One of the frequent questions I receive in my email concerns the formation of trans fatty acids in the typical cooking process. I was quite surprised the first time I received this question, for several reasons. I knew that there were several things that were necessary for the formation of the trans fatty acids. One was a tank of hydrogen; second was a closed container, which allowed an adequate vacuum to form; third, an appropriate catalyst was needed; and last, the heat that would allow the chemical changes to occur had to be sufficiently high in conjunction with the other components.
During my many years of analyzing foods for the presence of trans fatty acids, I had found numerous examples of used frying oil that had started out without being partially hydrogenated and did not have any trans fatty acids, and there was still never any trace of trans fatty acids in the used oil unless the oil had been used for frying foods that had been prefried in a partially hydrogenated oil.
I am not sure who started the rumor that frying or even just cooking or heating polyunsaturated oils would produce trans fatty acids in those oils; but it is just that, an untrue rumor. It was likely started by one of the many internet writers hired to fill space or by someone who thought he or she knew the reason that there was trans fat in a particular product.
The idea that cooking with heat damages the oils that are highly polyunsaturated is true and the warning against cooking or frying using fragile oils such as flaxseed oil is valid, but not because trans fats are formed. What is formed under harsh circumstances such as high-temperature cooking and frying is a polymerized oil, and this is because the heat has helped to form free radicals and then various breakdown products. (Flaxseed oil that is still in the ground seed can be heated in baking and it does not become damaged.)
A number of years ago, a dietitian/nutritionist told me about her experience trying to make trans fatty acids in an open pan on top of the stove. She wanted to make a video of the process to use for teaching purposes. She was unsuccessful with this venture, and she had contacted me to ask me why her project had failed. She had not actually known how the trans were formed to begin with and assumed from what she had been told that the raising of the temperature would cause the trans to form. The project had been undertaken in one of the laboratories in a local university, and the analysis was to be done by someone in the same laboratory who knew how to use the instrument for analyzing the oil.
Certain types of trans fatty acids could probably be formed from a highly polyunsaturated oil during deep fat frying in one of the new pressure cooker fryers, but these types of trans fats would be like those formed in high pressure deodorization. They would not be the broad range of trans fats with delta-6, 7, 8, 9, 10, etc. So far, none of the groups doing analysis have reported this. Very small amounts of trans fatty acids have been found in corn chip products formed by extrusion cooking. This is due to the high pressure and the presence of a type of alkaline catalyst; but those trans that are formed are reported only in trace amounts from omega-6 or omega-3 oils.
Those fats and oils that are appropriate for cooking or sautéing and will withstand fairly high temperatures are those that have been in use for thousands of years, including olive oil as well as the more stable saturated coconut and palm oils and the animal tallows. An oil such as sesame oil with its special heat-activated antioxidants can be blended with coconut oil and olive oil to form a very stable good cooking oil.
This article appeared in Wise Traditions in Food, Farming and the Healing Arts, the quarterly magazine of the Weston A. Price Foundation, Winter 2003.
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Thank you for this article, the trusted info backed by science is always enjoyable to read especially when my m.o.v.e. Coach at the veterans clinic who is a certified nutritionist,insists olive oil and others can turn into trans fats when heated. Please continue posting good info for those of us who like to research.
Thank you for your article. I too like Steve run into this question all the time. I am a certified nutritionist (AFPA), teach nutrition and cooking classes and this can be a very “heated ” topic. I am going to have your article on hand when the question and “Heat Turns” up again. Thank you! Amy
Hi,
according to these:
http://www.sciencedirect.com/science/article/pii/S0308814610006084
http://www.ncbi.nlm.nih.gov/pubmed/22668846
There is a *slight* (yet not significant) change in trans fatty acids.
I stumbled upon this article in search of learning the effects of heating unsaturated fats. Though it does make sense that heating unsaturated fats would not form actual trans fat, I do still have one concern. I am a student, and one of my textbooks affirms that heating unsaturated fat can cause this fat to “act as” trans fat once inside the body. I would like to hear your take on this and any supporting links you may provide. You may reply at the email provided.
Hi! Can you please supply me the name of the textbook so I can read it as well? Many thanks.
Thanks so much for this article. It has definitely clarified these very controversial issue for me.
My husband eats breakfast and lunch at restaurants everyday due to his work schedule. I never eat out—-or very rarely. He is 60 lbs overweight. I would like to lose 10 lbs, but it is not imperative. Occasionally, I will fry chicken or fish. I always fry in organic canola oil or peanut oil. Had read an article which claimed ALL fried foods had trans fats. So glad you have cleared this up—-which I always suspected was the truth. I do believe my husband’s weight problem is the restaurant food. He did not have this problem before he started eating out so much. Restaurants are worried about the bottom line—-not the health of it’s customers.
Donna, it’s not the restaurants making your husband fat, it’s the amount of calories he’s eating. Restaurants use a lot of calorie dense ingredients to make their food taste good, which is why it’s supposed to be a treat. Eating out everyday makes him fat because he’s eating too much food for the amount of energy he expends. Even if he ate the highest quality ingredients, if he eats more calories than he expends he will get fat. Calories in, calories out.
Matt, it’s not that simple. Calories in/out is a clumsy and ineffective measure. No scientist uses this expression in such a broad manner. The caloric load on the SYSTEM plays its part for sure, but it’s what FORM those calories are in and what HAPPENS to them once they’re inside our bellies that counts.
If Donna’s husband ate 2,000cal of BEEF every day for two months, and his clone ate 2,000 calories of CANDY per day for the same two months, one is going to be MUCH fatter (and have many more health issues) than the other.
But Donna, no…. (a) don’t deep-fry ANYTHING – there’s simply no need – but esp. don’t fry it in canola oil (and there’s honestly no such thing as ‘organic’ canola oil. Not in the ‘it’s healthy!’ way many duped people might think). Cook with coconut oil, lard, dripping, ghee, grass-fed butter. Natural oils only. Not something that could only come into existence via a factory.
But the oil is probably not the major issue here. He’s probably eating a lot of bread (and perhaps pasta and rice) at restaurants – who bulk out their meals with this cheap, nutritionless junk to save money – and it’s these grains, these pointless (but yep, often tasty) carb-laden foods with their rubbish proteins which bind to minerals and nutrients to render them useless, that are the big problems for most people when they eat out. A roll here, a bread stick there, a bit of pasta on the side. It adds up!
Beefwalker, you have no knowledge so it seems about Canola oil. Canola oil comes from the canola seed, and in turn is organic when taken out. The process in which it goes under maybe the issue. I have seen organic Canola Oil at my local grocery store, and I believe it to be what it says, especially that it has been labeled organic by the Organic FDA seal. There are so many lies when it comes to selling what’s good for you and what’s not. It’s not what you eat at home that kills you it’s what you eat outside, that you don’t know what type of oil they use. Cottonseed oil being the cheapest and worse.
Matt, however, has the same mentality of my doctor, who says from the foods you eat the liver will create glycogen which is stored in the liver to use for fuel in the muscles and liver, but when you take in more calories from carbohydrates the cells store the excess as fat. So in essence, if you don’t burn the calories the fat accumulates and the weight goes up. So excess calories in, excess calories out.
I was tired of reading articles that all say different things. I went to university so I can actually stomach reading scientific papers so I went and found some actual evidence, not internet heresay.
“ordinary frying process using unhydrogenated edible oils has little impact on TFAs intake from edible oils.” http://www.sciencedirect.com/science/article/pii/S0308814610006084
“No trans fatty acid formation was observed even after extensive heating of unhydrogenated and hydrogenated soybean oil for 24 h. This phenomenon demonstrated that trans fatty acids can only be formed under severe conditions.”- http://www.sciencedirect.com/science/article/pii/S0308814606008557
However I also found this- “After 7 days of frying, the amount of PUFA was reduced by half and the trans isomers contribution increased 2.5 times during frying at 215 °C”-https://link.springer.com/article/10.1007/s11746-008-1328-5
I would say that it doesn’t significantly increase trans fat at normal frying temperatures but be sure not to fry at above 200 degrees.
I wish I could read that last article in full and get its numbers, because if the initial trans isomer content of the oil was like 0.1g/100g, that would mean it only increased to 0.25g/100g which is still not significant for most people who will use a couple tablespoons a day.
Also, I’m interested to know what effect the degradation of PUFAs has on food. It’s often said this is a bad thing but why? Does it break down to sugars? Does it simply break down to CO2 and H2O? Would its fatty acids still be present and bioavailable? It’s just a common concern with oils but it doesn’t seem clear to me why and I’d love some clarification.
In the meantime, I’ll just avoid it.
Well, I don’t know about anyone else, but I am still confused—even more so after reading the comments and answers provided here after the article. Would it be too much to ask for specific kinds of fats/oils that actually do present a problem of breaking down into trans fats in frying?
Thanks.
Vegetable oils do turn to aldehyde’s and PUFAS are just flat out bad for you. Olive oil is fine cold. Stick to tallow, ghee, or coconut oil for cooking. If you want some debate on this look up and talk to bart kay. I’m sure he will shed some light on it so you can do another write up.
I thought I would add reference to a more recent study that does indeed show an increase in trans-fats caused by a heating process.
https://www.sciencedirect.com/science/article/abs/pii/S0308814616309141?via%3Dihub
This article should be updated or removed because there are plenty studies that clearly state the oposite here there is just one example of it
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4609978/