Some Background
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.