Sliced apples and pears make a healthful and tasty snack or 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 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 cutside 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 whitepartup 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 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!
To learn more about acids, bases, and pH, check out our science lesson.