What is the smoke point of an oil and why does it matter?

Smoke point gets quoted as a fixed figure per oil, which is misleading. It is a property of a particular bottle in a particular state, and the useful version of the idea is about margin: how far above your cooking temperature the fat sits, and how fast that margin is disappearing as you reuse it.

Safety

Step by step

1. Understand what is actually smoking.Fats break down under heat into free fatty acids and glycerol, and glycerol decomposes into acrolein — a sharp, irritant compound you can feel at the back of your throat. The visible smoke is that breakdown. It carries a burnt, bitter flavour straight into the food and it will not cook out.
2. Know that the number varies with refining.Unrefined oils keep more of the plant material they came from, and those fragments burn first. Extra virgin olive oil, unrefined sesame and cold-pressed rapeseed all smoke well below their refined equivalents. Refined or 'light' versions of the same oil sit substantially higher, which is what refining is for.
3. Know that butter is a special case.Butter smokes early not because of its fat but because of its milk solids and water, which brown and then burn. Clarified butter or ghee, with the solids removed, behaves like a normal high-smoke-point fat. This is why so many recipes pair butter with a splash of oil — the oil raises the working temperature.
4. Match the fat to the job with margin.Gentle sweating runs at 120–140°C, shallow frying at 160–180°C, deep frying at 160–190°C, searing meat at 200°C or more at the metal, stir-frying higher again. A fat whose smoke point merely touches the job will smoke as soon as the pan runs slightly hot. Pick one that clears it.
5. Watch the fat, not the clock.Oil in a pan tells you where it is: still and glassy when cool, moving and shimmering as it approaches frying temperature, then the first wisps of smoke. Shimmering is the signal to add food. Smoke is the signal that you have gone past it — take the pan off the heat and let it cool before adding anything.
6. Expect the smoke point to fall with reuse.Frying breaks oil into smaller fragments and drives water and food particles into it. Used oil smokes sooner, foams, darkens and smells stale. Strain it after every use, keep it in the dark, and discard it once it smokes early, foams heavily or smells rancid — typically after a handful of uses.

Tips

• The idea that heating olive oil makes it harmful is overstated, but extra virgin olive oil genuinely does smoke and taste burnt at frying temperatures. Use it for dressing, low sautéing and finishing; use refined oils for heat.
• A pan of oil that smokes the instant it is hot is telling you the oil is done, not that the hob is too high.
• Never judge oil temperature by how long it has been on the heat. The same hob setting takes very different times with different pans and different oil depths.
• Extractor on and window open before you start anything at frying temperatures — acrolein is an irritant to eyes and airways well before it is a fire risk.

Common mistakes

• Deep frying in extra virgin olive oil or in butter — Both break down inside or just above the frying range. The food tastes burnt, the kitchen fills with acrid smoke, and the fat is unusable afterwards.
• Topping up old oil with fresh instead of replacing it — The degraded oil contaminates the new. You get the low smoke point of the old batch and a larger volume to dispose of.

Questions people ask