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Fermentation

Introduction

If you were to ask someone what role fungi played in industry, it is likely that they would indicate use of yeasts in brewing, wine making and bread making. This response is probably related to their level of awareness of yeasts in brewing and baking, rather than their understanding of the importance of fungi in industry. Brewing and baking rely upon a simple principle. The basis of current industries has been around for many centuries. Fermentation commonly results in the release of alcohol, but lactate, glycerol and other organic molecules can also be released. The principles are simple and well understood.

Production of Alcohol

In the yeast Saccharomyces cerevisiae the presence of excess glucose represses respiration. In principle, materials rich in sugars (or starches) are then fermented resulting in the production of alcohol.

Alcohol is produced almost invariably by use of Saccharomyces cerevisiae or its close relatives. Ales and wine use S. cerevisiae, lager uses S. carlsbergensis, cider uses S. uvarum, and saki uses S. sake. The yeast hydrolises sugar, commonly sucrose, to pyruvic acid via the Embden-Myerhof-Parnas pathway, and then via acetaldehyde to ethanol. The reaction is exothermic, and unless the heat is dispersed, the reaction is slowed. Further, though up to 50% of sugar can yield alcohol, by weight, the solution rarely goes beyond 15% ethanol, because the fungus is sensitive to high concentrations of ethanol.

Chardonnay, Reisling, Shiraz, Port, Colombard, Grenache, Pinot Noir, Cabernet, Champagne

Nearly half the wine of the world is produced in Italy, France and Spain. Australia and USA are rapidly increasing production of high quality wines. The main grape used is Vitus vinifera. As the grape berry ripens, the concentration of sugar in the juice increases and acidity (especially malic acid) decreases. After harvest, the grapes are crushed and the juice (must) is either fermented in contact with skins (red wine from dark berries) or without (white wine).

Fresh berries are covered by various microbes, including yeasts. The contaminating microbes are suppressed by addition of sulphur or a compound that releases sulphur dioxide to the must. Alternatively, the must might be sterilised by other means. The starter culture of specifically selected yeasts is then added. The temperature is carefully controlled, and various treatments are used to influence the flavours that develop. After a period, the yeasts sink and the development of complexity is allowed by aging and maturation.

Beer, Ale, Lager, Stout

Ales and lager use cereal grains, commonly barley, as the basis of fermentation. Grain is dried, then germinated synchronously. The germination process involves release of amylases within the grain. These enzymes are then used to digest the starch of a batch of ground cereals mixed into a mash, resulting in the release of hexoses. The resulting wort is drained off. After various treatments, including addition of hops, the liquid is inoculated with specific yeasts and the mixture fermented for a short period.

Sake

Sake is a Japanese beverage made from rice. Steamed rice is inoculated with Aspergillus oryzae to produce koji. Further rice mash is lactic acid fermented using bacteria and yeasts. The mash and koji are mixed and fermented for around 20 days, when the alcohol concentration has reached around 18%. The product is filtered, pasteurised and stored before consumption.

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Bread Making


Loaf of bread.

Bread consists of a mixture of flour (usually from cereals especially wheat), with water, salt and sugar, leavened by yeast. Flour is mixed with the remaining ingredients and incubated at about 25 C. The yeast ferments the sugar forming carbon dioxide and alcohol. The released gas causes bubbles to form by elastic extension of gluten (a protein) in the flour. On baking, the alcohol evaporates. The length of leavening, the quantity of gluten in the flour, the constituents of the grain, and the temperature determine the texture and flavour of the bread.

Flour from wheat can be mixed with starch from a variety of sources. For instance, potato and pumpkin have been used successfully mixed with wheat flour to make a different style and flavoured bread.

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Conclusion

A variety of foods are fermented before consumption. By far, the largest industries in the west are based around production of alcohol and bread. Fermentation to produce soy sauce and similar products are more likely to be local industries in South East Asia.

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