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