Bring yeast to life, and discover how they ferment.
Genre
Life Science
Nutrition
Chemistry
Required Props
1 package of dry yeast from the supermarket
water
sugar
corn syrup
corn starch
measuring cups and spoons
a candy thermometer
3 six-ounce glasses (they can be plastic)
3 spoons
a large pot or basin
Setting the Scene
Yeasts are one-celled plants that are distant cousins of mushrooms. Like mushrooms and other plants that do not contain chlorophyll, yeasts cannot make their own food. They must get their food from their surroundings. When conditions are not favorable, yeasts become inactive; but they will spring to life when conditions are right. The dried yeast sold in packages in the supermarket is ready to show your students an amazing life process called fermentation.
The products of fermentation, alcohol and carbon dioxide, are of utmost importance to the winemaker and the baker. The winemaker is interested in alcohol production, and the baker is interested in the carbon dioxide, for that is the gas that makes bread rise.
This experiment is designed to show your students what type of food yeasts like most (which foods make the most favorable conditions for yeast growth). The experiment can be done in connection with the next experiment in the Show-Biz Science series, which I call Yeast Busters.
Stage Direction
This activity can be done as a class demonstration; or students can work in small groups. I prefer the small-group approach because the kids like doing this themselves.
Be sure to visit Vicki's Kids' Science Page for more great science fun, a complete list of her books, and information about how you can invite Vicki to come to your school!
PLOT
Act I
Stir the yeast into ½ cup of 110 degree (Fahrenheit) water. (The water is warm, but not hot.)
Divide the yeast mixture evenly into three glasses.
Add 1 tablespoon of sugar to the first glass, 1 tablespoon of corn syrup to the second glass, and 1 tablespoon of cornstarch to the third glass. Stir each glass with a different spoon.
In a large pot or basin, set up a warm-water bath for the yeast. Put enough water in the pot so the water level will come up half-way on the sides of the glasses of yeast mixture. Be careful not to let the water-bath water splash into the glasses. The bath water will cool slowly, but it will remain warm long enough to generate active fermentation.
Act II
Measure the fermentation activity by the size of the bubbles and how rapidly they form. Students can also sniff the mixture to smell the alcohol.
Which glass starts fermenting first?
(the glass with the corn syrup/yeast mixture)
Do all the mixtures ferment? (yes)
Which mixture is the slowest to get going? (the corn starch mixture)
Behind the Scenes
Glucose, a simple sugar, is the principle "food” of yeast use for baking. When yeast comes in contact with glucose, fermentation begins immediately. Glucose is present in corn syrup.
Yeast can also get glucose from table sugar (sucrose) and starch but it takes longer to get going. Sucrose and starch have to be broken down into simpler sugars before fermentation can begin.
The End
The next experiment, Yeast Busters, deals with inhibiting yeast action.
