Have you ever wondered why apple slices turn brown once you cut them or why a yellow banana gets dark spots over time? Both of these phenomena have the same cause: enzymatic browning triggered by an enzyme called polyphenol oxidase (PPO). In this activity, you will find out how this enzyme works by turning a banana from yellow to brown in just a matter of seconds. Then you will explore how you can keep your apple slices looking fresh!
Time: 45 minutes – 1 hour
Key Concepts: Biochemistry, Enzymes, Food
- Banana (yellow with no brown spots)
- Adult helper
- Cutting board
- Lemon Juice
- Distilled vinegar
- An additional one to two bananas (optional)
- Fridge (optional)
- Tape (optional)
- Other fruits and vegetables to test (optional)
- Fill a pot with tap water.
- With the help of an adult, place the pot on the stove and heat the water until boiling. Always use caution and adult help when working around very hot water.
- Take one of your bananas and look closely at its peel to observe its color.
- Carefully dip the bottom third of the banana into the boiling water for 30 seconds. Question: What happens to the banana when you submerge it in hot water?
- After the 30 seconds remove the banana from the boiling water and observe it for another three minutes. Question: What do you notice? Does the banana look different after a while? How?
- When the banana has cooled down peel the banana. Look at the fruit that was inside the peel. Question: Did you expect the banana to look like that?
- With the help of an adult cut two slices from the apple on a cutting board. Place each slice onto its side.
- Poke one of the apple slices with a fork several times. Then observe both slices for 15 to 20 minutes. Question: How do the apple slices change over time? Do you notice a difference between the two slices? If yes, can you explain why?
- Cut five more slices from the apple and place each slice on its side. Immediately after cutting, sprinkle milk on top of the first slice, distilled vinegar on the second slice, lemon juice on the third slice and water on the fourth slice. Keep the last slice as is. Then poke each apple slice several times with a fork.
- Observe all five apple slices for another 15–20 minutes. Question: How are the apple slices different after 15–20 minutes? What did each liquid do to the apple slice? Can you explain your results?
Were you able to change the color of your banana? Most likely, yes! You probably didn’t observe a big difference in the banana right after putting it into the boiled water, but within the next 30 seconds and after taking it out of the water it should have turned pretty dark. You should have noticed that the color change only happened where the banana was submerged in the hot water. This is because the boiling water caused heat stress to the cells in the outer layers of the banana peel and destroyed them. As the cells broke open, they released PPO and phenolic compounds, which then reacted with the oxygen of the air to form melanin. Only the peel should have been affected by enzymatic browning as the inner part of the banana was protected by the peel.
If you put a banana in the fridge, the whole banana should have turned brown. As the banana is a tropical fruit, it is evolved for warm temperatures, which is why the banana cells get damaged in the cold. If you taped parts of the banana, however, you should have noticed that underneath the tape the banana kept its yellow color. This is because the tape sealed the banana from the oxygen, which is necessary for the enzymatic browning reaction to happen.
When you cut an apple its tissue is damaged, and its cells are broken due to mechanical stress. This again triggers enzymatic browning, which you should have observed on the apple slices. When poking the apple slices with a fork, you damaged even more cells and released more enzyme and phenolic compounds, which is why this apple slice should have turned noticeably darker. The PPO content inside a fruit or vegetable determines the degree of its enzymatic browning. This is why some fruits or vegetables, even different types of apples that contain more of these compounds, become darker than others.
When you sprinkled, milk, lemon juice, vinegar, and water over your apple slices you should have noticed that acidic solutions such lemon juice prevented enzymatic browning. This is because PPO oxidase doesn’t work well in acidic environments, which means that the enzyme stops working or slows down considerably. So next time you eat an apple and don’t want it to get brown you know what to do!
Svenja Lohner, PhD, Science Buddies