Fall or autumn is full of apples – an apple for the teacher, apple picking, apple cider, etc.
Have you ever wondered why apples turn brown when you cut them up or take a bite and then let them sit for a while? Do all apples do this? Is there any way to stop this from happening? Let’s find out!
Oxidation is the reason the inside of the apple turns brown when exposed to air. Enzymes in the apple react with oxygen in the air and iron in the apple to form iron oxides – kind of like rust on metal. Do you think all apples react in the same way? Can you think of how this reaction could be stopped or slowed?
First, if you have more than one type of apple, you can compare how quickly each type turns brown and how brown each variety gets. Make some predictions, then have an adult cut a slice of each type. Leave the apple slices sitting out. Observe them from time to time and record your findings.
Second, choose one type of apple and have an adult cut several slices. Place each of the apple slices but one (referred to as the “control”) in a different substance and see how long it takes to turn brown and how dark it gets. Do you have any predictions for which substance will work the best to keep the apple from turning brown? Observe the slices from time to time and record your findings. Comparing each slice to the control slice, do you think any of the substances slowed or reduced the oxidation process? Which worked best? Which apple slices from the experiment do you think would taste the best? Can you think of any other ways you might be able to keep apple slices from turning brown?
Two ways to slow the oxidation of the apples are to reduce the exposure to oxygen and to reduce the pH of the fruit. Covering the fruit with an acidic liquid combines both of these approaches. The pH scale measures how acidic a substance is – the more acidic the substance is, the lower its pH is. For instance, lemon juice is acidic and has a lower pH than water, which is a neutral substance. Reducing the pH of the apple works because the enzymes that react with the iron in the apple and oxygen in the air are pH dependent – they won’t work as well or as quickly if the pH level is low. Some people prevent sliced apples from turning brown by slicing the apple, then holding it together with a rubber band. If you want to keep the experiment going, you could try this method, and compare it to the best results you got earlier!