Oils and Cooking Fats

TL;DR. Most of what matters about dietary fat is in Chapter 13; this chapter is about the bottle in your cupboard. How oils are extracted decides what survives in them: cold-pressed keeps the polyphenols and the flavour, refined keeps almost nothing but the fat and gains heat stability. Smoke point is the number everyone quotes and oxidative stability is the one that matters. Extra virgin olive oil is more heat-stable than its reputation, and reused frying oil is the genuine hazard in a domestic kitchen.

1. How oil gets out of a seed

Nearly all cheap seed oil is solvent-extracted and refined. Residual hexane in the finished oil is regulated and measured in parts per million; the toxicological concern is occupational rather than dietary. The nutritional cost of refining is the loss of the minor components: polyphenols, tocopherols beyond what is added back, phytosterols, and chlorophyll.

Deodorisation at high temperature also creates a small amount of trans fat (typically under 1 percent) and, in some oils, glycidyl esters and 3-MCPD, which is why palm oil in particular is now subject to limits on those compounds in the EU.

Don't be confused: "cold-pressed" and "unrefined" are not the same as "healthy," and "refined" is not the same as "bad." Refined oils are more heat-stable and better for high-temperature frying. Unrefined oils carry more of the plant's chemistry and more flavour and degrade faster. Use both, for different jobs.

2. The oils, compared

Approximate fatty acid composition and smoke point:

3. Smoke point versus oxidative stability

Smoke point is the temperature at which an oil visibly smokes, and it depends heavily on free fatty acid content, which rises as an oil ages or is reused. It is the number on every chart and it is a weaker guide than it looks.

Oxidative stability is what actually determines whether an oil degrades into harmful compounds, and it is set by how many double bonds the fat has. Each double bond is a site for oxidation. So:

• Saturated fats (coconut, ghee) are the most stable.
• Monounsaturated fats (olive, high-oleic sunflower, avocado) are next.
• Polyunsaturated fats (standard sunflower, soybean, corn, flax) are the least stable.

Plus antioxidant content: polyphenols and vitamin E slow oxidation from within, which is why extra virgin olive oil outperforms its smoke point.

A 2018 Australian study by de Alzaa and colleagues heated ten common oils and measured the degradation products, and found extra virgin olive oil produced the fewest polar compounds and was among the most stable overall, despite having a lower smoke point than several refined oils. That result has been replicated in similar work. The received wisdom that EVOO is unsuitable for cooking is not supported.

What actually forms when oil degrades: free fatty acids, polar compounds, polymers, and aldehydes, including acrolein (the acrid smell of burning oil) and 4-hydroxynonenal, which are the compounds of genuine concern in heavily reused frying oil.

4. Frying, and the real domestic hazard

In short: Reused, overheated frying oil is the one fat-related risk in a home kitchen that is worth taking seriously.

Oil degrades with each frying cycle. Commercial fryers monitor total polar compounds and are legally required in many countries to discard oil above 24 to 27 percent. Domestic kitchens monitor nothing.

Signs oil should be discarded:

• It has darkened substantially.
• It foams on the surface when food is added.
• It smokes at a lower temperature than it used to.
• It smells acrid, sharp, or fishy.
• It has become viscous or sticky.

Practical rules for frying:

• Use a stable oil: high-oleic sunflower, peanut, refined olive, or lard.
• Keep the temperature at 170 to 190 °C. Too low and food absorbs more oil; too high and the oil degrades fast.
• Strain the oil after each use and store it cool and dark.
• Reuse two or three times at most for shallow frying, and fewer if you fried anything strongly flavoured or battered.
• Do not top up old oil with fresh; the degradation products catalyse further degradation.

5. Storage, rancidity, and buying

Oil has three enemies: light, heat, and oxygen.

• Buy dark glass or tin, not clear plastic. Supermarket olive oil in clear bottles on a lit shelf is already degrading.
• Store in a cupboard, not next to the hob, which is where almost everyone keeps it.
• Buy sizes you will use within a couple of months of opening.
• Look for a harvest date on olive oil, not just a best-before. Olive oil is at its best within 12 to 18 months of harvest.
• Refrigerate the fragile ones: flaxseed, walnut, and toasted sesame oil belong in the fridge.
• Rancid oil smells of crayons, putty, or old paint and tastes sharp and bitter. Most people have become so used to mildly rancid oil that they do not notice; taste a fresh, good oil alongside an old one and the difference is obvious.

Adulteration is a persistent problem in olive oil specifically. Investigations and Italian prosecutions have repeatedly found oil labelled extra virgin that fails the chemical or sensory standard, or that is cut with cheaper oils. Defences: named estate or region, harvest date, dark glass or tin, a protected designation, and suspicion of prices that are too low.

The fridge test is a myth. Different real olive oils solidify at different temperatures depending on their fatty acid profile, and some adulterants also solidify. It proves nothing.

6. Butter, margarine, and spreads

Butter is roughly 80 percent milk fat, 16 percent water, and a little milk solids, and its 63 percent saturated fat content is why it raises LDL relative to unsaturated oils (Chapter 13).

Margarine has changed completely. Old hard margarines were made by partial hydrogenation and contained substantial trans fat, which was worse than butter. Since trans fat elimination (2018 to 2023 across most of the world), modern soft spreads are made by interesterification and blending, contain little or no trans fat, and are typically lower in saturated fat than butter and higher in unsaturated.

So the old advice to prefer butter over margarine, which was correct in the 1990s, is no longer correct for modern products. Read the label: the saturated fat figure and the absence of "partially hydrogenated" is what matters.

Plant sterol and stanol spreads (Benecol, Flora ProActiv) are a legitimate functional food: 2 to 3 g a day of sterols reduces LDL by around 8 to 12 percent by competing with cholesterol for absorption in the gut. They carry authorised health claims in the EU and US. They do slightly reduce carotenoid absorption, and there is no outcome trial showing they reduce cardiovascular events.

7. Palm oil

Worth a note because it is in an enormous share of processed food and because the issues are environmental rather than nutritional.

Nutritionally it is around 49 percent saturated, mostly palmitic acid, which raises LDL. Red (unrefined) palm oil is extremely high in carotenoids and tocotrienols; the refined version used industrially is not.

Why it is everywhere: it is semi-solid at room temperature without hydrogenation, which made it the obvious replacement when trans fats were banned; it is stable, cheap, and extremely high yielding, producing 4 to 10 times more oil per hectare than any alternative.

The environmental problem is deforestation and peatland conversion in Indonesia and Malaysia, with consequences for orangutans, carbon emissions, and Indigenous land rights. Boycotting it is not straightforwardly the answer, because substitutes need far more land for the same oil. The mainstream conservation position, including from the IUCN, is that certified sustainable production and preventing further forest conversion beats substitution. Certification schemes (RSPO) have real and contested effectiveness.

8. Practical guidance

• Two oils cover most needs: a good extra virgin olive oil for dressing and most cooking, and a neutral, stable, higher-smoke-point oil (rapeseed, high-oleic sunflower, refined olive) for high heat.
• Add a finishing oil if you enjoy it: toasted sesame, walnut, pumpkin seed, or a peppery early-harvest olive oil, used off the heat.
• Replace saturated fat with unsaturated fat, not with refined carbohydrate. This is the substitution that trials support (Chapter 13).
• Do not fear cooking with olive oil.
• Discard degraded frying oil. This is the one genuinely important safety habit here.
• Store everything dark, cool, and closed, and refrigerate flax, walnut, and toasted sesame.

9. The bottom line

• Extraction method decides what is in the bottle: cold-pressed keeps polyphenols and flavour, refining strips them and buys heat stability.
• Oxidative stability, driven by the number of double bonds plus antioxidant content, matters more than smoke point. Extra virgin olive oil is more heat-stable than its reputation and is fine for ordinary cooking.
• Never heat flaxseed oil; treat toasted sesame and nut oils as finishing oils.
• The real hazard in a home kitchen is reused, darkened, foaming frying oil. Strain it, limit reuse, and throw it out when it changes.
• Modern soft margarines no longer contain trans fat and are generally lower in saturated fat than butter, which reverses the old advice.
• Palm oil is a nutritional negative and an environmental problem that substitution would probably make worse, not better.

Sources and notes

Extraction, refining, and the RBD process follow standard fats and oils technology texts. Trans fat and 3-MCPD/glycidyl ester formation during deodorisation follow EFSA's 2016 opinion and subsequent EU limits. Fatty acid compositions and smoke points follow Codex standards and the Institute of Shortening and Edible Oils reference. Oxidative stability versus smoke point, and the comparative heating study finding extra virgin olive oil among the most stable, is de Alzaa, Guillaume, and Ravetti, Acta Scientific Nutritional Health, 2018. Aldehyde formation in reused frying oil follows Grootveld's work at De Montfort. Total polar compound limits for commercial fryers follow national food hygiene regulations. Plant sterol LDL reduction follows EFSA's authorised health claim and Katan et al.'s meta-analysis. Interesterification replacing partial hydrogenation follows food technology literature. Palm oil yields, deforestation, and the IUCN position that substitution would require more land follow the IUCN Oil Palm Task Force report, 2018. Olive oil adulteration follows Italian prosecutions and UC Davis Olive Center testing.

Open questions. Whether interesterified fats carry any metabolic disadvantage relative to natural fats is an open question with limited human data. How much aldehyde exposure from domestic frying actually matters to health has not been quantified in people.

👉 Next: dairy and eggs.