Activity – Slowing Apple Browning

By David A. Katz and Veronica I. Jaramillo

Safety Suggestions

• Ask an adult for permission to do the activity and for help when necessary.
• Read all directions and safety recommendations before starting the activity.
• Wear appropriate personal protective equipment (safety glasses, at a minimum), including during preparation and clean up.
• Tie back long hair and secure loose clothing, such as long sleeves and drawstrings.
• Do not eat or drink food when conducting this activity.
• Clean up and dispose of materials properly when you are finished with the activity.
• Thoroughly wash hands after conducting the activity.
• Do not eat or drink any of the materials used in this activity.

Disposal: All unused solutions can be poured down the drain with running water. All the apple pieces can be disposed of in the trash.

Introduction

Have you ever thrown away apple or avocado slices because they started turning brown? Many people find fruit slices with brown spots to be less appetizing. The browning can change the taste and texture of fruit, as well as how it looks. This browning is due to a chemical reaction with oxygen in the air, and it may be responsible for up to half of all food waste. If there were only a way to slow this chemical reaction down, it would prevent a lot of waste … and save a lot of money!

In this activity, you will test different ways of slowing down browning in fruit through chemistry. You’ll add different substances to apple slices to discover the most effective way to prevent the slices from turning brown.

Materials

• Apples
• Cutting board
• Plastic knife
• ¼-cup (about 60 mL) measuring cup
• 6 paper plates
• 5 plastic cups
• 5 spoons
• Lemon juice
• Vinegar
• Salt
• Sugar
• Water

Procedures

1. Label 5 plastic cups with these titles: Lemon Juice, Vinegar, Water, Salt Solution, and Sugar Solution.
2. Label 6 paper plates with these titles: Lemon Juice, Vinegar, Water, Salt Solution, Sugar Solution, and No Liquid.
3. Add ¼ cup (60 mL) water to each of the 3 cups that are labeled Water, Salt Solution, and Sugar Solution.
4. Make the solutions by adding 1 teaspoon (5 mL) of salt and 1 teaspoon of sugar to each labeled cup and stirring with a spoon to dissolve.
5. Pour ¼ cup lemon juice into the cup labeled Lemon Juice.
6. Pour ¼ cup vinegar into the cup labeled Vinegar.
7. With the help of an adult, cut the apple into at least 6 uniform slices. Lay one slice on its side on each of the six labeled plates. Use a spoon to dip each of the apple slices into the matching labeled cup for 30 seconds, remove the slice, and then place it on its labeled plate. The apple slice on the plate labeled No Liquid, should not be dipped in any liquid.
8. Check the apple slices at 10-minute intervals for up to one hour. Record your observations in the data table.

What did you observe?

Describe what the apple slices look like. Possible results could include no browning, slight browning, brown patches, halfway light brown, completely light brown, and completely dark brown.

How does it work?Look at your data, and compare the browning of your different apple samples to the control apple, which was the slice not dipped in any solution. Which apple browned the least? Which solution was best at slowing down the browning reaction?

An enzyme in apples speeds up the chemical reaction between the fruit tissue and oxygen in the air. When fruits or vegetables are peeled or cut, the enzymes in the plant tissue are exposed to the air. The oxygen in the air speeds up the conversion of some of the nutrients in the apple to a brown product. This browning can be prevented by making the enzyme stop working or slow down.

In your experiment, some of the treatments worked better than others to prevent browning. If they worked, it was because they interfered with the enzyme. Changes in acidity, saltiness, and oxygen content can all affect the enzyme action. Lemon juice and vinegar are quite acidic, and work by altering the pH of the solution. Salt can break up protein structure, which is what enzymes are made of. Sugar solutions can coat the surface of fruits, and prevent oxygen from getting to the surface. Review your results and decide which solution did the best job of slowing down the browning action of the fruit.

Just because something works to prevent browning doesn’t mean you want to use it on fruit you plan to eat. Which of the solutions would have the smallest effect on the flavor of your apple slices?

David A. Katz, Ph.D. is a Consultant of Chemistry Education in Wilmington, DE.
Veronica I. Jaramillo, Ph.D. is the Department Chair of Physical Science at Pasadena City College in Pasadena, CA.