Pyramids, Pasteur and Plastic Baggies What Makes Yeast Grow?
What makes yeast grow best?
Picture yourself living in ancient Egypt and imagine that it is your job to rise before the sun each
day to bake crackers for your family.
Mixing up ground wheat and honey one afternoon, you are distracted. Maybe you're watching a
pyramid being built just across the Nile. You forget to cover up the cracker dough. It sits all
night in an open window, caressed by a warm breeze carrying tiny life forms that are too small
to see.
When you wake the next morning, you find the dough puffed up and overflowing its
bowl. Everyone will be awake and hungry soon and you don't want to get in trouble, so you go
ahead and bake it.
The crackers are not hard and flat like usual, but emerge from the hearth light, puffy and
delicious. You have just baked the first bread in human history.
No one really knows how the ancient Egyptians discovered yeast, but we have learned from their
writings and artwork that they have been making bread for over 4,000 years.
How bread rose was a mystery though, until a famous scientist named Louis Pasteur proved that
tiny living organisms called yeast were responsible for making bread dough puff up.
Bread yeast is a type of fungus and is related to mushrooms. If you look at yeast cells under a
microscope, you will see that they are shaped like balloons and footballs. The single-celled
organisms reproduce themselves by making tiny buds that will become new yeast cells.
Bakers' yeast growing
Growing yeast love to eat sugar and starches, like the ones in bread flour. When they eat these
starches, some of the proteins in the flour, called glutens, swell up. Yeast eating starch make a
gas called carbon dioxide that forms lots of tiny bubbles in the bread dough. The tiny bubbles
pop during baking, but leave tiny holes where they were. You can see these holes in the bread
you eat.
The yeast you buy at the store is alive, but it is dried and can't grow until you add water to it.
Here's a fun experiment you can try to see what makes yeast grow best. All you will need are
some zip-lock bags, yeast, salt, sugar and water.
What makes yeast grow?
How to:
1. Label four bags as follows:
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

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Sugar + warm water
Sugar + cold water
Sugar + salt + warm water
No sugar + warm water
2. Add a package of yeast (or 2 tsp.) to each plastic bag. Add 2 tsp. of sugar to each of the bags
that say sugar and 1 tsp. of salt to the bag that says salt.
3. Carefully, add half a cup of water to each bag. The warm water should be warm, but not too
hot, or it will kill the yeast. The cold water can be room temperature.
4. Seal the bags, squeezing out as much of the extra air as possible and let them sit. (The yeast
will grow faster in a warm room than a cold one.)
5. Watch the bags to see what happens. You will know your yeast is growing if the baggie it is
in puffs up. Keep an eye on your experiment. If a bag gets so puffy that it looks like it might
explode, be sure to open it to let the pressure out!
What do you think?
1. Which ingredients help yeast grow best?
2. Did you find an ingredient that kept them from growing well?
3. Do yeast grow better in warm or cold water?
4. What is making the bags puff up and how does this tell you that the yeast is
growing? (Hint: the answer is in the paragraph above about how yeast makes bubbles in
bread!)