How Seed Oils Are Made
And Why That Matters for Your Health
Most people, if asked how cooking oil is made, would picture something fairly simple: seeds going in, oil coming out. Perhaps a press, perhaps some heat. A natural process yielding a natural product. The reality — for the vast majority of seed oils lining supermarket shelves and filling commercial kitchen fryers — is considerably more complicated, and considerably less wholesome than that picture suggests.
Understanding how seed oils are produced is not about demonising a category of food. It’s about having an accurate picture of what you’re actually consuming, and what that means for your body. Because the process that turns a sunflower seed or a soybean into a bottle of refined vegetable oil is an industrial operation involving high temperatures, chemical solvents, and multiple stages of treatment — each of which has real implications for the nutritional profile and biological effects of the oil that ends up in your food.
This article pulls back the curtain on that process, step by step. It explains what happens to the oil along the way, what the science says about the compounds that form as a result, and how you can use that knowledge to make genuinely better choices — whether you’re cooking at home or reading labels in a supermarket aisle.
Listen on Spotify — a deeper dive into the industrial process behind this article.
✦ Key Takeaways
- Most commercial seed oils go through an industrial refining process involving chemical solvents, high heat, and multiple decontamination stages — far removed from simple pressing.
- This process destroys naturally occurring nutrients including vitamin E, plant sterols, and polyphenols that would otherwise offer biological benefits.
- High-heat processing promotes oxidation of polyunsaturated fatty acids, generating potentially harmful compounds including aldehydes and lipid peroxides.
- Traditional fats like extra virgin olive oil, butter, and tallow are processed very differently — and retain a meaningfully different chemical profile as a result.
- Labels like “vegetable oil,” “refined,” or “RBD” are useful signals of heavily processed oils. Cold-pressed and extra virgin designations indicate minimal processing.
Step-by-Step: How Industrial Seed Oils Are Made
The term “RBD oil” — standing for Refined, Bleached, and Deodorised — describes the endpoint of a multi-stage industrial process that most commercial seed oils go through. Each of those three stages exists to solve a problem created by the previous one. Understanding the sequence helps you see why the final product is so chemically different from the seed it started as.
Before any oil can be extracted, the seeds — whether soybeans, sunflower seeds, rapeseed (canola), or corn kernels — are cleaned to remove debris and then conditioned using heat and moisture. This loosens the cellular structure of the seed and makes the oil easier to extract. Seeds are typically heated to temperatures between 60°C and 88°C (140–190°F) at this initial stage. Soybeans are often cracked, then rolled into thin flakes to maximise surface area for the extraction that follows.
In some operations, particularly for higher-oil-content seeds like sunflower or rapeseed, an initial mechanical press extracts a portion of the oil. This uses a screw press or expeller, which generates significant pressure — and, as a consequence, significant heat through friction. Temperatures during expeller pressing can reach 85–95°C (185–200°F), sometimes higher. This already begins the process of degrading heat-sensitive compounds in the oil. For lower-oil-content seeds like soybeans, mechanical pressing alone isn’t considered economically viable, so solvent extraction is the primary method.
This is the step that tends to surprise people most. After mechanical pressing — or instead of it — the seed material is typically bathed in hexane, a petroleum-derived solvent chemically similar to petrol. Hexane dissolves the remaining oil out of the seed material with high efficiency. The seed-and-solvent mixture is then heated to evaporate the hexane, leaving behind crude oil. While the food industry maintains that hexane residues in the final product are extremely low, it is worth noting that hexane is a neurotoxic industrial chemical, and its use is not disclosed on product labels. The environmental impact of hexane in processing facilities is also a concern that regulatory bodies continue to monitor.
The crude oil extracted at this point contains a range of compounds beyond just fatty acids: phospholipids (gums), free fatty acids, pigments, waxes, and various other plant compounds. While some of these — like phospholipids — have nutritional value, they affect the oil’s appearance, stability, and shelf life in ways that are commercially undesirable. Degumming uses water or acid to remove the phospholipids. Refining follows, using an alkali solution (typically sodium hydroxide — caustic soda) to neutralise free fatty acids, which are then separated and removed. This alkali treatment further strips the oil of naturally occurring beneficial compounds.
After refining, the oil is still coloured — typically yellow, brown, or green — due to remaining pigments including chlorophylls and carotenoids. From a commercial standpoint, consumers associate colour in cooking oil with impurity, so bleaching is performed using activated clay or charcoal adsorbents that bind and remove these pigments. The irony is that those pigments — carotenoids in particular — are antioxidants with genuine biological value. Bleaching removes them, producing a pale, visually “clean” oil that has been further stripped of its natural protective compounds.
The final and most thermally intensive stage. By this point the oil has an unpleasant smell — a result of volatile compounds formed during earlier processing, as well as natural odour compounds from the seed itself. Deodorising uses steam distillation under high heat — temperatures commonly ranging from 240°C to 270°C (464–518°F) — for periods of 30 minutes to several hours. This eliminates most volatile odour compounds. However, at these temperatures, the polyunsaturated fatty acids in the oil undergo further chemical changes. Some trans fatty acids can form. Oxidation byproducts accumulate. The oil that emerges is neutral in smell and taste — and significantly altered from its original state.
A bottle labelled simply “vegetable oil” or “sunflower oil” gives you no indication of how many processing stages it has been through, what temperatures it has reached, or whether hexane was involved in its extraction. RBD oils are the industry standard — but that designation rarely appears on consumer packaging.
What Happens to the Oil During Processing
Nutrients lost in translation
A whole seed is nutritionally rich. It contains not just fatty acids but a range of biologically active compounds: vitamin E (tocopherols and tocotrienols) which protect the seed’s own fats from oxidation; phytosterols which help modulate cholesterol absorption; polyphenols with antioxidant activity; and CoQ10, a compound involved in cellular energy production. These are not trace amounts — they are present in meaningful concentrations in unprocessed seeds and cold-pressed oils.
The industrial refining process strips most of them away. Vitamin E content is significantly reduced by the combined effects of heat, alkali treatment, and bleaching. Phytosterols are partially removed. Polyphenols are almost entirely eliminated. What remains after RBD processing is, in terms of micronutrient content, a shadow of the original seed. The oil provides calories and fatty acids — but very little of the protective nutritional complexity that whole food sources of fat retain.
Oxidation byproducts: the compounds of concern
Polyunsaturated fatty acids (PUFAs) are chemically reactive. Their multiple double bonds — the structural feature that defines them — make them vulnerable to oxidation: a process whereby oxygen reacts with the fat molecule and causes it to break down. Heat dramatically accelerates this process.
During the high-temperature deodorising stage in particular, PUFAs begin to oxidise and generate a range of breakdown compounds. These include lipid peroxides (early oxidation products), aldehydes (including the reactive compound 4-hydroxynonenal, or 4-HNE), and a range of other volatile and non-volatile degradation products. Some research suggests that even before a bottle of RBD seed oil reaches your kitchen — before you’ve applied any heat yourself — it may already contain measurable levels of these oxidation products from processing.
“By the time a refined seed oil reaches your kitchen, it has already been exposed to temperatures exceeding 250°C, a petroleum-derived solvent, caustic soda, and industrial bleaching agents. What remains is chemically very different from what began as a seed.”
Why This Matters Biologically
Oxidised fats and the body’s response
Oxidised lipids are not inert. When consumed, they enter the body’s metabolic pathways and can interact with cells, proteins, and DNA in ways that healthy, intact fats do not. 4-Hydroxynonenal (4-HNE), one of the most studied aldehyde byproducts of linoleic acid oxidation, has been shown in cell and animal studies to be cytotoxic at higher concentrations — capable of causing DNA damage, disrupting mitochondrial function, and triggering inflammatory signalling pathways. Oxidised LDL cholesterol — which forms when lipid peroxides react with LDL particles — is a well-established contributor to atherosclerotic plaque development.
It is important to note the same caveat that applies across much of this field: many of the most striking findings come from cell studies or animal models at concentrations that may not directly reflect typical human dietary exposure. The dose and the biological context matter enormously. What is established clearly is that chronically elevated oxidative stress — from whatever sources — is associated with worse health outcomes, and that consuming already-oxidised fats represents a meaningful additional oxidative burden on the body.
Chemical stability: why it matters for cooking
One of the most important but underappreciated properties of a cooking fat is its oxidative stability — how well it resists breaking down further when exposed to heat, light, and oxygen during cooking. This is determined largely by the fat’s chemical structure. Saturated fats, which have no double bonds, are extremely stable and very resistant to oxidation even at high temperatures. Monounsaturated fats — like oleic acid, the dominant fatty acid in olive oil — have one double bond and are moderately stable. Polyunsaturated fats, with two or more double bonds, are significantly less stable and degrade much more readily under heat.
Seed oils, which are high in PUFAs, are therefore inherently less suitable for high-temperature cooking than more stable fats — not because of their omega-6 content per se, but because their chemical structure makes them prone to generating oxidation byproducts when heated. This is not a fringe concern: it is a straightforward consequence of organic chemistry that is accepted across the food science literature.
Traditional Fats vs. Industrial Seed Oils: A Comparison
The difference between how traditional fats and modern seed oils are produced is not simply a matter of degree — in many cases, it is a categorical difference in the nature of the process itself. Here is how the most common fat sources compare:
| Fat / Oil | Production Method | Heat Stability | Processing Level |
|---|---|---|---|
| Extra Virgin Olive Oil | Cold mechanical pressing, no solvents, no bleaching or deodorising | Moderate — suitable for medium heat | Minimal |
| Butter | Churning of cream — a single mechanical process | High — suitable for sautéing and roasting | Minimal |
| Beef Tallow / Lard | Rendering (low-heat melting) of animal fat tissue | Very high — traditional deep-frying fat | Minimal |
| Coconut Oil (unrefined) | Cold pressing or centrifugal extraction | High — mostly saturated fat, very stable | Minimal |
| Cold-Pressed Sunflower Oil | Mechanical pressing only, no solvents or RBD treatment | Low — not recommended for high heat | Low |
| Refined Sunflower / Soybean Oil | Hexane extraction + full RBD refining at 240–270°C | Low — PUFA-rich, prone to oxidation | Heavy |
| Canola Oil (refined) | Hexane extraction + full RBD refining | Low-moderate — some trans fat formation possible during processing | Heavy |
| Corn Oil | Hexane extraction + full RBD refining | Low — very high in linoleic acid, unstable at heat | Heavy |
Extra virgin olive oil has been produced by essentially the same method for thousands of years: crush the olives, press the paste, separate the oil. No solvents, no bleaching, no temperatures above 27°C for cold-pressed varieties. The polyphenols, vitamin E, and oleocanthal (a natural anti-inflammatory compound) remain largely intact. This is why extra virgin olive oil occupies such a different position in the health literature from refined vegetable oils — it is, in processing terms, an entirely different category of product.
What the Science Says
Stability, oxidation, and lipid peroxidation
The food science literature is fairly consistent on the question of thermal stability: polyunsaturated fatty acids oxidise faster than monounsaturated or saturated fats when heated, producing a broader range of degradation compounds at lower temperatures. Research published in the journal Acta Scientific Nutritional Health found that when common cooking oils were tested at typical frying temperatures, extra virgin olive oil produced significantly lower levels of polar compounds — a class of oxidation byproducts — compared to refined vegetable oils. Coconut oil and butter performed similarly well on stability measures. Refined sunflower oil and corn oil generated considerably higher levels of degradation compounds under the same conditions.
Separately, a body of research has investigated the relationship between dietary oxidised fats and cardiovascular and metabolic health markers. While the picture is not yet complete — particularly in terms of long-term human dietary trials — the mechanistic evidence linking consumed lipid peroxidation products to oxidative stress, endothelial dysfunction, and inflammatory signalling is well-established in the scientific literature. The National Toxicology Program in the United States has formally identified 4-HNE as a compound of concern based on its demonstrated cytotoxicity in multiple model systems.
Trans fatty acids — which can form in small but measurable quantities during the high-temperature deodorising of seed oils — represent a separate concern. The health consequences of trans fat consumption are not disputed: they raise LDL cholesterol, lower HDL cholesterol, promote inflammation, and are independently associated with cardiovascular disease risk. Industrial trans fats from partial hydrogenation have been largely removed from the food supply following regulatory action, but the lower levels of trans fats that form during RBD deodorising remain less regulated and less discussed publicly.
How to Identify Heavily Processed Oils
Navigating food labels with any confidence requires knowing what to look for — and what manufacturers are not required to tell you. Here is a practical guide to decoding the language around cooking oils.
Reading Oil Labels: A Quick Guide
The restaurant and packaged food trap
Home cooking represents only a portion of the seed oil exposure for most people. Restaurants — particularly fast food and mid-range casual dining — cook almost universally in refined seed oils, most often soybean oil, corn oil, or sunflower oil. These oils are often stored in large fryers and heated repeatedly over the course of an operating day. Each heating cycle increases oxidation, accumulates degradation compounds, and reduces the smoke point of the oil. Studies measuring the aldehyde content of commercial frying oils at the end of a working shift have found levels significantly elevated compared to fresh oil.
Packaged and ultra-processed foods present an additional layer of complexity. The ingredient lists of biscuits, crackers, crisps, instant noodles, cereal bars, cooking sauces, ready meals, and fast food almost universally feature refined seed oils — often as one of the top three ingredients by weight. These products represent the primary vehicle through which most people in the developed world consume seed oils in quantity, and they do so in an already partially-oxidised state, often combined with refined carbohydrates, excess sodium, and minimal fibre.
Conclusion: Not All Fats Are Created Equal
The most important takeaway from understanding how seed oils are made is this: processing is not a trivial detail. It is central to what the product actually is, what it contains, and how it behaves in your body. A cold-pressed, unrefined sunflower oil retains its vitamin E, its natural flavour compounds, and its phospholipids. A RBD-refined sunflower oil — despite coming from the same seed — has been stripped of most of those compounds, exposed to industrial chemicals and extreme temperatures, and may already contain measurable oxidation products before it even reaches your kitchen.
The conversation about seed oils too often collapses these distinctions. Critics of seed oils are sometimes making legitimate points about industrial RBD oils used in deep-fryers and processed foods. Defenders of seed oils are often citing data on linoleic acid itself, or on studies using cold-pressed varieties. Both groups are frequently talking past each other because neither is clearly specifying what type of seed oil, at what processing level, in what culinary context, they are discussing.
The practical implications are straightforward, even if the science is nuanced. Use extra virgin olive oil for most cooking and all cold applications — it is the most robustly supported fat in the scientific literature, with a processing method that has changed little in millennia. Use butter, ghee, or coconut oil for higher-heat cooking where you want stability. Reduce your consumption of ultra-processed foods, which are the primary source of heavily refined, oxidised seed oils in most people’s diets. And when choosing packaged cooking oils, look for cold-pressed or unrefined labels and treat “vegetable oil” as the processed product it is.
This is not about dietary extremism. It is about understanding that how a food is produced affects what it becomes — and that in the case of refined seed oils, industrial processing creates a product that is meaningfully different from the natural food sources of fat that human bodies evolved alongside.
Frequently Asked Questions
Is hexane in cooking oil dangerous?
Hexane is a petroleum-derived solvent used in the extraction of many commercial seed oils. Food manufacturers and regulatory bodies maintain that residue levels in the final product are extremely low — typically below 1 part per million — and within what is considered safe for consumption. However, critics note that there is no comprehensive long-term data on the health effects of chronic low-level dietary hexane exposure, and that its use is not disclosed on consumer labels. Whether you consider this a meaningful concern or an acceptable industrial practice is ultimately a personal judgement call.
Is “cold-pressed” always better than refined?
For nutritional profile and oxidation status at the point of purchase, yes — cold-pressed oils retain more of their naturally occurring beneficial compounds and have not been subjected to high-temperature processing. However, cold-pressed oils are also less shelf-stable (because the antioxidants that would degrade in refining are still present but won’t prevent rancidity indefinitely) and should be stored away from heat and light, used within their shelf life, and generally not used for high-temperature cooking. Cold-pressed is not a guarantee of quality unless the oil is also fresh and stored correctly.
What is the best oil for frying?
For high-heat frying, you want an oil with a high smoke point and high oxidative stability — which means a fat that is predominantly saturated or monounsaturated. Refined avocado oil, ghee, tallow, lard, and coconut oil are all good choices for high-heat cooking. Refined olive oil (not extra virgin) is also more heat-stable than polyunsaturated seed oils. Extra virgin olive oil is suitable for medium-heat sautéing but not ideal for deep-frying. Polyunsaturated seed oils like sunflower, corn, and soybean oil should be avoided at high temperatures wherever possible.
Does olive oil go through the same industrial process?
Extra virgin olive oil does not. It is produced by mechanical pressing of whole olives at temperatures below 27°C, with no solvents and no RBD treatment — hence its distinctive green colour, pronounced flavour, and relatively high polyphenol content. “Pure olive oil” or “light olive oil,” however, is typically a blend of cold-pressed and refined olive oil, where the refined portion has been through a process similar to (though not identical to) the RBD process used for seed oils. Extra virgin is the designation to look for if you want minimal-processing guarantees.
Are there any seed oils that are processed minimally?
Yes. Cold-pressed flaxseed oil, cold-pressed hemp seed oil, and cold-pressed walnut oil are examples of seed-derived oils that are produced without chemical solvents or high-temperature refining. These retain their nutritional profiles — including beneficial omega-3 fatty acids — but are very heat-sensitive and should only be used cold (in dressings, smoothies, or drizzled over food after cooking). They should not be used for cooking. They are also considerably more expensive than refined commercial seed oils, which is largely why they remain niche products.
Sources & Further Reading
- De Alzaa F, Guillaume C, Ravetti L. (2018). Evaluation of chemical and physical changes in different commercial oils during heating. Acta Scientific Nutritional Health, 2(6), 2–11.
- Choe E, Min DB. (2007). Chemistry of deep-fat frying oils. Journal of Food Science, 72(5), R77–R86.
- Grootveld M et al. (2020). Healthy frying: challenges, opportunities and implications. Free Radical Research, 54(11-12), 906–941.
- Esterbauer H, Schaur RJ, Zollner H. (1991). Chemistry and biochemistry of 4-hydroxynonenal, malonaldehyde and related aldehydes. Free Radical Biology and Medicine, 11(1), 81–128.
- Simopoulos AP. (2008). The omega-6/omega-3 fatty acid ratio, genetic variation, and cardiovascular disease. Asia Pacific Journal of Clinical Nutrition, 17(S1), 131–134.
- Frankel EN. (1991). Recent advances in lipid oxidation. Journal of the Science of Food and Agriculture, 54(4), 495–511.
- Mozaffarian D, Rimm EB. (2006). Fish intake, contaminants, and human health: evaluating the risks and the benefits. JAMA, 296(15), 1885–1899.
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