This experiment was created by America’s Test Kitchen Kids. Have you ever looked closely at a slice of bread and seen lots of little holes in it? Those are the handiwork of microscopic (really tiny) yeast! In this experiment, you’ll discover what temperature yeast likes best.

Safety Considerations: Uses the microwave

Difficulty: Beginner

Time: 30 minutes, plus 1 to 1½ hours rising time

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 (Don’t substitute active dry yeast — it won’t work in this experiment!)
• 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

1. 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. 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.

(Don’t read until you’ve finished the experiment!)

Did your yeast at room temperature make enough carbon dioxide gas to totally inflate the bag — like a tiny, rectangular balloon? Was the cold bag slightly inflated, while the hot bag didn’t change at all? What made the yeast produce different amounts of carbon dioxide at different temperatures?

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.

Put your new knowledge about yeast to work by making these Pretzel Buns!