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Objective
To observe the effect of acid treatment on the enzymatic 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 surface gets a dark color? This color is not very attractive. It gives the appearance of food spoilage. This problem is also obvious in other foods such as sliced potatoes and avocado. So, how could you prevent this problem from occurring? First, you will need to understand the science behind what’s happening.
Fruits, including apples, have enzymes called polyphenol oxidases inside of them. 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 acts on is called a substrate. In the presence of oxygen, polyphenol oxidases react with certain substrates in fruits called polyphenols to produce dark melanin compounds. We call this process enzymatic browning. So, apples will naturally get brown after slicing unless you control at least one of the elements 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 to soak the slices in lime juice, lemon juice, or citric acid solution. This causes the apple to drop in pH, making it more acidic. Polyphenol oxidase, which has an optimum pH close to neutral, does not perform well in acidic conditions. In this lab, you will observe the effect of adding acid to apple slices.
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Materials
- Apple slices
- Lemon or lime juice
- Knife
Method
- Collect one apple, and slice it into 5-6 slices
- Place the slices on a kitchen counter
- Take another apple and slice it as you did before, but this time, immediately soak the slices in lemon or lime juice for 5 minutes.
- Remove the soaked slices and spread them on the kitchen counter
- Observe the color of the slices after 30 minutes and record your observations
Assessment
- Compare the color of the untreated apples and the ones soaked in lemon juice. What is the difference?
- Which pH is the most acidic?
- (a) 6.5
- (b) 12.0
- (c) 3.5
- (d) 7.0
- Identify the main factor responsible for controlling enzymatic browning
- (a) The pH of the apples
- (b) The moisture content of the apples
- (c) The variety of apples
- (d) The size of the apples
- If you do not have lime or lemon juice, which liquid would be a good substitute in this experiment?
- (a) Water
- (b) Sprite
- (c) Baking soda
- (d) Cow’s milk
- Based on what you learned about polyphenol oxidase, which treatment would be a suitable and effective alternative to soaking in acid to prevent enzymatic browning?
- (a) Dying the apples
- (b) Spraying the apples with bleach
- (c) Refrigerating the apples
- (d) Vacuum packing the apples
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