Vegetable oils supply energy-dense calories to your diet. They may also contain essential fatty acids and serve as a vehicle for absorption of fat soluble vitamins. Vegetable oils are derived from the seeds of a variety of plant sources, including canola, corn, cottonseed, olive, peanut, safflower, soybean, and sunflower. Vegetable oils, as with any food product that contains high levels of fats, have a shelf life. Understanding the effects of expired vegetable oils can help you decide whether to keep a product or discard it.
Vegetable oils consist of triglycerides: three fatty acid molecules bound to a glycerol. If the carbon chains of the fatty acids hold as many hydrogen atoms as possible, the fatty acids are called saturated fats. Unsaturated fats, on the other hand, have fewer hydrogens surrounding the carbons of the fatty acids. The greater the saturation level of a fat, the more shelf stable it is. Oils, which are liquid at room temperature, are unsaturated fats and are therefore more susceptible to rancidity than saturated fats, which are solid at room temperature.
Types of Rancidity
Expired vegetable oils can undergo either hydrolytic or oxidative rancidity. Hydrolytic rancidity involves the splitting apart of the triglyceride molecule into its three fatty acids plus glycerol. This process occurs in the presence of water and can result in the release of volatile free fatty acids. In oxidative rancidity, heat or light acts on the fatty acids of the vegetable oil in the presence of oxygen to create compounds known as hydroperoxides. Hydroperoxides then become oxygenated aldehydes. The effects of expired vegetable oils depend on whether they are hydrolytically or oxidatively rancid.
The volatile free fatty acids released during the hydrolytic rancidity process can smell unpleasant. For example, butyric acid is a short-chain volatile fatty acid with the characteristic rancid butter odor. These fatty acids, although they may smell and taste foul, are not harmful to your health. The oxygenated aldehydes produced during oxidative rancidity are toxic, however. These molecules create a condition of oxidative stress in your cells and may raise your risk for developing artherosclerotic and degenerative diseases. Expired vegetable oils may be not only offensive to eat but also hazardous to your well-being.
Manufacturers often add anti-oxidants to vegetable oils to delay the rancidity process. Protecting vegetable oils from light, heat, water and oxygen may also extend their shelf life. However, as you might not know what kind of rancidity your expired vegetable oil has undergone, your safest action is to discard any oils past their expiration date.