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Why cut apples turn brown in color?

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I was cutting the apples into pieces when kids entered the science class. Noticing that, they all assumed I was going to give it to them to eat :-) which I didn't. So, one of them asked me " Aunty, you had left the cut apple pieces out. Will they not turn brown in color?". I was glad that they had initiated the discussion for the day. Further, I asked them to think about things that turn brown when left outside. That triggered them to wonder 1. Why cut apple pieces  turn brown in colour? 2. Why copper vessels change color from pink to brown when kept out? 3. Why does Iron rust when exposed to moisture? Let us first check out why do cut apples turn brown in colour? Have you ever bitten into an apple, left it for sometime to find that it had turned brown in colour. When apple is cut or bruised, oxygen is introduced into the cells which then reacts with the enzyme (polyphenol oxidase) in the chloroplast to turn into brown colour. The fru