Fresh cut apples turn brown when iron-containing chemicals in the apple react with oxygen in the air. We see this every day when iron objects rust. The chemical reaction is called 'oxidation'.

There are several ways to inhibit oxidation.

Chemical anti-oxidants can be added to food. Lemon juice, for example, will inhibit the browning of a freshly cut apple because lemons are high in citric acid, an anti-oxidant. Sulphur dioxide, used in the commercial processing of many foods, does the same thing.

Removing the air (and thus the oxygen) from food can also inhibit oxidation. This is one reason canned food stays fresh so long.

Because the enzymes involved in oxidation need water to work, the reaction can be also be slowed by de-hydrating (removing water from) food.

Finally, the browning of apples and other fruit can be avoided by using clean, high-quality cooking utensils. Iron salts, such as those found on a steel knife corroded by organic acids, can act as potent catalysts to oxidation, and produce per-oxides and super-oxides.

As an interesting (and potentially useful) side note, the browning of apples releases ethylene gas which is known to induce flowering of many plants, including pineapples, and ripening of fruits such as tomatoes, citrus, and melons.

Flowering can be induced by placing cut apple and your plant under a clear plastic tent, such as those used to hold in humidity. Once the plant flowers, remove the apple or the flowers will wilt very quickly.

To ripen fruit, store it in a bowl or a paper bag with ripe apples or apple peels.

Don't store apples near lettuce (causes spots and wilting), broccoli, cucumbers (turns them yellow), bananas (makes them brown faster), watermelon (makes it pulpy), or leafy greens (they'll lose their colour).
The main acid that oxidises after cutting fresh fruit/veg is Ascorbic Acid, better known as Vitamin C. This is pretty damn good for you, and hence fresh fruit and vegetables should only be cut just before they are cooked/served, especially if they are being served raw. The whole TV Chef thing of having pre-prepared bowls of chopped fruit and veg is actually bad practice, unless cooking for a large number of people or in other speed critical situations, in which case blanched or acidiculated fruit/veg is recommended. (thanks to Jimnyo for that)

However, if you're going to boil the broccolli yellow or otherwise overcook the food, this is irrelevant. Overcooking, especially boiling, tends to create Ascorbic Acid Oxide which is subsequently lost into the water or other cooking medium.

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