4.12 The real nutritional value of fungi (contributed by Stephanie Ingram, 2002)
Fungi have been influencing human affairs for thousands of years, whether as a direct food source, as a medicine, or in a food process . Today food of fungal origin is consumed all over the world in vast quantities, and commercial production is part of a rapidly growing industry. Fungi are of excellent value nutritionally, and of great importance to vegetarians. Edible mushrooms have high protein content, and are an excellent source of fibre, vitamins, and some minerals. Efforts to combat anticipated world food shortages, led to the production of ‘single cell protein’, grown in industrial fermenters using yeast cells. The result is a protein extract with high amino acid content potentially favourable for use in human nutrition. One particularly successful model was that of mycoprotein
, marketed as Quorn™. Essentially the mycelium of Fusarium venenatum, its filamentous nature much resembles the fibres of meat. Quorn is now available in supermarkets, marketed as a high-protein, low-fat, cholesterol-free ‘meat alternative’. When it comes to fungi as a food source, many people are apprehensive and much education is needed before the true nutritional value of such a cheap, readily available food source can be fully realised.
Use of fungi in human affairs. Fungi have a profound influence on human affairs, and their use in food is a very ancient practice; mushrooms, truffles and puffballs must have been, in their season, part of every hunter-gatherer's collection. The written record starts with the ancient Greeks, as early as the fifth century BC, where records in the classical writings of Hippocrates and Euripides mention fungus poisoning; from which we can deduce sufficiently long and wide usage of collected field fungi for the few poisonous mushrooms to be distinguished from the non-poisonous. During the years of the Roman Empire, the death of the emperor Claudius was said to be due to the eating of a plate of poisoned mushrooms. Some of the fungi were even regarded as great luxuries with magical properties, considered food of the gods.
The cultivation of macrofungi to yield fruit bodies began to flourish in the beginning of the seventeenth century, and today over six million tonnes of edible mushrooms are produced commercially each year around the world. Many fungi are of considerable medical significance. For thousands of years, Eastern cultures have revered mushrooms as both food and medicine. Mushrooms or their extracts are made into a soup or tea, research suggesting they may aid in the treatment of certain types of cancer, boost the immune system and reduce the risk of coronary heart disease.
Other fungi are used to modify food to make it more nutritious or palatable. Tuber melanosporum, known to most as the truffle, has a taste and aroma so intense that it is used as flavouring instead of a separate dish. Soy sauce is also produced by growing a filamentous fungus on cooked soya beans. It is used extensively as a flavouring and condiment in Chinese cuisine.
Yeast cells and mould mycelium (known collectively as ‘single-cell protein’) grown in fermenters are widely used as nourishment. Modern developments include a novel food termed ‘Quorn’ - mycelium of the fungus Fusarium venenatum grown in fermenter vessels. The product compares favourably with meat and is the one significant product to have emerged from the efforts to produce ‘single-cell protein’ to combat the expected world food shortage.
Not only are fungi used as food, but they are also employed in food processes traced back to the times of the Ancient Egyptians. Saccharomyces cerevisiae, a yeast, is used in the production of ethanol: when added to glucose solution (or ‘wort’) in anaerobic conditions it will ferment sugars into beer. Another product of fermentation, carbon dioxide, is exploited in the bread making industry, where it leads to a well-risen loaf. A number of foods are preserved by fungal fermentation including several products produced in Asia from soya beans, and salami and cheese production in the West (particularly the introduction of Penicillium roqueforti into curds of cheese, producing the highly flavoured blue-veined cheeses) .
Today, food of fungal origin is consumed all over the world in vast quantities, and commercial production is part of a rapidly growing industry. From mushroom farmers to Quorn-makers, there are plenty of people anxious to sell you fungus as food, usually with a claim that their product represents healthy eating. But how far can these claims be justified? Ancient cultures believed edible fungi promoted healthiness and well-being. These people may have been right, but not because fungi share magical characteristics. Fungi are of excellent nutritional importance and deserve attention for their unique contributions to a healthy diet.
Mushrooms and other macrofungi
Cultivation. The word mushroom is thought to have been derived from the French mousseron, a term that included poisonous varieties as well as edible mushrooms. Today, the word refers only to edible fungi, and is generally applied to the above-ground portion, the fruiting body. Those without the typical stem and cap are identified by other specific names such as morels, truffles or puffballs.
Mushrooms have played an important role in the diet of many people for thousands of years. Ancient Egyptian hieroglyphics reveal that mushrooms were thought to bring immortality. Many later cultures believed that eating mushrooms could endow them with super-human strength, give them clairvoyance in locating lost objects and lead the soul to reside with the gods. Most, if not all, these imagined attributes presumably derive from the hallucinogenic properties of some common species of mushroom.
The cultivation of mushrooms in Europe has been going on for nearly 300 years. It began in the underground caves of Paris in the time of Louis XIV and flourished until the mid-twentieth century. Mushrooms are now being produced in over eighty countries around the world. In 1945, the British Mushroom Grower’s Association (MGA) was born and total production was estimated at some 450 tonnes.
Today total annual production is estimated at over six million tonnes. Britain is the fourth largest producer being surpassed only by the USA, France and Taiwan.In various countries in Asia, up to eighty distinct varieties of wild fungi are offered for sale in the markets. In Britain, the public are much more apprehensive when it comes to buying wild mushrooms. Even so, it is perhaps remarkable that success on a large scale has been achieved with the commercial production of a very few.
Agaricus bisporus. Known also as the white or button mushroom, this is the most commonly cultivated in Britain. It varies in colour from creamy white to light brown and in sizes from small (button) to jumbo. They are pleasingly mild and woodsy, their flavour intensifying when cooked. They were first cultivated on prepared horse manure in seventeenth century France, and in caves that provided the ideal stable environment in terms of temperature and humidity. Today they are strictly cultivated in rich compost in purpose-built mushroom houses where heat and humidity are carefully controlled – it being the most technically advanced horticultural industry as a result.
Lentinula edodes. More commonly known as shiitake mushrooms, L. edodes has been commercially cultivated since the species was originally imported into Europe from Japan around 1940; the species is now widely available in supermarkets as well as in Asian markets. Originally harvested from hardwood trees in their native country for at least two thousand years, they are best cultivated on artificial logs made of compressed wood chippings. Shiitake has a medium brown colour with a distinctive, thick, umbrella-shaped cap, and offer a rich, distinctly earthy flavour and chewy texture. Introduction of a mushroom more aromatic and flavourful than the common button mushroom has rapidly increased demand. After Agaricus, shiitake is the second most cultivated mushroom in the world. Healthfood stores, oriental and fine-dining restaurants are all targets for this speciality mushroom. A notable peculiarity, though, is that the mushroom is dried before sale in Asia (and this develops the full flavour), but is sold fresh in Europe.
Other macrofungi in commercial cultivation. Four other mushrooms are grown on a smaller commercial scale. Pleurotus spp., the oyster mushroom is usually a soft brown colour (although yellow and orange species have distinctive, peppery, flavours), it is a large mushroom with a shape reminiscent of an oyster shell. Oyster mushrooms have a delicate, mild flavour and velvety texture. Volvariella volvacea, the paddy straw mushroom (so-called because its substrate is a compost of rice straw), has been cultivated for 2000 years in China and South-east Asia. The others are Flammulina velutipes (or enokitake) and Pholiota nameko, cultivated in China and Taiwan. Enoki is a white mushroom with a unique long, slender stem and tiny cap when cultivated (it grows commonly in the wild as a fairly unremakable small brown mushrooms). Enoki is grown mainly in Japan.
The biggest change in the industry in the West during the last quarter of the twentieth century has been the increasing interest shown by consumers in a wider variety of mushrooms. Even in the most conservative of markets (like the UK) 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. Some also offer enokitake (Flammulina velutipes), buna shimeji (Hypsizygus marmoreus), shiroshimeji (Pleurotus ostreatus), and king oyster (Pleurotus eryngii) among others, most of which are are cultivated locally (as examples, visit the websites of The Mushroom Basket or Smithy Mushrooms in the U.K. [http://www.themushroombasket.com/the-mushroom-basket and http://www.smithymushrooms.co.uk/index.html], but the industry is truly international and mushroom cultivation is the next-biggest biotechnology industry after alcohol production (Table 1).
Table 1. World cultivated mushroom* production figures 1981-2002
Global production (millions of tonnes)
|*The species distribution is approximately 46% Agaricus bisporus (button mushroom), 26% Pleurotus spp. (oyster mushrooms), 15% Lentinula edodes (shiitake), 13% Auricularia spp. (wood-ear). Geographical distribution (as of 1997) was approximately 74% farmed in Asia, 16% in Europe, 7% in North America, and less than 1% in the rest of the world. Data from Chang, 2008 [ref. 2].
Another macrofungus worth mentioning is the truffle. Sometimes regarded as a mushroom, the truffle is a subterranean European macrofungus belonging to the order Tuberales with an underground fruiting body, usually round and pitted. Truffles have been collected for at least 3600 years. The most highly prized is the black truffle, Tuber melanosporum. Its tantalizing taste and aroma are so intense that it is used as flavouring instead of a separate dish. Truffles are extremely valuable; they can be worth up to £1000 per kilogram, with a single truffle 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 20 to 30 years .
Mushrooms and nutrition
Varying opinions have been expressed regarding the true nutritive value of edible mushrooms. Although mushrooms are a staple food in the diet of some human cultures (and many vertebrate and invertebrate animals ), edible mushrooms are usually considered for their flavour and condiment value. In the past, authors have dismissed mushrooms as a food of little nutritive value, some considering them devoid of nutrients. However, this naïve approach is simply inaccurate, as much research conducted on mushrooms suggests otherwise, showing mushrooms as a nutritionally sound food for everyone that are of even greater value to vegetarians.
To begin with, mushrooms have a fairly high protein content, typically 20-30% crude protein as a percentage of dry matter. However there is extreme variation among species (3.5% in Cantharellus cibarius, 44% in Agaricus bisporus). High protein content makes them an ideal food because they contain all the amino acids essential to human nutrition. There are about eight essential amino acids, that is, those which cannot be produced by the human body, and so must be consumed in the diet daily. Mushrooms can be an important dietary source of these amino acids.
Since fats and carbohydrates are rarely lacking in diets typical of the western (developed) world, protein is the most critical component contributing to the nutritional value of a food. It is therefore the component considered most important in nutritional evaluation. The amino acid content, specifically the content of essential amino acids is considered a reliable measure of nutritive value.
Anderson & Fellers (1942) , reported that rats given mushrooms as their only source of protein gained only 30% of the weight gained by control animals. This suggests mushroom protein alone is nutritionally inadequate. In 1959, Oser  proposed the use of an Essential Amino Acid Index (EAA Index) to rate dietary protein in terms of the ratio of the essential amino acids contained in a food relative to the essential amino acid content of a highly nutritive reference protein, whole egg protein in this case. On examination of EAA Indexes, although those analysed contained all essential amino acids, every species was limited in their availability of at least one essential amino acid, some in up to seven. Agaricus bisporus has EAA values highly comparable to whole egg protein. It was found that the most nutritive mushrooms rank in potential nutritive value with those calculated for meat and milk, the only difference being relatively low content of certain amino acids, namely isoleucine, leucine, lysine and histidine. On the other hand, relative levels of lysine and tryptophan were significantly higher than those for legumes and vegetables. Even the least nutritive mushrooms were comparable to some common vegetables. Mushroom protein appears to be intermediate in nutritional quality between meat and vegetable proteins.
After moisture, which accounts for 90% of fresh weight, carbohydrates are the main component of mushrooms (average of 4.2% of the fresh weight). Polymeric carbohydrates that occur include glycogen (an energy store compound also found in humans and comparable to starch in plants), and chitin or “fungus cellulose”, a polymer of N-acetylglycosamine, the structural component of the fungal cell wall. Chitin is not easily digestible and is considered to be the major constituent of the fibre content. The cell wall contains many other large carbohydrate polymers such as glucans, chitosans and mannans, and, including chitin, these polymers are linked together with covalent bonds that cannot be attacked by our digestive enzymes. Therefore, it is suspected that humans cannot utilise a large percentage of the carbohydrate in mushrooms as nutrients and so it functions only as roughage.
Because mushrooms cannot photosynthesise, simple carbohydrates are present in lower proportions than vegetables such as carrots and sprouts, and so provide only a fraction of the energy requirement. Mushrooms contain an average of 85-125 kJ per 100 g whereas an adult male needs about 10 000 kJ per day. This low energy value of mushrooms enables it to be used in low-calorie diets. The low carbohydrate value makes them an ideal food for diabetics.
Mushrooms are characteristically low in fat, comprising 2-8% dry weight. This crude fat includes representatives of all classes of lipid compounds including free fatty acids, glycerides, sterols, and phospholipids. Of existing fatty acids, a high proportion are linoleic acid (the only essential fatty acid required in the human diet), has been found to be 63-74% of total fatty acids. Sphingolipids, important in the brain and nervous system, have also been identified, but appear to represent only a small proportion of total glycolipids.
Like vegetables, mushrooms are a cholesterol-free food. This is promising in terms of health issues, as cholesterol is regarded as a risk factor of coronary heart disease and related conditions. A study carried out by Fukushima et al. (2000)  reported that some mushrooms in Basidiomycotina have the ability to lower serum cholesterol concentration. Using Agaricus bisporus, they found that rats fed a diet of the mushroom fibre led to lowered serum total cholesterol and lowered VLDL, IDL and LDL cholesterol concentrations, all thought to be atherogenic lipoproteins. Cheung (1998)  also described how edible mushrooms are an ideal food for the prevention of atherosclerosis due to their high fibre content. His study concluded that the inclusion of edible mushrooms into the diet has a hypocholesterolemic effect, perhaps due to dietary fibres such as β-glucans, which may increase intestinal motility, reducing bile acid and cholesterol absorption.
In the kitchen, mushrooms are popularly sautéed, roasted, grilled, boiled or fried in a variety of dishes giving rise to a whole selection of flavours. All these techniques, however, require the use of butter or oil, ultimately adding to its fatty value, thus increasing the amount of cholesterol consumed. Mushrooms are also commonly stuffed, again possibly adding to the fat content. It is therefore too straightforward to state simply that eating mushrooms contributes to a low-cholesterol diet, when clearly its fatty value is dependent upon the cooking method and the recipe.
There are many essential vitamins required daily in the diet. The fruit body of a mushroom is an excellent source of B-complex vitamins including riboflavin (B2) niacin, pantothenic acid, thiamin (B1) biotin, folate and vitamin B12. Mushrooms are a particularly rich source of riboflavin. One "Portobello" mushroom (the commercial marketing name for large open-capped Agaricus bisporus) provides nearly one-third our daily requirement (Table 2). Vegetarians should also be aware that mushrooms are one of the best non-animal-based sources of niacin available: 100g of fresh mushrooms provide more than a quarter of the adult daily requirement of this vitamin.
|Table 2. Important sources of riboflavin, according to USDA Nutrient Database for Standard Reference, Release 13, 1999
Riboflavin (mg) *
|Crimini mushrooms (i.e. closed A. bisporus primordia)
|Beef liver, cooked
|*The daily requirement of adult humans for riboflavin is set at 1.7 mg.
Mushrooms are unique in that they contain Vitamin B12, something that vegetables can't produce at all. Since B12 is mainly of animal origin, deficiency is commonly associated with vegetarian diets. Mushrooms were found to contain 0.32-0.65 mg per gram of B12, allowing just 3 g of fresh mushrooms to provide the recommended daily allowance of this vitamin. Vegetarians may find this a useful way of getting this important nutrient.
Outila et al. (1999)  found that ergocalciferol in mushrooms increased serum 25-hydroxyvitamin D concentrations as effectively as did supplements, allowing mushrooms to be reliably recommended as a natural vitamin D source. Pro-vitamin D is present in some mushrooms, particularly shiitake, and can be converted to vitamin D by the ultraviolet irradiation in sunlight. Vitamin A is uncommon although several mushrooms contain detectable amounts of pro-vitamin A measured as the β-carotene equivalent. Most cultivated mushrooms are believed to contain low amounts of the fat-soluble vitamins, K and E, and make only a small contribution to the daily requirement of vitamin C.
As far as minerals are concerned, in general, mushrooms contain significant quantities of phosphorus, potassium, and copper, Agaricus bisporus being especially high in these minerals. A serving of white mushrooms has more potassium than a tomato; a single Portobello mushroom has more than is found in a glass of orange juice. Approximately 104 mg of phosphorus can be supplied by 5 raw button mushrooms; 100 g of mushrooms would also supply more than half the daily requirement of copper. Mushrooms are also an excellent source of selenium. Foods of animal origin and cereal grains are sources of selenium, but among fresh produce, only mushrooms are a good source. This is good news for vegetarians, whose sources of selenium are limited. A serving of shiitake mushrooms provides about one-third of the recommended daily intake of selenium.
With their range of unique flavours, high protein value comparable to meat, and important contribution to the supply of vitamins and minerals in the diet, mushrooms are evidently a valuable food source.
Single cell protein
Yeast. As calculations of world food supply have become more pessimistic over the years, attention has focused on the potential of mass cultivation of microorganisms as sources of food. The dietary component in most short supply is protein. Single cell protein (SCP) is considered the most promising nonconventional protein source in augmenting the anticipated world protein shortage.
Many microorganisms are able to utilise cheap sources of nitrogen and abundant carbon sources (in many cases the latters being derived from agricultural wastes). The resulting biomass has a high protein content. Microbial growth rates are high; a compact fermenter can produce as much protein as a large area of agricultural land and can operate all year round. These considerations have led to massive industrial research aimed at production of microbial biomass for human consumption.
Various waste products, including cheese whey, starch, fruit-processing residues, animal waste, and petroleum hydrocarbons, have all been used to produce SCP. Yeast is an especially important fungus and the most promising source of SCP although its growth is not as rapid as that of bacteria. A study carried out by Lyutskanov et al. (1989)  on the applicability of protein extracts from a strain of Saccharomyces cerevisiae revealed the percentage of essential amino acids lysine and isoleucine were higher than that of soya bean and the chemical score of these amino acids higher than egg protein.
Protein extracts from yeast, however, always carry the problem of high nucleic acid content, which can cause gout and urinary tract stones. Successful methods of reduction led to a simultaneous decrease of carbohydrate content . This is also good news as existing results show allergic reactions are associated with high carbohydrate content of nonconventional protein sources. Its low nucleic acid and carbohydrate content make it a good candidate for large-scale production of high nutritional quality protein extracts, potentially applicable to human nutrition. One example could be to increase protein quality of cereal based food products.
The scale of the protein shortage problem is likely to be so large that its resolution is more a matter of economics rather than new technology. Any novel microbial protein requiring costly research and development would not be able to compete with cheaper alternatives. Yet one high-technology alternative model in the UK has been successful, and has emerged as a commercial product. Once described as a “twenty year overnight success story”  , mycoprotein
, marketed as Quorn™ and produced by Marlow Foods Ltd, now decorates the shelves of British supermarkets. Essentially the mycelium of Fusarium venenatum, mycoprotein
is grown in continuous culture on food-grade glucose derived from starch as the carbon source. The resulting product is a filamentous hyphal mass, much resembling a meat-like texture. The marketing behind Quorn centres on its filamentous structure, claiming that it simulates the fibrous nature, chewiness, and succulence of meat. The structure also allows the fabrication of products of different textures and forms.
Compared with many bacteria and yeasts, the relatively slow growth rate of most filamentous fungi is a disadvantage for biomass production. However, the slow growth rate generally leads to a lower nucleic acid content. Nevertheless the RNA content is still moderately high and so must be reduced by a heat-shock process. This has substantial economic penalties, as other constituents are also lost during the process, perhaps up to one third of the biomass dry weight .
Nutritionally, mycoprotein shares much of the value of the mushroom. Its net protein utilisation (NPU) is 75/100, comparable to beef (80) and cow's milk (also 75). When supplemented with 0.2% methionine, NPU is 100 (equal to egg protein). It has a protein content minimum of 44% and contains adequate proportions of all essential amino acids. mycoprotein
is high in dietary fibre, mainly due to chitin and β-glucans in hyphal walls. Complementary to beef, it is low in saturated fat, contains no cholesterol, and is of low calorie value (Table 3). It is a good source of the B vitamins, biotin (16 mg/100 g), riboflavin, niacin and B6 (content equivalent to lamb), although lacks in B12. mycoprotein
is also an excellent source of zinc, particularly useful to vegetarians.
|Table 3. Comparison of composition of braised beef and Quorn (data from reference 10)
|Component as % of total (by weight)
|Polyunsaturated fatty acids:
Still, the nutritive value does rely much upon what is added to the mycoprotein
as flavouring in production of the particular recipe which is put on sale. Marlow Foods state that “...if ever a conflict exists between optimum taste and maximum nutritional benefit, taste will always have priority”. For example, where Quorn is the ‘meal centre’ in, say, burgers, added fat maintains the desired succulence and, of course, adds to the fat content and number of calories the final food yields. It is therefore important again to say that composition depends more on the product recipe than on the mycoprotein
itself (Table 4).
|Table 4. Typical values for the same amount of three Quorn meals and their corresponding meat meals (data from www.quorn.com)
|of which sugars
|of which saturates
was originally developed to supplement the world’s protein shortage, by the early eighties when the production techniques had been fully established, and it had been approved for public sale, the expected global protein shortage had not materialised. Instead, Quorn was sold, first in the UK, as a high-protein, high-fibre, low-fat, cholesterol-free health food: a ‘meat alternative’, also available to vegetarians. However Quorn is more expensive than most meat products, suggesting it was to be sold to those who would pay the price for a top-quality health food, not those who couldn’t afford meat. Here the retail price reflects the perceived value created by marketing and not the actual nutritional value.
The first Quorn retail product was a savoury pie sold by Sainsbury’s in 1985. It was a traditional product rather than a new one to generate consumer interest rather than create apprehension concerning food of fungal origin. Today over 35 products are available in the UK and many other countries are now being introduced to this fungal health food (in particular, the USA in January 2002), making it significant as the only human food product to have emerged from the much-heralded SCP revolution.
It may hopefully be assumed that decisions about diet will be governed by sound nutritional knowledge. If it were as easy as identifying those foods of high nutritive value, and the corresponding integration into our diets would follow, then maybe many would eat a much more balanced diet. Such hopes are unfortunately often proved to be groundless, and knowledge in itself is not always effective in motivating our actions. In reality, underdeveloped countries don’t have the liberty to choose what they eat, and when they do, custom and prejudice usually prevail. The diet of those living in developed countries, although we don’t like to admit it, is dominated more by taste, palatability and price than nutritional value.
Many people are apprehensive concerning fungi as a food source. Ignorance has led many to become sceptical concerning a food produced by a microorganism, and the majority are cynical that food of fungal origin can hold any great nutritional importance. It seems much education is needed before we take full advantage of the true nutritional value of such a cheap and readily available food source.
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Updated December 7, 2016