All fats are susceptible to degradation, as this is what causes them to spoil. The more unsaturated fatty acids that make up a certain oil, the more quickly the oil will spoil. This process can also be accelerated by warmth, light, oxygen, metals (especially copper), enzymes and microorganisms. As the fatty acids begin to degrade, the oil takes on a lightly sour or bitter taste. In time, the oil tastes and smells unpleasant and goes rancid, at this point the oil is spoiled and should not be consumed.