Yeast--It’s Alive ... and it’s Making Bubbles in my Bread!

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Bread and crackers are both tasty treats. But, it’s hard to eat a sandwich on crackers. Have you ever wondered why crackers and break are so different? If you look at the ingredients for bread and crackers, you will notice that bread contains yeast and crackers do not. Yeast is responsible for breads’ softer texture.

Yeast is something familiar to most people; especially people who like to bake their own bread. Yeast comes in cakes, packages and jars and looks pretty simple—a beige powdery substance. But unlike most recipe ingredients that can be mixed into a recipe without much thought, yeast requires special treatment. Why is that? Because yeast is actually a living thing! It digests sugar (food) and turns it into carbon dioxide. When the yeast is living in dough, the carbon dioxide causes the bread to rise. The holes you see in a slice of bread come from the carbon dioxide bubbles.

Do you wonder if you’ve been eating living things in your bread? Don’t worry, the heat from baking kills the yeast before the bread is done. Yes, yeast is alive, and like all living things, it has certain requirements for food and living conditions. Yeast needs to be kept reasonably warm, but not too hot, and fed sugar. That is why bread recipes specify a temperature for liquid ingredients, and why yeast and sugar are often mixed together, into the warm liquid, before the rest of the recipe is completed.

This month, our experiment involves testing different living conditions for yeast. As you do the experiment you will observe the yeast creating bubbles of carbon dioxide gas. When you are done with the experiment, you can use the rest of the yeast to try baking your favorite bread recipe.

Growing yeast in your kitchen!

yeast (any form, but read the label to find the optimum temperature for the particular type)




clear containers

1. Decide what you want to test in your experiment. Possibilities include temperature or amount of sugar.

2. Set up a control—this is a system that has the proper temperature and food for the yeast to grow. Mix about 1/2 teaspoon of yeast with 1/2

teaspoon of sugar in 1/4 cup of warm water (check the yeast package for the optimal temperature for yeast growth—it depends on the type of yeast). Let this sit for about 10 minutes. Observe what happens as the yeast starts to digest the sugar. See how high the bubbles rise.

3. Set up your experiment. Make sure you only vary one parameter (temperature or amount of sugar) at a time. Label the cups to help make sure your data collection is accurate .

4. Start the experiment and make observations. Do the bubbles rise as high as in the control?

5. You can repeat this experiment using other variables—if you tried varying the amount of sugar, try varying the temperature (but keep the amount of sugar constant). at happens if you add salt to the yeast? Use juice or milk instead of water? The possibilities are endless!