Why Does Frying Oil Start to Smoke?

Frying oil starts to smoke once it reaches a temperature called the smoke point, where the oil's molecular structure begins to break down and a fine, visible smoke rises. Every oil has a different smoke point, depending on which plant it comes from and how it was processed.

Once oil starts smoking, it has already passed the ideal frying temperature; from that point on it starts imparting a burnt taste to food and breaking down quickly.

What exactly is the smoke point, and why does it vary by oil?

The smoke point is the temperature at which free fatty acids in a heated oil start breaking down into volatile compounds. Refined oils with a low free fatty acid content can stay smoke-free up to a much higher temperature than raw or lightly processed oils.

Refined sunflower oil, for example, can handle heat up to around 230°C without smoking, while extra virgin olive oil starts smoking at a much lower range, around 160 to 190°C.

How does reusing oil lower its smoke point?

Oil that has been used once for frying breaks down a little from the heat and from moisture carried over by the food; that breakdown raises the free fatty acid content with every use. As a result, the same oil starts smoking at a noticeably lower temperature on its second or third use than it did the first time.

That is why oil that has turned dark, foams up, or smells strong should not be reused for frying — both for flavor and to avoid smoking too early.

How does smoking oil ruin the taste of food?

Oil that goes past its smoke point turns bitter, and that bitterness transfers to whatever is fried in it; the oil itself also becomes more prone to breaking down with every further use. A strong, sharp burnt smell in the kitchen is a sign the oil has already crossed that point.

Oil that starts smoking should be taken off the heat and replaced right away; continuing to fry in it only makes the result worse.

Which oils work best for high-heat cooking?

Oils with a high smoke point, like refined sunflower oil, corn oil and refined canola oil, are better suited to high-heat jobs like deep frying. Oils with a lower smoke point, like extra virgin olive oil and butter, work better at low-to-medium heat or used raw.

If an oil has darkened in color or developed a sharper smell, its structure should be considered already breaking down, even if it has not started smoking yet.

What should you watch for to avoid smoking oil?

Heating oil gradually rather than leaving it on the highest setting the moment it goes on the stove keeps the temperature from suddenly overshooting the smoke point. Tracking the oil's temperature with a thermometer during frying is far more reliable than judging by eye.

Using a deeper pot instead of a wide, shallow one also reduces the oil surface's contact with air, cutting down on heat loss and lowering the risk of smoking; straining the oil after frying and storing it somewhere cool and dark also helps keep its smoke point higher the next time it is used.

Patting the food dry before it goes into the oil matters too; a wet piece hitting hot oil can cause sudden spattering and a burst of steam, which is both a safety hazard and something that pushes the oil toward breaking down sooner than it should.