system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does putting lemon/ lime juice on fruit keep it from browning?

I've always wondered why lemon juice slows the browning of fruits (apples, avocados). If this had something do do with the surface being exposed to air, wouldn't any type of liquid/ solid barrier work? 

Thanks!
gpt
Oxygen reacts with a chemical (polyphenol oxidase) in plants, this reaction leaves the fruit brown (kind of like rusting). Lemons and other acidic fruits have ascorbic acid which also reacts with oxygen. The oxygen reacts with the ascorbic acid instead of the polyphenol oxidase until the acid is used up.