Some fruit juices are fermented with the use of yeast.
Many science projects use the mixing of natural compounds to reveal chemical reactions. Students can craft projects that mix food products and then explain what is happening to the chemicals in each food. Popular science experiments deal with food and fermentation. Fermentation is a food-processing method whereby sugars and other carbohydrates are broken down by an organism such as yeast. Students can demonstrate fermentation with the use of yeast and fruits.
Fruit juice and Yeast
A major difference between fruit juices and wine is the fact that yeast breaks down carbohydrates into alcohol. When yeast is added to fruit juices, the process, known as fermentation, occurs, creating wine. Although a middle school student does not have the equipment (or legal) materials to create alcoholic beverages, a student can demonstrate how yeast eats the organic materials found in fruit juice. Typically, the student can display three to four glasses of different fruit juices, and add the same amount of yeast in each glass. Results should typically show a layer of foam-like substances rising to the top of the juice. This is the yeast consuming fruit sugars and causing fermentation.
Organic Decomposition
Students can also show how yeast or similar organic life forms dissolve dead organic matter. Liane D'Allessandro, of the Knight Foundation Summer Institute at Haverford College, used two bananas in a scientific study: one banana without yeast covering it; and another banana with yeast covering it. Both bananas are to be left out and exposed to the environment. According to D'Allessandro, the banana with yeast should decompose much quicker because the yeast feeds off the banana. A student can explain how the yeasts are consuming the organic product (the banana) and how the banana ripens quicker as a result.
Light and Yeast
Similar to the fermentation experiment, the student can focus on how the external environment impacts the consumption rate of yeast within fruit. Taking the similar variables, the student should place yeast within different fruit juices. However, the control will be a fruit juice sample that is not underneath ultraviolet light, while another fruit juice glass is given daily doses of ultraviolet light. The ultraviolet light should reduce the rate that the fermentation occurs within the fruit juice since ultraviolet harms the single cell structure of yeast. Also, ultraviolet light should reduce water levels in the fruit juice, reducing the amount of liquid in each glass. Fermentation will still occur, with signs of a foam-like residue at the top of the liquid forming, but the rate will be much slower.
Pasteurization
One of the best experiments for a student to create is to tell her audience the difference between fermentation and pasteurization. Pasteurization is the process that eliminates harmful pathogens from liquids. Yeast is one organism that is destroyed from the pasteurization process. The student, with adult supervision, can heat the yeast-ridden fruit juice to temperatures of 131 to 158 degrees F. The fruit juice should boil rapidly and then cool. Essentially, all yeast organisms in the fruit juice will be killed, allowing the fruit juice to be drinkable. This is the process of pasteurization: heating the liquid to high temperatures so all fungi (which yeast is) and bacteria from the liquid die. The student can inform the audience how pasteurization changed food processing and what happens to yeast in the experiment.
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