Activating Yeast
A Hands On Activity

Yeasts are unicellular organisms. They reproduce by budding, and can be activated in the classroom.

Materials
Zip-lock plastic sandwich bags  Sugar
Packages of Yeast Warm water
Heating pad or hot water bottle Towel

Procedure:

Each group receives a zip-lock bag.
Pour about a third of a cup of warm water into the bag.
Add three or four teaspoons of sugar. Seal the zip-lock bag.
Shake sugar and water together until the sugar is dissolved.
Open the bag and add about a half a teaspoon of yeast.
Push as much of the air as possible out of the bag. Seal the bag again.
Set the bag aside and let the yeast go to work.

Notes:
Yeast grows quickly in water that feels a little warm. If the water is colder, the yeast will grow more slowly. If the water is too hot, it will kill the yeast.

The flat shape of the plastic bag has a lot of area in proportion to its volume. It will come to room temperature quickly. If you want the yeast to work more quickly, you may want to wrap it in a towel and put it on top of a heating pad or hot water bottle. Be careful not to get the yeast too hot, as too much heat will kill it.

If the room is cool and you have no way to keep the yeast culture warm, wait until next morning to see the bags puff up!

Results:

As the yeast works, it will break down the sugar. About 45% of the sugar is broken down into carbon dioxide. The rest is turned into alcohol. The yeast cells will grow and bud, producing daughter cells.
Yeast will stop growing when it has used up the sugar or when the alcohol level reaches about 14 % of the liquid.

Follow-up Activities:

Now that you have a yeast culture started, you can stir in some flour and see how it makes bubbles in the dough. You could also pour the yeast starter into a plastic bottle, add some more water and sugar, and seal the bottle by putting the mouth of a balloon over the mouth of the bottle. As the yeast works, it will blow up the balloon. When fermentation stops, the balloon will gradually shrink again.



© Elizabeth Anne Viau, 1999. This material may be used freely for instructional purposes but not sold for a price beyond the cost of reproduction. Please inform the author if you use it at eviau@earthlink.net.

Return to .pdf Download Page