Fungi may be the food, or may make the food edible following processing. Processing may make it possible to consume the foodstuff by adding, modifying or removing components, including flavours, nutritional elements such as vitamins or colours to enhance the appeal of the food.
Fungi are a common contributor to the processing of foods. Their use dates back to the start of the civilisation, when breads and wines were first made deliberately. These days, the selection and use of fungi is a highly organised field of research and development in industry.
We tend to take a variety of foods and food additives for granted, without being aware of the processes which get them to the table. One such food is Soy Sauce and its partner bean curd.
Soy sauce (shoyu) is a dark brown, salty liquid, high in amino acids and with a meat-like flavour. It was first produced in Japan (a similar product is made in other east Asian countries), where some microbial cultures were used to ferment the unpalatable soy beans. The current industrial process is highly controlled, used around the world and is based on this original process.
Fermentation is in two stages. Initially, soy beans are soaked, cooked to remove contaminants, and then mixed with roasted wheat. The fungus Aspergillus oryzae is added to the mix, and the amended mix kept aerobically for 20 to 40 hours at 25 C. The fungus produces invertases, amylases and cellulases, which degrade the soy paste. The paste is then mixed and taken into the second phase of fermentation.
In deep vats, brine is added to the paste and the yeast Saccharomyces rouxii and lactobacilli are added. Anaerobic conditions develop quickly, preventing further growth of A. oryzae. After about a month, a sour liquid is apparent. The liquid contains large concentrations of amino acids, simple sugars and a range of vitamins. After separation and further storage, the liquid is sterilised, bottled and sold as Soy Sauce. Similar products are called Koji, Idli, Patu, Laochao or Ogi.
Production of cheese relies on diverse microbes. The cheese environment is dynamic and the specific biological interactions complex. Fungi play a role in ripening in two different ways: they may assist ripening from the outside of the cheese, and they can impart flavours from inside the cheese.
Various yeast and filamentous fungi colonise the surface of cheeses. The may be surface contaminants or deliberately inoculated. Their impact will rely on the temperature, water content, pH, salinity and redox of the substrate.
Soft-ripened cheeses are ripened from the outside in. Penicillium camemberti (=P candidum) is inoculated onto the surface of cheeses, typically Brie, Camembert and Neufchatel, where growth over 7 to 70 days imparts a flexible powdery white crust and contributes to the runny texture and intense flavours of the contents.
Various cheeses are sold that have been stab-inoculated with a strain of Penicillium roquefortii. The result is a blue streak or vein through the cheese. The fungus imparts a strong, pungent flavour due to the aerobic production of methyl ketones. Famous blue cheeses include: Roquefort, Gorgonzola, Stilton, and Danish Blue. The fungus is a widespread spoilage organism found in cool conditions. It can grow at low oxygen availability and tolerates acidic conditions. Thus the presence of blue cheeses in your fridge may lead to widespread contamination of products like bread that use acids as preservatives.
Cheese is susceptible to the growth of fungi. While a huge diversity are associated with unwanted contamination, some such as Geotrichum candidum have been expoited because they are thought to impart desirable flavours and assist with the ripening process.
While many flavours are produced by bacteria, fungi are responsible for a range of flavours including terpenes, menthol and lactones. Fungi also produce compounds that deodorise offensive and neutralise bitter flavours. At present, flavour enhancement is an unimportant area of the industrial use of fungi.
Fungi produce a range of compounds that alter the colour of food. For instance, Monoascus purpureus has been traditionally used for the production of red wine. The pigments are polyketides that are insoluble in acid conditions. Beta carotene is produced by a range of Mucorales. This can be added to a variety of foods. Concern with the potentially toxic or allergic characteristics of some artificial colours has led to a closer examination of colours from natural sources. LINK
Gow N. & Gadd G.M. (Eds)(1995) The Growing Fungus. Chapman Hall, London.
Pitt J.I. & Hocking A.D. (2009) Fungi and Food Spoilage (3rd edit). Springer.
Wainwright M. (1992) An Introduction to Fungal Biotechnology. Wiley, Chichester.