QUESTION: At a dinner with some friends, I learned that oils left unrefrigerated can become a health hazard because the fats can oxidize. I would like to refrigerate my cooking oils, but whenever I try, they become thick and seem almost to coagulate. How serious is this, and what exactly is at risk?
A.B., East Orange, N.J.
ANSWER: Oils, by definition, are liquid at room temperature, and fats are solid. (Room temperature is usually considered to be 68 degrees Fahrenheit.) As an oil chills, it's safe and natural for it to become thick and cloudy. The cloudiness is a sign that crystals are beginning to form – the first stage as the oil begins the change from liquid to solid. You have probably noticed that the oil clears up when it returns to room temperature.
With oils, the greater the degree of unsaturation, the colder the temperature needed to start the crystalline formation. This means that oils with high levels of polyunsaturates, such as corn or safflower oil, will still be clear when your monounsaturated olive oil has begun to cloud. Fats with higher levels of saturates, such as butter or lard, are in their solid, crystalline form at room temperature.
Storing oils in the refrigerator does not harm them. Refrigeration may actually be the preferred storage method for unfiltered, unrefined oils. (Most oils are refined. The label will usually say if the oil is unfiltered and unrefined.) Unrefined, unfiltered oils can contain compounds that make them less stable.
There is minimal risk in leaving refined oils out, provided you follow some simple guidelines.
The oils should be kept out of the sunlight and away from any source of heat. The container should be sealed when not in use. If you buy more than you tend to use in about four months, split the large size into smaller portions, refrigerating the unused bottles until needed. Whenever an oil is stored in the refrigerator, keep the container well-sealed to prevent its picking up any undesirable refrigerator odors.
If an oil is going to be reused, such as oil from a fryer that you want to store between uses, let the oil cool a bit and strain it through a few layers of cheesecloth to remove food particles.
Any oil can go rancid if subjected to excess heat or if stored incorrectly or kept too long. An oil that has gone rancid will have an unpleasant smell and taste, and it will ruin your food.
The higher the proportion of unsaturates, the greater the tendency to oxidize. (That is why fish oils and flaxseed oils tend to be sold in dark containers with instructions to refrigerate after opening.) Oils are the energy source for the seed, and they often have their own protectants, including polyphenols, tocopherols (vitamin E) or other antioxidants.
Ed Blonz, Ph.D., is a nutritional scientist based in Northern California. General-interest questions about nutrition can be mailed to: Ed Blonz, Focus on Nutrition, P.O. Box 120191, San Diego, CA 92112-0191, or sent via e-mail to UTFood@blonz.com.
