Vegetable oils

Choose a variety

By Rebecca Kessler

March 9, 2009

Take a stroll down the vegetable-oil aisle at your local supermarket. At one end squat opaque, gallon-size plastic jugs of cheap, mass-market oils; at the other end sit miniature glass jewels sparkling darkly with expensive elixirs. In between oozes a sea of olive oil in glass, plastic, and tin.

Some of their labels hint at how the oils inside were made: “extra virgin,” “cold pressed,” “expeller pressed,” “naturally refined.” Everyone knows extra-virgin olive oil is supposed to taste good and be good for you, but what about the rest of the bottles on the shelf? How are they made? And which should you buy, for both health and taste?

Labels on mass-market oils — typically corn, canola, “vegetable” (usually soybean), safflower, and sunflower oils — tend to be silent on the subject of manufacturing. That’s probably because conventional oil production sounds none too appetizing.

Here’s how conventional oil is produced: Oil is extracted from the seed or grain with hexane, a toxic, petroleum-based solvent. The hexane is removed, then the crude oil — yes, the lingo for edible oils is the same as for petroleum — undergoes a harsh refining process involving caustic chemicals and temperatures as high as 500 degrees, plus water and steam. The refining stabilizes, deodorizes, and bleaches the crude oil, and raises its smoke point. The end product is a light-colored, nearly flavorless and odorless oil that can be cooked at high temperatures and has a shelf life of several years.

By contrast, the oils at the other end of the supermarket shelf are loaded with boastful labels claiming all sorts of health benefits. These oils have generally undergone a much gentler extraction process, coaxed from the crop mechanically rather than chemically. Mechanical oil extraction usually involves an expeller (a type of screw press), a centrifuge, or a vacuum press.

No heat is applied in mechanical extraction, but the friction of the process can raise the temperature of the oil. Delicate oils are “cold pressed,” which means that the oils are kept below 120 degrees (and often lower) throughout the extraction process. The low temperatures maintain flavor, natural antioxidants, and vitamins.

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Many mechanically extracted oils, such as extra-virgin olive oil, go into the bottle without refining. But those destined for high-heat cooking are often “naturally” refined, a process that uses lower temperatures (around 250 degrees) than conventional refining and avoids harsh chemicals entirely.

Natural refining removes some of the flavor, color, vitamins, heart-healthy compounds, and antioxidants from the crude oil, but not as much as conventional refining does. (Conventional producers often add some antioxidants after processing.)

Does the manufacturing process make a difference to your health? Well, the rawer the oil, the higher its levels of natural antioxidants, nutrients, and heart-healthy compounds. But according to Tong Wang, a food scientist specializing in oil processing at Iowa State University, there’s unlikely to be enough of a difference between conventionally and minimally processed oils to have a significant impact on health. And the nutritional impact of oil processing has not been studied.

“Ideologically, [the minimally processed oils] are good, especially for people who like organic food or natural food,” says Wang. “But I don’t think there’s hard science data on how much better those oils are.”

Should you wish to make your oil purchases based on environmental sustainability, however, the choice is clear: minimally processed oils are more eco-friendly. They require less energy and water, and they don’t use hexane, a toxic compound that is released into the atmosphere during conventional oil making, says Mohammed Alam, the head of the Fats and Oils Program at Texas A&M University. And most mass-market oils — including soy, corn, and canola — come from genetically modified crops, which raise many environmentalists’ hackles.

Four vegetable oils, clockwise from top left: grapeseed oil, extra-virgin olive oil, unrefined peanut oil, and sesame oil.

Health-wise, it’s good to remember that all oils are energy-dense, so keep calorie counts down by using them in moderation. And pick an oil or two with heart-protective omega-3 essential fatty acids (EFAs), like soy, canola, walnut, and — the gold standard — flaxseed. (Some experts suggest avoiding omega-6 EFAs; we need them, but most Americans eat too many, and an overabundance has been linked to inflammatory diseases.)

Each plant oil, of course, has a different composition of fats and nutrients, says Sharon Akabas, a nutritionist at Columbia University, so use a variety of oils for health, flavor, and cooking versatility. Buy a few delicate, unrefined oils, such as walnut and hazelnut oils, to use straight from the bottle over salad or fresh bread. Olive oil is good for everyday uses from dressings and marinades to sautéing. And buy a few refined oils to use in higher-heat frying (such as peanut) and baking (such as canola).

The less refined an oil is, the faster it’ll go rancid, so store these oils in a cool, dark place, preferably in a dark-colored bottle. Stash unrefined nut and other fragile oils, such as flaxseed oil, in the fridge. And since all oils have a “smoke point” — the temperature at which they begin to burn and degrade, destroying nutrients, oxidizing the oil, and tasting nasty — keep a watchful eye on that skillet.

Here’s a guide, alphabetized by type, to the most common supermarket oils.

• Almond. A strongly-flavored oil good for dressings and drizzling. When refined, it’s great for high-heat cooking and baking.

• Apricot kernel. Typically refined, it’s useful for the high-heat cooking techniques of roasting, grilling, and frying.

• Avocado. Unrefined, it has a rich, buttery flavor, delicious in dressings and for drizzling. When refined, it’s good for high-heat frying and sautéing.

• Canola. Canola is the marketing term for rapeseed. A decent source of omega-3s, canola has a neutral flavor that makes it a good all-purpose cooking oil: dressings, sautéing, frying, and baking.

• Coconut. Solid at room temperature, coconut oil has an intense, lovely taste that’s great for Asian dishes.

• Corn. Refined corn oil has a neutral flavor, while unrefined corn oil tastes more strongly of corn.

• Cottonseed. A neutral-flavored oil commonly used for frying and other high-heat applications in food service or packaged food. Cottonseed oil must be refined to remove gossypol, a natural toxin.

• Flaxseed. A great source of omega-3s and antioxidants, flaxseed oil should only be eaten raw, as it doesn’t hold up to heat. It’s very unstable and should be kept in the fridge.

• Grapeseed. A neutral-flavored oil that’s becoming increasingly popular. Typically refined, it’s good for frying and sautéing.

• Hazelnut. A richly flavored oil, typically sold unrefined. Great for dressings, drizzling, light sautéing, and baking.

• Macadamia. A creamy-flavored oil, good for everything from sautés and dressings to drizzles and bread dips.

• Olive. Extra-virgin olive is the oil supreme, beloved for its high antioxidant content, delicious flavors, and versatility in sautéing, drizzling, dipping bread, roasting, and just about anything short of baking sweets that you can think of (though there are some delicious olive-oil cakes). “Light” olive oil is refined; it has little or no flavor and reduced antioxidants, but is serviceable for high-heat cooking. “Pure” is a mix of refined and unrefined oils. Buyer beware, however, because terms such as “extra virgin” and “virgin” carry no legal weight in the U.S., and the Italian olive-oil industry has suffered from adulteration problems. The USDA is considering whether to change its 1948 labeling standards, which could set the stage for tougher regulation. See Jim Dixon’s olive-oil label guide for more info.

• Palm. Like coconut oil, both palm and palm-kernel oils are high in saturated fats. Palm oil is more often found as a substitute for hydrogenated oils in commercial baked goods than in the home kitchen.

• Peanut. Great for high-heat cooking, like deep-frying and stir-frying. As with other oils, unrefined peanut oil has more flavor but withstands less heat than refined peanut oil.

• Safflower. A sunflower relative, safflower yields an oil that’s mild-tasting and good for frying, sautéing, and baking.

• Sesame seed. The dark/toasted version of sesame oil is very flavorful; it’s good for low-temperature cooking and great for sauces, dressings, and drizzling. The light/untoasted version is neutral-flavored. And refined sesame oil can be used for high-heat cooking.

• Sunflower. High in vitamin E, sunflower oil is good for medium- to high-heat cooking.

• Soybean/vegetable oil. Most “vegetable” oil comes from soybeans, but check the label to be sure. Soybean oil is the most widely produced oil in the world. It’s used for everything from deep-frying to sautéing to baking, and has a neutral flavor. It’s a decent source of omega-3s and, particularly when unrefined, high in vitamin E.

• Walnut. Like most nut oils, unrefined walnut oil has a strong flavor, whereas refined walnut oil is mild. It’s great for dressings, drizzling, and, when refined, baking. It’s also a good source of omega-3s and antioxidants. Nut oils tend to go rancid quickly, so store it in the fridge.

Rebecca Kessler is an editor at Natural History magazine and writes about science, the environment, and food. She lives in Providence, Rhode Island.