What happens when cooking oil gets too hot? And How Should You Handle It?

  • By: Emma
  • Date: August 13, 2022
  • Time to read: 5 min.

It is not uncommon that cooking oil becomes overheated during coking or frying. And if you have ever experience this, you may wonder what happens when cooking oil gets too hot?

Just like you, I had the same experience and I digged out the science behind this process. In this blog post, I have provided a detailed yet easy to follow answer to this question.

What happens when cooking oil gets too hot?

When cooking oils get too hot, physical, and chemical changes occur in it. Overheating oil results in thermoxidation, polymerization and hydrolysis which leads to foaming and altering free fatty acid (5.7-8.7). Also, the color of oils turns black and its viscosity increases (>750 mp) which leads to decrease in oil’s smoke point (<325 ◦F), affecting its quality and nutrition.

A study recently published in Arabian journal of Chemistry depicted that both the physical and chemical properties are affected by excessive heating of cooking oil.

The researchers observed that heating oil for 1-7 h leads to darkening of color and increases foaming. Changes in viscosity, free fatty acids, and peroxide values of oil due to overheating negatively affect the quality of the cooking oils.

The study concluded that excessive and repeated heating reduces the quality of the oil.

Types of oilCorn oilMustard oil
Density (bh)0.92230.9694
Viscosity (bh)112117.2
Peroxide value (bh)0.1620.83
Density (ah)0.84860.862
Viscosity (ah)94.36115.1
Peroxide value (ah)2.5691.838
Table 1: What happens when cooking oils get too hot? Comparison of chemical properties of oil before and after heating (bh = before heating, ah = after heating). Source: Nduka, J. K. C., Omozuwa, P. O., & Imanah, O. E. (2021). Effect of heating time on the physicochemical properties of selected vegetable oils. Arabian Journal of Chemistry14(4), 103063.

Even though most of us know that cooking oils shouldn’t be heated above a specific temperature, many people aren’t aware of the fact that frying above this temperature can damage the oil beyond repair.

When oils get heated above the smoking point, they react with the oxygen in the air to form dangerous compounds called oxides. This reaction creates off flavors in both oil and food and damages carcinogens.

When you fry or sauté foods in hot-tempered oils, these oxides are formed inside the bottle. As time passes and more and more heat is added to the bottle, it gets hotter and hotter until it finally bursts, usually resulting in a very expensive loss.

These reactions create chemical compounds that alter the flavor of oil and food, making them lose their original taste and smell.

If you’ve ever had fried chicken that smelled like kerosene or steak that smelled like rotten eggs after being stored in hot oil for a while, then you know what we mean!

What happens when cooking oil gets too hot? A flow chart showing the sequence of chemical changes.

Read more: How to keep oil from burning when frying?

What are the indications that cooking oil is going to become overheated?

You can tell if your cooking oil is overheating by the color. If you notice that your oil has turned from a light yellow to an orange or dark brown, then it’s time to throw it out and start over.

Another indication that your oil has heated too high is when you see bubbles in the bottle. You might also notice a white film on top of the container. When this happens, you must get rid of your cooking oil quickly before it becomes dangerous!

What fumes does burning oil give off?

When you fry food in oil heated past the smoking point, it produces several fumes. These include toluene, benzene, and n-hexane. The first two are considered to have carcinogenic effects, while the third is used as an industrial solvent.

The best way to prevent this issue is by using a deep-frying thermometer when cooking to keep your oil below the smoking point at all times.

Another solution is to use a coolant like ice cubes or water when cooking foods in hot-tempered oils. This coolant tempers down the temperature of the oil before it becomes too hot for frying or sautéing.

Is burnt cooking oil poisonous?

Cooking oil burnt beyond the smoking point can contain dangerous carcinogens and harmful compounds, which can be poisonous.

If you fry food in this type of oil, then you’re ingesting these chemicals into your body!

Read more: Can you saute onions without oil?

What oils are carcinogenic when heated?

Frying in hot-tempered oils like vegetable oil, peanut oil, and corn oil that contain high levels of saturated fats can produce dangerous compounds called nitrosamines. These compounds are formed when the oil gets heated over 350 degrees Fahrenheit.

These compounds also cause cancer, liver damage, and birth defects. We have all heard about the importance of using non-stick cookware, but sometimes it is necessary to break out the old frying pan for a special occasion.

Make sure you’re cooking with appropriate heat and oils!

How does hot oil affect the food’s carbohydrates, proteins, and fats?

The most essential oil compounds are fats, carbohydrates, and proteins. Oils that have been heated for an extended period of time also have a high content of free fatty acids and carboxylic acids.

These reactive compounds change the structure of food proteins, making them less digestible by breaking down their structures. These compounds are also responsible for altering the flavor and smell of food.

Furthermore, these reactions cause changes to the structure of lipids like phospholipids and triglycerides. This can significantly affect how one’s body reacts to ingesting certain nutrients from fats in oil-based meals.

How to handle hot cooking oil?

Heat-proof containers can be used to store hot cooking oil and prevent it from reacting. You can also check the temperature of your oil with a thermometer to ensure that the oil isn’t overheated.

When frying or sautéing in hot cooking oils, you should use a lower temperature setting and stir frequently. Otherwise, you risk damaging the oil and getting an expensive loss!

Top 5 tips to handle hot cooking oil


So, what happens when cooking oil gets too hot? To conclude, the oil that is too hot will cause the oil to be burnt on the pan, smoke, and can cause a chemical reaction that affects the food’s taste and texture.

Cooking oil comprises fatty and polyunsaturated fatty acids, which are stable at average cooking temperatures.

Cooking at high temperatures will transform these polyunsaturated fatty acids into free radicals, causing a reaction that alters the taste and texture of the food.

When the food is finished cooking, the free radicals will be neutralized in the presence of antioxidants.