Fungi—sing. fungus; from the Greek sphongis (sponge)—are nonphotosynthetic and thus must absorb nutrients from organic matter formed by other organisms. The great majority of fungi obtain their food from dead organic matter and hence are known as saprophytes; a relatively small percentage derive their food from other living organisms and are known as parasites. Fungi may be unicellular (yeasts) or multicellular (mushrooms) and their cell walls usually contain chitin or cellulose and bglucan. They may produce sexually or asexually by means of spores that are roughly comparable with the seeds of higher plants.
The fungal kingdom offers enormous biodiversity with over seventy thousand known species and an estimated 1.5 million species. According to molecular evidence (16S-like ribosome RNA sequences), the fungi may have originated from protozoan ancestors before the kingdoms Animalia and Plantae split; there is strong evidence that Fungi are closer to Animalia than Plantae (Hawksworth et al.). Fungi are associated with some of the earliest remains of land plants. Some scientists believe that lichens (a stable self-supporting association of a fungus and an alga) might be transmigrants, the earliest colonizers of land.
Fungi have contributed to the shaping of humankind’s welfare since the beginning of civilization. Fungi are recognized as both beneficial and harmful in their relationship to humans although this role is predominantly beneficial. They are responsible for a major portion of food deterioration in developing countries; however, the preservative effects of fermentation of foods and beverages with fungi are well-known benefits, including organic acids, alcohol, antibiotics, pigments, vitamins, growth regulators, immunomodulating agents, and enzymes. Finally, various types of edible mushrooms are consumed as an important part of human diets in many countries.
Fungi and Food Processing
Fungi used in food processing have been an integral part of the human diet since the beginning of civilization. In such foods, fungi are the agents responsible for imparting special flavors, textures, odors, or consistencies to food products. Fungi such as Aspergillus spp., Rhizopus spp., Penicillium spp., Neurospora spp., Cladosporium spp., and Mucor spp., as well as yeasts and many others have long been used to process a number of food products from soybeans to peanuts, rice, gram, maize, cassava, taro, and cacao beans.
Fungal enzymes : Food formulation using enzymes derived from fungi has undergone a rebirth in recent years. Enzyme suppliers have improved their ability to supply single-activity enzymes that do not have undesirable side activities (see Table 1 for a list of commercial fungal enzymes and their uses). Enzyme products have found increasing application for improving product clarity and yield and in replacing costly physical processes such as heating.
Cheese manufacture : Two general types of cheese are made with fungi as the ripening agents. Roquefort cheese is an example of cheese that is ripened primarily by growth of fungi (Penicillium roquefortii ) throughout the cheese mass. Brie cheese is an example of one type of soft cheese that is ripened by the growth of fungi (Penicillium camemberti ) on the outside of the cheese mass. In both types of cheeses, the fungi grow and release protein and fat-degrading enzymes that soften and ripen the cheese. Roquefort cheese requires about two months to ripen while Brie cheese requires only about one month to ripen.
Baker’s yeast : Leavening, a process whereby batter or dough is caused to rise via the production of gas, especially carbon dioxide, was first discovered in Egypt. Today, most of the bread, cakes, cookies, and the like consumed by the public are prepared from leavened batter or dough. Most cakes and cookies are leavened chemically (by using baking powder) while most bread is leavened by yeasts (such as Saccharomyces cerevisiae ). Yeasts develop and reproduce by producing buds on mother cells that subsequently enlarge and produce more buds. During growth, carbohydrates in the dough are metabolized to carbon dioxide that is trapped in the dough in the form of bubbles. During the leavening process, alcohol may accumulate in the dough to as high as 0.5 percent. The alcohol is driven off during baking and helps give the bread a pleasant aroma.
|List, source, and uses of enzymes derived from fungi for food manufacture|
|α -Amylase, amyloglucosidase||Aspergillus niger A. oryzae Rhizopus spp.||Hydrolysis of starch in production of beer, bread; manufacture of high-fructose syrups|
|α-Galactosidase||Mortierella vinacea||Hydrolysis of raffinose to sucrose and galactose during sugar refining|
|Catalase||Aspergillus niger Penicillium vitale||Remove excess hydrogen peroxide formed during cake baking or that may be added during pasteurization of milk and cheese|
|Cellulase||Aspergillus niger Trichoderma viride||Improve palatability of low-quality vegetables, accelerate drying of vegetables, alter texture of foods, increase flavor of commercial mushrooms|
|Hemicellulase||Aspergillus niger Trichoderma viride||Manufacture of instant coffee|
|Invertase||Yeasts Aspergillus spp.||Increases sweetness in confections; yields soft center in chocolate-covered candies|
|Lactase||Aspergillus niger A. oryzae||Hydrolysis of lactose in milk products, enabling their use by lactose-intolerant individuals; production of syrups for use as sweetening agents|
|Lipase||Candida spp. Aspergillus spp. Mucor spp. Rhizopus spp.||Used for flavor development in cheese, chocolate crumb, apple wine, and cooking fats; improved whipping properties of egg whites; fish processing|
|Naringinase||Aspergillus niger||Reduce bitter flavonone glycoside derivative found in some citrus products|
|Nuclease||Penicillium spp.||Flavor enhancers|
|Pectic Enzymes||Aspergillus niger Penicillium notatum Botrytis cinerea||Remove turbidity from fresh fruit juices; removal of pectins before concentrating juice; clarifying agent in wine|
|Protease||Aspergillus spp. Mucor pusillus||Meat tenderizer; remove bitter flavors, replace rennin in cheese manufacture, chill-proofing of beer; reduce elasticity of glutin proteins in bread|
|Rennet||Mucor spp.||Milk coagulation in cheese manufacture|
|Tannase||Aspergillus niger||Treat insoluable material that forms during manufacture of instant tea|
|SOURCE: Adapted from: Beuchat (1987) and Moore-Landecker (1995)|
Edible Mushrooms : Mushrooms have a long history of human consumption. Traces of puffball fungi have been found in Stone Age settlements. Over 4,500 years ago in ancient Egypt only pharaohs were permitted to eat mushrooms, which they believed were “sons of the gods” sent down to earth on lightning bolts announced by claps of thunder. The legend that mushrooms may have originated from thunder and lightning also existed among people of other ethnic groups. In Roman folklore, some fungi were believed to spring from the ground in places struck by a thunderbolt. In the Hindu tradition, there was a god named Soma that manifested himself to the priests in the form of hallucinogenic fluids. Some scientists believe that Soma was the fly mushroom, Amanita muscaria. A similar legend may have existed among the inhabitants of the highlands of Guatemala and Mexico, where even today the people refer to A. muscaria by a common name meaning thunderbolt (Lowy).
Cultivated species : The cultivation of edible mushrooms worldwide reached 6.16 million metric tons in 1997, up from 1.26 million tons in 1981 (Table 2; Chang). This represents a 12 percent annual increase. Six mushroom genera accounted for 87 percent of the total mushroom supply (Table 2). These were Agaricus (31.8%), Lentinula (25.4%), Pleurotus (14.2%), Auricularia (7.9%), Flammulina (4.6%), and Volvariella (3%). China produced 3.92 million tons of mushrooms in 1997, or 63.6 percent of the total world output. The major mushroom of commerce in China is L. edodes, which accounts for 35 percent of the total output for that country. China currently produces 88 percent of the total world production of L. edodes.
Agaricus bisporus (button mushroom) : The cultivation of the button mushroom originated in the Paris region in France. Melon growers in this region discovered how mushrooms could be grown and started cultivating them in 1650. By the mid 1700s it was discovered that A. bisporus could grow without light, and that very favorable conditions for growing mushrooms prevailed in subterranean tunnels and caves. successful culture was undertaken inside the numerous caves that were excavated for building stones and for gypsum. The caves presented, from a climatic point of view, several advantages over the previous growing conditions in open air. Factors such as temperature and relative humidity were much more constant in caves compared with aboveground conditions.
From France, mushroom cultivation spread to other parts of the world. The business grew and soon spread to England and other countries. By 1825, the first mushroom crops were being produced in caves in Holland. In 1865, mushroom culture entered the United States via England and the first mushrooms were grown on a small scale on Long Island, New York; by 1870 the industry had begun to develop.
The button mushroom is produced commercially on a selective substrate prepared by composting mixtures of wheat straw, hay, corncobs, horse manure, or combinations thereof. The finished compost should have a nitrogen (N) content of 2–2.5 percent, and to reach such a level, nitrogen-rich supplements must be added. Inorganic nitrogen supplements can be added but only to provide part of the necessary amount. Organic sources of nitrogren include oilseed meal, brewers’ grain, malt sprouts, and poultry manure.
Once the compost has been prepared, it is seeded with mushroom spawn that is prepared from a mother culture maintained by a spawn laboratory. Spawn is prepared by inoculating a pure culture of the mushroom onto steam-sterilized grain, usually rye or millet. Approximately one liter (500 g) of spawn is used to seed 0.5 m2 of production surface that is contained in trays or beds inside environment-controlled production houses. Spawn run (vegetative growth of the mycelium) lasts ten to fourteen days, then a layer of neutralized peat moss (casing) is placed on top of the colonized compost to stimulate production of mushrooms. Approximately ten to fourteen days after casing, mushrooms are ready for harvest.
Lentinula edodes (shiitake) : Production of shiitake worldwide increased more than sevenfold in the fourteen-year period from 1983 (207,000 t) to 1997 (1,573,000 t; Chang). Most of this increase occurred in China, where more than ten million part-and full-time farmers cultivate shiitake. Shiitake is widely consumed in China, yet one-third of production is exported. In 1997, China produced approximately 88 percent of the total world output (Chang). In the United States, production of shiitake is a relatively new enterprise, having begun only in the late 1970s. In 1990, the United States produced 1,123 tons of shiitake and by 1999 production reached 3,941 tons, a 3.5-fold increase (USDA). This increase in production was due, in part, to increased production efficiency and to increasing consumer demand. Farmers have learned to provide the specialized management this crop requires, thereby reducing production costs. The amount of controlled-environment production surface devoted to growing shiitake on synthetic logs has increased 2.9 fold from 1990 to 1999 (74,200 m2 to 212,400 m2, respectively).
Sawdust is the most popular basal ingredient used in synthetic formulations of substrate for producing shiitake in the United States, but other basal ingredients may include straw, corncobs, or both. Starch-based supplements (20–60 percent dry weight) such as wheat bran, rice bran, millet, rye, and maize may be added to the mix. These supplements serve as nutrients to provide a more optimal growth medium (Royse).
Pleurotus spp. (oyster mushroom) : Oyster mushroom production increased at a rapid rate worldwide during the 1980s and then decreased slightly during the 1990s (Table 2). From 1986 to 1997, oyster mushroom production increased from 169,000 tons to 917,000 tons (a 5.4-fold increase). China was responsible for most of the production increase. In the United States, production of oyster mushrooms was 1,647 tons in 2001, up 2 percent from the previous year (USDA).
In the United States, the primary ingredients used for Pleurotus spp. production are chopped wheat straw or cottonseed hulls or mixtures thereof. After completion of pasteurization (140°F [60°C] for one to two hours) the substrate is cooled and spawned with the desired strain. There are several species of oyster mushrooms cultivated, with various colors of fruiting body. In Japan, bottle production of oyster mushrooms is most common. Substrate is filled into bottles, sterilized, and inoculated with Pleurotus spawn. Upon completion of the spawn run, bottle lids are removed and mushrooms emerge from the surface of the substrate. After the mushrooms are harvested they are weighed and packaged for shipment to market.
Auriculariaspp. (wood ear mushroom) : Total production of Auricularia spp. in 1997 exceeded 485,000 metric tons (fresh weight; Table 2). This value is an increase of 366,000 tons or fourfold over 1986 levels (Chang). Auricularia spp. production now represents about 8 percent of the total cultivated mushroom supply worldwide.
Auricularia auricula and A. polytricha commonly are produced on a synthetic medium consisting of sawdust, cottonseed hulls, bran, and other cereal grains or on natural logs of broadleaf trees. For synthetic medium production of Auricularias, the substrate may be composted for up to five days or used directly after mixing. The medium is filled into heat-resistant polypropylene bags and sterilized (substrate temperature 240°F [121°C]) for sixty minutes. After the substrate has cooled, it is inoculated with either grain or sawdust spawn. Light intensity of more than 500 lux during the spawn run may result in premature formation of primordia. Temperature, light intensity, and relative humidity all interact to influence the nature and quality of the mushrooms.
Flammulina velutipes (enokitake) : Worldwide production of F. velutipes has increased from about 100,000 metric tons in 1986 to about 285,000 tons in 1997. Japan is the main producer of enokitake. In the United States, enokitake production has increased at an estimated rate of 25 percent or more per year for the last four years. However, only about 60 tons of enokitake were produced in the United States in 2001.
Production of most enokitake in Japan is based on synthetic substrate contained in polypropylene bottles. Substrates (primarily sawdust and rice bran; 4:1 ratio) are mechanically mixed and filled into heat-resistant bottles with a capacity of 800 to 1,000 ml. Sawdust primarily from Cryptomeria japonica, Chamaecyparis obtusa, or aged (nine to twelve months) Pinus spp. appears to offer the best yields. In the United States, a sterilized, bran-supplemented medium, consisting primarily of corncobs, serves as the primary medium. When the substrate is fully colonized, the original inoculum is removed mechanically from the surface of the substrate and the bottles may be placed upside down for a few days.
To further improve quality during fruiting, temperatures are lowered to 37° to 46°F (3 to 8°C) until harvest. As the mushrooms begin to elongate above the lip of the bottle, a plastic collar is placed around the neck and secured with a Velcro® strip. This collar serves to hold the mushrooms in place so that they are long and straight. When the mushrooms are thirteen to fourteen cm long, the collars are removed and the mushrooms are pulled as a bunch from the substrate. The mushrooms then are vacuum packed and placed into boxes for shipment to market.
Grifola frondosa (maitake) : Japan is the major producer and consumer of maitake. Commercial production of maitake in Japan began in 1981 (325 t) and by 1997 reached 32,000 tons (a 98-fold increase). Maitake is produced primarily in the Japanese provinces of Niigata, Nagano, Gunnma, and Shizuoka. Other countries, such as the United States, began maitake production in the early 1990s. Maitake production in the United States in 2001 was estimated at about 84 tons.
Most maitake is marketed as food. However, maitake has been shown to have both antitumor and antiviral properties. Powdered fruit bodies are used in the production of many health foods such as maitake tea, whole powder, granules, drinks, and tablets. Maitake also is believed to lower blood pressure, reduce cholesterol, and reduce the symptoms of chronic fatigue syndrome.
Commercial production of most G. frondosa is on synthetic substrate contained in polypropylene bottles or bags. A common substrate used for production is hardwood sawdust supplemented with rice bran or wheat bran in a 5:1 ratio, respectively. Other formulas include hardwood sawdust (70 percent based on oven dry weight basis) supplemented with white millet (20 percent) and wheat bran (10 percent). Some growers may add soil to the mix to stimulate fruit body formation. For production in bags, the moistened substrate is filled into micro-filtered polypropylene bags and sterilized to kill unwanted competitive microorganisms. After cooling (sixteen to twenty hours), the substrate is inoculated and the bags are heat-sealed and shaken to uniformly distribute the spawn throughout the substrate. Spawn run lasts about thirty to fifty days depending on strain and substrate formulation.
Volvariella volvacea (straw mushroom) : Cultivation of V. volvaceae is believed to have begun in China as early as 1822. In the 1930s, straw mushroom cultivation began in the Philippines, Malaysia, and other Southeast Asian countries. Production of the straw mushroom increased from 54,000 tons in 1981 to about 181,000 tons in 1997 (about 3 percent of the total mushroom supply).
Many agricultural by-products and waste materials have been used to produce the straw mushroom. These include paddy straw, water hyacinth, oil palm bunch, oil palm pericarp waste, banana leaves and sawdust, cotton waste, and sugarcane waste. Volvariella is well suited for cultivation in the tropics because of its requirement for higher production temperatures. In addition, the mushroom can be grown on non pasteurized substrate, which is more desirable for low-input agricultural practices.
In recent years, cotton wastes (discarded after sorting in textile mills) have become popular as substrates for straw mushroom production. Cotton waste gives higher and more stable biological efficiencies (30 to 45 percent), earlier fructification (four days after spawning) and harvesting (first nine days after spawning) than that obtained using straw as a substratum. Semi-industrialization of paddy straw cultivation on cotton wastes has occurred in Hong Kong, Taiwan, and Indonesia as a result of the introduction of this method.
Wild mushrooms : In many developing countries, the collection and sale of wild edible mushrooms has become an important source of income for many people in remote forested regions. Despite a relatively short growing season, wild mushrooms provide many families with 50 to 100 percent of their income. World trade in wild, edible mushrooms is estimated at more than $7 billion annually (Arora). The global trade in matsutake (Tricholoma matsutake ), the most expensive wild mushrooms after truffles, is estimated at $3 to $5 billion. Matsutake may sell for as much as $200 apiece in Tokyo markets. The King Bolete (Boletus edulis ; also known as porcini, cepe, borovik, etc.) is the most popular wild mushroom of Europe. These may be served fresh in some upscale restaurants. Dried boletes are famous for their concentrated flavor and choice aroma and are available year round from almost anywhere in the world. Other wild mushrooms available on world markets include chanterelles (Cantharellus cibarius ), morels (Morchella spp.), hedgehog mushrooms (Hydnum repandum ), lobster mushrooms (Hypomyces lactifluorum ), candy caps (Lactarius fragilis ), and cauliflower mushrooms (Sparassis crispa ).
WARNING: Collecting and ingesting wild mushrooms without the presence of an expert to correctly identify specimens can be very dangerous and should be discouraged since there are several deadly mushrooms that look like edible wild ones.
Mycotoxins are chemical compounds produced by fungi growing on organic substances such as corn, cottonseed, or peanuts that, when ingested, have some undesirable effect on humans or on an animal consuming them. Adverse effects can range from vomiting to weight loss, various types of tumors, and in some cases, death. Over one hundred toxic compounds produced by fungi have been identified, and about forty-five of these occur in grain crops. Some mycotoxins are rare in occurrence while others such as aflatoxin are common in some years. The seriousness of the mycotoxin problem varies with the year, the crop being grown, and the intended use of the crop product. Most mycotoxins affect the blood, kidneys, skin, or central nervous system, and some may cause cancer.
The genera of fungi of greatest importance to humans with respect to natural poisoning outbreaks are Aspergillus, Penicillium, and Fusarium. The Aspergillus flavus group produces aflatoxins (at least eighteen types known) that are considered the most important from the viewpoint of a direct hazard to human health. Aspergillus flavus is a common fungus that is found in soil, air, and decaying plant residues. Infection by A. flavus and subsequent aflatoxin production can occur in the field, in transit, or in storage. Most reports indicate that infection occurs in the field, while aflatoxin production can occur whenever the product is exposed to favorable conditions, either in the field or in storage.
Control of aflatoxin includes prevention of fungal growth, removal of toxins, and inactivation of toxin. Most control efforts have been directed toward control of aflatoxins in peanuts and corn. Hand picking, electronic sorting, and air classification accomplish control of aflatoxin in processed peanut products. Removal of shriveled, rancid, or discolored kernels has proven the most practical way of limiting aflatoxin contamination in peanuts.