types of mushrooms require different types or substrate/compost. Agaricus bisporus grows on fermented compost which is traditionally developed from wheat straw mixed with horse manure, and it requires higher nitrogen content.
Composting. composting refers to the piling up of substrates for a certain period of time and the changes due to the activities of various micro-organisms, which result in the composted substrate being chemically and physically different from the starting material. This is sometimes referred to as a solid state fermentation.
Two types of composting are commonly described. One type involves the decomposition of heaps of organic wastes and the subsequent application of the residue to the soil. The aim of this type of composting is to reduce, in a sanitary manner, both the volume and the Carbon-Nitrogen ratio of the organic waste so that is it suitable for manuring the soil to improve the growth of plant crops. When given directly to the soil without composting, organic waste with a high C:N ratio (such as straw) can give rise to a temporary nitrogen deficiency which will then result in a reduction in yield of the plant crop.
The second type of composting is also a process of microbial fermentation, but in this case the substrate is used for the cultivation of edible mushrooms. Through composting, a mixture of rich organic materials is converted into a stable medium which is selective for the growth of a particular mushroom but is not suitable, or is less favourable, for the growth of competing microorganisms.
The competitors exist in uncomposted materials, and often in partially composted materials, but they are far less active in well-composed mushroom substrates. Actually, this type of composting is derived mainly from the Agaricus mushroom-growing industry, in which a composting technique which renders wheat straw with horse manure specific for the growth of the Agaricus mushroom has been developed.
It should be noted that the treatment of substrates for growing other mushrooms can be regarded as “composting”, but the procedures followed in composting and the nature of the product can be quite different. This is because the starting materials and the lengths of time accompanying the various changes in the substrates vary from mushroom to mushroom.
As stated above, the role of composting is the production of a selective substrate that will preferentially support the growth of the mycelium of the mushroom. The basis of this selectivity, however, cannot be attributed to one factor or one aspect of the entire system. The physical, chemical and biological aspects of composting are fundamentally interrelated, but can be artificially separated for the convenience of investigation and discussion.
Mushroom growers like to practice the liberal use of the sense of sight, smell and touch to evaluate the progression of the composting process and the quality of the final product. The gross characteristics of compost, usually referred to as “structure”, result from a number of complex physical, chemical and microbial processes.
The overall goal of composting is to produce selective nutrient media for the growth of the mushroom. These selected nutrient-rich substrates should support a high yield of good quality mushrooms. The general aspects of the achievement of composting are summarized as involving:
1) straw softening and other structural changes;
2) modification of plant materials so that nutrients are made available to mushroom growth and development;
3) building up of an appropriate biomass and a variety of microbial products( some of these can serve as nutrient sources for the mushroom);
4) establishment of selectivity, i.e. the compost promotes the growth of the mushroom over competitor organisms;
5) modification of compost structure so that it holds more water; and
6) building up of compost moisture content to serve as a water reservoir for the mushroom crop. The substrate left after the mushrooms have been harvested is known as spent compost. This is present in large amounts, and raises the question of what can be done with it. It is certainly not desirable to leave it as a possible source of pollution.
It is known that there still remains in the spent compost a considerable amount of lignocellulosic material in addition to the mushroom mycelia and also other products formed by the metabolic activities of the mycelium. Thus, the spent compost should be capable of supporting further biological activities, e.g., the growth of another species of edible mushroom; use as fodder for livestock; as a soil conditioner and fertilizer; and also in bioremediation.
The material on which the mycelium of the mushrooms grows is called substrate. Agricultural wastes like wood chips/sawdust, sugar cane bagasse, and different types of straw can be used as the main ingredients in the substrate for oyster mushrooms.
The properties of a substrate determine which mushrooms and microbes can grow in it. The more selective it is, the better the substrate meets the demands of a specific mushroom and the less suitable it is for others.
After mixing and adding certain supplements, the substrate undergoes a heat treatment to give the desired mushroom mycelium an environment with few competitors.
Preparation of the substrate
Oil drums and plastic bags are all you need to prepare the substrate.
A cemented floor is the preferred underground for mixing and moistening the sawdust (or straw) and a fork for mixing the ingredients.
Substrate preparation equipment:
- substrate mixer (optional); the mixing of the substrate ingredients can be performed just as well manually
- a steam source or heating equipment like an oil drum
For the substrate:
- raw substrate materials, like sawdust, rice bran, wheat straw, dried banana leaves, dried elephant grass, dried grass pieces etc.
- substrate containers (plastic bags or bottles)
- depending on the type of bags/bottles: additional plugs and plastic rings and/or rubber bands
Mixing the substrate
The aim of mixing is to distribute the different ingredients, including water, as evenly as possible. If adding a small quantity of one component like chalk, then it is better to mix it first with some of the substrate and only then add it to the large heap. Otherwise its distribution will probably remain non-uniform. Moreover, lumps might occur and the very high concentration of nutrients at these spots will result in contamination.
Mixing is also very important for the moisture distribution. The correct amount of water should be available everywhere in the substrate.
After mixing, the moisture content should be 60 – 65%.
|Sometimes a better distribution can be achieved if the substrate ingredients are mixed in a dry state (e.g. in “sterilized” substrates containing sawdust and supplements); the water is then added later.|
.A batch of up to 2000 kg can be mixed by hand on a cemented floor, similar to cement making. Two people are capable of manually mixing 2 tons of substrate per day. More people, however, should do the filling.
|Sterilize the substrate as quickly as possible after mixing in the supplements. Storage of the mixed ingredients for a period longer than 6 hours should be avoided to prevent fermentation of the substrate.|
The sawdust (or other bulk substrate material) has to be stacked on a heap and moistened. By keeping the heap moist, the sawdust will soften. This will ease the absorption of water. Usually the sawdust is stacked for only one or two days.
If only fresh sawdust is available, like sawdust from recently felled trees, it should be stacked for a much longer period: up to several weeks.
The sawdust substrate should be free of splinters or bigger pieces of wood. These may damage the bags, offering contaminants easy access after sterilization. On the other hand, several growers feel that a combination of fine and coarser sawdust or wood chippings provides the best starting material. Very fine sawdust should be avoided as it clogs the airflow when moistened.
Moisten the finely chopped substrate ingredients and apply the squeeze test to determine whether the substrate is moist enough.
Fill small substrate containers (usually plastic bags) with the substrate before sterilizing.
The aim of the heat treatment is to kill competing microorganisms and to get rid of soluble nutrients. Most substrates are given a heat treatment before spawning. It is an important measure to control pests and diseases.
In this article three methods are dealt with:
- pasteurization by immersion in hot water
- pasteurization by steam
Advantages and disadvantages of different heat treatments.
Immersion in hot water
This method is a form of pasteurization. The hot water will kill contaminants. Different types of straw can be treated in this way for the cultivation of oyster mushroom (Pleurotus)
The method is very easy: only hot water, containers and the means to keep the water hot are required.
Materials and equipment required:
- substrate material
- substrate containers (e.g. plastic bags or trays)
- Containers for hot water and means to keep the water hot (fuel, solar energy, steam, etc.)
- wire mesh to let the substrate drain
The substrate is put in wire mesh cylinders in hot water. The water has to be kept at 70 °C for at least 15 minutes, but 30-60 minutes is safer.
|Immersion in water at lower temperatures and for periods shorter than 15 minutes is insufficient to kill all contaminants.|
The size of the water containers depends on the scale of the operation. A 240-litre container can hold about 90 kg of wet straw substrate.
The same container can be used 2-3 times a day, because the actual immersion time is only about 30 minutes to one hour.
|The same batch of water should not be used for more than two or three batches of substrate.|
Draining and cooling
Drain the heat-treated substrate and let it cool on a clean plastic sheet on a table or on the floor inside the farm.
Bulk pasteurization by steam
This method kills the unwanted organisms but keeps the favourable ones alive. To achieve this, a temperature of 60 ºC to 70 ºC has to be sustained for at least 8 hours; after which most pests and diseases (contaminants) will be eliminated.
- Materials and equipment required:
- substrate material
- substrate containers (e.g. plastic bags)
- oil drum and burner
Put a rack in the oil drum, with a fine mesh wire to prevent straw from falling through. Fill water below the rack to a height of 20 cm. Then add the moistened straw on top. Steam the straw for at least 8 hours.
Take care that the straw has cooled down to 30 C before spawning.
|Allow the steam to escape through small openings to prevent explosion of the drum.|