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Just as oxygen makes iron rust, it makes apple slices go brown and other cut food discolour. To get around this problem, a housewife might add lemon juice to apple slices - which is effective in the shorter term.
Oxygen, however, also breaks down fats and oils in food, eventually making them rancid, and also diminishes vitamins, especially vitamin C.
To minimise this problem, the food industry has developed strong antioxidants - these are substances that neutralise "free radicals" - the most commonly found being BHA (butylated hydroxyanisole - E320) and BHT (butylated hydroxytoluene - E321). There has been a worry about the latter (it is in fact banned in Germany, and it was banned here for a short while in the mid-60"s) but both are effective in very small quantities, and it"s considered that the benefits far outway any small long-term risks - although both can damage the kidneys, particularly the latter. Propyl gallate is also an effective antioxidant, but expensive so generally combined in minute quantities with one of the above two.
Tocopherol, or vitamin E, which occurs naturally in vegetable oils, is still used a little; so is ascorbic acid and its salts and erythorbic acid and its salts.
The addition of these chemicals has never been so controversial as the use of artificial sweeteners and flavourings, as they are so much more necessary. The alternative - of having more "free-radicals" in the body damaging cells, far outweighs the use of antioxidants. See Free Radicals