It's fun to ferment

4th July 2003 at 01:00
Our GCSE pupils have a link with Youngs' brewery in Wandsworth, south London, thanks to a Royal Society Partnership Grant to find out about brewing and "what makes yeast cells happy". But in our first practical sessions nothing worked and the yeast just sulked at the bottom of the tubes.

The brewery advised us to buy an enzyme to speed up cutting the carbohydrate into the bite-sized sugar molecules that yeast likes so much, and we used it with cornflour, rice flour and potato starch. Two per cent solutions of starch were digested with 0.5ml AMG enzyme. This enzyme works best at 75oC and 30 minutes is long enough to achieve an effective concentration of sugars. These solutions can then be refrigerated until the next lesson. The next steps can be low or high tech.

* A comparison of yeast growth in different media can be achieved by adding 2g of yeast and collecting the carbon dioxide in a burette over water.

* To follow the course of aerobic growth until the end of fermentation you can use a bubble logger on a fermentation lock. This will produce a computer record of the rate of gas production. (It's fiddly to set up and we have yet to make this method totally reliable.) The fall in mass can readily be logged if you tie up a balance for 36 hours. We found Datamass software easy to use.

Now we were in business. The yeast had a field day and we made superb graphs from serial measurements of the carbon dioxide produced. To make alcohol fast, use potato starch.The procedure is feasible for a class practical over two lessons. Those involved gained Creativity in Science and Technology awards (www.csiro.aucrest).

Judy Machin, head of science, Gumley House Convent School, Isleworth, Middlesex

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