The stages of mushroom growing

In a commercial operation, mushroom cultivation is in temperature and humidity-controlled windowless greenhouses, tunnels or caves. There are five major steps to readying a mushroom for your pot.

1. Substrate preparation. Substrate is the organic material that the mushroom uses as its food source. Carefully preparing this determines both the size and quantity of the crop. Different cultivated species flourish in different substrate.

All substrates are high in cellulose, which the mushroom organism breaks down and uses as food energy. Few other living organisms are able to break down cellulose. (Exceptions are wood-eating termites and hay-eating mammals.)

Most substrate is inexpensive straw which the growers keep damp for about two weeks, then add specific material that helps the straw compost, such as chicken manure, gypsum, cotton seed hulls and other organic material. After a few weeks of composting they use steam to pasteurize the substrate and to remove the ammonia. Some growers use huge pressure cookers at high temperature to provide a sterile substrate before they introduce the spawn, so that no other organisms can take over.

A successful mushroom farm is kept scrupulously clean and sterile, just like a microbiology lab, so you need not worry about where a mushroom has been before it turns up in your kitchen. Even if it is manure, it is a sterile manure. Contamination only comes in subsequent handling.

2. Preparation of fungal culture and spawn. The fungal culture is the initial growth from mushroom spores. They nurture spores in a petri dish under sterile laboratory conditions. Once the fungal culture covers the dish, they transfer it to a larger food source, usually moist rye or millet grains in a jar, to give the culture a good, healthy start.

Eventually the fungus colonizes the grain, penetrating it fully. This is called the spawn or mycelium. Sometimes mushroom farms buy this already developed from spawn companies ready to inject into their prepared substrate. But many farms develop their own, using spores from a particularly nice crop, or culture an exceptional specimen they have found elsewhere.

3. Spawning. This is the process of introducing the spawn into the substrate. It is a simple physical process. If the substrate is wood, they drill it full of holes, place a little bit of spawn in each and they seal off the hole. If the substrate is loose organic material, they pack it into large containers lined with sterile plastic sheeting, and they inject the spawn with sterile needles or some other means, making sure no other organism can find its way into it. In larger facilities, special spawning machines do the spawning that mix the compost and spawn mechanically, sometimes also blending in additional nutrients.

4. Mushroom production. When the entire substrate is completely penetrated with the still growing and spreading spawn, it is time to introduce a change to trigger the growth of the mushrooms. Depending on the species, the change may be lowering or raising temperature and humidity, turning fans on for air movement, or turning on strong lights. Often, they use a combination of these. It is an art.

For our common button mushrooms, for instance, they move the blocks of substrate to production rooms, and they lower the temperature of the compost by 2°F (1°C) every day. They water the blocks twice a day to increase the moisture content. As the mycelium gets colder and wetter, it starts shooting rootlike projections toward the surface of the compost. Each of these (called rhizomorphs) is tipped with a mushroom pin, an incipient mushroom that begins to grow. This stage takes about 3 weeks.

5. Harvesting the mushrooms. Mushrooms continue to develop for 5 to 6 weeks until they exhaust the food source. The temperature of the compost, relative humidity and carbon dioxide content of the air are all important for maximum yield and growers must continue to carefully control them throughout the harvesting phase. If carbon dioxide is too high, for example, mushrooms stop growing. If humidity is too high, the mushroom caps become sticky and clammy, and begin to deteriorate before harvesting. If too low, the mushrooms start drying out.

If the grower neglects the slightest detail, there may be no mushroom growth at all. Pure scientific knowledge of the fungus is important in the mushroom-growing process, but experience and intuition are equally necessary. No wonder my childhood neighbor in the story above had such poor success in his second attempt.

Continue reading here: Mushrooms in the kitchen

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