The Science of Yeast

by Alice Knisley Matthias

Tischenko Irina/Shutterstock

Ask your students what they know about bread and how it is made. Can they name the ingredients in a recipe for bread or rolls? If someone mentions “yeast” as an ingredient, you can guide the conversation from there. If not, introduce the word and a definition.

Yeast are living, single-celled organisms and are found everywhere in the world around us. Varieties of yeast are found on our skin, in the air, and on the skins of fruits and vegetables. Read here to learn about how the same grapes from different parts of a vineyard can change the taste of the resulting wine due to the yeast found on the grape skins:

In bakeries and home kitchens, there are several categories of commercial baking yeast available. They are compressed cake yeast, (also known as fresh yeast), active dry yeast, quick-rise yeast, and instant yeast.

The scientific name for the active dry yeast, that commercial and home bakers use, is called Saccharomyces Cerevisiae, otherwise known as “sugar-eating fungus.” This is a strong variety of yeast and it is responsible for fermentation. Fermentation is the step when the yeast makes carbon dioxide in the bread making process.

For this lesson, tell your students they will focus on the active dry yeast which is available in commercial markets. Two popular companies, who have been producing baker’s yeast for years, are Fleischmann’s and Red Star.

Any bread recipe needs a “leavener” to help the ingredients work together. You can easily demonstrate how yeast works as a leavener with a few ingredients. (Don’t worry; you don’t need the baking skills of a famous baker to guide your students in this demonstration.)

Mix some flour, water, and salt and you have a wet flour mixture for your students to observe. Nothing much there. Now, explain that in order for a bread product to be made, these ingredients need a “leavener.” A leavener can be baking soda, baking powder or yeast. A leavener is what the dough needs to begin to rise. According to Webster’s New World Dictionary, the word “leaven” comes from the Latin word “levare,” a word that means “lift up, to make light and rise.”

Yeast gets energy from its favorite source of food in the form of sugar. This can be pure sugar, honey, molasses, maple syrup, or fruit juices containing natural sugars.

The yeast love to be able to eat their favorite sugar in a warm, moist environment where they will multiply in numbers. As the yeast munch away on their sugar, a process called anaerobic fermentation begins to take place. The by-products of this process are alcohol and carbon dioxide. During this time, the carbon dioxide is trapped by a series of strands of gluten in the rising bread. This is what causes the bread dough to rise, or expand on the surface, leaving behind a series of air pockets in the dough. The yeast eventually dies off, from the heat when baking, and any remaining alcohol evaporates. The air pockets left behind are what give baked bread its crunchy goodness. Tell your students to rip apart their bread now, and identify the air pockets that formed when the bread was baked.

Active dry yeast comes in packets, and the dry granules are the size and color of sand. Yeast packets will be marked with an expiration date, but it has a decent shelf life in a kitchen cabinet, making it a favorite product for home bakers.

Tell your students the science they will take part in is a process called “proofing” the yeast. This means they will “feed” the yeast its sugar source and watch the yeast start to go through the fermentation process in a glass. The following activity calls for a few items including some packets of yeast. Share with your students that this is a test many bakers use to make sure the yeast is still active before proceeding with a recipe.

Ask your students to observe the scent of the yeast, as it mixes with the warm water and sugar in a glass. It already smells like the aroma of bread being made!


Some flour, water and salt for demonstration

A loaf of crusty bread for observation

Depending on class size, you can work in groups with 4-5 students to a cup/bottle with materials

Each group should have:

Packet of yeast

Measuring glass (or any clear glass to see results)

Two tablespoons sugar (not substitute)


1/4 cup warm water for each measuring glass (between 105-115 degrees)



Same as above

Clear bottle



-Pour water in glass.

-Add packet of yeast.

-Add sugar and gently stir.

Observe as the yeast mixture will start to double in size as it begins to “proof.”


-Mix the same yeast mixture and pour into a bottle.

-Fit a balloon on the opening of the bottle and set aside.

As the yeast starts to expand it will cause the balloon to start to “rise.”


Alice Knisley Matthias writes about food, garden, family and education. Her work appears in The New York Times, Allrecipes, Taste of Home, Food Network, Washington Post, Eating Well, The Kitchn, Delish, Birds and Blooms, Woman's Day, Redbook, Good Housekeeping, America's Test Kitchen, Boys' Life and Parade. Her book, "Tasty Snacks in a Snap!" is published by Scholastic for young readers. You can read her work at Home / Alice Knisley Matthias ( and follow her at Alice Knisley Matthias (@aliceknisleymatthias) • Instagram photos and videos and @AKnisleyMatth ( / Twitter (