Paheli watched her grandmother making mango pickle. After she bottled the pickle her grandmother poured oil on the top of the pickle before closing the lid. Paheli wanted to know why oil was poured. Can you help her understand why?

Hint: Oils are unsaturated lipids that are viscous at ambient temperatures and have high carbon and hydrogen content which are usually flammable. Some edible vegetable and animal oils are used for cooking and food preparation.

Complete answer:
The oil poured by the grandmother on the pickle starts forming a layer of protection between the air and pickle from the bacteria which is present in the air. It increases the shelf life of the pickle and does not let bacteria to deteriorate the quality of the pickle. Preserving the shelf life of food by immersion in vinegar or by anaerobic fermentation is pickling. Several vegetables can be preserved commercially or in households by pickling. Preventing the growth of microorganisms by introducing bacteria or fungi and slowing down the oxidation process which causes rancidity of fats in food preservation. Preservation is important for creating nutritional value and maintaining the texture and flavor of various food materials.
Picking can be done in two ways which include the preservation of pickles in vinegar and the pickles that are soaked in Salt. The preservation of pickles in vinegar constitutes a strong acid in which a few bacteria can survive and the bottled pickles are available in the supermarket which is preserved in vinegar. Pickles that are soaked in Salt increase fermentation which is the growth of good bacteria which makes food less vulnerable towards bad spoilage-causing bacteria. It changes the taste and texture of the food in a profusion of interesting and yummy ways. The preservative which is most common in acidified foods is sodium benzoate which is present in foods such as fruit juices, syrups, jams, pickles, jellies, fruit cocktails, and sauerkraut.

Note: Natives of the Pacific Islands pickle their food in the ground in holes with the banana leaves which they used as food reserves in case of storms. The guys can't get a girl in Fiji without showing their parents pickle pits. An average American eats 8.5 lbs of pickle a year which is estimated by the department of agriculture.