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A slice of apple becomes reddish or brownish after a few minutes because of a chemical reaction called oxidation brought about by the oxygen in the air.
Apples and many other fruits contain chemicals called polyphenolic compounds. Phenols are colourless, but on oxidation under the influence of an enzyme called polyphenol oxidase (PPO) â€” also known as tyrosinase â€” they turn brown in colour.
Once you cut the fruit, you open up some of the cells. This breaks the structure of the enzymes which are otherwise coiled. The enzyme then has access to the oxygen in the air and produces quinones, which turn the cut surface brown. It is also for this reason that brown spots appear on apples when you drop them.
The easiest way to prevent browning is to put the sliced apples in water so that the enzyme does not have access to oxygen. Also, the enzymes can be deactivated by heating or adding lemon juice. One can also use preservatives like sulphur dioxide for reducing the amount of available oxygen.
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 darken on exposure to air.
In humans, tyrosinase is important because it helps create melanin, which causes tanning. The lack of tyrosinase in humans leads to albinism. So in humans, the browning that tyrosinase helps cause is actually a good thing!
Source: The Telegraph (Kolkata, India)