What happens inside the fat or oil is that it starts to break down.
This also changes the flavour, usually for the worst. Mustard oil is an exception: Indians always heat mustard oil to its smoking point before using it, because the flavour improves.
The oils also become less nutritious for you, because “free radicals” are released. The exception, again, is mustard oil — it actually becomes healthier for you at the smoke point.
Refined oils and saturated oils tend to have the highest smoke points (though they’re also said to be the worst for our bodies.)
It’s okay to strain, save and re-use oil for deep-frying, but its smoking point will decrease with use.
Some stir-fry experts say that despite every thing you hear, stir-frying is best done once the oil you are using has reached its smoking point
