Question

why does edible oil show rancidity when stored for a long time

Answer 1

Rancid oil may also lose some of its healthy properties, like the antioxidants found in olive oil. Oils go rancid through a chemical reaction that causes the fat molecules in the oil to break down. The whole process is sped up by exposure to air, light and heat.

Answer 2

because it gets oxidized with the atmospheric oxygen