DEAR DR. BLONZ: I read that the minute a jar or bottle of cooking oil -- such as safflower, canola, peanut or olive -- is opened, oxygen enters and the oil starts to go rancid. Will refrigeration stop this? I know that people won't die from using unrefrigerated oils, but are there negative consequences, healthwise? I read the label on a bottle of canola oil and it said nothing about needing to be refrigerated. And what about fish oil in capsules? -- J.S., Phoenix
DEAR J.S.: Any oil can go rancid if stored in the wrong way. Rancidity occurs when the oil reacts with oxygen. Aside from having an odor and giving foods an "off" taste, the consumption of oxidized oil does represent a health risk.
With refined oils, there is only minimal risk in leaving oils at room temperature, provided you follow some simple guidelines. Oils should be stored out of the sunlight and away from any source of heat. Since air, and therefore oxygen, gets in whenever the container is opened, keep the container sealed when not in use. This doesn't mean you need to get frantic and cap the container instantly, but it does make sense to close it when you are done, especially if it takes you months to get through the entire container. If you buy more oil than you tend to use in a few months, consider splitting the large size into smaller portions and refrigerating the unused bottles until needed.
Whenever an oil is stored in the refrigerator, keep it well-sealed to prevent it from picking up any undesirable refrigerator odors. Some oils may become cloudy when refrigerated, but this disappears when they return to room temperature, and is not a reflection on the oil's wholesomeness.
It is an oil's points of unsaturation, the double bonds in its fatty acids, which are most vulnerable to attack from oxygen. This means that the greater the degree of unsaturation, the greater the tendency to oxidize. As produced in nature, oils are blends of fatty acids with varying degrees of unsaturation, but we tend to classify them by their predominant type. Monounsaturates, such as olive oil, mostly contain one double bond. Polyunsaturates, such as soy, corn and safflower, have two. These types are more stable, but they should also be well-sealed and stored away from sun and heat. With flaxseed oil, over half the fats contain three double bonds, so it should be chilled.
Fish oils have four or five double bonds, so they are among the most susceptible to oxidation, and should always be refrigerated when in bottles. As dietary supplements, however, fish oil tends to be dispensed in air-tight capsules, which can be kept at (cool) room temperature.
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