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Procedure (continued):
5. Observe the color of both apple halves, then place them white part up on the plates, with the corresponding labels nearby.
6. Observe the apples again at 10- minute increments, up to 30 minutes. Note any color changes and/or differences
in appearance.
7. Look at the apples again periodically throughout the day. What do you find?
Expected Results:
When an apple is cut open, the polyphenol oxidase enzyme is released from the apple cells and reacts with the oxygen
in the air. This reaction causes the fruit to turn brown, like rust forming on metal.
However, lemon juice acts as a preservative (a substance that prevents decomposition by microbial growth or by chemical
changes). The lemon juice is full of ascorbic acid (an isolate of Vitamin C) and has a low (acidic) pH level.
Ascorbic acid reacts with oxygen before it reacts with polyphenol oxidase. Ascorbic acid is like the zinc coating on stainless steel,
which prevents the steel from rusting. Additionally, lemon juice has a very low pH of about 2 (that means it is very acidic). Most
enzymes, including Polyphenol oxidase, only work well within a specific range of pH. For polyphenol oxidase, this range is about
5–7. Therefore, by lowering the pH at the surface where polyphenol oxidase is being released, the lemon juice helps slow browning.
Follow up:
• Repeat the experiment except try dipping slices of apple in a variety of substances. These include water, milk, orange juice,
lemon-lime soda, cold water, or commercial products designed to preserve fruit (like Fruit Fresh).
– Observe how the different substances impact oxidative browning.
• Try different types of fruits and vegetables like bananas, avocados, potatoes, or peaches.
• Explore other methods designed to disrupt the browning process like deactivating the enzymes with heat or sealing out
oxygen with plastic containers.
• Explore how refrigeration interferes with the growth of microorganisms that harm food
Teaching notes:
This lesson introduces acids, bases, and the pH scale as a measure of acidity. You can extend this lesson to introduce proteins,
amino acids, and enzymes.
Questions and observations for students:
Before beginning, ask students what they think will happen? Which mixture will prevent browning the most? Have students write
down their predictions.
• What happens when you cut into a piece of fruit and leave it on the counter?
• What did you notice happening in this experiment?
• Why do you think this happened?
• How did the apple slices change in their appearance?
• Which group of fruits or vegetables had the least amount of browning?
• Which had the most?
• Ask students to identify the ingredient that prevented the apples from turning brown the most.
Disposal/Clean-Up:
Discard leftover paper products in the trash. You can compost leftover fruit.
Why Do Apple Slices Turn Brown?
Understand the Core Principles of Oxidation and pH (continued)
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