A color-changing science project from Science Buddies

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!

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

Thomas Frank and E&E News

Chelsea Harvey and E&E News

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