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    Experiments with Yeast ­ Part III of III

    Kids, did you make your own bread from yeast according to the last few columns? We hope you did, but if not you can still do quite a few experiments with store bought yeast. The first experiment here tests how sugar effects the growth of yeast. Fill two 1-cup glass measuring cups with 1/2 cup warm water. In one cup, add 1 tsp sugar. Put 1/4 ounce package of active dry yeast in each cup, stir, and wait 10 minutes. Which cup has more yeast foam and why? Is sugar necessary for the growth of yeast and why?

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    Is yeast alive? Make a yeast solution using 1/2 cup warm water, 1 tsp sugar, and 1/4 ounce package active dry yeast. Each day, transfer 1 tsp of original yeast solution to a solution of 1/2 cup warm water and 1 tsp sugar. Make another sugar solution and add 1 tsp water daily. Keep a record of observations for five days. Does the yeast culture continue to multiply even though it is diluted by the daily transfer?

    When flour, sugar, water, and yeast are mixed, what happens? Get two empty 1-liter soda bottles and two balloons. Fill each soda bottle with a 1/4 ounce package active dry yeast, 1 tsp sugar, and 1 cup room temperature water. In one bottle, add 2 Tbsp all-purpose flour. Secure a balloon on top of each soda bottle. Record and time what happens to the balloons. What is the difference between them? Does flour make a difference in the length of time the fermenation works and why?

    What effect does temperature have on the fermentation of yeast? Again get two empty 1-liter soda bottles and two balloons. Fill each bottle with a 1/4 ounce package active dry yeast, 1 tsp sugar, 2 Tbsp all-purpose flour, and 1 cup room temperature water. Set one bottle in a vessel with warm water. Set the other bottle in a vessel with ice water. Secure a balloon on top of each soda bottle. Observe and record results. What effect does temperature have on the fermentation of yeast? When was the difference most noticeable? Read over the last two columns for interesting facts concerning yeast and they will also help you answer the questions asked here.

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    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    kcarrado@anl.gov
    February 2001

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    Reference: www.redstaryeast.com ("The Science of Yeast" webpage). For microscopic photos of budding yeast cells check out: http://goodquestion.net/beer/info/yeast.html or www.bath.ac.uk/Departments/BiolBioch/wheals2.html