Does cooking with oil create trans fats?
Q. How exactly is trans fat created? If I bake with some (say, 1/4 cup) oil in a batter, does that make trans fats?
A. We’ve all read a lot about the dangers of trans fats. Most of the trans fat in our diet comes from artificially hydrogenated oils that are still widely used in food manufacturing. But did you know trans fatty acids also occur naturally? The natural trans fatty acids found in milk and other foods are not considered to be a health threat. In fact, one naturally-occurring trans fatty acid called conjugated linolenic acid, or CLA, is thought to have many beneficial effects in the body.
The trans fats you need to watch out for are the ones created by artificial hydrogenation. These “Franken-fats” are polyunsaturated fatty acids (PUFAs) that have been twisted into an unnatural configuration called a trans-isomer. This means that the molecule contains the exact same number and type of atoms as it did before, but they are arranged in a different shape.
But what a difference that small change makes! Polyunsaturated fats and their trans-isomers produce completely opposite effects in the body. While PUFAs help to improve cholesterol profiles and reduce your risk of heart disease, trans fats contribute to heart disease by fueling inflammation, raising bad cholesterol and lowering good cholesterol.
What does it take to create a “Franken-fat”?
Artificial hydrogenation involves mixing polyunsaturated oils with hydrogen atoms under very high pressure--not something you could accidentally do at home! High heat can also cause PUFAs to trans-isomerize into trans fats, if it is applied for long enough. For example, you have to heat a polyunsaturated oil to 240 dgrees F for 16 hours before it begins to trans-isomerize into trans fats--and even then, only a very small percentage (less than 1%) are converted.
As you can see, normal cooking temperatures and times are not nearly enough to create any significant amount of trans fats in the foods we cook at home.
Reference: Wolff, R. L. 1993. Heat-induced geometric isomerization of alpha-linolenic acid: effect of temperature and heating time on the appearance of individual isomers. Journal of the American Oil Chemists Society 70(4): 425-430.
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Posted by: Hayley | Oct 7, 2009 12:24:34 PM
can you use applesauce instead of cooking oil for cakes and brownies? And does the applesauce make a a difference in the food?
Posted by: shelly | Jul 20, 2009 11:13:30 AM
cooking with PUFA's may not be a transfat hazard, BUT: cooking with poly fats is a huge problem because it creates toxic chemical changes that are far more dangerous than the small tfa's created. Coconut oil is the only safe one to cook with. look it up . . .
Posted by: Riz Din | Jun 10, 2009 8:56:47 AM
Superb, I've been looking far and wide for a scientific based answer to the question of what it takes to make healthy PUFAs go bad.
We use a refined rapeseed oil for frying. I just checked the ingredients and an anti-foaming agent has been added. I think it's 'hexane'. Should I be at all concerned?
Posted by: Bert | Jun 8, 2009 7:25:10 PM
Thanks for the excellent explanation. I was worried that I was creating trans fats when I was cooking with EEOO. Now, I know that is not the case.
Posted by: Dienna | Jun 5, 2009 12:08:15 PM
Thank you for settling our fears about creating trans-fats during cooking. But what about cooking oils being "oxidized"? That is what I am still confused about. Like if I leave a bottle of olive oil out on the counter without the lid on, will it go rancid? Should I refrigerate olive oil like I do my fish oil capsules? Thanks!
Posted by: darya | Jun 4, 2009 2:06:20 AM
Another great article, Monica. I have a request though, can you add a ShareThis button or other social networking/bookmarking link? I love sharing your info with my readers but your blog makes it harder than most websites.
Posted by: maryann | Jun 3, 2009 10:43:44 PM
Mike, yes, the trans form is more (thermodynamically) stable, but that does not mean it does not require high temperature to convert.
The barrier to conversion is very high (kinetic stability). To isomerize from cis to trans does require a means of overcoming this barrier, whether it is high temperatures (in this case 240 degrees), pressure, or hydrogenation catalysts (or a combination of these)
Posted by: Mike | Jun 3, 2009 8:56:10 PM
That is false. The trans form of fat is more stable. It takes less than 240 to convert a Cis fat to a trans.
Posted by: Bill | Jun 3, 2009 6:02:31 PM
Can you please explain in a scientifically correct way why 'bad cholesterol" is bad and why 'good cholesterol' is good?
Posted by: Sameer | Jun 3, 2009 12:36:35 PM
You said - For example, you have to heat a polyunsaturated oil to 240 dgrees F for 16 hours before it begins to trans-isomerize into trans fats--and even then, only a very small percentage (less than 1%) are converted.
And if this is the case then why is it considered as a bad thing-no-no to reuse frying oil again and again? I have heard that it should not be done because it breaks it down. But at times feel bad to throw away remaining frying oil and feel tempted to use it the next day. I am sure it does not get re-used up to the point to make it reach 16 hrs. If that's the case then reuse is OK or is there some other reason for not reusing it that I am missing?
BTW, I read all your nutrition/diet blogs and listen to podcasts religiously. I love them all. Full of useful information and common sense/logical approach that I love and almost always agree with. Thanks a lot. You are doing a great service to the society.
Thanks,
Posted by: darla | Jun 3, 2009 12:32:25 PM
Monica, you are doing a fantastic job writing this blog. I am consistently impressed by the clarity, the details, the research references, and the total lack of sensationalism. I look forward to reading every post and I always learn something. Thank you!
Posted by: Katrina | Jun 3, 2009 11:47:37 AM
Does anyone know the health benefits of grape seed oil? How does it compare to olive oil? (I don't see this food listed on NutritionData website.)
Monica's Response: Here's the nutrition facts for grapeseed oil: http://www.nutritiondata.com/facts/fats-and-oils/579/2
