When we think of fungi in the context of food, the first image that springs to mind is probably mushrooms. The second is usually how to prevent food going mouldy and mould inhibitors have found a very useful place in the market, reducing spoilage, prolonging shelf-life and keeping down prices in the fresh produce and bakery sectors. However, delve beneath the surface and fungi can be found to have an enormous number and range of uses, though nowhere near approaching the 100,000 or so species of the fungi themselves. Their capacity to grow anywhere, from the richest of soils, to the bleakest of environments, from air conditioning systems to the Antarctic have required them to employ a remarkable range chemicals in order to survive.
Aspergillus niger is used in fermentation processes
More than mushrooms and mould
In food, fungi have enjoyed a special place in fermentation since ancient times; more recently giving us tofu and Quorn®, which are good sources of dietary protein and fibre; tempeh from grains (by fermenting them with Rhizopus oligosporus) and some of our best-loved and most pungent cheeses (what would blue Stilton be without Penicillium roquefortii or camembert without P. camembertii?). then there’s miso and soy sauce, both produced by fermentation with Aspergillus oryzae or A. sojae.
Mushrooms and filamentous fungi are widely cultivated in China (and indeed China still grows around 60% of the world’s mushrooms), where their medicinal properties are highly valued. This alone gives us something of an insight into the potential value of fungi for non-nutritive health-promoting substances. Recent studies into medicinal mushrooms have identified biological properties that include haemagglutination and antitumour agents, all of which have potential pharmaceutical value.
Less well known is their place in the manufacture of a whole host of food additives. The majority of the world’s citric acid supplies, for example, are now made by fermentation by strains of Aspergillus niger, which is also a good source of ascorbic acid. Furthermore, the waste mycelium from citric acid production is also a good source of chitosan.
Chitosan is proving such a versatile product of fungal fermentation that in recent years industrial microbiologists have been looking at optimising the fermentation process, using different species of fungi (Rhizopus oryzae, Mucor rouxii and Aspergillus niger are the most common), different substrates and both submerged and solid state fermentation processes. Also obtained chemically from the shells of arthropods and marine invertebrates like shrimps and crabs, there is always a market for ‘vegetarian’ sources of chitosan. It is relatively easy to extract the finished product from fungal mycelium and from there it is used as a source of dietary fibre, in the clarification and purification of fruit juice and wine, as an antioxidant and is widely used as an antibacterial and ironically, antifungal, coating on all sorts of fresh produce, including fruit, vegetables, meat, cheese and, of course, sliced mushrooms!
Other functional and medicinal fungal products working away behind the scenes include lipases, poly- and oligosaccharides, dietary fibres, triterpenoids, peptides, proteins, alcohols, phenols, minerals and vitamins; as well as technological agents like emulsifiers, stabilisers and flavours.
Given these widespread applications for fungi and their components, it is hardly surprising that the scientific community is scrutinising the fungal world for new and better sources of nutrients, nutraceuticals and food additives. And where better to start the search than the world’s collections?
Special relationships bear fruit
Mycologists at CABI in the UK have something of a special relationship with fungi. Home to the UK’s national collection since 1947, CABI currently supports some 28,000 live and 400,000 dried specimens. This collection of spores, mycelia and fruiting bodies are specimens of the strains used widely in food, ingredients and additives, but, as Dr. David Smith, mycologist and Director of Biological Resources at CABI explains, they are only as much use in the search for potential ingredients as the experts searching. Smith’s team of around a dozen microbiologists are charged with the unenviable task of maintaining and documenting this enormous collection, making them available to other researchers hunting for pharmaceuticals, nutraceuticals, enzymes, antioxidants, colourings and flavourings, to name but a few. What the human element of the relationship brings to the search is an intimate knowledge of fungi, their taxonomy, natural habitats, and preferred substrates and, importantly, what types of chemicals they produce at different life stages. Marry this expertise to the DNA fingerprinting and genetic screening technology available in the lab, and you already have a formidable partnership. Throw in some chemical analyses for good measure and you can look for any substance of interest or even find things you might not have expected.
The search for a new compound, or, more likely, a new source of a familiar compound, could start anywhere within the collection but, as Smith says, the days of random screening are over. ‘Random screening has been done to death,’ he explains. Instead, ‘we are trying to use our knowledge of taxonomy and physiology. Fungi do different things at different life stages,’ he says. ‘If you understand that you have a head start.’
Most often, the search takes the form of a hunt for a novel, fungal source of a known substance. A classic example is that of a very recent Ph.D. research collaboration between CABI and the Royal Holloway, University of London that identified some novel fungal sources of omega-3 and omega-6 fatty acids. Omega-3 fatty acids, in particular, have proven health benefits that range from preventing and treating cardiovascular diseases, improving cognitive development and have also been investigated in relation to conditions such as depression and autism. While omega-3 fatty acids are also present in nuts and some plant oils, these sources are already heavily in demand as foods in their own right and for biofuel production. The best-known source, fish oil, while abundant right now, is not without problems – the ongoing controversy regarding PCB’s, organochlorines and methyl-mercury levels in oily fish have caused some people to avoid eating fish altogether and miss out on the benefits of thei omega-3 content. So, while the ink is not yet dry on the Ph.D. student in question’s thesis, it won’t be long before the race is on to maximise productivity by manipulating fungal fermentation.
Future for fungal finds
The specimens at CABI are stored under a variety of conditions – while most can be found either preserved in liquid nitrogen or freeze dried in the drawers upon drawers of glass vials (both are current favourite storage techniques for maximising genetic stability), there are also dried samples dating back to the original collection and shelves of specimens stored under oil, a method that was the cutting edge of preservation technology back in the 1950’s. The samples under oil are the subject of current speculation – although it is widely agreed nowadays that oil is not the ideal preservative, sometimes it inadvertently aids the search. Scanning the shelves of vials, some appear slightly yellowing. A sign of age, maybe? Not to the experienced mycologist. Yellow, or better still orange, is a fairly sure sign that a fungus can produce carotenoids – and thus might be a good candidate in a search for antioxidants or colourings. Clearly, this isn’t the standard screening method, but a record of findings like these is nonetheless helpful.
Other useful information includes the environment the original organism was found. Given that the specimens in the core collection originate from some 142 different countries, this might as easily be the UK as China or even the Antarctic. CABI’s latest partnership, with the British Antarctic Survey has led to the addition of more than 450 ‘extremophiles’ – unique organisms that thrive at low temperatures and may hold novel enzymes or nutraceuticals, including omega-3 fatty acids. All that is needed to reap the benefits is to find them.
Setting up the fungi funnel
David Smith is quick to point out that the search for new fungal sources of compounds doesn’t end with CABI’s own collection. He is actively involved in a pioneering new enterprise that aims to help access the world’s fungal collections.
Given that Smith’s estimate that ‘taxonomists are describing more than 1000 new species a year across the globe, the use of fungi in food could, quite literally, mushroom.
Sarah Mellor is Content Editor, Human Nutrition and Food Sciences at CABI. firstname.lastname@example.org
This article originally appeared in Food Ingredients, Health and Nutrition (August/September 2008 issue)