Year 6 Make Bread
What's the point of yeast?
The picture above shows some yeast cells magnified 400 times. They don't look particularly impressive but without them we wouldn't be able to eat nice fluffy loaves of bread. We'd also not have beer and wine.
Method
Firstly, the production of carbon dioxide gas by yeast was observed. Dried yeast was activated by putting it into a solution of water and sugar and warming it to 40°C in a conical flask. The gas produced by the yeast was delivered to a test tube containing limewater. The gas bubbled through the limewater. It was observed that the limewater slowly changed colour from clear and colourless to become milky white. This is the test for carbon dioxide - thus yeast makes carbon dioxide gas.
Two bread doughs were prepared. One containing flour, milk, water, butter, sugar and yeast; the other containing flour, milk, butter and sugar (i.e. no yeast).
All the equipment and ingredients were assembled.
The ingredients were mixed well.
The doughs were kneaded well for 10 minutes.
The two doughs were left for 20 minutes to rise. In the photo, above, the dough on the right hand side contained yeast.
The two batches of dough were made into 6 bread rolls. The rolls were baked in an oven at 200°C for 15 minutes.
Results
After baking the rolls were removed from the oven and compared. In the photo, above, you can see that the rolls on the right hand side (made with yeast) are bigger in volume.
When the rolls were cut open it could be clearly seen that the rolls made with yeast had many bubbles inside them. Whereas the rolls without yeast had no bubbles in them at all.
Conclusion
Yeast cells produce bubbles of carbon dioxide gas. These bubbles are evenly spread throughout the bread dough. When the dough containing yeast is cooked the bubbles of gas expand and cause the bread to rise. The heat of the cooking process kills the yeast making it perfectly safe to eat. Without yeast the bread is said to be 'unleavened' and will not rise.