英語時間｜Why do cut apples turn brown?【611期】

更新時間：00:00 2022-02-17
發佈時間：00:00 2022-02-17

You have probably seen it yourself – after you cut an apple open and leave it alone for a few minutes the apple flesh will turn brown, making it quite ugly to look at. Why?

Many adults and even‘science’websites would claim it is because apples contain a lot of iron. But this is not the truth!

In fact, apples are not rich in iron – they contain some, but not that much. Therefore, the reason why apple flesh turns brown is not because of rust – ie, iron oxide, which is iron combined with oxygen. The substance responsible is an enzyme called polyphenol oxidase（PPO）. When an apple is cut, oxygen in the air will have a chemical reaction with PPO to produce a brown substance called melanin, which is why the cut apple flesh turns brown.

So, how can we prevent the cut apple flesh from turning brown? You may have been told to squeeze lemon juice on it. This actually stops the browning effect! This is because the enzyme PPO does not work well in an acidic environment（lemons have acidic acid）. Another method is to wrap the apple slices with plastic wrap, or coat them with sugar or oil. This prevents oxygen in the air from contacting the apple flesh.

Terms to know
rust（銹）：a reddish-brown substance formed on some metals after touching water and air

enzyme（酵素）：a substance that helps a chemical change happen more quickly without being changed itself

chemical reaction（化學反應）：a chemical change produced by two or more substances acting on each other

acidic（酸性的）：containing acid

Vocabulary
claim (v)聲稱

combine (v)結合

substance (n) 物質

melanin (n)黑色素

prevent (v)預防

coat (v)塗

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