Toxic and carcinogenic, the harm is comparable to gutter oil! Many families still use it to cook

The Spring Festival is coming soon, and we will definitely prepare some food for frying, but how to deal with the leftover oil? In order to uphold the principle of not wasting, I believe most people will keep it for cooking. But this is really not recommended!

Oil that has been used to fry food and is then used repeatedly for high-temperature cooking is called "recycled oil". Eating such oil frequently can cause serious harm to your health.

There are so many dangers of recycled cooking oil!

1. Produce carcinogens:

After high-temperature treatment and repeated use of oil, it is easy to produce a variety of carcinogens, such as acrylamide, polycyclic aromatic hydrocarbons, heterocyclic amines, etc. Some data mentioned that the content of malondialdehyde in oil that has been used repeatedly for 7 times can increase to more than 30 times.

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Many people may be unfamiliar with MDA. It is actually a product of oil rancidity and has certain toxic side effects. Excessive intake of MDA will not only reduce animal physical fitness, damage nerves, and destroy intestinal cell structure, but also have mutagenic and carcinogenic effects.

A study on recycled cooking oil and mouse health led by scientists from the University of Illinois at Urbana-Champaign (UIUC) found that long-term intake of reused cooking oil changes the gene expression in mice, promoting the development of breast cancer. As for the reason, the researchers believe that the triglycerides in the oil that is repeatedly used at high temperatures will break and oxidize the free fatty acids, causing the malondialdehyde level in the oil to rise. The carcinogenic properties of malondialdehyde cause changes in the gene expression levels of mice, which at least to some extent promotes the metastasis of breast cancer.

2. Contains trans fatty acids:

Trans fatty acids in cooking oil are mainly formed from the isomerization of unsaturated fatty acids after frying. This component will reduce the content of high-density lipoprotein cholesterol (good cholesterol) in the human body and increase the content of low-density lipoprotein cholesterol (bad cholesterol), thereby increasing the risk of coronary heart disease and causing a variety of chronic diseases, which seriously endangers human health.

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The content of trans fatty acids in oils and fats will increase with the number of times the recycled oil is used, and the content of trans fatty acids produced by different types of oils and fats varies.

Experiments have shown that when soybean oil is used for frying, the change in trans fatty acids is not obvious; when sunflower oil is used for frying, the change in trans fatty acids is most significant, and the content of trans oleic acid, trans linoleic acid and trans linolenic acid gradually increases with the extension of frying time; when rapeseed oil is used for frying, the change in trans fatty acids is also quite significant, and accumulates with the increase of time and number of times; when peanut oil is used for frying, although the change in trans fatty acids is quite obvious, the total amount of trans fatty acids is not high, and will not accumulate in large quantities with the increase of frying time and number of times.

3. Prone to oxidation and rancidity

Repeatedly used cooking oil will be affected by oxygen and moisture in the air, which will promote oxidation and rancidity.

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During the process of oil oxidation and rancidity, aldehydes, ketones and certain carboxylic acids are produced, which give the oil a special pungent odor, also known as rancidity. At the same time, it will also destroy essential fatty acids, fat-soluble vitamins and other nutrients, and cause gastrointestinal discomfort. In addition, the peroxides and free radicals produced by oil oxidation and rancidity will also accelerate the aging of the human body.

Acid value is an indicator used to evaluate the degree of rancidity of edible oil. The higher the acid value, the more serious the rancidity of the oil. A study compared the acid value of unfried edible oil with that of edible oil fried once, twice, three times, four times, and five times (each time frying for 2 hours). It was found that the more times the oil was reused, the higher the acid value and peroxide value were. The acid value and peroxide value of the oil fried five times were 7.4 times and 5.6 times that of the oil fried once, respectively.

Image source: Reference [2]

4. Reduced nutritional value

Nutrients such as linolenic acid, linoleic acid, and oleic acid in the oil used for cooking will be greatly reduced, and the longer the frying time, the more they will be destroyed. A study mentioned that the content of unsaturated fatty acids such as linolenic acid, linoleic acid, and oleic acid in peanut oil is over 85%, but only 17% remains after 2 hours of frying. 70% to 90% of the vitamin E in vegetable oil will be destroyed after high-temperature frying, and "used oil" will cause even more serious damage.

How to make reasonable use of it if you don’t want to waste it?

There is a lot of oil left after frying food, and it is a waste to throw it away. If you want to reuse it, you should pay attention to how you use it and not cook it at high temperature again, which is healthier.

-Cold dishes and soups: When making cold dishes or soups, add 1 teaspoon of leftover oil to enhance the flavor.

-Mix the stuffing: When we make dumpling stuffing/steamed bun stuffing, we have to add some oil, which is not only convenient for stuffing, but also can lock the moisture of the vegetables in the stuffing, and the taste is better. Using cooking oil that has been fried will not waste and can increase the flavor.

-Blanching vegetables: For vegetables with peculiar smell or high oxalic acid content, they should generally be blanched before cooking. Adding 1 teaspoon of oil during blanching can not only keep the color of the vegetables, but also reduce the loss of some nutrients.

Save the remaining oil and pay attention to these details

Although the oil left over from frying food can be reused, the prerequisite is that the following three points must be done well.

-Oil selection: If you want to reuse the leftover oil from frying, it is recommended to choose cooking oil with a higher smoke point, such as first-grade soybean oil, first-grade sunflower oil, first-grade rapeseed oil, and first-grade corn oil. At the same time, do not fry for too long, the shorter the time, the better.

-Let it cool: After frying, turn off the heat immediately and let the oil sit to cool before transferring it to other containers for storage.

-Filter the residue: There will be a lot of black impurities in the oil used to fry food. These ingredients are harmful to health. You can put a piece of cheesecloth or gauze at the mouth of the container to filter the oil.

Summary: Recycled oil is very harmful, so it is best not to eat it, and it is also best not to eat fried food. The oil left over from frying food occasionally can be used to make cold dishes, soups, stuffing, blanching vegetables, and making flower rolls, but don't use it for stir-frying or frying food!

References

[1] Sun Changhao. Nutrition and Food Hygiene. 8th edition. People's Medical Publishing House. 2018

[2] Li Dujuan. Detection of rancidity index and food safety of repeatedly fried edible oils[J]. Henan Science and Technology, 2021, 40(5): 127-129

[3] Yuan Meijuan, Xue Wentong. Verification of the quality of cooking oil by high performance liquid chromatography[J]. Food Science and Technology, 2012, 37(8): 283-285289

[4] Yang Yuexin, Ge Keyou. Chinese Nutrition Encyclopedia. People's Medical Publishing House. 2019

[5] Yang Xuelian, Zhang Xiangyu, Xie Jianchun, Wang Chengtao. Comparison of trans fatty acids in frying oils of some foods[J]. Journal of Chinese Institute of Food Science and Technology, 2021, 21(1): 327-333

[6] Zhai Xiaohu, Yang Haifeng, Chen Huiying, et al. Research progress on the toxic effects and detection technology of malondialdehyde[J]. Shanghai Journal of Agriculture, 2018, 34(01): 144-148. DOI: 10.15955/j.issn1000-3924.2018.01.26

[7] Cam, Anthony etal. "ThermallyAbusedFryingOilPotentiatesMetastasistoLunginaMurineModelofLate-StageBreastCancer." Cancerpreventionresearch(Philadelphia, Pa.) vol.12,4(2019):201-210.doi:10.1158/1940-6207.CAPR-18-0220

[8]https://www.wikihow.com/Reuse-Cooking-Oil

Author: Xue Qingxin, registered nutritionist

Reviewer: Ruan Guangfeng, Director of Science and Technology Department, Kexin Food and Nutrition Information Exchange Center

Zhang Yu, MD, researcher at Chinese Center for Disease Control and Prevention