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