Objective: To observe the effect of lime juice on enzymatic browning of apples

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. In this lab you will be observing this effect by applying lime juice to apple slices.

Materials

  1. Apples
  2. Cutting board
  3. Knife
  4. Lime juice
  5. Beaker

Method

  1. Slice an apple in half
  2. Slice each half in about quarter-inch slices
  3. Spread one half on the counter with the surface exposed to the air
  4. Soak the other half in lime juice for 5 minutes
  5. Remove the slices and spread them on the counter as you did with the other half
  6. Observe the color of the slices after 30 minutes
Sliced apples soaked in lime juice (right) and unsoaked (left)

Lab Questions

  1. What are enzymes? (2 points)
  2. Name the enzyme responsible for browning of apples (2 points)
  3. What is the substrate and the final product of enzymatic browning? (2 points)
  4. How did the lime juice affect the color of the apples? (2 points)
  5. Based on what you have now learned about enzymatic browning and its causes, describe another strategy that you could use to prevent enzymatic browning in apples. (10 points)
Courtney Simons
Administrator
Courtney Simons is a food science professor. He holds a BS degree in food science and a PhD in cereal science from North Dakota State University.
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