Today, we’re talking about cooking oils, and how we actually get the oil from the fruit, vegetable, or seed. Somehow, we need to “extract” it, but how we go about that can affect the texture, flavor, and nutrient content.
That brings us to the process of extracting and refining the oil. We can do it in a number of different ways. Which is best? Which is most healthy?
Types of Extraction
To start off, we have to get the oil out of the source material. There are three basic ways to do that:
- Use heat only: Called “rendering,” this process applies heat—usually in the form of sunlight—to fruits like olives. Harvesters pile the olives in the sun and collect the oil that exudes from them over time. Palm fruits may be boiled in water, after which the oil is skimmed from the surface.
- Use chemicals: The source material is ground or rolled and then purged or washed with a chemical solution that typically includes hexane. (Hexane is a hydrocarbon chemical made from crude oil.) The solution releases the oil in the seed. The oil is then heated to separate the oil from the chemical solution. (The solvent evaporates, leaving the oil behind.) Theoretically, the heat is supposed to remove all solvent from the oil, but microscopic portions can remain. How much depends on the efficiency of the processor. This is how most vegetable oils in the conventional food industry are extracted.
- Press it out: This method uses a mechanical press to actually press seeds or nuts through a barrel-like cavity. Using friction and pressure compresses the seed material, and oil seeps through openings too small for fiber solids. The result is said to be cleaner and more pure, higher in natural colors and flavors. “Cold-pressed” oil does not use any heat at all, and is said to be even purer with a better flavor then oil expressed with heat.
Types of Refining
Once the oil is extracted, it may also go through a refining process, which is simply a way to create a cleaner and more pure oil. Manufacturers also use refining processes to create a more uniform oil that lasts longer on the shelf. Refined oils are said to have less nutrients and flavor than unrefined, but they are typically more stable and last longer on the shelf. They are also more resistant to smoking, and are good choices for high-heat cooking and frying.
Different types of refinement include:
- Unrefined oil: Filtered only lightly to remove large particles. These oils may appear cloudy or contain visible sediment at the bottom of the bottle, but this doesn’t typically compromise quality. Some people prefer unrefined oils as they have more pronounced flavors, colors, and fragrances, and are thought to have more nutrient content. They don’t last as long on the shelf as refined oils, however, and must be replaced more frequently. In addition, the natural resins and other particles burn easily, so they’re best used in low-heat cooking or unheated, such as in salad dressings. Examples of oils often unrefined include peanut oil, sesame oil, and macadamia nut oil.
- Naturally refined: Naturally refined oils are more thoroughly filtered and strained than unrefined. Typically, no chemicals are used, and they are more stable for high-heat cooking. They may also be preferable for cooking when you don’t want the flavor of the oil overwhelming the flavor of the food. Coconut oil is a good naturally refined oil.
- Distilled: The oil is heated to high temperatures to evaporate off chemical solvents (used after chemical extraction).
- Degummed: This process passes hot water through the oil to get rid of any gums and proteins, considered “impurities.” The “dirty” water is then discarded.
- Neutralized: This oil is treated with sodium hydroxide or sodium carbonate, which pulls out fatty acids and other fats, pigments, and waxes.
- Bleached: Oil that may contain off colored impurities may be bleached with fuller’s earth (clay-like earthy material), activated carbon, or other clays, followed by heating, filtering, then drying to recoup the oil.
- Dewaxed: The oil is cooled to low temperatures, then manufacturers remove any solids that form, leaving behind a clearer oil.
- Deodorized: Oil is treated with high-heat pressurized steam to evaporate less stable compounds.
- Preserved: Oils that have been made less stable because of high-temperature processing may need preservatives to last on the shelves—BHT or BHA may be added.
Manufacturers like chemically extracted and refined oil because it’s fast, cheap, and lasts a long time on the shelf. On the other side of the spectrum are advocates of natural oil, who prefer the nutrient content and flavor of oil that is more gently treated.
Some oils are easier to extract with expeller pressing than others. Olive, avocado, and walnut oils, for example, come from soft fruit or nuts that easily give up their oil when pressed. Hard seeds like soy or canola usually require some heat in the form of steam to release the oil—and though chemical extraction may be easier, pressing the oil also works with a little assistance from heat.
There are a number of concerns about chemically extracted, refined oils. These include:
- Loss of nutrients: Even in oils that are simply heated, there can be a loss of valuable nutrients, including tocopherols, sterols, and antioxidant carotenoids. That’s why many prefer cold-pressed oils when possible, as they are more likely to retain the maximum amount of nutrients. As cold pressing does not reduce pesticide content, however, it’s important to purchase organic oils when going cold-pressed.
- Disturbs fats: High temperatures and chemicals can also weaken the bonds of unsaturated fatty acids, which can lead to the creation of damaging free radicals. Though tocopherols and carotenoids protect against free radicals, these may be lost during high-heat processes as well, which can leave the oil much less likely to nurture the body’s cells, and in a state where it may even cause harm.
- Stripped of antioxidants: Natural oils contain a lot of healthy antioxidants, but heat, chemicals, and bleach destroy these free-radical fighters.
- Preservatives: BHT and BHA are not considered good for the human body. BHT is listed by the Centers for Science in the Public Interest (CSPI) as either increasing or decreasing the risk of cancer in various animal studies. BHA has been linked to cancer in animal studies as well. The U.S. Department of Health and Human Services considers BHA to be “reasonably anticipated to be a human carcinogen.”
- Remember that the refining process—whether natural or chemical—removes impurities, which, in general, increases the smoke point. So if you’re using high temperatures for cooking, choose a naturally refined oil like avocado, coconut, red palm oil, and almond. Unrefined oils are best for low- and moderate-heat cooking.
- Oils high in polyunsaturated fats (such as corn, soy, sunflower, and other seed oils) form a harmful compound when heated—even if they’re refined. Use these only for low-heat cooking. These oils are also likely to go rancid sooner—always check them for color and smell before using them.
- Cheaper oils are typically chemically extracted and refined. If you want expressed, unrefined oils, be willing to pay a little more.
- For the most natural oil, choose organic, expeller-pressed, unrefined oils when possible. Just be cautious about the type of oil you’re using when cooking to avoid creating unstable fats.
- Consume it within three months. Natural oils go bad more quickly than refined oils. Buy smaller containers, keep out of the light, and discard after three months.
- Consider the fats. Coconut and palm oils are rich in saturated fats, so they’re more stable and better for high-heat cooking. Polyunsaturated fats like those in vegetable, soybean, sunflower, safflower, flaxseed, grapeseed, and sesame are higher in omega-6 fats, which we already get too many of in our modern Western diet and which have been linked to heart disease. Choose olive, almond, and avocado oil when you can to get more of those omega-3 fats.
- Choose extra virgin. This indicates the oil was derived from the first pressing of the olive, in most cases. Research the company, however, as sometimes manufacturers will simply add a splash of extra virgin oil into a bottle of regular oil and then charge the higher price. Make sure you’re buying from companies you trust.
In essence, you can think of chemically extracted and refined oils as white bread—a product that started out full of nutrients and healthy antioxidants, then was processed, stripped, treated, and heated until it lost most of those healthy things and became a relative “nonfood,” after which synthetic preservatives are added to extend shelf life.
What to Do
If you’d like to be sure the oil you’re using is healthy and natural, check out our post on “What’s the Best Oil to Cook With?,” then consider the following steps:
How do you choose the healthiest oils? Please share your tips.
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Carol Ness, “How refining affects edible oils,” SFGate, March 12, 2003, http://www.sfgate.com/food/article/How-refining-affects-edible-oils-2664089.php.
“Understanding Oil Extraction Methods” Expeller pressed vs. solvent extracted oils,” Spectrum Ingredients, http://www.spectrumingredients.com/product/gen_info/oil_extract.html.
“Oil extraction,” http://www.britannica.com/EBchecked/topic/426152/oil-extraction.
Fats Oils in Human Nutrition: Chapter 5: Processing and refining edible oils, Agriculture and Consumer Protection, http://www.fao.org/docrep/v4700e/v4700e0a.htm.