Mushroom cultivation is a technical process. As mushroom professionals often talk in a technical language, a few of these terms will first be explained. In the early years of mushroom culture in the Netherlands, compost was scooped into the mushroom trays and then inoculated with spores. A nine week wait followed, until the mycelium spawned sufficiently, flushing started and the grown mushrooms could be harvested by hand.
Phase 1: Composting
The growing cycle of mushrooms starts with compost. Compost preparation starts with horse manure. The compost factories get the horse manure from large horse breeding companies that pay the compost factories to collect the manure. Straw, gypsum, chicken manure and water are added to the horse manure. The straw improves the structure, gypsum ensures the proper acidity and the two manures are the nutrients. The compost is produced in tunnels in order to prevent the smell from becoming a nuisance. As manure emits ammonia, compost factories purify the air with ammonia wash to prevent the emission of gases. The indoor fresh compost looks like earth from a forest. Dark brown, full of trampled bits of straw.
The compost is steaming, due to the composting process: heat is generated which digests the components. What’s left is a very fertile, nutritious source for mushrooms. On one batch of compost, two to three flushes of mushrooms can be grown. A square metre of compost (which is equal to 90 kilos) yields a maximum of 35 kilos of mushrooms. After that it’s no longer lucrative to reuse the compost. The leftover compost can still be used as a soil conditioner in other agricultural companies.
The largest producer of mushroom compost in Europe is Walkro, with production facilities in Belgium, Germany and the Netherlands.
Phase 2: Spawning
In a tunnel, the indoor fresh compost is pasteurized at 57-60 degrees Celsius. This kills all possible bacteria. The compost stays in the tunnel to mature for six days, after which the compost is mixed with spawn that will produce the mushrooms: the mycelium. The compost is then moved to another tunnel where the mycelium can spread through the compost. The mycelium grows quickly; after two weeks it has completely permeated the compost, which means that is has reached the point that it is ready for the growers. At this time the compost looks like light brown peat.
Most mushroom growers do not produce their own spawn, as it is a very sophisticated process. Specialized companies produce the spawn by inoculating grain with spores. The grain is sterilized first to prevent infection and it’s kept moist, exactly the way mushrooms like it. Ten kilo of spores (22 pounds) provides about five hundred kilos of inoculated grain (1100 pounds). The grain is incubated in a bag for two weeks at 25 degrees Celsius (75 degrees Fahrenheit), then transferred to a refrigerator at 2 degrees Celsius (35 degrees Fahrenheit) to harden it. In this way, the spawn gets a shelf life of 6 months without the mycelium losing its vitality.
Phase 3: Casing
The matured compost is spread onto long stainless steel boxes, the mushroom beds. The beds are inside special dark rooms called cells. The temperature in the cells is kept nice and warm, at about 23 degrees Celsius. A layer of peat casing material is added on top of the compost to keep the compost moist. Over a period of six days, 20 to 25 litres of water is sprinkled on each m2 in each cell because more moisture is needed. After this, the fungus has two days to grow through the covering layer of casing soil.
Phase 4: Pinning
Mushrooms only grow in the wild in autumn. However, they can be cultivated year round by recreating autumn conditions. Therefore, the temperature in the cell is gradually lowered from 23 to 17 degrees Celsius over four days. The mushroom grower starts to lower the temperature once he sees that the mycelium has grown to its full extent. The temperature shock is a sign for the mycelium to start sprouting the mushrooms.
The same thing happens in nature. Mycelium grows well in mild autumn weather, and after an October storm, the mushrooms will start appearing. The mycelium starts to form little buds, which will develop into mushrooms. Those little white buds are called pins. In this phase, air temperature and humidity can influence growth. Low air temperature and low humidity produce more buds, which yield smaller mushrooms. Higher air temperature and humidity produce fewer but larger mushrooms.
Phase 5: Harvesting
After this, the temperature is kept steady at 18 degrees Celsius. Mushrooms grow best at this temperature; they’ will grow 3 cm (1 inch) in a week, which is the normal size for harvesting. In week 3 the first flush is harvested. Mushrooms destined for selling fresh are still harvested by hand; mushrooms destined for preserving are being picked and sorted mechanically. Although hand-picking is a lot of work, it offers the best guarantee that the mushrooms will be removed from the beds undamaged.
On average, a picker can harvest between 18 and 30 kilos of mushrooms an hour. The mushrooms are picked from the beds with a rotating motion and sorted by the pickers based on quality, size and weight. Nine days after the first flush, the second flush will be harvested. The second flush often consists of larger, but fewer mushrooms than the first flush.
After the second flush of mushrooms has been picked, the cells need to be cleaned. First the cell is pasteurized with steam to kill any remaining fungus to ensure that there is no transfer from cycle to cycle. During steam-cleaning, the temperature in the cells reaches 70 degrees Celsius for eight hours. After steam-pasteurization, the compost is removed from the beds. The empty cell is thoroughly cleaned one more time and then it is ready to be filled again.
