Cooking oils are said to double their oxidation rate for every 10℃ increase in temperature. Oxidation accelerates rapidly at high temperatures of 160℃ or more, which are common for deep-frying, causing the oil to degrade quickly.
Oils That Oxidize Easily vs. Those That Don’t
An oil’s tendency to oxidize varies greatly depending on the type of fatty acids it contains.
- Oils that oxidize easily: These are rich in polyunsaturated fatty acids. Their molecular structure is unstable, making them prone to oxidation from light and heat. They are best suited for dishes that aren’t heated, such as salads.
- Examples: Linseed oil (flaxseed oil), perilla oil, grapeseed oil, and some vegetable oils (e.g., soybean oil, corn oil).
- Oils that don’t oxidize easily: These are rich in saturated fatty acids or monounsaturated fatty acids. Their stable molecular structure makes them resistant to heat and oxidation. They are suitable for cooking with heat.
- Examples: Coconut oil, butter, lard, olive oil, rice bran oil, sesame oil, and canola oil.
Signs of Oil Degradation
When oil oxidizes, it shows the following signs:
- Color: It loses its transparency and becomes darker.
- Viscosity: It becomes stickier and thicker.
- Foam: During frying, excessive foam appears and doesn’t dissipate easily.
- Odor: It develops an unpleasant, rancid, or sour smell.
- Smoke Point: The smoke point of oxidized oil is lower, so it begins to smoke at a lower temperature.
Continued use of oxidized oil can not only ruin the flavor of food but may also have negative health effects. When you notice these signs, it’s time to replace the oil.
The Relationship Between Oil Oxidation and Temperature
Oil oxidation doesn’t happen suddenly at a specific temperature; it gradually accelerates as the temperature rises.
- At Room Temperature (20℃ to 30℃): “Auto-oxidation” occurs slowly due to exposure to light and oxygen. Oils rich in polyunsaturated fatty acids are susceptible to oxidation even at room temperature.
- During Heating (150℃ to 200℃): During cooking, especially deep-frying, oil temperatures reach 150℃ to 200℃, causing oxidation to proceed rapidly. This is called “thermal oxidation,” which produces different compounds than auto-oxidation. A sign of this degradation is a lowered smoke point. While fresh oil has a smoke point of around 230℃, oxidized oil can have a smoke point as low as 170℃.
- For Storage (15℃ to 25℃): The optimal temperature for storing oil is 15℃ to 25℃ in a cool, dark place. Avoid storing oil in the refrigerator, as it can solidify and condensation may form on the container, which can degrade the oil’s quality.
Oil Types and Their Heat Resistance
The table below summarizes the heat resistance of different types of oils based on their fatty acid content.
| Fatty Acid Type | Characteristics | Examples of Oils | Suitable for |
| Saturated Fatty Acids | Stable molecular structure; highly resistant to oxidation. | Coconut oil, butter, lard | All types of heated cooking, including deep-frying and stir-frying. |
| Monounsaturated Fatty Acids | More stable than polyunsaturated fatty acids; moderately resistant to oxidation. | Olive oil, rice bran oil, sesame oil, canola oil | All types of heated cooking, including deep-frying and stir-frying. |
| Polyunsaturated Fatty Acids | Unstable molecular structure; prone to oxidation. | Linseed oil, perilla oil, grapeseed oil, vegetable oil (soybean, corn) | Unheated preparations like dressings. |
