Yeast Chemistry - Part I of III

    Kids, did you know that yeast is a tiny living fungus and that, like all living things, they need to eat? Here you will make your very own bubbly, gooey yeast for baking bread. The biochemical process is called fermentation, which begins as yeast eats the sugars in fruit and grain. This releases enzymes that decompose the food to make alcohol and carbon dioxide gas. Make a chart to keep track of the daily changes in color, smell, and texture (frothy, pasty, gooey). (Note: don¹t use metal bowls or spoons because metal interferes with the chemistry; wooden spoons are okay).

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

    Remove 10 grapes from their stems, wash well to remove pesticides (which kill yeast), and tie them in a double layer of cheesecloth. Or use unwashed organic grapes. Using your hands, fold 4 cups of lukewarm water and 3-3/4 cups unbleached white bread flour in a large glass bowl until it¹s like lumpy papier-mâché. Swish the grape bag through the mixture and push it to the bottom. Either cover the bowl tightly with plastic wrap or transfer the mixture to a plastic container with a tight lid. Store at 70-75oF. Remove the lid every day to record your observations. Fermentation has begun when you see bubbles and smell an odor. On day 4 you should see a yellow liquid. Any purple or brown spots are beneficial bacteria that also add unique flavor. Feed the yeast a cup of bread flour (which supplies sugar) and a cup of water. Mix well. On days 5-9 watch for any green, fuzzy mold. If it forms, remove it and add another cup of flour and water. On day 10, take out the grape bag and divide the mixture into two-cup "starter" portions. These can be given to friends and relatives, keeping one for yourself.

    In a large glass bowl, add one cup of flour and one cup of water to your starter. Mix well. Cover loosely with a towel for about 8 hours. A lot of CO2 gas will be given off here so do not use a tight-fitting lid. Repeat this step and ferment 12-14 hours. Add flour and water in this manner twice a day through day 14. During these feedings the dough should rise as enzymes from the yeast break down the sugars in flour and release CO2 gas that is then trapped in the gooey gluten. In next month¹s column we¹ll bake the bread. If needed you can store your starter in a refrigerator, where it will become dormant.


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    December 2000


    Reference: Nancy Lang, Scientific American Explorations magazine, Fall 2000, p. 14.