Olive oil turns rancid when oxygen, light and heat attack the fatty acids over a long time. Rancid oil smells of old nuts, putty or wax crayons and leaves a greasy feeling in the mouth. Its freshness is gone.
A slightly rancid oil is not dangerous, but it no longer tastes good. Preventing it is simple: store it cool and dark, close it well and do not leave an opened bottle standing too long.
