Tanoide substances give taste and shelf life of apples. Apple slices, exposed to air immediately after cutting, are turning brown because oxidation of tannins (polyphenol oxidase).
This enzyme, polyphenol oxidase, is inside of apple's cells and it reacts with oxygen from the air.The answer to reaction with oxygen from the air is a layer of dark brown color,a layer that appears in other vegetables / fruits that contain this enzyme. This enzyme is released when apple cell destruction begins, which happens when apple is peeled or cut in slices.
So, the conclusion would be: For apple damaged cells begin the process of oxidation, they need light, water and oxygen, therefore, to slow down the oxidation process, apples without peel or cut, should be kept in a cool and dark place. Also, the ascorbic acid from the lemon juice, reacts with oxygen from air, thus preventing that the oxygen to reacts with polyphenol oxidase from the apple. So, the more apple surface has an acid nature, the less oxidation process will happen.
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