Yeast and Brewing: A* understanding for iGCSE Biology 5.5

Humans first discovered how to make beer around 7000 years ago and brewing has been an integral part of human civilisations ever since.  Yeasts are a family of single celled fungi that can use the sugars in fruits and seeds as a source of energy for respiration.  Yeast can respire both aerobically and anaerobically and you should know the equations for these two processes.

Aerobic respiration

Glucose + Oxygen ——-> Carbon Dioxide + Water

Anaerobic respiration (aka Fermentation)

Glucose ——> Ethanol + Carbon Dioxide

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So when yeasts respire anaerobically they produce ethanol and carbon dioxide as the waste products.  Ethanol is also known as alcohol and humans learned a long time ago that alcohol is a drug that changes the way you think or feel, often in a pleasurable way in moderate doses.  Making drinks that were alcoholic also helped to kill potentially harmful bacteria and other pathogens in pre-industrial times when drinking water was not readily available.

If you add yeast to a source of sugar in anaerobic conditions, the yeast cells will ferment the sugars into alcohol and carbon dioxide.  In order to make beer, the source of sugar comes from germinating barley seeds.  Hops (dried flowers of a hedgerow plant) are added later to give the bitter flavour beer drinkers seem to like…..

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The flow diagram above shows the stages in making beer.  I can’t imagine you would be expected to know the details.  Any question on this topic would presumably focus on the anaerobic respiration of the yeast rather than the details of the brewing process.

In case anyone is interested, I am not a huge fan of beer although I can occasionally be forced by peer pressure into consuming one or two.  My preferred fermentation reactions happen not in the copper tuns of English breweries but in the beautiful Northern Rhone valley in France, where skilled wine-makers can take Syrah grapes grown under the influence of the cooling mistral wind and turn them into beautiful Cote Rotie or Hermitage.  Now there’s a happy thought for a cold November evening……..

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2 comments

    • Paul Gillam

      Not really: yeast is the organism in which fermentation reactions can take place. The catalysts for these reactions are the enzymes found in the cytoplasm of the yeast cells.

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