Edible Microbes Edible Fungi

One does not usually associate the words edible and microbe. Instead “microbe” evokes images of disease-causing bacteria or viruses as seen through the lens of a microscope. Edible microbes, however, encompass a wide variety of organisms ranging from the edible fungi to others such as yeast, algae, and bacteria. Some of these microbes are themselves edible, while others are consumed in products that the microbes have helped to generate such as cheeses, yogurt, or bread.

One of the largest groups of edible microbes is from the Kingdom Fungi,the mushrooms. A mushroom is a fleshy fruiting body of a fungus that produces spores for reproductive purposes. (1) Of the thousands of wild mushrooms, many species are edible including the morels, chanterelles, boletes, oyster mushrooms, agarics and many more. (2) However, some fungi such as those from the genus Amanita produce mycotoxins and are poisonous. (2) For this reason, most individuals are encouraged to purchase species of mushrooms that have been cultivated for human consumption. (2) One of the most commonly cultivated species is Agaricus bisporus. Approximately 30-35% of the cultivated fungi worldwide belong to this species. (3) In the United States this species comprises approximately 90% of the mushrooms cultivated for commercial use. (4) Commercial production involves inoculating beds of soil enriched with organic matter with the pure-cultured fungal spawn. Masses of fungal hyphae form mycelium within the bed and it is then covered with special “casing soil”. (1) Under the correct moisture and temperature conditions a flush of growth covers the bed. The mushrooms are harvested at maturity, packaged, and sent to market. (1) Other species of cultivated mushrooms include Shiitake and oyster mushrooms.

Quorn, a meat substitute derived from the hyphal growth of the fungus Fusarium venenatum, is produced by Marshall Foods. (5) This meat substitute has been sold in England for over a decade and has recently begun to gain popularity in the United States after being approved by the FDA as a “Generally Regarded as Safe” product. (6&7) Quorn is produced through a fermentation process that allows for a copious amount of fungal hyphae to develop. From this, mycoprotein is extracted and treated with heat to remove excess nucleic acid residues. (6) This mycoprotein is then dehydrated, mixed with the albumin from chicken eggs as a binder, and then pressed into desired shapes such as ground, links, or patties. (6)

From its first debut as a meat alternative in the United States, Quorn has met with opposition from several groups. First, the product is labeled as mushroom in origin’. (5) This labeling has been criticized by groups such as The American Mushroom Institute and the Center for Science in the Public Interest. (6) Mushrooms are commonly consumed as mentioned previously. The mycoprotein derived from Fusarium venenatum on the other hand is better described as ” . . . a processed cellular mass from the filamentous fungi Fusarium.” (5) It is important to note that fungi are primarily composed of masses of hyphae called mycelium. This highly filamentous network is the source of Quorn. (8)

Secondly, it is now a focus of concern that the Fusarium venenatum when grown under certain conditions may produce secondary metabolites or mycotoxins that could prove deadly. The genus, to which it belongs, Fusarium, has long been known to contain many species that are pathogenic either to animals or to plants. (9) Some of the diseases for which it is responsible include Root Rot, Wilts, Kernel Rot and many others in agricultural crops. (9) Some of the human and animal diseases linked to Fusarium mycotoxins include Kashin-Beck disease, esophageal cancer, “hemorrhagic, estrogenic, emetic, and feed refusal syndromes”, and many others. (10) In Fusarium venenatum, the mycotoxin that has been found under certain fermentation conditions is diacetoxyscirpenol or DAS. (11) Environmental conditions such as oxygen level, pH, water balance, and temperature can affect the production of this and other toxins by Fusarium. (11) In a study carried out by Miller and Mackenzie in 2000, it was shown that Fusarium venenatum, especially strain DAOM 212262, the main Quorn producing strain, has the potential to produce large quantities of DAS and other mycotoxins under specific fermentation conditions. (11) Since environmental and fermentation conditions are known to influence the production of mycotoxins, strict regulations and procedures are in place to control and assure the quality of the product produced. (6&7)

Another microbe that can be used as a food source, food supplement, or for food production is yeast. Yeast is a unicellular fungus. It is extremely important in the production of alcoholic beverages and bread. The main species of importance are Saccharomyces ellipsoideus and Saccharomyces cerevisiae. These species are used for fermentation into alcoholic beverages of fruit juices and grains and for food supplements, respectively. (1) Yeast cell culturing is carried out in large fermentation chambers in a molasses medium. To grow cells instead of produce alcohol the molasses levels are kept low and added slowly to the growth chamber. Yeast cells are separated from the growth medium through centrifugation and processed to form either a dry powder such as bakers yeast or mixed with additives to formed yeast cakes. (1)

Other microbe-derived products include cheeses, yogurt, vinegar, and products produced from algae such as thickening agents and ice cream. Bacterial species such as Lactobacillus are often utilized for the production of fermented dairy products like cheese and yogurt. This fermentation process differs from the alcoholic process described above. Instead, lactic acid fermentation is carried out by these microbes Starter cultures are added to milk and allowed to ferment. Different products can be produced based on which microbes are used, the temperature at which the culture is grown, the time allowed for fermentation, and a variety of other variables. (1)

Overall, edible microbes are a fascinating group and are of tremendous importance commercially. They provide a wide and delicious variety of products that would otherwise not be available for human consumption.

1)Madigan, M. & Martinko, J. 2006. Brock Biology of Microorganisms, Eleventh Edition. Pearson Prentice Hall.
2)Laessoe, T., Lincoff, G., and Del Conte, A. 1996. The Mushroom Book. DK Publishing.
3)Beelman, R., Royse, D. and Chikthimmah, N. Bioactive Components in Agaricus bisporus. http://www.foodscience.psu.edu/Research/RBB_ISMS_03.pdf
11)Miller, J.D. and S. Mackenzie (2000) with accompanying tables 1 and 2. Secondary metabolites of Fusarium venenatum strains with deletions in the Tri5 gene encoding trichodiene synthetase. Mycologia 92:764-771