The Spring Festival is coming soon, and it is inevitable to prepare some fried foods, but what to do with the remaining oil? In order to adhere to the principle of not wasting, I believe most people will stay and continue cooking. But this is really not recommended! Oil that has fried food and continues to be used for repeated high-temperature cooking is classified as "reheated oil". Frequent consumption of such oil is very harmful to the body.
There are so many dangers of reheated oil!
1. Produce carcinogens:
After high-temperature treatment and repeated "reheating" use of oil, it is easy to produce a variety of carcinogens, such as acrylamide, polycyclic aromatic hydrocarbons, heterocyclic amines, etc. Some data mention that the content of malondialdehyde in reheated oil that has been used repeatedly for 7 times can increase to more than 30 times.
Many people may be unfamiliar with malondialdehyde. It is actually a product of oil rancidity and has certain toxic and side effects. Excessive intake of malondialdehyde not only reduces animal physical fitness, damages nerves, and destroys intestinal cell structure, but also has mutagenic and carcinogenic effects.
A study led by scientists at the University of Illinois at Urbana-Champaign (UIUC) on cooking oil and mouse health found that,After long-term intake of repeatedly used cooking oil, the gene expression in mice will change, promoting the development of breast cancer.
As for the reason, researchers believe that the triglycerides in oil that are used repeatedly at high temperatures will break and oxidize free fatty acids, causing the level of malondialdehyde in the oil to increase. Malondialdehyde's carcinogenic properties caused changes in gene expression levels in mice that, at least in part, promoted breast cancer metastasis.
2. Contains trans fatty acids:
After frying in cooking oil, the formation of trans fatty acids mainly comes from the isomerization of unsaturated fatty acids. This ingredient 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, causing a variety of chronic diseases, and seriously endangering human health.
The content of trans fatty acids in fats and oils will increase as the number of times the recooked oil is used increases. Different types of fats and oils produce different levels of trans fatty acids.
Experiments have shown that when soybean oil is used to fry food, the change in trans fatty acids is not obvious; when using sunflower oil, the changes in trans fatty acids are the most significant during the frying process, and the contents of trans oleic acid, trans linoleic acid and trans linolenic acid gradually increase with the extension of frying time; so that When frying with rapeseed oil, the changes in trans fatty acids are also more significant, and they accumulate with the increase in time and frequency; when using peanut oil for frying, although the changes in trans fatty acids are more obvious, the total amount of trans fatty acids is not high, and it will not accumulate in large amounts with the increase in frying time and frequency.
3. Easily oxidized and rancid
Reheated oil that is used repeatedly will be affected by oxygen and moisture in the air, promoting oxidative rancidity.
During the oxidative rancidity of oil, aldehydes, ketones and certain carboxylic acids are produced, giving the oil a special pungent odor, which is the so-called hala smell. At the same time, it also destroys essential fatty acids, fat-soluble vitamins and other nutrients, and can also cause gastrointestinal discomfort. In addition, the peroxides and free radicals produced after the oxidation and rancidity of oils will also accelerate the rate of human aging.
The 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 cooking oil and cooking oil fried once, twice, three times, four times, and five times (frying for 2 hours each time). It was found that the more times the oil is reused, the more acid value and peroxide value it produces. The acid value and peroxide value of oil fried five times are 7.4 times and 5.6 times higher than those fried once.
Image source: Reference[2]
4. Reduced nutritional value
Nutrients such as linolenic acid, linoleic acid, and oleic acid in the reheated oil will be greatly reduced, and the longer the frying time, the more they will be destroyed. Some studies have mentioned that the content of unsaturated fatty acids such as linolenic acid, linoleic acid and oleic acid in peanut oil is more than 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 the damage of "reheated oil" is even more serious.
If you don’t want to waste it, how can you use it wisely?
There is a lot of oil left after frying food, and it would be a waste to throw it away. If you want to reuse it, you should pay attention to how you use it and don't cook it at high temperatures again, so that it will be healthier.
-Cold dishes and soups: When making cold dishes or soups, add 1 teaspoon of leftover oil to enhance the flavor.
- Mixing stuffing: When we make dumpling stuffing/steamed bun stuffing, we usually put some oil in it. It is not only convenient for mixing the stuffing, but also locks in the moisture of the vegetables in the stuffing, giving it a better taste. Using fried cooking oil is not wasteful and can increase the flavor.
-Blanching vegetables: For vegetables that have a peculiar smell or have high oxalic acid content, they generally need to be blanched before cooking. Adding 1 teaspoon of oil when blanching can not only maintain the color of the vegetables, but also reduce the loss of some nutrients.
Save leftover oil and pay attention to these details
Although the oil left over from frying food can be reused, the following three points must be done.
-Choice of oil: If you want the remaining oil from frying to be reused, 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 better.
- Leave to cool: After frying food, turn off the heat source immediately and let the oil sit. After cooling, transfer it to other containers for storage.
-Filter residue: There will be a lot of black impurities in the oil used for frying 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.
Summarize:
Twice-cooked oil is very harmful, so it is best not to eat it and eat less fried food. The oil left over from frying food occasionally can be used to make salads, soups, stuffings, blanched vegetables, and rolls, but don’t use it for stir-frying or frying!
References
[1] Sun Changhao. Nutrition and Food Hygiene. 8th Edition. People's Medical Publishing House. 2018
[2] Li Dujuan. Detection of rancidity indicators and food safety of edible oil from repeated frying [J]. Henan Science and Technology, 2021, 40(5): 127-129
[3] Yuan Meijuan, Xue Wentong. Use high performance liquid chromatography to verify the quality of reheated oil [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 some food frying oils [J]. Journal of Chinese Food Science, 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]. Journal of Shanghai Agricultural Sciences, 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
Planning and production
Author丨Xue Qingxin registered nutritionist
Review丨Ruan Guangfeng Director of Science and Technology Department of Kexin Food and Nutrition Information Exchange Center
Zhang Yu, PhD, researcher at the Chinese Center for Disease Control and Prevention
Planning | Fu Sijia
Editor丨Fu Sijia