DEAR DR. BLONZ: Do oils that are left at room temperature become more susceptible to oxidation? When I began to refrigerate my cooking oils, they became thick and seemed to almost coagulate. How serious is this, and what exactly is at risk? -- A.B., Philadelphia
DEAR A.B.: Oils, by definition, are liquid at room temperature, whereas fats are solid. (Room temperature is typically considered to be 68 degrees F.) It's natural for an oil to become thick and cloudy as it chills. This results from the crystals that form as the oil changes from liquid to solid. No doubt you have noticed that the oil clears up when it returns to room temperature.
The greater an oil's degree of unsaturation, the colder the temperature needed to start crystalline formation. This means that oils high in polyunsaturates, such as corn, sunflower, soy or safflower, will be clear when your monounsaturated olive oil has begun to cloud. Fats with higher levels of saturates, such as butter, lard or coconut oil, will be in a 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.) Unrefined, unfiltered oils can contain compounds that make them less stable, and thus benefit from refrigeration.
The higher the proportion of polyunsaturates, the greater the tendency to oxidize. Highly polyunsaturated oils such as fish oil or flaxseed oil tend to be sold in dark containers with instructions to refrigerate them after opening.
The risk is minimal when you leave refined oils out, provided you follow some simple guidelines.
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 expect to use in four months, consider splitting 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 it from picking up undesirable odors. If an oil is going to be reused, such as storing frying oil between uses, let the oil cool a bit and strain it through multiple layers of cheesecloth sufficient to remove all food particles.
Understand that 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 a noticeable and unpleasant smell and taste, and it will ruin your food.
As a nutritional aside, plant oils are the energy source for the seed, and, especially when unrefined, they have their own protectants. These include polyphenols, tocopherols (vitamin E) or other antioxidants. You will find more of these in oils that are unrefined or unprocessed, such as extra virgin olive oil.
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