Objectives: To determine the best treatment to prevent browning of apple slices

Background: Have you ever noticed that when you slice or bite an apple and then leave it sitting out for a while, the inside surface gets a dark color? This color is not very attractive. It gives the appearance of poor quality. This problem is also obvious in other foods such as sliced potatoes and avocado. So how could you prevent this problem from occurring? Well, first you will need to understand the science behind what’s going on. Fruit, including apples have enzymes inside of them called polyphenoloxidases. Enzymes are proteins that speed up the rate of biochemical reactions without being changed themselves. Enzymes control millions of chemical reactions in plants and animals, allowing them to survive and grow. The substance that an enzyme breaks down is called a substrate. The substrate that polyphenoloxidases break down are natural compounds called polyphenols. When polyphenoloxidase react with polyphenols in the presence of oxygen, a dark compound called melanin is produced. Have you ever heard that word before? It’s the same compound that determines our skin color. This browning of apples in the presence of an enzyme and oxygen is called enzymatic browning.

Therefore, apples will naturally get brown after slicing, unless you control at least one of the elements that are involved in the reaction. For example, you can prevent access to oxygen, or you can inactivate or slow down the enzyme. One way to slow down enzymatic activity is by soaking the slices in lime juice, lemon juice, or citric acid solution. This causes a drop in the pH of the apple, making it more acidic. Polyphenoloxidase, which has an optimum pH of close to neutral (6.5), does not perform well in acidic conditions. Other treatment conditions such as soaking in water, salt, sugar or baking powder may have an effect on enzymatic browning. In this lab you will observe and determine what those effects are.

Materials

  1. Water
  2. Sugar
  3. Lemon (or about two table spoons of lemon or lime juice)
  4. Salt
  5. Baking powder
  6. Knife
  7. Cutting board
  8. Table spoon (measuring spoon)
  9. ½ t-spoon (measuring spoon)
  10. Measuring cup
  11. Drinking cups
  12. Stirrer (e.g. a spoon)
  13. Two apples

Method

  1. Collect 5 drinking cups
  2. Measure and add 1 cup of water to each
  3. Label cups: water, salt, lemon juice, sugar, baking powder
  4. Measure, add and stir into to the corresponding cups:
    1. 1 table spoon salt
    1. 3 table spoons sugar
    1. ½ t-spoon baking powder
    1. juice from one squeezed lemon
  • Slice up two large apples and add three slices in each cup for 10 minutes. Set three slices aside for exposure to the air i.e. no liquid treatment
  • Remove slices and place them in front of their corresponding cups
  • Leave for 30 minutes
  • After the 30 minutes, observe and score color of slices  

Lab questions

  1. What caused the apples to get brown after slicing? Explain the science behind this.
  2. Rate all six sets of apple slices on a 1 to 5 scale from lightest (1) to darkest (5)
  3. Explain why the color shades were in this order.
Courtney Simons
Administrator
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
Courtney Simons on FacebookCourtney Simons on Linkedin