Processed oils are cooking oils that have been mechanically and chemically refined to remove color, flavor, odor, and impurities from the crude oil extracted from seeds or plants. Most liquid vegetable oils on grocery shelves, including soybean, canola, corn, sunflower, and safflower oil, go through this multi-step refining process. The result is a neutral-tasting, long-lasting oil that looks identical regardless of the plant it came from, but one that has lost a significant portion of its original nutrients along the way.
How Oil Gets Extracted From Seeds
The processing journey starts with extraction, the step that separates oil from the seed or plant material. There are two main approaches: mechanical pressing and solvent extraction.
Mechanical pressing (sometimes labeled “expeller-pressed” or “cold-pressed”) physically squeezes oil out of crushed seeds using high pressure. This method is simpler and leaves more of the oil’s natural compounds intact, but it doesn’t capture all the available oil from the seed.
Solvent extraction is far more common in large-scale production. Seeds are crushed and then mixed with an organic solvent, typically n-hexane, which dissolves the oil out of the seed material. The solution is then heated to evaporate the solvent, which gets condensed and reused. The EPA classifies n-hexane as a hazardous air pollutant, and it’s the chemical emitted in the largest quantity from vegetable oil extraction facilities. This method is used for soybeans, cottonseed, canola, corn germ, sunflower, safflower, peanuts, and flax. After extraction, what you have is crude oil. It’s dark, strong-smelling, and contains compounds that would make it go rancid quickly. That’s where refining begins.
The Four Stages of Refining
Crude oil goes through a sequence of steps known collectively as RBD processing: refining, bleaching, and deodorizing. Each stage strips away specific compounds.
Degumming
The first step is an acid wash, often using citric acid, with the oil heated to around 176°F (80°C). This causes phosphorus-containing compounds and other water-attracting substances to clump together so they can be filtered or spun out of the oil. Degumming removes about 83% of phospholipids from the oil. It also strips out 88% to 90% of certain minerals like calcium and magnesium.
Neutralizing
Free fatty acids in crude oil contribute to off flavors and reduce shelf life. Neutralization uses an alkaline solution to bind with these acids so they can be separated from the oil. This step also removes 4% to 16% of the oil’s vitamin E content.
Bleaching
Bleaching doesn’t use liquid bleach. Instead, the oil is mixed with a special absorbent clay that attracts and holds pigments, residual soaps, and oxidation products. Activated clay grabs more compounds than natural clay (organic oils use natural clay). This step eliminates up to 96% of chlorophylls and roughly 77% of carotenoids, the plant pigments that give crude oil its color.
Deodorizing
Deodorizing is the final and most aggressive step. The oil is heated to very high temperatures, typically between 220°C and 270°C (430°F to 518°F), under vacuum to strip away volatile compounds responsible for taste and smell. This leaves the oil completely flavorless and odorless. It also removes an additional 4% to 26% of remaining vitamin E, depending on the method used. More importantly, the extreme heat can chemically alter the fatty acids in the oil.
What Processing Does to the Oil’s Nutrition
Each refining stage removes something, and the cumulative losses are substantial. Research on sunflower oil illustrates the scale. Total phenolic content, a measure of protective plant antioxidants, drops from about 19 mg per 10 grams of oil down to roughly 1.8 mg. That’s a loss of over 90%. Total vitamin E (tocopherols) decreases by 14% to 34% through chemical refining and by as much as 55% through physical refining. Phytosterols, plant compounds that can help manage cholesterol levels, drop by about 60%. Beta-carotene, a precursor to vitamin A, falls by 80%. Squalene, another naturally occurring antioxidant, decreases by roughly 33%.
What remains after full RBD processing is almost pure fat. The oil still has calories and its basic fatty acid profile (omega-6, omega-3, monounsaturated), but the protective minor compounds that came packaged with those fats in the original seed are largely gone.
Trans Fats and Oxidation Byproducts
The high temperatures used during deodorizing don’t just remove compounds. They can create new ones. When oils rich in certain polyunsaturated fats (particularly alpha-linolenic acid, an omega-3) are heated above 220°C, some of the fatty acid molecules flip into a different shape, forming trans fatty acids. In rapeseed oil, the extent of this conversion ranged from about 4% to 45% of the linolenic acid depending on temperature and time. A survey of commercial linseed oils found that 9% of samples exceeded 2 grams of trans fats per 100 grams of oil.
Trans fats raise LDL (“bad”) cholesterol and lower HDL (“good”) cholesterol, increasing the risk of heart disease and stroke. On ingredient labels, you won’t see them listed as “trans fats.” Instead, look for the terms “partially hydrogenated oil” or “hydrogenated oil,” which indicate fats that were intentionally converted. The trace amounts formed during deodorizing are harder to spot because they don’t require separate labeling if they fall below certain thresholds.
There’s also the question of oxidized fatty acids. Linoleic acid, the dominant omega-6 fat in most processed seed oils, is prone to oxidation. When oxidized linoleic acid ends up in LDL cholesterol particles, it triggers a chain reaction: immune cells called macrophages absorb the damaged LDL, forming foam cells that build up in artery walls. This is a core mechanism of atherosclerosis. Oxidized linoleic acid metabolites also directly damage blood vessel linings by promoting inflammation and increasing permeability.
Why Processed Oils Have Higher Smoke Points
One practical advantage of refining is that it raises an oil’s smoke point, the temperature at which oil begins to break down and release visible smoke. Free fatty acids are a major factor in lowering smoke points, and refining removes them. Virgin olive oil, for example, has a smoke point between 350°F and 410°F, while refined olive oil ranges from 390°F to 470°F. This makes refined oils more predictable for high-heat cooking like frying and sautéing. It’s one of the main reasons food manufacturers prefer them.
How to Identify Processed Oils on Labels
Most cooking oils in clear plastic bottles at the grocery store are fully refined unless the label states otherwise. A few label terms help you distinguish processing methods:
- Cold-pressed or expeller-pressed: Oil was extracted mechanically without chemical solvents. It may still be refined afterward, but the extraction step was gentler.
- Unrefined or virgin: The oil skipped the RBD process entirely and retains its natural color, flavor, and nutrients.
- Refined: The oil went through the full degumming, bleaching, and deodorizing sequence.
- Partially hydrogenated: The oil was chemically altered to be more solid at room temperature, creating trans fats in the process.
- Hydrogenated or interesterified: The fat structure was modified, often to replace partially hydrogenated oils in processed foods. These are heavily engineered fats found mostly in packaged goods.
If a bottle of vegetable oil is very light in color, has no discernible smell, and tastes completely neutral, it has almost certainly gone through the full refining process. Unrefined oils are darker, have a distinct flavor tied to the plant they came from, and typically have a shorter shelf life because the natural compounds that were stripped during refining are the same ones that eventually cause rancidity.