11.5 Fermented foods

As well as being used directly as food, fungi are also used in the processing of various food products. In these applications the fungus is primarily responsible for the production of some characteristic odour, flavour, or texture and may or may not become part of the final edible product; production is dealt with in Chapter 17, CLICK HERE to view now. Indonesian tempeh is produced by fermentation of partially cooked soybean cotyledons with Rhizopus oligosporus. The fungus binds the soybean mass into a protein-rich cake that can be used as a meat substitute which is being increasingly widely sold into the vegetarian market. There are a variety of other fermented products of this sort. Ang-kak is a rice product popular in China and the Philippines which is fermented using Monascus species. Monascus purpureus produces the characteristic pigments and ethanol which are used for red rice wine and food colouring. The pigments are a mixture of red, yellow and purple polyketides and about ten times more pigment is obtained from solid state fermentation than from submerged liquid fermentation.

Soy sauce is, in its traditional form, a fermentation product. Soybeans are soaked, cooked, mashed and fermented with Aspergillus oryzae and A. sojae. When the substrate has become overgrown with the fungus, the material is transferred to brine and inoculated with the bacterium Pediococcus halophilus and 30 days later with Saccharomyces rouxii. The brine fermentation takes six to nine months to complete, after which the soy sauce is filtered and pasteurised.

Cheese could be considered the occidental equivalent of the fermented soya products which are popular in Asia. Cheese is a solid or semisolid protein food product manufactured from milk. Before the advent of modern methods of food processing, particularly refrigeration, cheese manufacture was the only method of preserving milk. Although basic cheese making is a bacterial fermentation, there are two important processes to which filamentous fungi contribute; these are the provision of enzymes for coagulation on the one hand, and mould-ripening on the other.

Cheese production relies on the action of enzymes which coagulate the proteins in milk, forming solid curds (from which the cheese is made) and liquid whey. Traditional cheese-making uses animal enzymes, specifically chymosin and pepsin, extracted from the stomach membranes of unweaned ruminants. Rapid expansion of the cheese-making industry caused attention to shift to alternative sources of such enzymes and moulds like Aspergillus spp. and Mucor miehei have supplied these to the extent that around 80% of cheesemaking now uses non-animal coagulants. Very recently, animal enzymes produced by genetically-modified microbes have entered the market, but for the moment most industrial cheese production still depends on enzymes from filamentous fungi for the coagulation step. Mould ripening is another matter, being a traditional method of flavouring cheeses which has been in use for at least two thousand years.

Blue cheeses, like Roquefort, Gorgonzola, Stilton, Danish Blue, Blue Cheshire, use Penicillium roquefortii which is inoculated into the cheese prior to storage at controlled temperature and humidity. The fungus grows throughout the cheese, producing methyl ketones, particularly 2-heptanone, as the major flavour and odour compounds. Camembert and Brie are ripened by Penicillium camembertii, which changes the texture of the cheese rather than its flavour (in some varieties, secondary bacterial growth changes the flavour). P. camembertii grows on the surface of the cheese producing extracellular proteases which digest the cheese to a softer consistency, working from the outside towards the centre (see Section 24 in Chapter 17 for more details; CLICK HERE to view now).

Updated December 17, 2016