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.

Coagulating enzymes are an absolute necessity to produce all cheese varieties. The traditional coagulant was derived from the dried stomachs of unweaned calves; later, concentrated or dried enzyme extracts from that source, called calf or animal rennet, were developed for more controlled cheese making. But by the 1970s demand for coagulating enzymes started to exceed the supply as world cheese production increased (by a factor of about 3.5 between 1961 and 2010 according to FAO statistics) while reduced availability of calf stomachs decreased the rennet supply.

The most important rennet substitutes include enzymes of microbial origin; recombinant (animal) proteases produced by genetically modified microbes, and plant proteases (proteolytic enzymes called cardosins, from Cynara cardunculus (cardoon thistle) are traditionally used to curdle the milk in some Spanish cheeses). With one exception, all commercial clotting enzymes are aspartic proteases that cleave the Phe105–Met106 bond of bovine casein; the exception is a protease from Cryphonectria parasitica (causal agent of chestnut blight) which cleaves the Ser104–Phe105 bond. Genetic engineering could provide unlimited amounts of appropriate coagulants, but these products meet consumer resistance and food regulations in various European countries prohibit the involvement of GMOs (genetically modified organisms) in food manufacture (Jacob et al., 2011). Today, only 20-30% of the demand for coagulants can be satisfied by animal rennet so up to 80% of cheese making now uses non-animal coagulants, mainly enzymes from filamentous fungi like Aspergillus spp. and Mucor miehei.

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, and Blue Cheshire, use Penicillium roqueforti which is inoculated into the cheese prior to storage at controlled temperature and humidity. The fungus grows throughout the cheese (Fig. 17.41), 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). Penicillium 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 Fig. 17.42).

Updated July, 2018