Science corner

30th April 2004 at 01:00
Sophie Duncan rises to the challenge of yeast growing

Experimenting with yeast helps pupils find out why it is used to make bread.

Take a glass bottle and fill it with warm water. Pour the water away and add a packet of dried yeast powder and a spoonful of sugar. Stir with a long-handled spoon and then add half a cup of warm water.

Blow up a balloon and release some of the air. Place the balloon over the top of the bottle and secure it with an elastic band.

Place the bottle into a bowl of warm water and wait. You should see bubbles forming in the mixture, and after a while you should find that the balloon inflates.

Yeast is a type of fungus. The yeast-sugar mixture reacts to form carbon dioxide gas, and the pressure in the bottle increases, causing the balloon to inflate.

Yeast is often used to make bread. When yeast is added to flour and water, fermentation occurs. The starch is broken down to glucose, and some of the glucose is converted into carbon dioxide gas. These bubbles cause the dough to rise. When the bread is baked in the oven, these trapped bubbles expand, giving the bread a light airy texture.

Pupils usually like to make bread dough - fresh bread smells and tastes great. You can explore its texture, noting the holes.

A striking experiment to show how much air there is in bread is to take a container that is the same size as a loaf of bread. Buy at least six economy loaves, and push them into the container. They will all fit in.

Alternatively, squash one loaf as thin as possible to show the same effect.

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