Why do cut Apples turn brown?

We might have observed that fruits like Apples and Bananas turn brown within in no time after cutting them. This is because of a chemical change that occurs due to the oxidation of some of the compounds present in these fruits.

In detail, once we cut the Apples the tissues are exposed to the oxygen in air resulting in an oxidation reaction. Apples and other fruits contain an enzyme (catalyst) Polyphenol oxidase (tyrosinase) that oxidises phenolic compounds in the apple to form orthoQuinones. These o-quinones polymerise to form a brown color pigment.

The best way to prevent this is to either prevent the oxidation process or decrease the activity of the PPO enzyme. Keeping the cut Apples in water slows down the oxidation process, as it would take longer time for oxygen to reach the tissues. The other way is to add anti-oxidants (which are plenty in citrus fruits like lemons and oranges) on the surface of cut Apple. Vinegar (Acetic acid) would decrease the pH below the working range of PPO enzyme thereby reducing the oxidation process and thus browning of Apples would take.

                                                                                      Hemachandra K
                                                                                                Research Scholar, TIFR, Mumbai

Published date : 27 Apr 2013 06:39PM