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Saccharomyces cerevisiae

Beer is proof that God loves us and wants us to be happy.
Benjamin Frankin

Sometime between 10,000 and 15,000 years ago, a Mesopotamian farmer discovered that the water some grain had been soaking in had developed a funny taste. He woke the next day having made two important discoveries:

The first written records of brewing come from Sumeria about 6,000 years ago. But all that drinking was making people hungry, so in Egypt around 5,000 years ago, they started making bread (or at least, wrote down the recipe). Before that, bread was tough, dry stuff that tended to break your teeth and made your jaw ache. Bread made with yeast was wonderful, light, tasty stuff. The secret? Saccharomyces cerevisiae:

Budding S. cerevisiae

Yeasts such as Saccharomyces cerevisiae are single-celled fungi which that multiply by budding, or in some cases by division (fission), although some yeasts such as Candida albicans may grow as simple irregular filaments (mycelium). They may also reproduce sexually, forming asci which contain up to eight haploid ascospores. If you look closely at the video, you can see examples of budding cells (arrow, left). Saccharomyces cerevisiae has thick-walled, oval cells, around 10 µm long by 5 µm wide.

Fermentation:

FermentationSaccharomyces cerevisiae is commonly known as "bakers yeast" or "brewers yeast". The yeast ferments sugars present in the flour or added to the dough, giving off carbon dioxide (CO2) and alcohol (ethanol). The CO2 is trapped as tiny bubbles in the dough, which rises.

Why does yeast do this?
To gain energy from the breakdown (fermentation) of carbohydrates, as in the diagram opposite.
The fermentation of beer and wine was originally caused by naturally occurring yeasts present in the environment. Some wineries still use natural yeast strains, however most modern brewers use highly cultured isolates, e.g. Saccharomyces carlsbergensis, named after the Carlsberg Brewery in Copenhagen. The bubbles in sparkling wines such as Champagne are trapped CO2, the result of yeast fermenting sugars in the grape juice. One yeast cell can ferment approximately its own weight of glucose per hour, giving rise to large volumes of CO2. The same process occurs in bread dough - as the CO2 from fermentation is trapped, the dough rises:

 


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