Yeast Experiment for Kids | America’s Test Kitchen Kids

activity

All those holes inside a soft, chewy loaf of bread are the handiwork of microscopic (really tiny) yeast! Find out what temperature yeast likes best in this bubbly experiment.

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Did you know that yeast is alive? It’s a tiny creature—so small that just ½ teaspoon contains millions of them! Yeast get their energy from starch and sugar, just like you get energy from the food you eat. As yeast eat starch or sugar, they “burp” carbon dioxide gas. That gas causes yeast doughs to rise. All those holes inside a loaf of bread? That’s the work of yeast!

In this experiment, you’ll investigate how yeast works at different temperatures. You’ll add yeast, sugar, and hot, cold, or room temperature water to zipper-lock bags. Then, you’ll discover which temperature makes the yeast the most active by seeing which bag inflates with carbon dioxide gas!

Prepare Ingredients

Ice

Water

1½ teaspoons

instant or rapid-rise yeast

1½ teaspoons

sugar

Gather Equipment

Large glass

Marker

3

snack-size zipper-lock bags

½-teaspoon measuring spoon

1-tablespoon measuring spoon

Rimmed baking sheet

Small microwave-safe bowl

Oven mitts

Fine-mesh strainer

Get Started!

1

Ice

Water

Fill large glass with ice and water. Set aside.

2

Use marker to label 1 zipper-lock bag “Cold,” second bag “Room Temperature,” and third bag “Hot.”

3

1½ teaspoons

instant or rapid-rise yeast

1½ teaspoons

sugar

Add ½ teaspoon yeast to each bag. Then, add ½ teaspoon sugar to each bag.

4

Add 2 tablespoons room-temperature water to bag labeled “Room Temperature.” Seal bag, squeezing out as much air as possible. Place bag in center of rimmed baking sheet.

5

Fill small bowl about halfway with water. Heat water in microwave until steaming, 30 seconds to 1 minute. Use oven mitts to remove bowl from microwave. Add 2 tablespoons hot water to bag labeled “Hot” (ask an adult for help). Seal bag, squeezing out as much air as possible. Place bag on right side of baking sheet. Discard remaining hot water.

6

Hold fine-mesh strainer over now-empty small bowl. Pour ice water through fine-mesh strainer. Discard ice. Add 2 tablespoons ice water to bag labeled “Cold.” Seal bag, squeezing out as much air as possible. Place bag on left side of baking sheet.

7

Set baking sheet aside in a place where it won’t be disturbed. Make a prediction: At which temperature do you think the yeast will be the MOST active (make the most carbon dioxide gas)? Why do you think so?

8

After 1 to 1½ hours, observe your experiment. Which bag inflated the most? Those yeast were the MOST active. Which bag is the flattest? Those yeast were the LEAST active. Check out “Food for Thought” to learn why it’s important to let yeast doughs rise at just the right temperature.

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Food for Thought

The Results

(Don’t read until you’ve finished the experiment!) When we did this experiment in the Young Chefs’ Club lab, the yeast at room temperature made enough carbon dioxide gas to totally inflate the bag—like a tiny, rectangular balloon! The cold bag was slightly inflated, but the hot bag didn’t change at all. Were your results similar?

Hot
If the water is hot—130 degrees or above—it can kill the yeast, which means they won’t produce any carbon dioxide gas and the dough won’t rise at all. (At the very least, hot water can make the yeast work TOO quickly. That gives breads a sour flavor and makes them rise less.)

Room Temperature
Most bread recipes call for room temperature water and letting the dough rise on the counter for several hours—that’s because yeast is pretty busy at 70 degrees (room temperature). The yeast creates enough carbon dioxide gas to make the dough inflate quickly, usually in just a few hours.

Cold
If the water is cold, the yeast still produces carbon dioxide gas, but at a much slower pace. Bakers sometimes let dough rise in the refrigerator for one to three days. Because the yeast works slowly at cold temperatures, it has more time to create lots of special molecules that add flavor to the dough.