Why Does Frying Oil Smoke?

Frying oil smokes when it reaches its smoke point; at this point, the oil’s molecular structure starts to break down and a thin, visible smoke rises. Every oil has a different smoke point, depending on the plant it comes from and how it is processed. Smoking means the oil has exceeded the ideal frying temperature; from then on, a burnt flavor starts to transfer to the food and the oil begins to deteriorate quickly.

What exactly is a smoke point, and why does it vary by oil?
A smoke point is the temperature at which the free fatty acids in an oil begin to break down and turn into volatile compounds when heated. Refined oils with a low proportion of free fatty acids can remain smoke-free at much higher temperatures than raw or minimally processed oils. For example, refined sunflower oil can remain smoke-free up to around 230°C / 445°F, while extra-virgin olive oil starts smoking at a much lower temperature, in the range of 160–190°C / 320–375°F.

How does reusing oil lower its smoke point?
Oil used once for frying deteriorates somewhat from the heat and the moisture released by the fried food; this deterioration increases the proportion of free fatty acids with each use. As a result, the same oil starts smoking at a much lower temperature on its second or third use than on its first. For this reason, it is recommended not to reuse oil that has darkened, foams, or has a strong smell, both for flavor and because of the smoke risk.

How does smoking oil spoil the taste of food?
Oil heated beyond its smoke point turns bitter, and that bitterness transfers to the fried food; the oil itself also becomes more prone to deterioration with each use. A strong, sharp burnt smell in the kitchen is a sign that the oil has already passed this point. Oil that starts smoking should immediately be taken off the heat and replaced; continuing to fry with the same oil makes the result worse.

Which oils should be used at high heat?
Oils with high smoke points, such as refined sunflower oil, corn oil, and refined canola oil, are more suitable for high-heat methods such as deep-frying. Oils with low smoke points, such as extra-virgin olive oil and butter, are better for low-to-medium-heat cooking or uncooked use. If an oil has darkened or developed a sharp smell, its structure may have started to deteriorate even if it is not yet smoking.

What should you keep in mind to prevent oil from smoking?
Instead of leaving the burner on its highest setting as soon as the oil is placed on it, heat the oil gradually to prevent its temperature from suddenly exceeding the smoke point. Monitoring the oil temperature with a thermometer while frying is much more reliable than estimating by sight. Using a deeper pot instead of a wide, shallow one reduces the oil’s surface contact with air and can somewhat reduce heat loss and therefore the risk of smoking. Also, straining the oil after frying and storing it in a cool, dark place helps its smoke point remain higher for the next use. It is also important to dry the surface of the food thoroughly before frying; when wet food touches hot oil, it can cause sudden splattering and a burst of steam, creating a safety risk and contributing to the oil’s premature deterioration.