The quality of fats and oils is greatly affected by their freshness. They become rancid when exposed to light, warmth, and air for long periods of time. Rancid fats and oils will usually smell and taste bad.
All fats and oils are subject to rancidity, but at different rates depending on their type and the way they are processed. Unsaturated fats and oils will generally become rancid sooner than saturated fats and oils.
Most unrefined vegetable oils are rich in Vitamin E (tocopherol) which acts as an antioxidant and natural preservative. Wheat germ oil, corn oil, and safflower oil are high in Vitamin E. In the refining of oils, about 1/10 to 1/3 of the Vitamin E present is removed. Solvent Extraction destroys some of the oil’s natural Vitamin E. Therefore, manufacturers of refined oils often extend the products’ shelf life by adding preservatives such as BHA, BHT and Citric acid.
Heat and oxygen promote rancidity, so the best place to store fats and oils is in a tightly closed container in your refrigerator. Dark or amber coloured bottles will protect oil longer than clear glass bottles. Natural or unrefined oils will keep from 4 to 6 months or longer if stored properly. Refined and heavily processed oils will keep at least twice as long.