Fruits Gone Bad? Discover Enzymatic Browning

Key Concepts
Biology
Chemistry
Chemical reactions
Enzymes
Food

Introduction
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!

Background
Tons of fruits and vegetables are produced, processed and shipped on a daily basis so that we can buy them fresh. Many of these pieces of produce, however, never make it into stores. This is because some fruits and vegetables such as apricots, mushrooms, lettuce and pears degrade over time because of enzymatic browning. You can see this for yourself if you observe a banana for several days or weeks. Would you buy a brown banana? Enzymatic browning is one of the largest causes of quality loss in fruits and vegetables—even though it does not make the food harmful to eat. So what exactly happens during enzymatic browning?

The process occurs over several steps. The enzyme responsible for the browning is called polyphenol oxidase (or PPO). In the presence of oxygen the PPO enzyme changes substances known as phenolic compounds (through a process of oxidation) into different compounds called quinones. The quinones then react with other compounds to form melanin. Melanin is the same dark brown pigment that colors hair, skin and the irises of our eyes. It also turns fruit and vegetables brown. This reaction, however, usually does not happen within fresh fruits and vegetables because the PPO and the phenolic compounds are separated in produce plant cells.

The enzymatic browning process is only triggered when PPO, phenolic compounds and oxygen come in contact with one another. This is exactly what happens when a fruit is cut, falls or is knocked around too much. When fruit tissue is damaged because of heat, cold, age or mechanical stress, its cells break open and the phenolic compounds and the enzyme are released and mix with oxygen in the air. As a result the damaged tissue turns brown almost immediately. In this activity you will see this browning process for yourself and investigate how this reaction can be avoided. So grab some fruit and get started!

Materials

• Banana (yellow with no brown spots)
• Stove
• Pot
• Water
• Timer
• Adult helper
• Apple
• Cutting board
• Knife
• Lemon juice
• Distilled vinegar
• Milk
• Additional one to two bananas (optional)
• Fridge (optional)
• Tape (optional)
• Other fruits and vegetables to test (optional)

Preparation

• 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.

Procedure

• Take one of your bananas and look closely at its peel. What color is it? Do you see any brown spots?
• Carefully dip the bottom third of the banana into the boiling water for 30 seconds. 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. 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. 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. How do they look?
• Poke one of the apple slices with a fork several times. Then observe both slices for 15 to 20 minutes. 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. What do you think these liquids will do to the apple?
• Observe all five apple slices for another 15 to 20 minutes. How are the apple slices different after 15 to 20 minutes? What did each liquid do to the apple slice? Can you explain your results?
• Extra: Instead of heating the banana, try exposing it to cold. Place one banana in the fridge for several days. Take a second banana and cover parts of the banana peel with duct tape. Get creative with the pattern of the tape! Then put that banana in the fridge as well. Check on both bananas every day. Do both bananas change color? How does the second banana look when you remove the tape after a couple of days? What do you think happened?
• Extra: Beside bananas and apples other fruits can also undergo enzymatic browning. Test different fruits or vegetables to see if they are prone to enzymatic browning. Or try the same fruit but test different kinds of that fruit. How do different kinds of apples compare?

Observations and 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 as vinegar and lemon juice prevented enzymatic browning. This is because PPO 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!

Cleanup
If you would like to and if you used clean implements, you can eat the experimental fruit. You can compost uneaten fruit.

More to Explore
Why Do Apple Slices Turn Brown after Being Cut?, from Scientific American
Why Does Bruised Fruit Turn Brown?, from Scientific American
Enzymatic Browning, from Food-Info, Wageningen University
Prevention of Enzymatic Browning in Fruit and Vegetables, from European Scientific Journal
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies