Soda Science

    Kids, here you'll be dabbling in the science of drinkable bubbles by making your very own root beer soda pop. Most sodas use pressurized carbon dioxide for the bubbles, but that would be very difficult to mimic at home. So instead we'll be using yeast to carbonate the brew. Last year we discussed the use of yeast in baking bread (12/00-2/01). The scientific process is called fermentation, where yeast eats sugar and makes carbon dioxide and alcohol by-products. (It's only a little alcohol here: an entire 2-liter bottle of root beer has less of it than is in one really ripe banana!).

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

    What you'll need is a scrupulously clean and dry 2-liter plastic bottle and cap (sterilized is best), active dry yeast packets, tepid tap water, sugar, and root beer or vanilla flavoring. Wash your hands and all utensils very thoroughly. Fill a 2-cup glass measuring cup with tepid (room temperature to warm) tap water. Dissolve one tablespoon of sugar in the water. Add 1/8 teaspoon of yeast to the sugar water. Stir gently and let stand 5 minutes. This is called "proofing", and you are proving that the yeast is still alive. In a 2-quart glass bowl combine 6 cups warm water, 1 cup sugar, and 1 tablespoon root beer flavoring. Stir thoroughly. If the yeast has a thin layer of froth (tiny bubbles) on top, it can be added to the bowl now. (If your yeast had no froth, try slightly cooler or warmer water, or buy a new supply). Stir thoroughly and pour the mixture into the bottle. Add more water until the liquid reaches about one inch below the bottle's neck. Cap, and let the bottle stand at room temperature for 2-4 days. You'll know it's ready when enough CO2 has formed to expand the bottle and make it feel rigid, like it was just bought at the store. Now refrigerate for at least 2 days to stop the growth of the yeast. After this you can drink your very own soda. It is best consumed within one month.

    People used to make soft drinks at home this way all the time using wild yeast, long before commercial brands were available. People still use wild yeast today to make sourdough bread starter. Root beer extract can be found now in large grocery stores near vanilla and other extracts in the spice aisle. In later batches, try to change the taste a little by changing from sugar to other sweeteners like brown sugar, honey, or molasses. Enjoy!


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    March 2002


    Reference:   Beth Robelia, "Root Beer Chemistry" in Scientific American Explorations magazine, Winter 2002, page 12.