Even refined oils, devoid of enzymes, are subject to deterioration. Highly unsaturated ones are the least stable, especially those which contain appreciable amounts of linolenic acid (e.g. soya oil), which is the most troublesome of the common fatty acids.
Oils react with oxygen from the air. This can cause oxidative rancidity: a series of chemical reactions which releases compounds with an unpleasant smell and taste. Fish oils, which are highly unsaturated, and again soya oil, are especially prone to this spoilage.