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When an apple is sliced, damaged cells expose compounds inside the fruit to oxygen.
An enzyme called polyphenol oxidase helps drive reactions that eventually produce brown-colored compounds.
Acidic liquids such as lemon juice can slow the reaction by creating conditions less favorable for the enzyme.
That's why treating cut fruit can keep it looking fresh longer.
The brown color isn't simply the apple “drying out”—it's active chemistry triggered by cutting it.
© Institute of Food Technologists