The smoke point of an oil is the temperature at which it starts to smoke. When an oil smokes, it begins to decompose, and - say some experts - many of its unsaturated fatty acid molecules become saturated. What is known for sure is that the chemical breakdown of the gylcerol molecules in the fat creates acrolein, an obnoxious-smelling compound that can inflame the cook's respiratory system.
Knowing an oil's smoke point can also save you money because each time you deep-fry with an oil, you lower its smoke point irreversibly. If you buy an oil with a smoke point not very far above 375oF (the normal deep frying temperature), chances are its smoke point will drop to below 375oF after its first use. If you want to save money by reusing an oil as many times as possible, select one with the highest smoke point.
The smoke point for any given vegetable oil varies from brand to brand, but approximate averages for the leading oils are 510oF for safflower, 495oF for soybean, 475oF for corn, 440oF for peanut, 420oF for sesame, and 375oF for olive. The figure is roughly 375oF for vegetable shortening (solidified oil).
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