A few days ago, I saw a hot search "A humble cooking habit can cause cancer." It was about burning oil until it smokes when cooking.

In fact, this hot search is half right. "Burning oil until it smokes when cooking is bad for your health." This is correct advice.

However, what is said in the hot search "high temperature will cause oxidative cleavage and isomerization of unsaturated fatty acids in dishes, resulting in the production of trans fatty acids with trans configuration" is the logic of "shoot first and draw the target later". It doesn't make sense.

  The amount of trans fatty acids formed when cooking oil is boiled until it smokes is very small.

  First of all, to convert unsaturated fatty acids into trans fats, "high temperature + catalyst" is required to proceed efficiently.

If there is no catalyst but only high temperature, it is not easy to produce trans fat.

During the refining process of vegetable oil, the temperature is much higher and the duration is longer than that of stir-frying. The trans fat formed is generally only about 2%.

Even if you burn oil for cooking until it smokes, the trans fat formed is very small and can be ignored.

  Fume from cooking is an important source of PM2.5 at home

  However, "burning oil until it smokes" does have considerable harm, but it has nothing to do with trans fats.

  There are some impurities in the oil, such as free fatty acids, lecithin, etc.

When the oil temperature reaches a certain level, these substances will generate various volatile substances and turn into "smoke".

These "smoke" particles are very small and are an important source of PM2.5 in the home.

If you put an air purifier or PM2.5 detector in the kitchen, you can see that if you burn oil until it smokes and fry a dish, the PM2.5 will soar from dozens to hundreds.

The fumes produced when cooking are equivalent to creating haze in the kitchen and inhaling it yourself.

  There are many harmful components in oil smoke, which have different adverse effects on the human body.

  1. Acrolein, a harmful component in oil smoke.

It is a strong irritant to the eyes and respiratory tract, and was even used as a chemical weapon in World War I.

  2. There are also polycyclic aromatic hydrocarbons in oil smoke, which enter the lungs through breathing and increase the risk of lung cancer just like smoking.

  A survey shows that chefs and middle-aged and elderly women who are often exposed to oil fume have significantly higher levels of carcinogens in their blood than those who are not exposed.

Even if they do not smoke, their lung cancer incidence rate is significantly higher than that of non-exposed people.

  3. There are also various free radicals in oil fumes.

  Oil smoke contains a large number of free radicals, and long-term and frequent exposure will accelerate skin aging.

  4. Kitchen fumes are particularly harmful to pregnant women and can affect the development of babies.

  Therefore, pregnant women and breastfeeding women need to stay away from oil fumes.

  "Self-pressed oil" and "stupidly pressed oil" have low smoke points and can easily produce oil smoke.

  The temperature at which oil begins to smoke is called the "smoke point."

Smoke point is closely related to the impurities in the oil.

Unrefined "self-pressed oil", "earth-pressed oil" and "stupid-pressed oil" are called "crude oil" in the food industry.

There are many impurities in crude oil, so the smoke point is relatively low. For example, the smoke point of soil-pressed sunflower oil is less than 110℃, and the smoke point of soil-pressed soybean oil and peanut oil is about 160℃. Relatively speaking, the smoke point of soil-pressed oil is relatively good. The smoke point of sesame oil can be close to 180℃.

  After refining, various impurities such as free fatty acids and lecithin in vegetable oil are almost completely removed, and the smoke point is greatly increased.

Generally speaking, the smoke point of refined vegetable oil is above 230°C.

  When cooking, the oil is usually heated to "70% hot" or "80% hot", and the corresponding oil temperatures are roughly 210°C and 240°C.

In other words, those unrefined "self-pressed oils", "earth-pressed oils" and "stupid-pressed oils" basically smoke when they cannot reach the temperature required for stir-frying, while refined oils basically do not.

  To avoid the harm of oil fumes, you can consider the following three things:

  Smoke from stir-frying is harmful to health, and this hazard mainly affects people who cook.

To avoid the harm of oil fume, you can consider the following three things:

  1. Reduce the cooking methods of frying and stir-frying, and use more cooking methods that do not produce oil smoke such as stewing and steaming.

  2. If you want to fry and stir-fry, use highly refined vegetable oil with a smoke point instead of "self-pressed oil", "earth-pressed oil" or "stupid-pressed oil" with a low smoke point.

  3. Install a powerful range hood, turn it on before it starts burning oil, and continue to turn it on for a while after cooking to fully discharge the oil smoke.

  Text/Yun Wuxin (Ph.D. in food engineering, popular science writer)