Demystifying Oil
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Demystifying Oil

Once upon a time, buying cooking oil was an easy task. You walked into the grocery store, went down the baking aisle, and pulled a bottle from the shelf. There were no options; there was no confusion.

Today, entire aisles are devoted to cooking oil. It seems oil can come from just about anything, including avocados, almonds, soybeans, canolas (wait...what's a canola?). And to make it more confusing, you can get oil flavored with anything from chili peppers to rosemary to lemon.

How do you know which oil is best for the sizzling vegetable stir-fry, the perfect pumpkin muffins, or the savory balsamic salad dressing? By taking the mystery out of oil. Getting to know a little bit about oil will help you decide which oil is right for your cooking adventure.

Oil From Just About Anything

Oil can be made from a variety of sources:

  • Seeds: safflower, sesame, sunflower, genetically modified rapeseed (source of canola oil--the name comes from "Canada")
  • Nuts: almond, walnut
  • Grains: corn
  • Beans: peanut, soy
  • Fruits: avocado, olive, coconut

The first step in processing is to remove the oil from the seed, nut, grain, bean, or fruit. The extraction process can be chemical or mechanical. When done chemically, the oil source is soaked in a petroleum compound, usually hexane. The oil then requires further refining to remove this toxic solvent. This method is efficient, provides a high yield, and is more common than mechanical extraction.

Mechanical pressing, also called expeller-pressed, uses no chemicals. The oil is derived from its source by squeezing it in a mechanical press. The process can raise the temperature to anywhere from 120° to 190°F. Cold pressed means that no additional external heat is added during the processing. Oil purists believe that unrefined, cold pressed oil retains the most flavor, aroma, color, and nutrients.

Understanding Oil's Structure

In order to know which oil to choose, it is important to understand a little bit about the chemistry of oil. Oil is made up of fatty acids. A fatty acid is a chain of carbon atoms. Every carbon on the chain has places that hydrogen atoms can fill. If each carbon on the chain has all the available slots filled with hydrogen atoms, it is a saturated fatty acid (SFA).

If the fatty acid chain is not holding all the hydrogen that it can, it is considered unsaturated. When there is one point of unsaturation, the fatty acid is considered monounsaturated (MUFA). If there are two or more points of unsaturation, the fatty acid is polyunsaturated (PUFA). Specially modified margarine-like fatty acids are known as trans fats.

Trans fats occur only to a small degree in nature. An oil may contain all other three types of fatty acids, but in different proportions. An oil is named for the type of fatty acid present in the largest amount. For example, olive oil is 74% monounsaturated oil; therefore it is called a monounsaturated oil.

Saturated fats raise total blood cholesterol as well as LDL (“bad”) cholesterol. However, they lower blood triglycerides and may raise levels of HDL (“good”) cholesterol, leading to an overall mixed effect. In contrast, monounsaturated and certain polyunsaturated fats may improve most or all aspects of cholesterol profile. Trans fats, on the other hand, worsen most aspects of cholesterol profile and should be avoided.

Properties of Oil

The amount of saturation also controls certain properties of an oil. For instance, a saturated fat will stay solid at room temperature, whereas a monounsaturated fat will be liquid at room temperature but congeal in the refrigerator. And a polyunsaturated fat will stay liquid both at room temperature and in the refrigerator.

In addition, the more unsaturated an oil is, the more susceptible it is to oxidation. Oxidation causes rancidity, which produces an off-flavor in the oil. Rancidity can also produce an off aroma. So, if the oil doesn't smell right, pitch it.

Oil should be used within 6 to 12 months after opening. To help preserve the flavor and quality, store your unopened oil in a cool, dark cupboard. Once the oil is opened, you can keep it in the refrigerator. Olive oil, however, will congeal in the refrigerator—so only keep it in there if you are using it infrequently.

Smoke Point Determines Use

If you are cooking with oil and it begins to smoke, you have reached its smoke point. At the smoke point, the oil begins to emit unpleasant odors and impart unsavory flavors to your meal. Watch out for the smoke point signs. Getting to it means you are getting close to the flash point, which is when the oil can erupt into flames.

Knowing the smoke point is important when determining which oil you are going to use (see table below). Oil with a low smoke point is good for salad dressings, wine sauces, and seasoning, while the higher smoke point oils should be used for sautéing, baking, or frying.

The Goodness of Oil

Oil contains essential fatty acids, which your body needs to survive but can't make and therefore must be obtained through food. Oil is a source of vitamin E as well. In addition, oil is crucial for transporting the fat-soluble vitamins A, D, E, and K throughout the body.

Regardless of the source, one tablespoon of oil has 120 calories. Because oil is a fat, it has more calories per gram than carbohydrate or protein. So no matter which oil you choose and no matter what you choose it for, remember to use it in moderation. And in accordance with the American Heart Association's recommendations, opt for an oil high in monounsaturated or polyunsaturated fatty acids.

Type of oilPercentage of fatty acidSmoke point
SFAMUFAPUFA
Almond86917430°F
Avocado117013520°F
Canola76132468°F
Corn132958453°F
Coconut9172350°F
Olive157510428°F
Peanut194633471°F
Safflower81577468°F
Sesame143941410°F
Soy152362465°F
Sunflower121672464°F
Walnut92263400°F

RESOURCES:

American Dietetic Association
http://www.eatright.org

American Heart Association
http://www.americanheart.org

Canola Council of Canada
http://canola-council.org

Canolainfo
http://www.canolainfo.org

CANADIAN RESOURCES:

Canadian Council on Food and Nutrition
www.ccfn.ca

Dietitians of Canada
http://www.dietitians.ca/

References

American Heart Association website. Available at: http://www.americanheart.org. Accessed February 18, 2008.

Dietary fat chart. Canolainfo website. Available at: http://www.canolainfo.org/html/health/dietfat.html. Accessed February 18, 2008.

Duyff RL. The American Dietetic Association's Complete Food & Nutrition Guide. 3rd Edition. Hoboken, NJ: John Wiley and Sons, Inc; 2006.

Food oil smoke points. Canolainfo website. Available at: http://www.canolainfo.org/html/recipes/smokepoint.html. Accessed February 18, 2008.



Last reviewed February 2008 by Maria Adams, MS, MPH, RD

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.


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