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You get a beautiful apple, slice it, put it in a plastic bag in your child's lunch. That afternoon your child comes home from school and there in the lunch pail is the uneaten sliced apple, all brown and mushy. When asked when he or she didn't eat it, you child says "It looked brown and gross!"
When you slice an apple the cut cells of the apple are exposed to oxygen in the air. Enzymes, called polyphenol oxidase (PPO), in the apple's chloroplast cells react to this apparent injury by converting some naturally occurring phenolic compounds into another type of compound called o-quinones. Then these colorless o-quinones react with other naturally occurring compounds in the apple to create the brown-colored secondary compounds.
While most plant tissues contain PPO, the level of PPO and the phenolic compounds, varies between varieties of fruits. This is why some varieties like Granny Smith brown less and les quickly than others, like Red Delicious. Also PPO levels vary due to weather, growing conditions and the maturity of the fruit.
There are several ways:
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