Fungi
Page 13
Brewing and baking
The industrial applications of fungi are recent innovations compared with the ancient uses of yeast in brewing and baking. Brewing and baking are the original biotechnologies.
Humans have brewed beer from cereal grains and made wine from fruits and tree sap for millennia. Some archaeologists have proposed that cereal cultivation was driven by our species’ love of beer rather than any pressing need to grow grains for their nutritional value. Whether beer came before bread, or vice versa, the associated development of cereal cultivation encouraged human settlement. The earliest brewers relied on fermentation by wild yeasts that bloomed on the grains and fruits used in their recipes. Domestication of Saccharomyces cerevisiae began when brewers developed methods for deliberate transfer of the fungus from one fermentation to the next. With the introduction of this practice, yeast strains were set on novel evolutionary tracks where they were provided with an abundance of food in return for their effectiveness at producing alcohol. Genetic profiling of yeast strains suggests that this began more than 10,000 years ago. Contrary to the general pattern of genetic isolation of agricultural plants and animals, resulting in very limited genetic variation, yeast strains used in beer brewing and winemaking continue to vary according to geography.
Saccharomyces cerevisiae and other yeasts obtain their energy from sugar metabolism and generate carbon dioxide and water as waste compounds when oxygen is available. This is aerobic respiration and provides the greatest energy yield from the consumption of sugars. When oxygen levels fall, or if the yeast is growing in very high concentrations of sugar, cellular metabolism is redirected to a different pathway that releases carbon dioxide and ethanol. This fermentative mechanism is less efficient than aerobic respiration and leaves a lot of ‘unburned’ calories in ethanol. This simple switch by the yeast to an inefficient burn is the foundation of the great variety of alcoholic drinks.
The basics of brewing and winemaking are covered in so many books and online sources that it seems more interesting to consider the role of fungi in other brewing practices in this chapter. Apple cider and palm wines have huge regional significance. Cider apples are classified according to sweetness, ranging from bittersharp (high in acidity and tannin) to sweet (low acidity and low tannin). The apples are crushed and the pulp is pressed to release the juice. Modern ‘industrial’ cidermaking involves inoculation with specific yeast strains and addition of sulphur dioxide to exclude other microorganisms. Traditional cidermaking relies upon yeasts carried on the apple skin or clinging to the machinery between pressings.
Studies of traditional cider factories show that numerous yeast species are involved in the fermentation. Yeasts associated with the harvested apples multiply in the juice for the first few days and drive the fermentation until they are overcome by the rising levels of alcohol. As the number of these pioneers declines, the community of fungi changes and Saccharomyces cerevisiae becomes the principal species. This is explained by the higher alcohol tolerance of Saccharomyces. A third blend of yeast species controls the final maturation phase of cidermaking. Similar successions of microbial communities occur in traditional fermentations that produce other alcoholic drinks. Bacteria accompany yeasts in many fermentations. Lactic acid bacteria convert the tart-tasting malic acid into the softer lactic acid in cider, beer, and wine, but can also spoil fermentations by producing unpleasant flavours.
Palm wines produced by fermenting the sap from palm trees are common in many countries in Africa and Asia. These potent brews are fermented by complex communities of yeasts and bacteria, but wild strains of Saccharomyces cerevisiae always seem to dominate these drinks. Fifteen different strains of this ubiquitous yeast were identified in a metagenomic study of palm wine from Cameroon. Palm wines are among the oldest alcoholic drinks, whose discovery was an inevitable consequence of the attraction of natural populations of wild yeast cells to palm sap. Yeasts operated in the same way long before human evolution. We find, for example, that mammals that pollinate palm trees drink natural alcoholic nectar fermented in the inflorescences of these plants. Yeast has been making alcohol for animals for millions of years.
Saccharomyces cerevisiae has been used to ferment dough made from wheat and rye flour for more than a millennium. Froth from beer vats filled with top yeast was used for breadmaking by Romans and this method was widespread in the 19th century. By starting fresh dough with a small quantity of the mix saved from the previous batch of bread, called leaven in the Bible, bakers engaged in the unconscious selection of vigorous yeast strains. The first commercial food yeast was produced in Holland and sold as a cream in the 18th century. This was superseded by compressed cakes of yeast in the 19th century and granulated dry yeast became the choice of bakers in the 1940s. Baker’s yeast is manufactured today using fed-batch fermentation with molasses supplemented with a variety of nutrients. Fed-batch fermentation allows the producer to add nutrients to the reaction intermittently, or continuously, to control the metabolic activity of the yeast and generate high cell densities. These methods are used to produce 200,000-litre batches of starter yeast in huge stainless steel fermenters that fill entire buildings. The ethos is quite different for the growing number of artisanal bakers who exchange small quantities of starter dough with their friends.
Cheesemaking involves many microorganisms, beginning with bacteria that ferment lactose in milk. Yeasts and filamentous fungi play subsidiary roles in flavouring and ripening cheeses. The white rinds on Brie and Camembert are formed from the dense white mycelium of Penicillium camemberti. Penicillium roqueforti proliferates in blue-vein cheeses including Roquefort, Gorgonzola, Stilton, and Danish blue. Fungi play a primary role in the fermentation of traditional Asian foods. Tempe, originating in Java, is manufactured by inoculating cooked soybeans with spores of the zygomycetes Rhizopus and Mucor. A similar method is used to produce furu or sufu, which is a cheese-like food made from soybeans in China. Soy sauce is made by inoculating a mixture of boiled soybeans and roasted wheat with Aspergillus species to form ‘koji’. After a few days, the koji mash is mixed with brine to produce ‘moromi’, and this is fermented by yeasts and bacteria for several months to produce this staple condiment of Asian cuisine.
Quorn is a popular meat substitute produced by Fusarium venenatum that is grown in connected pairs of fifty-metre tall air-lift fermenters that hold 230 tons of broth. The vessels are connected at the top and the bottom to form a continuous loop. Compressed air and ammonia are pumped into the bottom of the first vessel, called the ‘riser’, oxygenating the culture and circulating the liquid containing the fungus towards the top. Carbon dioxide from the respiring fungal cells is released through a vent, and the liquid falls through the second ‘downcomer’ vessel, where it is infused with fresh nutrients. A heat exchanger at the bottom of the system maintains the temperature at 30°C and the culture is harvested at a rate of thirty tons per hour. Quorn production is completed by heating, drying, mixing with egg white, and the addition of flavourings and colourings.
Bioethanol and bioremediation
Saccharomyces cerevisiae is used to generate bioethanol from sugar cane in Brazil and from corn in the United States. Juice separated from sugar cane fibre is concentrated to make sugar and molasses. The fibre is burned as a source of energy for the biofuel plant, the sugar is refined for the food industry, and ethanol is produced from the molasses. Corn is a more complicated ‘feedstock’ for bioethanol production because its seeds are rich in starch rather than sucrose and other sugars. This adds a step to the production process, because the starch must be converted into sugars using enzymes. Some of these enzymes are fungal products. The efficiency of bioethanol production would be revolutionized if fungi could be used to release sugars from agricultural waste containing fibrous lignocellulose polymers. Natural decomposition of woody plant debris by white rot basidiomycetes is the model for the development of this industrial process. Pilot studies in which rice straw is milled and fed to mycelia of the oyster mus
hroom, Pleurotus ostreatus, and other white rot fungi are promising, with significant breakdown of lignin, cellulose, and other polymers. The next step in this ‘second generation’ biofuel production is the addition of yeast to ferment the sugars into ethanol. Agricultural wastes would provide a limitless carbon-neutral supply of fuels, but we are decades away from enjoying the benefits of this emerging technology.
White rot fungi also have significant potential as decontamination agents in soils polluted by oil and gas extraction, the mining industry, chemical companies, and agriculture. Interest in this field, called ‘bioremediation’, is encouraged by the effectiveness of the wood-rotting enzymes secreted by fungi at breaking down toxic organic compounds. Decomposer basidiomycetes are able to decontaminate wood chips impregnated with hydrocarbons and chlorophenols in experimental trials. Other filamentous fungi and yeasts degrade pesticides, dyes, toxic solvents, and explosives in culture. Scaling up these processes for larger clean-up projects is a considerable challenge for investigators.
Appreciation of the importance of fungi can lead to wishful thinking about the ability of these extraordinary microorganisms to counteract the environmental damage caused by our species. Because fungi sustain forest trees through mycorrhizal interactions and by enriching the soil through wood decomposition, it is tempting to think that they can restore productive ecosystems after the original habitats have been ruined. This is a simplistic view of ecology. Human civilization is supported by the biological activities of the fungi that have been introduced in this book, but there are evident limits to the ability of the fungi to ‘save the planet’. In closing this short introduction to mycology, I leave you with two unassailable facts: fungi are everywhere, and will outlive us by an eternity.
Further reading
Academic journals
Mycology is such a vibrant area of research that any book that aims to be current is certain to miss important findings within a few months of publication. Academic journals are the best source of current information and the following periodicals showcase current mycological research: Fungal Biology, Fungal Biology Reviews, and Fungal Ecology are published by the British Mycological Society (
Textbooks
C. J. Alexopoulos, C. W. Mims, and M. M. Blackwell, Introductory Mycology, 4th edition (New York: Wiley, 1996).
S. C. Watkinson, N. P. Money, and L. Boddy, The Fungi, 3rd edition (Amsterdam: Elsevier, 2016).
J. Webster and R. W. S. Weber, Introduction to Fungi, 3rd edition (Cambridge: Cambridge University Press, 2007).
The following website is a useful supplement to these books because it presents a clickable evolutionary tree that directs readers to details on individual groups of fungi:
Books for non-specialists
Books written for non-specialists include E. Bone, Mycophilia: Revelations from the Weird World of Mushrooms (Emmaus, PA: Rodale Books, 2013), N. P. Money, Mr. Bloomfield’s Orchard: The Mysterious World of Mushrooms, Molds, and Mycologists (Oxford: Oxford University Press, 2002), and N. P. Money, Mushroom (Oxford: Oxford University Press, 2011).
Amateur mycology societies
The North American Mycological Association (NAMA) is a vibrant society of mycological enthusiasts that organizes a popular annual foray (
Mushroom identification guides
G. A. Lincoff, G. H. Lincoff, and C. Nehring, National Audubon Field Guide to North American Mushrooms (New York: Knopf, 1981).
K. McKnight and V. McKnight, Peterson Field Guide Series: A Field Guide to Mushrooms of North America (Boston: Houghton Mifflin, 1998).
J. Petersen, The Kingdom of Fungi (Princeton: Princeton University Press, 2012).
R. Phillips, Mushrooms and Other Fungi of North America (Buffalo, NY: Firefly Books, 2010).
P. Sterry and B. Hughes, Collins Complete British Mushrooms and Toadstools: The Essential Photograph Guide to Britain’s Fungi (London: Collins, 2009).
These can be supplemented with regional guides and books on particular groups of fungi. Online resources for mushroom identification include:
Index
A
Acremonium chrysogenum 119
aeciospores 71–3
aflatoxins 95
Agaricus bisporus see white button mushroom
allergies 31, 109
Allomyces 25 Allomyces macrogynus 51
Amanita 61 caesarea see Caesar’s mushroom
muscaria see fly agaric
phalloides see death cap
ambrosia beetles 54, 56, 57
Amylostereum areolatum 56–7
anaerobic fungi 13, 25, 35, 84
animal infections see mycoses
antibiotics 118–19
antifungal agents 5, 100, 103–4, 106
ants 12, 57–8, 67
apothecia 33, 85
appressoria 80–1
aquatic fungi 4–5, 12, 18–19, 51–2, 88–9
arbuscules 34–5, 62, 63–4
Armillaria solidipes 11, 93
artillery fungus (Sphaerobolus stellatus) 21–2, 24, 29
artist’s fungus (Ganoderma applanatum) 90–1
Ascobolus stercorarius 85
Ascomycota (ascomycetes) defined 24–6, 31–4
life cycles 39–44
ascospores 33, 41–2, 79 discharge 31–2
formation 31, 41–2
asexual reproduction 33
ash dieback 69, 77–8
Asian soybean rust (Phakopsora pachyrhizi) 75–6
aspergillosis 106
Aspergillus 25, 33, 95, 120, 124 flavus 95
fumigatus 103
niger 7, 95, 120
parasiticus 95
sydowii 107
terreus 119
asthma 109
Auricularia auricula (Auricularia auricula-judae) see wood ear
B
baker’s yeast (Saccharomyces cerevisiae) 3, 7, 11, 15, 24, 39–43, 121–3, 124
baking 120–3
Basidiomycota (basidiomycetes) defined 24–31
life cycles 44–7
basidiospores 33, 71–3 discharge 26–8, 31, 48
Batrachochytrium dendrobatidis 25, 35, 36, 51, 107
bioethanol 124–5
biofilms 101–2
bioluminescence 28, 48–9
bioremediation 124–5
biotechnology 15, 17, 113–25
biotrophs 74
birch polypore (Piptoporus betulinus) 117
bird’s nest fungi 29–30, 31
black stem rust (Puccinia graminis) 74
black truffles (Tuber aestivum, Tuber melanosporum) 38–9, 114
Blastocladiomycota 25, 35, 51
Blastomyces dermatitidis 104
blastomycosis 103–4
bluestain fungus (Chlorociboria) 33
Blumeria graminis 78
boletes 28, 61, 94, 114–15
Boletus edulis see porcini
Botrytis cinerea 79
brewing 120–3
British soldiers (Cladonia crista
tella) 67
brown rot 91–2, 94
Buller, Arthur Henry Reginald 48–9
C
Caesar’s mushroom (Amanita caesarea) 114
Candida albicans 11, 101–2, 106
candidiasis 101–2, 106
Cantharellus cibarius see chanterelle
carbon cycle 84–97
cell wall 5–6
cellulose decomposition 84, 87–93, 96–7
cephalosporins 119
Chain, Ernst 119
chanterelle (Cantharellus cibarius) 114
cheesemaking 123
chestnut blight 69, 74, 77
Chlorociboria see bluestain fungus
cholesterol 5, 100
Chytridiomycota (chytrids) 4–5, 25, 35
cider making 121–2
Cladonia cristatella see British soldiers
clamp connections 45
classification 22–4, 33
Claviceps purpurea see ergot fungus
cleistothecia 33
climate change 69, 96–7, 107
Clusius, Carolus 22
coal formation 98
Coccidioides immitis 104
coccidioidomycosis 103–4, 106
coffee rust (Hemileia vastatrix) 75–6
commensalism 53–4
common stinkhorn (Phallus impudicus) 30
conidia 33–4, 78–9
Coniophora puteana 93
conservation 115
Coprinopsis cinerea 85
Coprinus comatus see lawyer’s wig mushroom
coprophilous fungi 14–16, 19–21, 84–7
cords 92
Cordyceps militaris 107
corn smut (Ustilago maydis) 77
Cortinarius see webcaps
Cryphonectria parasitica see chestnut blight
cryptococcosis 103–4
Cryptococcus 106 gattii 103
neoformans 103