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Fermentation as a Function of Sugar Content in Fruit Juice

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Author: Cy Ashley Webb is a science writer. In addition to having worked as a bench scientist and patent agent, she judges science fairs in the San Francisco bay area. She loves working with kids and inspiring them to explore the world through science.

Grade Level: 6th - 8th; Type: Biology

Objective:

Students will learn about how sugar content of juices powers the fermentation process.

Research Questions:

  • What causes fermentation?
  • What happens chemically during fermentation?
  • What is the relationship between sugar content and fermentation?

Fermentation involves the conversion of sugar to alcohol. Vintners rely on this process to make wine. Unfortunately, not all fermentation is as productive as winemaking, as much produce is lost this way. Students may be familiar with stored apples fermenting or juices turning bad in the fridge. This too, is evidence of fermentation.

In this experiment, we encourage fruit juice to ferment by adding yeast and leaving the mixture at room temperature. While students cannot quantitatively measure the amount of fermentation that takes place because this experiment does not lend itself to measuring the amount of CO2 produced in the fermentation process, using a Brix meter does allows students to measure how much sugar has been converted. Farmers customarily use Brix meters to measure the amount of sugar in produce. Knowing the optimal sugar content in produce allows farms to harvest crops at the optimal time. Students use a Brix meter to measure how much sugar converted to other products and compare this to their visual estimate as to which samples show the most evidence of fermentation.

Materials:

  • Grape Juice
  • Apple Juice
  • Orange Juice
  • Brix meter
  • Nine clean baby food jars (other small jars will work as well)
  • Small four-ounce jar of baker’s yeast
  • Camera
  • Graph paper

Experimental Procedure:

  1. Label three baby food jars OJ1, OJ2, and OJ3. Repeat this labeling the other jars AJ1, AJ2, AJ3, GJ1, GJ2, and GJ3. OJ stands for orange juice, AJ stands for apple juice, and GJ stands for grape juice. OJ1, AJ1 and GJ1 are your controls.
  2. Fill each jar with 120 ml (0.5 cup) of the corresponding juice.
  3. Read the directions on the Brix meter or get someone to show you how to use it accurately. Use the Brix meter to measure sugar content. Record your results.
  4. Add 0.5 tsp yeast to OJ2, OJ3, AJ2, AJ3, GJ2 and GJ3 and set all the jars aside at room temperature for 72 hours.
  5. After 72 hours, measure the sugar content with a Brix meter and record your results.
  6. Visually inspect all the juice containers. How did they compare to the controls? Which appear to ferment the most? The least? Take pictures of your fermented samples.
  7. Subtract the value you obtained in step 5 from the value you obtained in step 3. Chart this. Which juice lost the most sugar? How did this compare with your visual observations?

Students can repeat the experiment using samples that have been stored in the refrigerator to explore the role of temperature on fermentation.

Terms/Concepts: Fermentation; CO2 production; Relationship between sugar content and fermentation; Using a Brix meter

References:

Websites

Cellular Respiration and Fermentation

http://biology.clc.uc.edu/Courses/bio104/cellresp.htm

Encyclopedia Britannica: Fermentation

http://www.britannica.com/EBchecked/topic/204709/fermentation

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