GURPS Action 5: Dictionary of Danger

[malloyd]

Originally Posted by evileeyore

It has everything to do with thermal breakdown and solid dissolution/suspension (the same happens to motor oil and transmission fluid). Basically the oil undergoes a chemical change and can (in extreme cases) stop being a liquid at room (and even higher) temperature(s).

The major mechanism is the oils polymerize - the unsaturated (carbon-carbon double bonds) bonds open up and opened bonds link with each other - either directly or through a bridging oxygen or sulfur atom. In some polyunsaturated oils this can happen reasonably fast even at room temperature - things like linseed oil (which is mostly double and triple unsaturated linolenic acid) "dry" this way, but like most chemistry it happens faster when you add heat. Incidentally doing this on purpose, to form a plastic like film, is also what you are doing when you "season" cast iron cookware.

You could minimize it by cooking in more, or fully, saturated oils, but the problem there is they're less liquid to start with. Partial hydrogenation - that is converting many of the double bonds to a single bond and two bonds to hydrogen - is the standard way of converting a vegetable oil into a semi-solid shortening.