Cultivation of the Common White Mushroom

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The methods used for the cultivation of Agaricus bisporus are essentially similar to those pioneered by French horticulturists (9), but the old practices have been gradually refined and modified to the point where mushroom production is a closely controlled and scientific process. Mushroom spawn for each cultivation was simply transferred to an existing mushroom bed. A problem with this straightforward approach was the tendency for the spawn to gradually lose its vigor with repeated transfers from one bed to the next. Other improvements in the process have mainly concerned the development of specialized mushroom houses in which ventilation, temperature, and humidity can be carefully controlled to provide the best conditions for fruiting. In fairly deep caves the temperature remains remarkably stable, and for certain stages of mushroom development this advantage is obvious. Specially built mushroom houses are, however, more efficient, especially when it comes to preparing the mushroom beds or harvesting the crop.

The first stage in cultivating Agaricus bisporus is the germination of spores (5). This is carried out by taking spores or fragments from a mature fruiting body selected for desirable qualities such as color, taste, and vigor. The spores or fragments are then germinated on sterile nutrient medium to give a master culture. The master culture is used to prepare the spawn, which is then distributed to growers. This spawn was produced by inoculating a suitable sterile medium, such as horse-manure medium, but today a variety of spawn types are available, including grain spawn. Grain spawn is now almost universally used in houses; the grain may be wheat, rye, sorghum, or millet, in decreasing order of size. Whatever the medium, spawn production is merely a process of bulking up the initial mycelium to the point where it can be conveniently introduced into the culture beds themselves (Fig. 1, Fig. 2).

The fermentation process removes most of the simple sugars from the medium and leaves insoluble substances such as cellulose and lignin, which are readily utilized by Agaricus bisporus. The point of the composting step is to create a medium that both favors the mushroom mycelium and discourages its natural rivals. The composting process is usually carried out in the open, with the piles being regularly turned to ensure an even fermentation that results in the whole pile becoming hot. The length of time required to get the compost into the right condition varies, depending on the age of the manure, but normally it is at least 2 weeks. Composts for growing in mushroom houses vary according to the nature of the locally available materials and the type of pasteurization employed. Most trays now in use have short legs, which require the addition of spacing blocks for cropping. However, some farms use longer legs, so that the trays are always stacked at the full dis-

Figure 1. Gram spawn of Agaricus bisporus, with the fungus growing on sterilized wheat grains.
Primitive Grew Grain
Figure 2. Agaricus bisporus growing in a commercial bed.

tance apart, as for cropping. Although the principle involved is the same in all cases, different growers often have their own minor modifications designed to improve the end product because their different ingredients and preparations require adjustments to the control of temperature and aeration. Correct timing is all-important. Undercom-posted manure tends to overheat in the beds and leads to poor mycelial growth. On the other hand, overcomposting will remove many of the nutrients essential for production of a good crop. Once the compost has reached the correct stage of fermentation, the beds themselves can be prepared.

It has been known since mushrooms were first cultivated that the beds must be covered with a casting of soil or other suitable material to induce the change from vegetative growth of mycelium to fruiting. The virtues of different types of soil as casing materials are frequent subjects of argument among growers. Clay soils are reputedly better than sandy ones, but interest has also been extended to artificial substitutes, such as mixtures of peat and vermiculite (an artificial potting medium).

For many years the reason for casing was completely unknown. Recently research has revealed some of the effects produced by the soil. The change in growth conditions accompanying the addition of soil induces the formation of tiny primordia, which subsequently mature into mushrooms, but the exact action by which casing stimulates fruiting is still unknown.

The final stages in mushroom cultivation are cropping and packing. Following the first crop of mushrooms, several additional periods of fruiting usually occur at intervals of 2 or 3 weeks. The time at which the mushrooms are picked is important. It is agreed that commercial growers usually harvest their mushrooms before the veil has broken. The major consideration is the longer life of slightly immature mushrooms once they have reached the shop. In common with many other food items, the prevalent belief that a clean, uniform product is somehow superior to a more varied and possibly tattered product has no doubt contributed to the monopoly of the button mushroom. Once the beds are finished, the composting is removed and sold as manure for various horticultural activities.

Mushrooms are picked by gripping the cap and bending or twisting to break off the stem at the base. This is easily done with closed mushrooms; open ones are fragile, and more care is needed to ensure that the stem breaks at the bottom, not at the top.

The stage of growth at which mushrooms are picked is important. All open mushrooms, whatever their size, are picked. In many countries open mushrooms are considered extremely low grade, and in most they are cheaper in the market than closed cups or buttons. After opening, the stem grows but the weight of the cap does not increase appreciably, so that even disregarding quality and price, there is no further increase in yield. Up to the point of opening, the weight of the mushroom increases very rapidly, doubling in 24 h at its peak rate, so leaving closed mushrooms to grow as long as possible raises their picked weight. With tight picking, the more frequent flushes compensate for the loss of weight on the individual mushrooms. Closed mushrooms also have a better shelf life in the shop, particularly in warm weather.

Picking mushrooms has always been a major factor in wage costs. With closely packed trays or shelves, picking is carried out under cramped conditions, usually with poor lighting, so that selection and grading of mushrooms are difficult (5).

Two recent developments have promised to improve and accelerate the picking operation. One is development picking lines, in which the trays are transported to the pickers, where mushrooms can be picked under ideal conditions. The other is carrying of trays by forklift truck from the cropping room to a destacker at the input end of the line. They are automatically destacked and passed along by a conveyor system at a controlled speed that can be adjusted to rim fast if there are not many mushrooms and slower for a heavy flush. Pickers stand on both sides of the line with baskets handy and pick as the trays pass in front of them. The speed of the conveyor is adjusted so that the trays are cleared just before they reach the end of the line. There may be conveyor belts for stumps and the like and another conveyor for filled baskets of mushrooms. There is also an overhead spray line at the end of the conveyor so that trays can be automatically watered as they pass along.

Figure 3. Cultivated Pleurotus eryngii growing in reused plastic bottles.

Figure 3. Cultivated Pleurotus eryngii growing in reused plastic bottles.

The other development is mechanical harvesting. With pickers' wages becoming an increasingly important factor in production costs, the possibility of harvesting mushrooms by machine has interested many growers. A machine that travels over the bed, cuts mushrooms with a reciprocating blade just over the casing, draws the cut mushrooms back over the blade, and delivers them to one side behind the machine has been described. Patents have also been filed for a machine that locates mushrooms by a number of sensor units, grips them on the top of the cap by suction, and plucks them off the bed (5). Mushrooms on a bed must all, or nearly all, be at the same stage of development, because they must all be cut at once. Any mushrooms that are higher or lower in the bed will be cut with too much stalk or through the cap,and any that are crooked will also suffer. Harvesting machines already in existence can pick from shelves or could be mounted over picking lines. It is necessary to provide for the machine to run on the beds or side boards or to be fitted with sensors that control its height above the casing. This may require the provision of bare strips of bed at each side of the strip to be harvested. A harvesting machine produces ungraded mushrooms. Postharvest grading for size is most easily done in the water flumes used in canneries so that the first application of these machines may be postharvest.

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