Yeast and carbon dioxide

Experiment # 1 

Materials

Baker's yeast, water, sugar, 3 test tubes, a tea spoon, three balloons, warm water (about 40°C)

Procedure

• Label the test tubes with the numbers 1, 2 and 3
• Half fill each test tube with warm water (40)°
• Pour a tea spoon of sugar into the test tube #1 and fix tightly a  balloon over its mouth
• Pour a tea spoon of yeast and a tea spoon of sugar into the test tube #2 and fix tightly a  balloon over its mouth
• Pour a tea spoon of yeast into the test tube #3 and fix tightly a  balloon over its mouth
• Wait for few minutes. What do you notice?

What happens?

The balloon over the test tube #2 swells up because the yeast ferments, producing a gas, carbon dioxide (we can detect it with the carbon dioxide sensor).

Fermentationis a chemical process by which microorganisms such as yeast  break down sugars (in this case sucrose) into carbon dioxide and alcohol.

Sugar carbon dioxide + alcohol

Experiment # 2  

 Materials

100g flour, Baker's yeast, water, sugar, a cup, a bowl, a spoon, a towel,

Procedure

• Dissolve 1tea spoon of yeast in 1/2 cup warm water, add ½  tea spoon of sugar and wait for few minutes. What do you notice?
• Pour the flour into the mixing bowl.
• Add the mixture with the yeast to flour, stirring with the spoon until blended.; if necessary, add more flour, until the dough is too stiff to stir with a spoon
• Turn the dough out onto a floured surface and knead it for few minutes until it is smooth and elastic.
• Sprinkle the bowl with flour, put again the dough into it and cover with a towel
• Set in a warm spot for almost 15 minutes. What do you notice?

What happens?

When we dissolve yeast and sugar in water, the mixture bubbles, because the yeast makes sugar ferment.

The reaction is:

Sugar carbon dioxide + alcohol

Yeast makes also dough rise; in this case the  carbon dioxide is produced by the fermentation of the starch in the flour (starch is polymerized glucose, a sugar). 

Starch carbon dioxide + alcohol