I’ve been hearing about this science demonstration for years, and finally decided to try it! If you do it at home, kids should wear safety goggles or sunglasses to protect their eyes, and adults should pour the 3% hydrogen peroxide into the bottles.
a tray or cookie sheet
3% hydrogen peroxide (available at most pharmacies and discount stores)
liquid dish soap
dry yeast (2 packets)
empty 16 oz bottle
What to do:
1. Pour 1 cup hydrogen peroxide into an empty 16oz bottle. (A funnel helps!)
2. Add 2 Tbs. liquid dish soap to the bottle and mix well with the hydrogen peroxide.
3. Put 8 drops of food coloring into the bottle and swirl to mix.
4. Position the bottle on the tray.
5. Pour 2 packets of yeast into a paper cup and pinch the cup’s lip to make a pouring spout.
6. Quickly pour the yeast into the bottle, while swirling the liquid vigorously to mix well. The better you mix it, the better the experiment will work!
7. Set the bottle down on the tray before the foam emerges from the top.
8. Watch the chemical reaction between catalase in the yeast and the hydrogen peroxide create oxygen bubbles in the soap!
9. When the reactions has stopped, have an adult clean up the mess by pouring everything down the sink and rinsing the tray with water. (Normally kids should clean up, but for this one, I’d recommend an adult do it.)
The Science Behind the Fun:
Hydrogen Peroxide (H2O2) is a common household chemical that is often used to disinfect wounds and bleach hair. Certain chemicals can break it down into water (H2O) and Oxygen (O).
Dry yeast is a living fungus that produces a molecule called catalase. Catalase is very good at breaking down hydrogen peroxide quickly. When you add yeast to hydrogen peroxide that’s been mixed with liquid soap, the soap traps the oxygen and makes bubbles that push their way out of the bottle.
You may notice that the bottle feels warm. That’s because the chemical reaction produces heat and is called an exothermic reaction.
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!
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 experiment. If 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!