Elementary students can test yeast properties by observing the effects of yeast on decomposing bananas. In this experiment, students compare a banana without yeast on it to a banana with yeast on it. Have your student cut two lengthwise slices of equal size from a banana and place each slice in a separate plastic bag. Place about a half-teaspoon of yeast on top of one banana slice. Seal and label the bags. Have your student record an observation each day for the next several days, describing what the contents of each bag look like.
Elementary students can test whether yeast grains have one of the characteristics of living things -- the ability to grow. Obtain a Petri dish for your student with yeast growth medium in it (see Resources). Spread 10 to 12 grains of yeast across the dish and add several drops of water. Have your student inspect the dish each day and record any signs of growth in the dish. Have him sketch what he sees.
Elementary students can test whether yeast have another characteristic of living things -- the ability to use energy, or metabolize. Specifically, they can test whether yeast produces carbon dioxide gas when sugar is available and when no sugar is available. Set up four test tubes with equal amounts of yeast dissolved in warm water. Add equal amounts of sugar to two test tubes and dissolve it in the water. Cover the test tube openings with balloons to catch any gas formed. Have your student observe the test tubes and record her observations every five minutes for 30 minutes. Older students can do a more advanced metabolism experiment by measuring the carbon dioxide output of yeast under aerobic and anaerobic conditions. The hypothesis is that yeast extracts more energy from sugar when oxygen is present, so your student should add sugar to all test tubes and aerate the dissolved yeast solution in two tubes prior to and during the experiment.
Middle school students can conduct various experiments to explore fermentation, for example, fermenting different fruit juices, using different types of sugars in fermenting, or the effects of different kinds of light on fermentation. An experiment to study the effects of ultraviolet light on yeast fermentation could include constants such as the amount and type of yeast, the quantity and type of juice, and the temperature. Have your student subject two test tubes of yeast to UV light during fermentation while leaving two test tubes unexposed. Measure and record the amount of water displaced by the CO2 to see if there is a difference between the test tubes.