Browning is due to a chemical reaction known as Enzymatic Browning
According to the online journal eduqas and in general terminology, Enzymatic browning is a chemical process that occurs in fruits and vegetables. It causes the flesh to discolour, most commonly in a brown colour — the effect produced by a naturally occurring enzyme found in fruits and vegetables, polyphenol oxidase.
Browning of Apples. Image courtesy scientificamerican.com
Enzyme browning can be examined in fruits such as apricots, pears, bananas, grapes and avocados, and vegetables such as aubergines, potatoes, lettuce. Once the fruit or vegetables are cut, some of the cells opened up to the air. The enzyme polyphenol oxidase then has access to oxygen in the air, and a reaction occurs, which results in the fruit or vegetable turning brown.
Browning of Plum fruit. Image Courtesy farmer’s weekly.
How can this be controlled?
Using acid or heat method
Acid method:-Add some lime juice or vinegar to a bowl of water and add these vegetables of fruits. Let them soak to reduce the discolouration. Choose acid according to the type of fruits and vegetables. Choose an acid which complements the flavour of the fruit or vegetable.
Heat method:- Blanch/heat/microwave them. This is applicable when these served wholly cooked and not appropriate when these are to be eaten raw. The heat method can destroy the enzymes, but please note this may also end up cooking your fruit or vegetable.
Using water or anti-oxidants
Water method:- Immersed the cut fruit or vegetable in a bowl of water to prevent them from turning brown. Preferable to add some solutes- sugar or salt depending on the flavour combination. The method will limit the oxidation process on the cell surfaces and slow down the enzymatic browning.
Anti-oxidants:- Some hotels dip the cut fruits in an anti-oxidant mixture which that slows down the process — anti-oxidants like Vitamin C — ascorbic acid.
According to the European Scientific Journal, sugar or salt coating on the cut fruits and vegetables can easily prevent and slow down the process of browning.
You cut or peel only when needed
You can refrigerate, but refrigeration slows down the process, but the discolouration still occurs.
Storing these copper, iron or open tin cans easily speed up the process and deteriorate the condition of the product. Using iron knives can speed up the enzymatic browning. Metals be strictly avoided.
Do not leave any of the cut products open.