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Why Do Apple Slices Turn Brown?
Understand the Core Principles of Oxidation and pH
Recommended Grade Level(s):
Appropriate for: Middle school and High school
Time Requirements:
Activity Time: 1 hour and 15 minutes
Teaching Topics & Concepts:
• Students explore the chemical reaction behind apple browning: Polyphenol Oxidase + O2 =>Melanin.
• Scientific inquiry; properties of matter; chemical reactions; acids and bases; food chemistry.
• Students apply solutions with different acidity levels to apple slices to determine what level of acidity works best
to keep the apple slices from browning.
Background:
Enzymic browning is an oxidation reaction in some foods, mostly fruit and vegetables, causing the food to turn brown. Oxygen
in the air can cause sliced fruit like apples to brown, called enzymic browning (an oxidation reaction). Phenols and the enzyme
phenolase are found in apple cells, and when these are exposed to oxygen, the oxygen causes a reaction. The phenolase changes
the phenols into melanin, which has a brown color. Melanin is also the pigment that gives human hair, skin, and eyes their color.
To stop the oxidative reaction, the phenolase enzymes need to be denatured. Students can conduct this activity to observe how
these enzymes brown fruit. Then they can explore how acids keep the fruit looking fresh!
Materials:
• An apple or pear
• Sharp knife
• Lemon juice
• Small bowl (big enough to fit half the apple or pear)
• Clock/timer
• Paper plates
• Pen and paper
Procedure:
Preparation for Activity:
1. Use your pen and paper to make two labels, one reading "control" and the other reading "lemon."
2. Pour lemon juice, so it completely covers the bottom of the bowl.
3. With adult help or supervision cut the apple in half from top to bottom.
4. Take one apple half and place it cut side down into the bowl of lemon juice. Leave it for two minutes. The other apple half
without lemon juice is the 'control' sample that shows what normally happens to a cut apple.
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Safety
• Wear lab safety gloves, goggles, and an apron or lab coat.
• Get adult help or supervision when using any sharp objects (such as a knife).
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