Yeast Experiment: Pyramids, Pasteur and Plastic Baggies

 - by KitchenPantryScientist

This month has found me ridiculously busy teaching microbiology and writing, while trying to keep up with my kids’ activities. To keep you busy doing science, I thought I’d repost this yeasty microbiology experiment from last year, since bacteria, viruses and fungi have been on my mind (and in my house a few times in the form of colds and stomach bugs.) If you’re starved for more projects, on January 31st, you’ll find me on Kare11 morning news (Minneapolis/St. Paul) demonstrating how to extract DNA from strawberries at your kitchen table!

I demonstrated this experiment on Kare11- you can watch it here.

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 are 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.  The kind of yeast used to make bread is called Saccharomyces cerevisiae (sack-a-roe-MY-seas sair-a-VIS-e-ey).  Saccharomyces means “sugar fungus” and the word cerevisiae comes from the name of Ceres, who was a goddess of farming in Roman mythology.  Here’s what they look like under the microscope.

Growing yeast cells 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 cells 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 baggies, yeast, salt, sugar and water.

1. Label four baggies as follows:

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 ½ cup water to each baggie.  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 cells are growing if the baggie containing them puffs up. Keep an eye on your experimentIf a bag gets so puffy that it looks like it might explode, be sure to open it to let the pressure out!

Which ingredients help yeast grow best? Did you find an ingredient that kept them from growing well?  Do yeast cells grow better in warm or cold water?  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!)

Try coating the yeast with oil before adding the sugar and water.  What happens if you add fruit juice to the bags?  Honey? Lemon juice?  What happens if you put the bags in the refrigerator just after adding the yeast?

It’s fun to try the same experiment in bowls, but you won’t be able to see the carbon dioxide gas puffing the bag up!

I wrote this post as an article for INGREDIENT magazine, a magazine for kids curious about food.  The January/Feb. issue contains lots of great articles and teaches kids to bake bread!

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