11.6 Industrial cultivation methods

Having just included cheese production in this account, we could complete the ploughman’s lunch with bread and beer, but we prefer to leave baking and brewing, not to mention Quorn™ production, to later discussion of biotechnology in Chapter 17, and turn instead to mushroom cultivation (Moore & Chiu, 2001; Dupont et al., 2017). Whether calculated in terms of value of product, mass of product, number of people involved in the industry, or geographical area over which the industry is practised, mushroom cultivation is by far the biggest non-yeast biotechnology industry in the world. The technicalities of other sorts of solid state fermentation are discussed in Chapter 17; here we will deal with the industrial aspects of mushroom farming. In the European tradition mushroom farming has come to mean cultivation of a mushroom crop on composted plant litter (Junior Letti et al., 2018).

The mushroom industries of the world all depend on some form of solid state fermentation; but there are two traditional cultivation methods. In the European tradition the mushroom crop is cultivated on compost. Similar approaches were developed for Oyster and Paddy straw mushroom cultivation in the Orient, largely by peasant farmers; whilst in the Japanese and Chinese traditions the typical approach is to use wood logs to cultivate the crop of choice (Lentinula, called shiitake in Japanese or shiangu in Chinese.

The European mushroom industry is said to have originated in the caves beneath Paris at the end of the nineteenth century. It probably emerged from the food provisioning functions of the kitchen gardens on the estates of the European aristocracy. Some of the surviving records of such estates refer to manured and composted plots set aside for mushroom production. The compost used, and its preparation, would very definitely be familiar to competent gardeners of those days.

The current industry depends on compost which is very selective for the crop species Agaricus bisporus. Although widely distributed in nature, this fungus is rarely noticed because established mycelia produce a few isolated mushrooms very infrequently. Today’s industry is the result of a ‘joint evolution’ during which otherwise ordinary horticultural compost was developed that achieves high cropping densities by an otherwise unremarkable and not very abundant mushroom (Buth, 2017).

Composting proceeds in two phases: in phase 1 the straw, manure and other components are mixed into large heaps. After the water is added the heaps are thoroughly mixed by mechanical compost turning machines. This ‘pre-wetting’ treatment continues for a few days and then the machines arrange the compost into long stacks about 2 m wide, 2 m high and many metres long. Within a few days bacterial activity heats the stack to around 70°C in the centre, though it is considerably cooler at the surface. Higher temperatures, which would kill the microorganisms, are avoided by regular ‘inside-out’ turning of the compost heap. As well as heat, the bacterial degradation process releases large amounts of ammonia. An important aim in phase 1 of composting is to achieve uniformity by thorough mixing (so that all of the compost spends some time within the hotter core of the stack). A week after the stack was first laid it is mixed, or 'turned', by large, self-propelled ‘turning’ machines. It is left for a further week, then turned again. Three weeks after the process was started, the compost is ready for phase 2.

Phase 2, also known as peak-heat, pasteurisation or sweat-out is a continuation of the composting process but without further mixing and under more controlled conditions. The compost may be treated in bulk or loaded into the eventual growing containers. In either case the process is done in a building which allows air to be circulated around the growing containers or through the bulk of the compost. To begin with, air and compost temperatures are raised to about 60°C for several hours. This pasteurisation stage is usually completed in a day and then the amount of ventilation is increased and compost temperature is kept at about 50°C for 4 - 6 days. The beds are then allowed to cool to around 25°C and are ready for use. Natural drop in temperature and absence of free ammonia are signs that the composting process has been completed.

Although the basics of mushroom production are the same however the crop is produced, growing containers differ and the process can be separated into specialised stages. Mushrooms may be produced in large wooden trays, in beds on shelving and in plastic growing bags (Dhar, 2017).

  • Trays, made of wood in sizes varying from 0.9 × 1.2 m to 1.2 × 2.4 m and 15 to 23 cm deep, are arranged in tiers three or more high, separated by wooden legs. Fork lift trucks are needed to move the trays and some sort of mechanised tray handling line is necessary.
  • Shelving is usually made of metal and arranged to give four to six layers of fixed shelves in a cropping room with centre and peripheral access gangways. Each shelf is about 1.4 m wide and extends almost the whole length of the room. Special machinery for compost filling, emptying, spawning, casing and other cultivation operations is necessary.
  • Growing bags of about 25 kg are usually supplied to the farm with the compost completely colonised by the mycelia of the mushroom crop and may be arranged on the floor of the cropping house or on tiered shelving (this is phase III compost).

For a commercial mushroom farmer, the use of phase I compost gives the most flexibility to optimise farm conditions for cultivation of any mushroom strain. Purchase of phase II compost enables a farmer to choose which mushroom strain to spawn. The use of phase III compost, though more costly, guarantees the production of a crop in a short time and requires the least prior investment in facilities for mushroom production.

Spawning is the process that introduces the mushroom mycelium into the compost. This is generally done with some form of carrier that can be easily mixed into the compost, fungus-coated cereal grains (often barley) being the most usual (Moreaux, 2017). About 5 kg spawn per tonne of compost (0.5% w/w) is used. From the inoculation centres that the spawn grains provide the mycelium grows out to invade the compost (this is called ‘spawn running’), filling the compost bed after 10-14 days at a compost temperature of 25°C.

Phase III compost is completely colonised by the mushroom mycelia. Nowadays, the mushroom production industry comprises spawn makers, phase I compost, phase II compost and phase III compost suppliers.

Casing is the process that encourages fruiting of Agaricus, the spawn run compost (which is compost completely permeated by mycelium) must be covered with a ‘casing layer’ which was originally simply a layer of garden soil but now is most usually a mixture of moist peat and chalk. The chalk is used to adjust the otherwise acid pH to a neutral one. Slow-release nutrients are sometimes also included (Pardo-Giménez et al., 2017).

The optimum depth of the casing is 3 - 5 cm and it should be an even layer applied to a level compost surface. The mushroom mycelium grows into the casing layer but reaches the upper surface of this layer as strands and these are a necessary start to the fruiting process. To encourage completion of fruiting the growing room is ventilated to lower the concentration of carbon dioxide (usually to <0.1%) and to help reduce the temperature to 16 - 18°C.

Throughout these processes the casing layer is kept moist by mist-spraying with water, as required, because moisture, temperature and atmospheric gases all have to be closely controlled. After allowing 7 to 9 days for the Agaricus mycelium to grow into the casing layer, a machine with rotating tines is run across the mushroom bed to mix the casing layer thoroughly. This is called ‘ruffling’ and it serves to break up the mycelial strands and this injury encourages the mushroom mycelia to colonise the surface of the casing layer where it forms the mushroom initials. Casing is needed only by Agaricus, the procedure is not necessary when cultivating other species on composted straw such as Volvariella spp., Pleurotus spp., Auricularia spp. and Lentinula edodes.

A few days after ruffling, the injury and change in microclimate on the surface of the casing soil sensed by the mushroom mycelium together trigger the formation of Agaricus mushroom primordia. The first, called ‘pins’ or ‘pinheads’ are more or less spherical and have a smooth surface, are seen about 7 to 10 days after casing, and 18 to 21 days after casing marketable mushrooms can be harvested.

Successive crops of mushrooms (called flushes) then develop about 8 days apart, and each taking about 5 days to clear from the beds. During the cropping period, the casing needs to be kept moist and the air temperature must be maintained in the 16 to 18°C range. Ventilation must also be maintained to keep carbon dioxide levels low. Accurate balance is required here: humidification is essential to minimise desiccation, but too high a level of humidity encourages disease.

Growers expect to harvest between three and five flushes from each spawning cycle, with a total yield of around 25 kg m-2 of growing tray. After the final pick (seven to ten weeks after spawning) the compost is spent, and the cropping room is emptied, cleaned, sterilised and filled with the next crop. On most large commercial farms a new crop is filled every one or two weeks throughout the year. So a mushroom farmer is likely to see more crops in one year than a cereal farmer will see in a lifetime.

Commercial production of mushrooms produces a total crop of several million metric tonnes each year. In the mid-1970s the button mushroom (Agaricus) accounted for over 70% of total global mushroom production. Today, it accounts for something closer to 45% even though production tonnage has increased at least ten-fold in the intervening years (Table 1) (Royse et al., 2017). At averaged-out prices this total crop currently has a retail value of about 50 billion US dollars.

Table 1. FAO data for Production Quantities of mushrooms and truffles 1994-2016

Year

Global production (millions of tonnes)

1995

2.776

2000

4.190
2005

5.270
2010

7.443
2012

9.647
2014

10.409
2016

10.791
Geographical distribution (average 1994-2016) was approximately 69% farmed in Asia, 22% in Europe, 8% in the Americas, 0.8% in Oceania and 0.2% in Africa. Average production in the Asia region 1994-2016 was 4.231 million tonnes; mainland China alone produced an average of 3.972 million tonnes per year across this time period. Data from FAOSTAT website [http://www.fao.org/faostat/en/#data/QC/visualize].

The biggest change during the last quarter of the twentieth century was the increasing interest shown by consumers in a wider variety of mushrooms. Even in the most conservative of markets (like the United Kingdom) so-called ‘exotic mushrooms’ have now penetrated the market and supplies of fresh shiitake (Lentinula) and oyster mushroom (Pleurotus) are routinely shelved alongside Agaricus in local supermarkets (Royse et al., 2017). Many also offer Enoki (Flammulina velutipes), Buna shimeji (Hypsizygus marmoreus), Shiroshimeji (Pleurotus ostreatus), and King Oyster (Pleurotus eryngii) among others, most of which are cultivated locally (Stamets, 1994), but the industry is truly international and mushroom cultivation is the next-biggest biotechnology industry after alcohol production.

The production of Pleurotus (oyster mushroom) differs from that just described for Agaricus because the needs of the organism are much less stringent. Pleurotus will grow vigorously on both composted/pasteurised and sterilised/but uncomposted preparations of a wide range of substrates including sawdust, wood chips, cereal straw, and so on. Casing is not required. The crop can be adapted to different countries depending on their climates by growing different species of oyster mushrooms, e.g. Pleurotus pulmonarius (misnamed as P. sajor-caju) in India; P. ostreatus (commercially called P. florida, another inaccurate name) in Europe.

One reason for the remarkable increases seen in production of certain mushrooms at reasonable price has been the use of substrates which are waste products from other industries. For example, oyster mushroom species (Pleurotus ostreatus, P. cystidiosus, P. pulmonarius) are all easily grown on cotton wastes.  Similarly, although the straw mushroom (Volvariella volvacea) is traditionally grown in South-East Asia on rice straw, it too can be grown on cotton waste. Cotton waste gives higher yields and is also more widely available than is rice straw so it is a far cheaper substrate (the higher cost of rice straw does not derive from any intrinsic value but in the cost of transporting it to a non-rice-growing region). Cotton waste substrates are generated by the textile and garment industries and are produced in bulk by recycling schemes around the world.

Disposal of an abundant bulky solid waste coupled with currency earning by sale of a mushroom crop is a good example of an organic farming system integrated with a waste treatment system. The concept of using mushroom cultivation as a waste remediation has become a popular model in recent years, and we have already mentioned that agriculture generates enormous waste because so little of each crop is actually used (95% of the total biomass produced in palm and coconut oil plantations is discarded as waste, 98% of the sisal plant is waste, 83% of sugar cane biomass is waste, etc.).

Pleurotus spp. in particular grow readily on so many lignocellulose agricultural wastes that it becomes an attractive notion to use the fungus to digest the waste and by so doing produce a cash crop of oyster mushrooms. Even more attractive is that after the mushrooms have been harvested the ‘spent compost’ can be useful (Ferraro et al., 2020):

  • as animal feed (the mushroom mycelium boosts its protein content),
  • as a soil conditioner as it is a compost still rich in nutrients and with polymeric components that enhance soil structure and serve as a biofertiliser (Yu et al., 2019),
  • and even used to digest pollutants (like polychlorinated phenols) on land-fill waste sites because it contains populations of microorganisms able to digest the natural phenolic components of lignin (see Section 10.7).

Some care is needed because Pleurotus accumulates metal ions in the fruit body (Sakellari et al., 2019; Siwulski et al., 2019). If waste used as substrate comes from an industrial source contaminated with heavy metals (cadmium is a particular problem in many industries), then the mushroom crop may be unsuitable for consumption (Moore & Chiu, 2001; Kumhomkul & Panich-pat, 2013; Covaci et al., 2017; Mohd Hanafi et al. 2018). Harvesting the mushrooms would still be an effective way of removing the heavy metal contamination, though, and activity of the mycelium will remediate the rest of the waste.

Although European farming methods are used around the world, the Asian tradition for commercial mushroom production tends to favour more natural substrates. Lentinula edodes (shiitake) is traditionally grown on evergreen hardwood logs (oak, chestnut, hornbeam) and is still very widely grown like this in the central highlands in China. To put this statement into perspective, the traditional log-pile approach is still the most frequently used method in China over a growing region which covers an area about equal to the entire land area of the European Union. Logs suitable for shiitake production are over 10 cm diameter and 1.5 to 2 m long and normally cut in spring or autumn to minimise pre-infestation by wild fungi or insects. Holes drilled in the logs (or saw- or axe-cuts) are packed with spawn, and the spawn-filled hole then sealed with wax or other sealant to protect the spawn from weather. In this case the spawn may be mycelium grown on rice or other cereal grains, but is more likely to be mycelium grown on wooden dowels which can then be hammered into holes drilled in the production log (Royse et al., 2017).

Inoculated logs are stacked in laying yards on the open hillside in arrangements which permit good air circulation and easy drainage and provide temperatures between 24° and 28°C. The logs remain here for the 5 to 8 months it takes for the fungus to grow completely through the log. Finally, the logs are transferred to the raising yard to promote fruit body formation. This is usually done in winter to ensure the temperature shock (12 - 20°C) and increased moisture which are required for fruit body initiation. The first crops of mushrooms appear in the first spring after being moved to the raising yard. Each log will produce 0.5 to 3 kg of mushrooms, each spring and autumn, for 5 to 7 years.

This traditional approach to shiitake production is expensive and demanding in both land and trees; for these and other reasons more industrial approaches are being applied to shiitake growing. Hardwood chips and sawdust packed into polythene bags as ‘artificial logs’ provide a highly productive alternative to the traditional technique, and the cultivation can be done in houses (which may only be plastic-covered enclosures) in which climate control allows year-round production.

Volvariella volvacea (paddy straw mushroom) is grown mainly on rice straw, although several other agricultural wastes make suitable substrates. Preparation of the substrate is limited to tying the straw into bundles which are soaked in water for 24 to 48 h. The soaked straw is piled into heaps about 1 m high which are inoculated with spent straw from a previous crop. In less than one month, a synchronised flush of egg-like fruit bodies appears. These immature fruit bodies (in which the universal veil is intact and completely encloses the immature fruit body are sold for consumption just like the young fruits (‘baby buttons’) of Agaricus, though this is not the case with oyster and shiitake mushrooms which are sold mature. Comparatively low yields of V. volvacea are generated from the substrate, and it is difficult to maintain a good quality in post-harvest storage. Within 2-3 days the crop turns brown and autolyses even in cold storage. These factors restrict production of the crop. Efforts to identify genes involved in rapid growth of V. volvacea mycelium using comparative transcriptome analysis have identified four heat-shock proteins and up to 14 transporter genes that showed enhanced expression in a commercial straw mushroom strain (known as V9) that had a shorter growth cycle and higher biological efficiency than other commercial strains. This implies that improved ability to cope with stress and environmental variability, together with improved efficiency of import and/or export of metabolites and xenobiotics are key features of rapid mycelial growth (Liu et al., 2020).

Ganoderma lucidum is a cultivated mushroom which is unique in being consumed for its pharmaceutical value (real or imagined) rather than as a food. Under the names lingzhi or reishi in Asia, several Ganoderma spp. of the G. lucidum complex provide various commercial brands of nutriceuticals, in the form of health drinks, powders, tablets, capsules and diet supplements. Ganoderma is highly regarded as a traditional herbal medicine, though the claims made for it are clinically unproven. It is cultivated by being inoculated into short segments of wooden logs which are then covered with soil in an enclosure (such as a plastic-covered ‘tunnel’) which can be kept moist and warm. Fruit bodies emerge in large number quite close together and the conditions encourage the fungus to form the desirable long stemmed fruit body (Moore & Chiu, 2001).

The morphology of Ganoderma fruit bodies varies greatly. At least some of the reported variation is likely to be due to misidentifications as the taxonomy of the Ganoderma lucidum complex has been described as ‘chaotic’. Analysis of 32 collections of the complex from Asia, Europe and North America using both morphology and molecular phylogenetics recovered a total of 13 taxonomically distinct species within the complex (Zhou et al., 2015). In sharp contrast, a survey of the molecular phylogenetics of 20 specimens of the related clade, Ganoderma sinense, from China were found to exhibit varied fruit body morphology, even though they possessed identical nucleotide sequences (Hapuarachchi et al., 2019). Evidently, the phenotypic plasticity (= varied fruit body morphology) of a specimen or strain of Ganoderma can be influenced greatly by extrinsic factors, such as climate, nutrition, vegetation, and geographical environment rather than being associated with genotypic variation.

Truffles are extremely valuable; they can be worth £1,000/US$1,400 to £3,000/US$4,200 per kg, with a single truffle potentially weighing over 200 g. Truffle cultivation is different from that of the other fungi so far described because the truffle is the underground fruit body of one of the Ascomycota that is mycorrhizal on oak (Quercus), so it is dependent on its host tree. Traditionally, truffles are found using pigs or dogs trained to detect the volatile metabolites produced by the fruit body. Truffle ‘cultivation’ was first achieved in France early in the nineteenth century when it was found that if seedlings adjacent to truffle-producing trees were transplanted, they too began producing truffles in their new location. Truffières or truffle groves have been established throughout France in the past hundred years and the value of the crop is such that the practice is now extending around the world. Truffières are started by planting oak seedlings in areas known to be rich in truffle fungi; the seedlings can be grown-on in greenhouses after infection with Tuber melanosporum. Seedlings can be colonised artificially with the related T. magnatum (a white truffle). Truffles begin to appear under such trees 7 to 15 years after planting out, and cropping continues for twenty to thirty years (Moore & Chiu, 2001).

Updated May, 2021