What's That in My Root Beer?

The role of microoganisms in the production of foods was not understood until the middle of the 19th century, although their activity had been utilized in food and beverage production from ancient times. A small list of foods dependent on microorganisms for natural processing include: bread, root beer, fermented sausage, cabbage, olives, cucumbers, cheese, yogurt, tempeh, sour cream, chocolate, wine, sauerkraut, and coffee. The organisms that perform these complex chemical conversions cannot be seen without a microscope, but their activity can be safely observed outside of the laboratory.

Explore the microbial world by examining different microorganisms involved in the processing of food.

1. Fill two clean 330 milliliter soda bottles half full with milk that has been heated to a light boil.

2. After it cools for one-half hour, add a teaspoon of a commercial yogurt (one with active yogurt culture on its ingredient list) to one bottle. Add one-eighth of a teaspoon of commercial baker's yeast to the second bottle.

3. With a rubber band, firmly attach an uninflated balloon around the mouth of each bottle.

4. Leave the bottles at room temperature for 12-24 hours.

Carbon dioxide gives root beer its fizzle.

Which bottle's balloon filedl with gas? The second bottle with the baker's yeast should have filled up the balloon.Why? Because yeast cells produce the gas CO2 as a byproduct, the same compound that causes bread to rise (C02 is also the gas found in root beer!).

The balloon on the first bottle, the one with the yogurt mixture, should not have inflated. The microorganisms in yogurt don't produce CO2 but they do produce lactic acid, the same compound found in sour cream, buttermilk, and some cheese. If pH paper is available, the acidity of each mixture can also be determined. Which fermentation is the most acidic? Baker's yeast produces a small quantity of acid, while the bacteria in yogurt produce a significant amount of lactic acid.

At the end of the experiment, safely wash all ingredients down the sink.

This activity was adapted from "Classroom Lessons on the Frontier of Microbiology" produced by the American Society for Microbiology. Please visit their homepage at:


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