Cut an apple in half and you begin a race against time. Every second you waste is a second that your apple uses to turn into a flaccid brown mess. Why do apples try so hard to foil your quest for vitamins? Science has the answer.
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The Browning Enzyme
Certain fruits and vegetables seem to have a self-destruct switch on them. The moment a knife pierces their surface, they set about destroying themselves. They lose their texture and color, turning mushy and brown. They do this by, basically, rusting. Read up on the nutrients in these plants, and you'll find some iron. When you cut through the skin of the fruit, you are putting oxygen in contact with iron, and it rusts.
Iron alone won't get the job done. Plenty of foods have iron in them, but not all of them rust while we watch. The process is helped along by a little enzyme called polyphenol oxidase. Polyphenol oxidase, or PPO, is present in the chloroplasts of plant tissue. Injuring plant tissue spills it out into the air, where it grabs oxygen and foists that oxygen on the compounds around it. Oxygen is the town bicycle of the chemical world, willing to combine with a lot of different atoms and chemicals. When the compounds in the apple get an oxygen atom, they become more likely to combine with the other compounds around them. This rampant combination changes the texture of the apple, and generates melanin - which is responsible for the browning effect.
The Two Ways To Prevent Browning
There are two ways to stop the browning. Either take out the oxygen, or take out the enzymes. You can cut off the supply of oxygen by simply tossing the cut apples into a bowl of water until you're ready to use them. Cutting off the oxygen is also the reason why people coat apples with syrup, or the famously decay-proof honey. Coating the apples with sugar will put a layer between the apple tissue and the outside world, but more importantly, the sugary solution reduce the diffusion of oxygen through the apple cells. The oxygen, trapped on the outside, is prevented from ever coming into contact with PPO and the iron in the plant cells.