All edible fats primarily comprise of fat molecules which in turn are made up of fatty acids. The fatty acids are made up of chains of carbon atoms that are held together mostly by single chemical bonds. But every now and then a double bond appears on the chain. The majority of the fatty acids in olive oil (typically 75%+) contain only one double bond, whilst the fatty acids that dominate most other edible oils contain a higher number of double bonds.
The double bonds in fatty acids allow them to be ‘kinked’. Normally they kink in one particular direction (known as the cis form). Upon heating however, a small proportion of these fatty acids kink in the opposite direction (known as the trans form) as the latter form is the most chemically stable. Our bodies have evolved to be able to use the cis type of fat for normal bodily functions, but not the trans type, the consequence being that consumption of the trans form results in cardiovascular disease.
What’s Been Done
A number of scientific studies have looked at the amount of trans fat formation in edible oils during heating. However, most used either temperatures way above those needed to fry, or the oil was heated for lengths of time that you would only use if you accidentally left oil on the stove before heading out for a vacation. These studies were conducted to replicate the type and length of heating used in commercial deep frying operations.
What’s Been Found
I have included all the studies where the temperature was kept to a recommended frying temperature 180-200C and where there was no possibility of the food that was being fried (if that was the case) contained trans fats themselves. Unfortunately, in all the studies excepting one, the length of time that the oil was heated for far exceeded a typical domestic frying time. However, the data they provided do allow estimates of how much trans fat is formed when cooking for shorter periods of time that are more typical of domestic frying.
Not many of these studies used EVOO as it is an oil that is rarely if ever used for continuous deep fat frying in a commercial environment (comparing how fats perform when heated for long periods of time as occurs in commercial fat frying operations is the purpose of most studies, as the results have big $ implications). However, the same basic chemistry of isomerisation apply to all oils, so I have also summarised the results from these.
Table 1: Trans fat formation in edible fats after heating
|Type of oil||Time (mins)||Frying Temp
|Estimated increase* in trans fats
after 20 mins of frying (mg/kg)
|Extra virgin olive||120||180||0||Oven||Albi et al. 1997|
|Rapeseed||10||180||0||Frypan||Hrncirik & Zeelenberg 2013|
|Sunflower||10||180||30||Increase not statistically significant||Hrncirik & Zeelenberg 2013|
|Olive oil||45||230||6||In cooked dough||Coponio 2003|
|Corn||15||180||5||Yang et al. 2012|
|Peanut||360||180||6||Kala et al. 2011|
|Sunflower||480||180||0||Steel pan||Rani et al. 2010|
|Rice Bran||240||180||0||Tsuzuki 2012|
*Estimated by linear interpolation at t=20 min. ** 1mg = 1/1,000th of a gram (which is about 1/50th the weight of a drop of water)
The level of trans fats of most of the edible oils, including olive oil, did not increase by heating using typical temperatures needed for frying. In olive oil, the largest increase was 5 x 1,000th of a gram per kilogram of oil, and in EVOO no increase was observed.
EVOO is theoretically more resistant to trans fat formation, as being mostly monounsaturated it contains fewer double bonds than that of polyunsaturated fats such as sunflower or vegetable (soybean) oil. Also studies including those presented here have shown that natural antioxidants such as tocopherol and polyphenols which are abundant in EVOO inhibit the fat transing to the dark side.
But for some perspective – The average amount of trans fats in a 170 gm serving of french fries made by two major fast food chains (sampled from 20 countries) was found to be 4 grams (Stender et al. 2006). While this high level may be the result of prolonged high temperature heating, it is far more likely due to the high trans fat content (up to 25%) of the partially hydrogenated oils (1) that are commonly used in the fast food industry.
Based on the worst case result shown in Table 1, i.e. a 5 mg/kg increase in trans fat after heating, you would have to eat 310 servings of olive oil cooked French fries (53 kg or 140lbs worth) to get the same trans fat fix as a single serving cooked in an average fast food establishment (2).
Stender and colleagues in the New England Journal of Medicine stated that….
“Owing to the very high content of industrially produced trans fatty acids in certain fast foods, in many countries it is possible to consume 10 to 25 g of these trans fatty acids in one day and for habitual consumers of large amounts of this food to have an average daily intake far above 5 g”
Based on the studies above, heating 1kg of olive oil yields between 0 and 5mg of trans fat.
5 grams = 1,000 x 5 mg !
Albi, T., Lanzo, A., Guinda, M.C., Perez-Camino, C. and Leon, M. (1997) Microwave and conventional heating effects on some physical and chemical parameters of edible fats. J. Agric. Food Chem. 45, 3000-3003.
Caponio, F., Pasqualone, A. and Gomes, T. (2003) Changes in the fatty acid composition of vegetable oils in model doughs submitted to conventional or microwave heating Int. J. of Food Sci. Tech. 38, 481–486
Hrncirik, K. and Zeelenberg, M. (2014) Stability of essential fatty acids and formation of nutritionally undesirable compounds in baking and shallow frying. J Am. Oil Chem. Soc. 91, 591–598 DOI 10.1007/s11746-013-2401-2
Rani, A.K.S., Reddy, S.Y. and Chetana, R. (2010) Quality changes in trans and trans free fats/oils and products during frying. Eur Food Res Technol. 230, 803–811. DOI 10.1007/s00217-010-1225-7123
Stender, S., Dyerberg, J. and Astrup, A. (2006) High levels of industrially produced trans fat in popular fast foods. New Eng. J. Med. 354, 1650-1652.
Kala, A.L., Joshi, V. and Gurudutt, K.N. (2012) Effect of heating oils and fats in containers of different materials on their trans fatty acid content. J. Sci. Food Agric. 92, 2227-2233.
Tsuzuki. W. (2012) Study of the formation of trans fatty acids in model oils (triacylglycerols) and edible oils during the heating process. JARQ 46, 215-220.
Yang, M., Yang, Y., Nie, S. Xie, M. and Chen, F. (2012) Analysis and formation of trans fatty acids in corn oil during the heating process. J. Am. Oil Chem. Soc. 89, 859–867. DOI 10.1007/s11746-011-1974-x
(1) Hydrogenation is an industrial process mostly applied to polyunsaturated oils to make them semi-solid, and to increase their resistance to oxidation/rancidity. A by-product of partial hydrogenation is trans-fat production.
(2) given the reasonable assumptions of 1) oil retention of 15%, and 2) EVOO that had been heated for 20 minutes at 180C
Disclaimer: The information provided above is intended as general, and is not meant to be viewed as specific health advice.