Why do apples turn brown?
See it happen, first, in the experiment below.
Then, find out how chemical reactions work to keep your apples (or pears) fresh even after they’ve been cut!
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, 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 your ‘control’ sample that lets you see what normally happens to a cut apple.
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?
When an apple is cut open, an enzyme (proteins that cause chemical reactions) called polyphenol oxidase is released from the cells of the apple and reacts with the oxygen in the air. This reaction causes the fruit to turn brown, similar to rust forming on metal.
Almost all plants contain polyphenol oxidase. It’s believed plants use it for defense. When a plant is damaged, the browning of the affected area may discourage animals and insects from eating it. It may also work to heal the plant. The browning creates an antibacterial effect, limiting the spread of destructive germs.
Lemon juice, however, acts a preservative. Why? Because it’s 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. Even when the ascorbic acid is used, the pH of the lemon juice prevents browning. That’s because polyphenol oxidase works best in the 5 to 7 range on the pH scale. Lemon juice’s pH level is about 2, which keeps the polyphenol oxidase inactive.
In addition to lemon juice, lime juice and cranberry juice also have a pH below 3.0. Concord grape juice and grapefruit juice also have a low pH (not quite as low as the others), but will help delay the browning process. You may want to try several of these juices and find a tasty to way to serve sliced apples and pears in the process!
To learn more about acids, bases, and pH, check out this science lesson.