Smoke Point.
Extra-virgin olive oil smokes near 200 °C, refined peanut oil near 225 °C. Fry in either ten times and both smoke well below where they started.
Heat an oil until a thin blue smoke rises: that is its smoke point. What smokes first are the small molecules in the oil — free fatty acids split off the triglycerides, traces of water, and the flavour compounds that make an unrefined oil taste of something — and the breakdown products of glycerol, including acrid acrolein. Above the smoke point flavours turn harsh and the oil degrades fast; further up lie the flash point and, beyond it, fire (IX.8).
The smoke point is not a fixed property. Refining removes free fatty acids and flavour molecules and raises it; that is why refined rapeseed, peanut and sunflower oils smoke around 220–230 °C and unrefined or extra-virgin oils lower. Butter smokes near 150 °C because of its milk solids; clarify it and it smokes near 230 °C (IX.5). And every frying session splits more triglycerides into free fatty acids, so a reused oil smokes lower each time. The workshop follows an oil through repeated frying.
Below: the smoke point of four oils against the number of frying sessions, with deep-frying temperature marked, and how many sessions the oil you choose lasts before it smokes at 180 °C (published starting values; the fall with use is a teaching rate).
- Explain what smokes at the smoke point.
- Explain why refining raises it.
- Understand why reused oil smokes sooner.
- Read published smoke points as ranges.
A range, and a falling one
In this model refined rapeseed oil starts near 225 °C and smokes at 177 °C after ten sessions; it can be used about 8 times before it smokes at a 180 °C frying temperature. Extra-virgin olive oil starts lower, but it already holds more free fatty acids, so each session adds proportionally less: 193 °C after three sessions and about 11 uses in all — and its antioxidants slow the damage further, which is why frying studies find good olive oil steadier than its smoke point suggests. The case for frying in refined oil is cost and neutral flavour more than safety.
Signs that a frying oil is finished matter more than counting: it smokes at its usual temperature, foams persistently, has darkened and thickened, or smells rancid or fishy. Strain oil through a fine sieve after each use to remove crumbs, which burn and speed the breakdown; store it covered, cool and dark (IX.4); and never top up old oil with new, which only spreads the damage. Published smoke points disagree because oils differ by batch and laboratories measure differently: treat them as ranges.
Warm a little extra-virgin olive oil and a little refined oil in two small pans over the same gentle heat, with a thermometer if you have one. Note the temperature at which each first shows a wisp of smoke, then turn off the heat.
Terms
- Smoke point
- The temperature at which an oil gives off a steady thin smoke.
- Free fatty acids
- Fatty acids split from triglycerides; they lower the smoke point.
- Refining
- Processing an oil to remove free fatty acids, colour and flavour compounds.
- Acrolein
- An acrid, eye-stinging breakdown product of overheated fat.
Sources
- Smoke points vary with refining and free fatty acids; typical published ranges: unrefined extra-virgin olive oil about 190–210 °C, refined rapeseed and peanut oils about 220–230 °C, butter about 150 °C, clarified butter about 230 °C (H. McGee, On Food and Cooking, 2004).
- A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.