Mushroom Distribution: A Complex Network Of Growers And Suppliers

Mushrooms are a nutritious food source, distinct from plant- and animal-derived foods. They are a good source of B vitamins and vitamin D, and one of the highest sources of selenium. Of the 2000 edible mushroom varieties, 30 have been domesticated and 10 are grown commercially. The most common commercial species include shiitake, straw, oyster, enoki, and more. Oyster mushrooms, for example, are farmed in tubular plastic bags, while shiitake mushrooms are often grown on logs. The distribution of mushrooms is an important topic, as evidenced by the multi-state outbreak of Listeria monocytogenes affecting 36 people linked to enoki mushrooms, resulting in 30 hospitalizations and 4 deaths.

Commercial cultivation

Commercial mushroom cultivation is a complex process that involves several stages and specific conditions to ensure optimal growth and yield. Here is an overview of the key steps and considerations in commercial mushroom cultivation:

Substrate Preparation

The first step in commercial mushroom cultivation is preparing the substrate, which provides the necessary nutrients for mushroom growth. Common substrates include straw, stalks from crops, invasive plants, wood chips, sawdust, and even used coffee grounds. The substrate is typically shredded to increase the surface area, allowing the mycelium to move through it more easily. This step is crucial for efficient mushroom production.

Spawn Inoculation

The active mushroom culture, known as mycelium or spawn, is then introduced into the substrate. This process, called inoculation, can be done using sterilized grains, such as rye or millet, or by drilling holes in logs and filling them with the mycelium. The holes are then sealed with hot wax. This step ensures the mycelium colonizes the substrate, creating an ideal environment for mushroom growth.

Environmental Conditions

Mushrooms require specific environmental conditions, including temperature, humidity, and light control. During the spawn run, the compost temperature is maintained at around 75°F, and humidity is kept high to facilitate spawn growth. After casing, the temperature is gradually lowered by 2°F per day until young mushrooms, or pins, start to form. Controlling these conditions is essential for successful mushroom cultivation.

Indoor vs. Outdoor Cultivation

Commercial mushroom cultivation can be done indoors or outdoors, each offering distinct advantages. Indoor cultivation provides more control over the environment, excluding contaminants and pests, and enabling consistent year-round production. Outdoor cultivation, particularly using logs, has been practised for centuries and offers a natural and sustainable approach. However, it accounts for less than 5% of commercially sold mushrooms due to unpredictability and seasonality.

Harvesting and Flushes

Mushrooms are typically harvested in stages or flushes. The first flush accounts for about 60% of the total yield, followed by the second flush at 30%, and the third at 10%. Lowering the temperature signals the spawn to develop fruit bodies (mushrooms) and drop spores, allowing for continuous production. Harvesting techniques vary depending on the species and the chosen cultivation method.

Species Considerations

Different mushroom species have specific requirements and growth patterns. Agaricus bisporus, for example, goes through distinct stages of composting, fertilizing, spawning, casing, pinning, and cropping. Oyster mushrooms, the second most common crop, require more humidity and fresh air, and they often grow in tubular plastic bags. Shiitake mushrooms, the third most common, are often cultivated on hardwood logs, with young oak yielding the highest production.

Wild harvest

Wild mushroom harvesting is a popular pastime in many countries, and is sometimes commercially important. However, it is important to harvest mushrooms in a sustainable way that does not disturb the local ecosystem or harm the mushrooms. Here are some tips for wild mushroom harvesting:

First, it is important to be able to identify mushrooms. This is crucial for safety, as some mushrooms are poisonous, but it is also important for sustainability. Knowing how to identify mushrooms will help you avoid overharvesting certain species, and leave behind enough mushrooms to ensure future growth. Chanterelles, for example, are frequently found around specific host trees, as they may be symbiotic partners or opportunistic parasites. In Michigan, you would look for morels around dying ash trees or apple orchards, whereas in Colorado, you would look for big conifers, especially firs, for porcini and chanterelles, and avoid aspens.

Second, it is important to consider the spores. Mature mushrooms release spores into the air that are essentially mushroom "seeds". To help disperse these spores, collect mushrooms into a basket or porous cloth bag, with their gills/pores facing down. This will allow the spores to disperse as you move, helping to propagate the fungi.

Third, be considerate of other mushroom pickers. Saving some mushrooms for other pickers is more than just a courtesy; it can help ensure that there are enough mushrooms for future harvests. When you see a big cluster of mushrooms, leave half for the next person!

Fourth, avoid trampling native plants and compacting the earth. Trampling can damage native plant species and cause erosion. It can also reduce the number of fruit bodies in the rest of the season. To avoid trampling, walk lightly and carry small baskets to a central location, instead of dragging heavy harvests out of the bush or around with you as you pick.

Finally, do not litter or leave anything behind. Leave no trace of your time in nature.

Distribution in nature

Mushrooms are the fruit of the mycorrhizal network fungus, which connects trees through a network of tiny threads called mycelium. This network enables trees to share water and nutrients. Mycelium, a network of microscopic fungal threads, is vital to ecosystems worldwide. It helps plants share nutrients and communicate through chemical signals.

Mushrooms are distributed in nature through spores. A spore falls in a favourable spot and produces strands (hyphae) that grow in all directions, eventually forming a circular mat of underground hyphal threads. Fruiting bodies are produced near the edge of this mat, and they may widen the ring for hundreds of years. This is how the fairy ring is formed.

Some mushrooms grow on living or dead trees, and some grow on dead or decaying matter in nature. For example, the artist's conk (Ganoderma applanatum) is a wood-decaying fungus that primarily grows on dead or dying trees. Shelf fungi or bracket fungi (order Polyporales) also grow on trees, sometimes as destructive pests. Many of them renew their growth each year and produce annual growth layers that can be used to estimate their age. Examples include the dryad's saddle (Polyporus squamosus) and the beefsteak fungus (Fistulina hepatica).

Mushrooms are also distributed in nature through wild harvesting, which remains the largest source of commercially important mycorrhizal species such as truffles, chanterelles, and morels. Seasonal species such as morels (Morchella esculenta) and chanterelles (Cantharellus cibarius) are gathered in the wild and sold at retail stores and farmers' markets.

Some mushrooms are distributed through commercial farming. Agaricus is the leading mushroom crop worldwide and accounted for approximately 97% of total US mushroom production from 2017 to 2018. Oyster (Pleurotus spp.) and shiitake mushrooms rank second and third, respectively, in worldwide production. Oyster mushrooms are farmed in tubular plastic bags, while shiitake mushrooms are farmed using natural or synthetic oak logs.

Health and safety

The health and safety of mushroom distribution are critical for both consumers and producers. Mushrooms are highly perishable and delicate, requiring careful handling and short shipping distances, making them ideal for local farms. Here are some key considerations for ensuring the safe distribution of mushrooms:

Contamination Prevention

Mushrooms are susceptible to contamination by microorganisms, including spoilage and illness-causing bacteria, yeasts, and moulds. These contaminants can be present in the air, soil, water, manure, human handling, and harvesting or processing equipment. To prevent contamination, it is essential to maintain sanitary conditions and implement good agricultural and handling practices. This includes worker training on proper handwashing techniques, wearing gloves, and regular sanitization of tools and surfaces.

Water Quality

The quality of water used in mushroom cultivation directly impacts the safety of the crop. Whether growing indoors or outdoors, clean water with zero detectable Escherichia coli (E. coli) is essential for humidifying grow rooms and cleaning. All surfaces and containers that come into contact with mushrooms must be sanitized to prevent the growth of spoilage yeasts, moulds, and illness-causing organisms.

Food Safety Regulations

In the context of food safety, it is crucial to follow guidelines such as the Food Safety Modernization Act's Produce Safety Rule (FSMA PSR) in the United States. This regulation establishes science-based minimum standards for the safe growing, harvesting, packing, and holding of produce, including mushrooms, intended for human consumption. Similar regulations may exist in other regions to ensure the safety of mushroom distribution and consumption.

Toxic Element Monitoring

Some wild-grown mushrooms, such as Boletus edulis and Xerocomus badius, may contain toxic elements like mercury (Hg), lead (Pb), cadmium (Cd), and arsenic (As). While these mushrooms are available for sale in certain regions, the health risks associated with their consumption are a concern. Monitoring the content of toxic elements in wild-grown mushrooms and establishing legal maximum residue levels (MRLs) for these elements are essential to ensure consumer safety.

Safe Handling and Storage

Proper handling, washing, preparation, and storage of mushrooms are crucial to prevent food poisoning. Consumers should choose fresh and firm mushrooms, avoiding those with signs of bruising or spoilage. Prepackaged mushrooms with proper ventilation prevent the growth of harmful bacteria. Fresh mushrooms should be consumed promptly and can be stored in the refrigerator for a few days. For longer storage, mushrooms must be cooked before freezing.

International trade

The global mushroom market was valued at USD 50.3 billion in 2021 and is expected to grow at a compound annual growth rate (CAGR) of 9.7% from 2022 to 2030. The increasing demand for vegan and protein-rich diets is a key factor driving this growth. Mushrooms are considered a superfood due to their high nutritional content, including selenium, vitamin D, glutathione, and ergothioneine, which offer health benefits such as mitigating oxidative stress and reducing the risk of chronic conditions.

China is the world's largest producer of cultivated, edible mushrooms, with Lentinus edodes being the leading variety. Together with Lentinula and four other genera (Pleurotus, Auricularia, Agaricus, and Flammulina), they account for 85% of the world's total supply of cultivated edible mushrooms. The average consumption of mushrooms is about 5 kg per person per year, and this is expected to increase as consumers become more aware of the health benefits associated with mushrooms.

The distribution of fresh mushrooms is challenging due to their limited shelf life. In developed regions like Europe and North America, supermarkets and hypermarkets are the dominant distribution channels, accounting for 38.3% of revenue share in 2021. However, the growth of these traditional channels is being hampered by the increasing competition from online grocers offering discounted prices, contributing to a shift in consumer behaviour. It is expected that distribution through online stores will witness the highest revenue-based CAGR of 10.3% over the forecast period, as consumers increasingly prefer the convenience and price benefits offered by online channels for their fresh produce purchases.

Frequently asked questions