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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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.
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.
Enzymes are essential to life, but they're relatively delicate. Taking them out, or at least slowing them down, requires just a little adjustment to their environment. Lemon juice on an apple will change the pH of the environment enough so that the PPO doesn't function as well as it once did. (Lemon juice plays a double game. It's an antioxidant, meaning it will snap up the available oxygen itself, and keep the oxygen from reaching the apple tissue.)
The real enzyme-stopper is heat. Enzymes are proteins, and heat quickly and permanently alters the structure of proteins, in a process called denaturing. Think of the change in the proteins of an egg when it is boiled. When apples are cooked, or as some chefs recommend, briefly blanched in hot water, the enzymes in them, including PPO, are permanently reconfigured. The reconfigured enzymes are inert and useless, and are not grabbing oxygen any time soon. The apple can spoil in other ways, but it can't go through the same browning process.
If none of this works for you, take heart. Scientists have come up with a genetically modified apple that never browns when cut. As the apple grows, it is engineered to destroy the gene sequences that code for PPO, so no PPO is present in the apple, and it stays light and juicy.