Food Source Information Specialty Mushrooms

A mushroom is a fungus that produces a macroscopic fruiting body. Although button mushrooms (Agaricus bisporus) are the most commonly sold mushroom and account for about half of the world’s total production, specialty mushrooms are increasing in popularity. The United States Department of Agriculture (USDA) delineates specialty mushrooms as any species not belonging to the genus Agaricus which includes button, crimini, and portabella mushrooms. These specialty types include Shiitake (Lentinula edodes), oyster (Pleurotus spp.), straw (Volvariella volvacea), and enokitake (Flammulina velutipes), among others (Cornell Agriculture).

The fleshy spore-bearing body of fungal mushrooms are typically produced above ground on decaying wood or soil. Mushrooms are produced from spores that form by the thousands in the gills or pores beneath the parent plant’s cap. When these spores come to rest in a suitable environment, they germinate to form root-like filaments called hyphae. For the mushroom or fruit of the fungus to form, the hyphae must combine with hyphae of other compatible spores.

Specialty mushrooms contain some protein and provide fiber, vitamin C, vitamin B, and essential minerals. Some species of mushrooms can accumulate cadmium, selenium, and other heavy metals, while others can contain toxins such as heat-labile cardiotoxic proteins, volvatoxin and flammutoxin (Ware).

There is growing interest in mycotherapy (Wasser). Medicinal mushrooms (MMs) are reported to have numerous pharmacological actions such as antimicrobial, anti-inflammatory, immunomodulatory, antidiabetic, cytotoxic, antioxidant, hepatoprotective, anticancer, antioxidant, antiallergic, antihyperlipidemic, and prebiotic properties, among others, which requires a strong commitment from the scientific community to expand clinical trials and to ensure these claims are safe and effective (Zhao, et. al. 2020, Elkhateeb).

Between 2000 and 2020, at least 4 wild mushroom-associated outbreaks were reported to CDC’s National Outbreak Reporting System (NORS), causing 9 illnesses, 4 hospitalizations, and no deaths. Wood Ear mushrooms have specifically been attributed to 1 outbreak, 55 illnesses, 6 hospitalizations, and no deaths.

According to the Centers of Disease Control and Prevention (CDC), the following outbreaks with specialty mushrooms occurred in 2020:

Salmonella Stanley infections linked to Wood Ear mushrooms.

Wood ear mushrooms are commonly used in Chinese cuisine.  They are a type of edible tree fungus, wood ear mushrooms (Mu’er/木耳) are also known as wood jellyfish, tree ears, Jew’s ear, black fungus. Fresh ones (or rehydrated dried ones) have a curved, half circled, ear-resembling look and a light brown or brownish-black color. Wismettac Asian foods recalled the dried fungus due to possible contamination with Salmonella Stanley on September 23, 2020. Epidemiologic, laboratory, and traceback information showed that wood ear mushrooms from this distributor were the likely source of this outbreak due to Salmonella Stanley. A total of 55 people infected with the outbreak strain of Salmonella Stanley were reported from 12 states. There were six hospitalizations reported and no deaths. No cases were reported after June 9, 2020 and the outbreak was declared to be over by the CDC. However, Listeria remains an important cause of serious, life-threatening illness in the United States. The outbreak could continue to occur if restaurant managers/chefs are unaware of the recall and continue to serve recalled wood ear mushrooms because of their long shelf life (CDC, 2020).

Listeria monocytogenes and enoki mushrooms

Enoki mushrooms are a long, thin, white mushroom, usually sold in clusters. They are especially popular in East Asian cuisine and are also known as enokitake, golden needle, futu, or lily mushrooms. The FDA, along with CDC and state and local partners investigated a multistate outbreak of Listeria monocytogenes infections linked to enoki mushrooms from Green Co, LTD. of the Republic of Korea (US FDA 2020).

Thirty-six people were infected with this bacterial strain from seventeen different states. The number of hospitalizations due to this outbreak was thirty-one. Four deaths were reported; two from California and one case each from Hawaii and New Jersey. Finally, there were six cases associated with pregnant women and two fatalities were reported due to this outbreak.  The FDA has closed their investigation as of June 20, 2020 and the CDC declared the outbreak over (US FDA 2020). FDA recommends that anyone who received recalled products use extra vigilance in cleaning and sanitizing any surfaces and containers that may have encountered these products to reduce the risk of cross-contamination.

There has been an increase in the demand for specialty mushrooms, with sales increasing 4% per year in 2016 and 2017. Mushroom production begins with a single microscopic spore, one of thousands of spores created by the parent mushroom. The spore must join with compatible hyphae, which is a root-like filament. The network or mat that hyphae form is a mycelium. The already-growing mycelia are called spawns, which most mushroom cultivators use for mushroom production. Specialty mushrooms can be grown indoors or outdoors, providing each type of mushroom’s specific environmental requirements have been met.

Growing specialty mushrooms requires finding the correct substrate or growing medium for the mushroom mycelium which allows the spawns to grow and develop. There are several steps that are required based on the substrate needed. Specialty mushrooms use carbon rich materials such as woodchips, sawdust, coffee grounds, or grain hulls. It should be noted that Shiitake mushrooms are different and are grown on logs and not compost

Substrates can include but are not limited to logs, stumps, woodchips, straw, sawdust, coffee grounds, grain hulls, and other carbon-rich materials. Inoculation might be necessary depending on the substrate being used. The spawn will require different amounts of time to grow depending on the substrate but the mushrooms must be kept at consistent temperatures if they are being grown indoors and kept in shade if grown outdoors. It is extremely vital that nutrients and the environmental conditions are suitable, or the mycelium will not fruit. If the mycelium does not fruit, no mushrooms will be harvested from that spawn. Growing mushrooms is not an easy task. It is labor-intensive and requires considerable effort. Mushroom initials develop after rhizomorphs have formed in the casing. The initials are extremely small but can be seen as outgrowths on a rhizomorph. Once an initial quadruples in size, the structure is a pin. Pins continue to expand and grow larger enlarges to a mushroom. Harvestable mushrooms usually appear 18 to 21 days after casing.

Most specialty mushroom production occurs on small farms and harvested mushrooms are sold at local markets and to local restaurants. Producing mushrooms at a rapid rate and a high yield requires specialized facilities with ideal conditions. Mushroom production can occur 365 days a year and can be completed indoors.

When producing specialty mushrooms, contamination with pathogens is concerning. Listeria monocytogenes and Salmonella are the two pathogens associated with recent outbreaks, but another potential pathogen is Clostridium botulinum, which can thrive if mushrooms are improperly packaged, creating an aerobic environment. C. botulinum is the spore-forming bacteria which produces botulinum toxin associated with botulism. To prevent C. botulinum from growing in mushrooms, consumers must prevent spoilage by storing them in a paper bag or plastic bag with holes for aeration in the refrigerator, thus preventing moisture from being trapped (Harrison et al., 2020).

In the summer of 2015, a Listeria outbreak in Canada was associated with meat slicing equipment used for commercial mushroom production. A Pennsylvania State University professor, warned mushroom processors of the potential of a Listeria outbreak with mushrooms because the cold and wet conditions in mushroom operations make an ideal environment for Listeria and many use slicing equipment which can be difficult to clean properly.

In 2020, an outbreak of Listeria infections was linked to enoki mushrooms imported from Korea. To prevent Listeria outbreaks, mushroom operations that use slicers and conveyors need to perform daily sanitation of the slicer heads and conveyor belts by removing the slicer head from the frame to properly clean and sanitize with heat; this is to remove residue material that collects inside the mechanisms of slicing equipment and conveyors where Listeria can be found.

Correct categorization and better understanding are essential for the safe and healthy consumption of mushrooms as part of cooking functional foods as well as for their medicinal use. Cases of serious human poisoning generally caused by the improper identification of toxic mushroom species are reported every year. Toxicity studies of mushroom species have demonstrated that mushroom poisoning can cause adverse effects such as liver failure, bradycardia, chest pain, seizures, gastroenteritis, intestinal fibrosis, renal failure, erythromelalgia, and rhabdomyolysis. (Jo, 2014) Mushroom toxins have been divided into seven main categories: amatoxins (cyclopeptides), orellanus (Cortinarius species), gyromitrin (monomethylhydrazine), muscarine, ibotenic acid, psilocybin, and coprine (Lin, 2004).

The most common specialty mushrooms consumed in the U.S. are shiitakes, although oysters and enokis are quite popular in Asian territories (Harrison et al., 2020). In recent years, the per capita consumption rate in the U.S. has reached approximately three pounds. This amount will likely continue to rise as the population becomes more eager to incorporate specialty mushrooms into their diets. Their appeal is only enhanced by the incredible variety of ways that mushrooms can be prepared and included in meals.

Different cultures have found their own uses for mushrooms from all corners of the world, whether it be for salads, pasta, pizzas, soups, stir-fry, sandwiches, or simply raw. European countries have shown a distinct attraction for mushroom sauces and pates. Certain mushrooms have also been marketed as replacements for meat products due to their relatively high protein level, coupled with their similar texture and savory umami attributes.

Mushrooms have a low-calorie content and zero fat, making them more appealing since they contain protein, fiber, and carbohydrates in proportion. They are also unique because of all the known fruits and vegetables only mushrooms can be a source of vitamin D. This is because – like humans – they too can produce this vitamin when exposed to sunlight, particularly ultraviolet B light. Specialty mushrooms, such as shiitake and oysters, both naturally contain vitamin D (Phillips). They are also rich in minerals like selenium, potassium, copper, and others necessary for the human body. Minerals and vitamins aren’t the only benefits of these fungi. Certain species have been found to provide medicinal properties, like antioxidants, metabolites, and antimicrobial compounds (Harrison et. al., 2020).

A lesser-known specialty mushroom is Cordyceps sinensis, which has been used an an alternative to caffeine purportedly supplying the body with energy without caffeine effects. C. sinensis has been described as a Chinese and Tibetan medicine. It is a rare combination of a caterpillar and a fungus and found at altitudes above 4500m in Sikkim (Panda, Swain, 2011).

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