system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do apples turn brown/red after cutting and leaving them in room temperature for a bit?
gpt
When an apple is cut open, an enzyme called polyphenol oxidase is released from the cells of the apple and reacts with the oxygen in the air. This causes it to turn brown. It's similar to rust forming on metal. But, if you put lemon juice on the slices, it helps slow down the browning. Lime juice works too. Both are high in vitamin c (ascorbic acid) and have a relatively low pH (acidic). Oxygen will work on the ascorbic acid in the juice before it works on the polyphenol oxidase in the apple, slowing down the browning of your apple slices.