How Does Acid Keep Apples Fresh?

Apples and pears are great for snacks or to have as a side dish for dinner. But keeping them looking white and delicious after they have been sliced can be tricky. Try this experiment to see how chemistry can keep your apples and pears fresh even after they have been sliced.

Materials:

• An apple or pear
• Sharp knife
• Lemon juice
• Clock
• Adult help

What To Do:

1. With adult help, cut the apple in half from top to bottom.
2. On one apple half, lightly coat the white part of the apple with lemon juice. Leave the other half uncoated. (The uncoated half is your "control" sample and lets you see what normally happens to a cut open apple.)
3. Observe the color of both apple halves, then place them white-part-up on a counter or tabletop.
4. Observe the apples again after 30 minutes. Notice any color changes and/or differences in appearance.
5. Look at the apples again periodically throughout the day. What do you find?

What's Happening?

When an apple is cut open, an enzyme 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, and it is believed plants use this enzyme as part of a defense mechanism. When a plant is damaged, the browning of the affected area is thought to discourage animals and insects from eating the plant any further. It also might help the plant heal because the browning creates an antibacterial effect, preventing germs from destroying the plant even more.

Lemon juice helps keep the apple from browning, because it is full of ascorbic acid (Vitamin C) and it has a low (acidic) pH level. Ascorbic acid works because oxygen will react with it before it will react with the polyphenol oxidase. However, once the ascorbic acid gets used up, the oxygen will start reacting with the enzyme and browning will occur. Lemon juice's low pH level also helps prevent browning. Polyphenol oxidase works best when the pH level is between 5.0 and 7.0. However, below a pH level of 3.0, the enzyme becomes inactivated. The pH of lemon juice is in the 2.0 range, making it very effective against browning.

Besides 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!

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