Hi Avery! Apples brown due to a chemical reaction called oxidation. You may recognize the “oxi-“ part of that word. Oxygen is what causes this reaction to occur, which is why it’s part of the name of the reaction.

There are special chemicals in the apple that are naturally occurring and these react to oxygen when they are exposed to it. When you cut open an apple you allow a lot of oxygen in. The special chemicals are colorless but when there is a lot of oxygen, these chemicals become brown in color.

Like you said putting something like lemon juice on the apple prevents browning from occurring. This is because the lemon juice contains other chemicals that stop the browning reaction from occurring.

When you cut apples and leave them out for a while, one of the enzymes inside the apple is exposed to oxygen and turns the apple brown. The reaction between the apple flesh, enzyme, and oxygen turns the apple brown. One way to stop this reaction from happening is to take away all the oxygen from the environment or provide a barrier so the oxygen can't touch the apple flesh. Putting citrus on the slices is a great way to prevent the oxygen from touching the apple flesh! The citrus juice also is acidic which slows the rate of browning of the apple flesh.