Q: How do the various cooking oils differ in their boiling points, and what are the consequences for the cook?
A: I don't think you mean boiling point, because in spite of the poetic and sadistic appeal of the expression "boiled in oil," oil doesn't boil.
Long before it gets hot enough to bubble, a cooking oil will decompose, breaking down into disagreeable chemicals and carbonized particles that will assault your taste buds with a burnt flavor, your nostrils with an acrid smell and your ears with a shrieking smoke alarm. If you mean the highest cooking temperature for an oil, it is limited not by a boiling point, but by the temperature at which the oil begins to smoke.
The smoke points of common vegetable oils, which come mostly from plant seeds, can range anywhere from 250 to more than 450 degrees. In spite of the supposedly precise values listed in some books, exact smoke-point temperatures cannot be given because a particular type of oil can vary quite a bit, depending on its level of refinement, the seed variety, and even the climate and weather during the plant's growing season.
But according to the Institute of Shortening and Edible Oils (there's one for everything, isn't there?), the approximate smoke-point ranges of some common cooking oils are: safflower oil, 325-350 degrees; corn oil, 400-415 degrees; peanut oil, 420-430 degrees; cottonseed oil, 425-440 degrees; canola oil, 435-445 degrees; sunflower and soybean oils, 440-450 degrees. Animal fats generally smoke at lower temperatures than vegetable oils.
(You've always wondered what kind of a plant a canola is, haven't you? Well, so have I. It turns out there is no such thing as a canola. The oil comes from the seeds of the rape plant, a member of the mustard family. Canadian plant breeders developed an improved variety of rapeseed and wisely renamed it: "can-" for "Canadian" and "-ola" for oil. And while we're playing word games, Mazola, a corn oil, derives its name from maize-ola.)
Olive oil is a special case, because there are so many kinds that generalizations are hard to make. Its smoke point can be anywhere from 280 to 400 degrees. All the other vegetable oils, with the exception of a few peanut oils, are valued by American cooks for their blandness, for their lack of any possibly intrusive flavor. But olive oil is prized for its complex flavors, which can range from nutty to peppery and grassy to fruity, depending on the country and region from which it comes, the variety of olive and its stage of ripeness at harvest. Mediterranean cuisine owes its unique qualities largely to the almost exclusive use of olive oil in everything from baking to deep frying. The oil is a flavor component of the recipes, not just a cooking medium. And I've never heard a Spaniard or an Italian complain about a wisp of smoke in the kitchen.
When heated to around 600 degrees--not likely in the kitchen unless you decide to take a nap with the burner going full blast under the french fries--most cooking oils will reach their flash points, the temperatures at which their vapors can be ignited by a flame. At even higher temperatures of around 700 degrees, most oils will reach their fire points and burst into flame spontaneously.
Fortunately for smoke-averse cooks, the smoke points of several common cooking oils are higher than the most desirable range of temperatures for frying, which is 350 to 380 degrees. Deep-fat frying can reach up to 400 degrees, however, so there isn't much of a non-smoking cushion there. But except for the lowest-smoke-point cooking fat of all, unclarified butter, which begins to smoke at only 250-300 degrees, smoke shouldn't be a problem in sauteing and pan-frying unless you have a very heavy hand on the burner control. (What burns and smokes in butter isn't the butterfat itself, but the milk solids.)
It's important to note that all of the smoke points above are for fresh oils. When oils are heated or oxidized, they release free fatty acids, which both lower the smoke point and taste acrid. Reused deep-frying oil, or any oil that has been exposed significantly to heat or air, will therefore smoke more readily and take on a disagreeable flavor. Moreover, hot oils tend to polymerize--their molecules join together into much bigger molecules that give the oil a thick, gummy consistency and darker color. And finally, hot oils can break down into free radicals and other unhealthful chemicals.
All things considered, discarding deep-frying oil after one or at most two uses may be the safest and best route for both health and palate.
But where, oh where do you discard used cooking oil? Restaurants subscribe to disposal services that sell the stuff to soap and chemical companies. But for small at-home quantities, I pour it into an empty tin can and keep it in the freezer, where it freezes solid. On trash day I wrap it in a plastic bag and put it into the garbage, hoping that it won't melt and leak out until it's far, far away and untraceable.
Can any environmentalist out there recommend a less guilt-inducing method, short of cooking up a batch of soap? And don't talk to me about oil lamps; they're much too hard to read by.
Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Didn't Know--Scientific Answers to Everyday Questions." Send your food questions to firstname.lastname@example.org.