KnowHOW team explains: A slice of apple becomes reddish on exposure to air for a few minutes because of a chemical reaction called oxidation brought about by oxygen in the air.

Apples and many other fruits contain certain chemical substances called polyphenolic compounds. Phenols are colourless, but on oxidation they produce quinones which impart colours. In apples, this oxidation of polyphenolic compounds in presence of oxygen (in air) is catalysed by some enzymes called polyphenol oxidase or tyrosinase. Enzymes have a coiled structure and when an an apple is cut, the structure is broken. The exposed enzyme on the surface catalyses the oxidation of polyphenols.

The reaction can be slowed or prevented by inactivating the enzyme with heat (cooking), increasing the acidity on the surface of the fruit (by adding lemon juice or another acid), reducing the amount of available oxygen (by putting cut fruit under water or vacuum packing it), or by adding certain preservative chemicals (like sulphur dioxide).

On the other hand, using cutlery that has some corrosion (as is seen with lower-quality steel knives) can increase the rate and amount of the browning by making more iron salts available for the reaction. A similar effect is noticed with cut potatoes, pears, bananas and peaches, which also redden on exposure to air.

The question was sent by Razaurahman from Shibpur