Microbiology

The use of Fungi in Making Beer



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Beer is brewed from barely grain. The process involves six stages:

1. During malting, dry grain is soaked (steeped) in water and then layered in a melting tower or spread on a melting floor. This allows the grain to germinate partially so that its store of starch is converted into sugars (including maltose). Malting also converts proteins to amino acids. These breakdown products are used by yest for fermentation.

In general sense, the yeasts include all unicellular fungi that reproduce asexually by budding. They occur commonly on faeces, in the soil, and on the surfaces on plants and animals. The most familiar and industrially important yeast is Saccharomyces cerevisiae. The tiny cell of this yeast are very active metabolically. They usually respire aerobically, but when deprived of oxygen they switch to anaerobic metabolism, producing carbon dioxide and ethanol (alcohol) as waste products. It is these waste products that are important to us industrially.

Germination is usually accelerated and controlled by adding gibberellins (plant growth substances) and amylases. The process is stopped by slowly heating the grains to 80C.

2. Milling breaks down the grains produced by malting (called malted grain) into two or three small pieces. The crushed grain or gist is then mashed.

3. Mashing takes place in a 'mash run' in which hot water trickeles through the grains, softening them and releasing wort, a nutrient-rich liquid.

4. Hops (dried ripe flowers from hop plant) are added to the wort to give flavor and for their antiseptic properties. Boiling involves heating the mixture of wort and hops to a high temperature. The resulting concentrated, sterile liquid (called mash) is strained to remove the hops and then allowed to cool before fermentation.

5. In fermentation, brewer's yeast is added to the mash. Two commonly used yeast species are Saccharomyces cerevisiae and Saccaromyces carlbergensis; the latter is used to make larger. Fermentation usually takes place in large conical batch fermenters. During this process, the yeast converts sugars into ethanol and carbon dioxide as in bread-making. However, in beer making, the fermentation is mainly anaerobic and the ethanol is kept in the brew. Fermentation is generally completed after about five days, when the ethanol content has reached 4-6per cent, but beer with a higher or lower ethanol content can be made by varying the brewing process.

6. In the final stage, finishing, the beer is separated from the yeast and packaged for sale. Bottled and canned beers are usually pasteurized at 60C or higher to kill off any microorganisms in then brew. Aqueous extracts of brewer's yeast are used as a food, rich in vitamin B.

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