Why Do Apples Turn Brown?

Have you ever bitten into an apple, set it down for a few moments, and found that it has turned brown? This is caused by an enzyme (polyphenol oxidase and catechol oxidase are two common examples) that reacts with oxygen in the air and an iron or copper cofactor in the fruit. A cofactor is a component that is necessary for a certain enzymatic reactions to happen. The fruit starts to oxidize, when electrons are lost to another molecule (in this case the air), and the food turns brown. In other words, it’s like edible rust on your food!

Oxidation can be prevented or slowed down by not allowing oxygen to get to the surface of the fruit. To accomplish this, you can cook the food, which destroys the enzyme. It is also possible to prevent browning without cooking by covering the fruit (preventing air from reaching the fruit), or by lowering the pH on the surface, making it more acidic.

Problem:
Which liquid do you think will prevent the food from turning brown the best? Why?

Materials
Baking sheet or wax paper
Labeling tape
Pen or marker
Tongs
Bowl
Knife
Lemon juice
Vinegar
Clear soda
Olive oil
Water
Saltwater
Potato
Apple
Banana
Avocado
Any other liquid you want to test
Any other fruit or vegetable you want to test
Camera (optional)

Procedure
Use the tape to create labels for each type of liquid you will test.
Place your labels on the baking sheet or wax paper. This is where you will set down your samples for observation.
Cut each fruit or vegetable into slices at least 1-cm thick. Make sure you have as many slices of each food item as you do liquids to test, plus one morefor a control. Have an adult help you cut your samples! Why should the food samples be cut?
Set out a slice of each food item on the baking sheet or was paper under the heading “Control.” Why is it important to have a control?
Fill the bowl with enough liquid to fully submerge each sample.
For each fruit or vegetable you are testing, dip a slice into the liquid with tongs. Be sure to cover the whole slice! Let the extra liquid drip off before placing it under the correct label on the baking sheet or wax paper.
Rinse out the bowl and repeat until you have made samples with each liquid.
Record all your observations, taking note of the time. You can also take pictures to document how the food turns brown over several hours.
Results
Lemon juice, Vinegar, clear soda will all prevent food from turning brown quickly. These liquids are acidic, so they will lower the pH of the food surface. Olive oil, will also prevent food from browning, but is less effective than the acids. Water and salt water will also slow the browning of foods.

Lemon juice is the most effective, and by chance, also the most delicious!

Why?
Having a control group is necessary because it is important to know how long the food samples will take to turn brown without any liquid added.

The food’s skin protects the inside “meat” of the fruit or vegetable from damage and debris. When a fruit or vegetable is dropped and the skin is poked or broken, the food often goes bad faster. The reason fruits and some vegetables go brown when they are cut is because the part containing the oxygen-reactive enzyme is exposed. There is then a lot of surface area for the air to come in contact with the food. For the most part, brown fruits and vegetables still taste fine, they just do not look very appetizing.

Acids prevent browning because they react with the oxygen that comes into contact with the surface of the sample. Once all the acid (or whatever else is covering the surface) has reacted with the oxygen or the acid hasdegraded or washed off, then the sample will start to brown again. Stronger acids, like lemon juice, can even denature the enzyme. This means that the enzyme can no longer perform its original function because of its environment.

Digging Deeper
Why doesn’t dried fruit turn brown? What about the fruit drying process prevents enzymatic reaction with oxygen?