Smoke point

The smoke point, also referred to as the burning point, is the temperature at which an oil or fat begins to produce a continuous bluish smoke that becomes clearly visible, dependent upon specific and defined conditions.¹ This happens when one or multiple substances in the oil start to chemically react with oxygen and burn, which can include the oil itself, proteins, sugars, or other organic material.² It is distinct from the flash point and fire point, which denote the temperatures at which the oil itself (specifically, vaporized oil, which is distinct from the smoke produced at the smoke point) begins to burn.²

Smoke point values can vary greatly.³ The most important factor determining the smoke point of an oil is the amount of proteins and free fatty acids (FFAs).² Higher quantities of these lower the smoke point.² The FFA content typically represents less than 1% of the total oil and consequently renders smoke point a poor indicator of the capacity of a fat or oil to withstand heat, in a non-cuisine related sense.⁴⁵⁶ Virgin (raw) oils, which contain various flavorful organic compounds, have lower smoke points than refined oils because the organic compounds burn.² Animal-based fats and oils tend to have lower smoke points than vegetable-based ones, as well.² Oils made of polyunsaturated fats have lower smoke points, those made of monounsaturated fats have middling smoke points, and oils made of saturated fats have even higher smoke points.² The level of refinement, seed variety, and climate and weather of growth of the source plants also significantly affect its smoke point.²

Factors unrelated to the oil's composition are also important, such as the volume of oil utilized, the size of the container, the presence of air currents, and the type and source of light.³ And practically, even when smoke is cooked in ovens set to above its true smoke point, moisture and other objects can prevent it from reaching the full temperature.² The smoke point also decreases over time when oil is reused.² Cooks in practice tend to avoid the smoke point by noticing when the oil begins to shimmer, which happens just before it begins to smoke; adding food (to absorb heat) or lowering the temperature will prevent smoking.⁷

Acrolein, a potential carcinogen, is often present in the smoke, but this is only an issue to, for example, line cooks burning large quantities of food who breathe in large quantities of smoke over long periods, and not for home cooks.² This is because oil chemically decomposes into free fatty acids and glycerol, and at sufficiently high temperatures glycerol with burn to form acrolein.⁷ Free radicals produced by the high temperatures, although much reported on, are not dangerous.²

The smoke point of an oil correlates with its level of refinement.⁸⁹ Many cooking oils have smoke points above standard home cooking temperatures:¹⁰

• Pan frying (sauté) on stove top heat: 120 °C (248 °F)
• Deep frying: 160–180 °C (320–356 °F)
• Oven baking: Average of 180 °C (356 °F)

Smoke point decreases at a different pace in different oils.¹¹

Considerably above the temperature of the smoke point is the flash point, the point at which the vapours from the oil can ignite in air, given an ignition source.

The following table presents smoke points of various fats and oils.

Hydrolysis and oxidation are the two primary degradation processes that occur in an oil during cooking.¹¹ Oxidative stability is how resistant an oil is to reacting with oxygen, breaking down and potentially producing harmful compounds while exposed to continuous heat. Oxidative stability is the best predictor of how an oil behaves during cooking.³⁴³⁵³⁶

The Rancimat method is one of the most common methods for testing oxidative stability in oils.³⁶ This determination entails speeding up the oxidation process in the oil (under heat and forced air), which enables its stability to be evaluated by monitoring volatile substances associated with rancidity. It is measured as "induction time" and recorded as total hours before the oil breaks down. Canola oil requires 7.5 hours, for example, whereas extra virgin olive oil (EVOO) and virgin coconut oil will last over a day at 110 °C (230 °F) of continuous heat.¹⁰ The differing stabilities correlate with lower levels of polyunsaturated fatty acids, which are more prone to oxidation. EVOO is high in monounsaturated fatty acids and antioxidants, conferring stability. Some plant cultivars have been bred to produce "high-oleic" oils with more monounsaturated oleic acid and less polyunsaturated linoleic acid for enhanced stability.¹⁰

The oxidative stability does not directly correspond to the smoke point and thus the latter cannot be used as a reference for safe and healthy cooking.³⁷