The Real Chef Boyardee

Every fat or oil will turn from liquid to solid -- that is, freeze -- at some temperature or other. That's why refrigerated cooking oils can turn cloudy if the temperature -- usually about 40 degrees -- is lower than the oil's freezing point. But several factors confound our ability to say at exactly what temperature a given oil will freeze.

First of all, our cooking oils consist of mixtures of several -- often dozens of -- fats and fatty acids, and they all may have different freezing points. So some of them may freeze and make the oil cloudy, while others may not.

Second, every batch of olive oil may consist of a different mixture of fats and fatty acids, depending on the variety and ripeness of the olives and how the oil was processed. Extra-virgin oil is always the cream of the crop, but which crop, and how was it treated?

Third, we can't nail down a definite freezing point for even a pure, single fat or fatty acid, because unlike water, which freezes at precisely 32 degrees, fats freeze gradually over a range of temperatures. They don't suddenly turn hard, like ice; instead, they go through a vague, jellylike stage. So we can't say exactly when they are "frozen." Cottonseed oil, for example, changes from liquid to a waxy solid over the entire temperature range of 60 to 50 degrees.

Fourth, the very same fat can have different crystal forms that solidify (crystallize) at different temperatures.

Nevertheless, a few generalizations can be made. Saturated fats freeze at higher temperatures than unsaturated fats. In fact, most animal fats are already "frozen" solid at body temperatures. On the other hand, unsaturated fats generally stay liquid down to lower temperatures; the shorter-molecule ones have lower freezing points than the longer-molecule ones.

What does all this mean to your olive oils? It's just your luck that the average freezing point of olive oils is 42 degrees, right about refrigerator temperature. So even small variations in fatty acid composition can determine whether they will freeze when refrigerated.

Does your expensive olive oil perhaps contain slightly more of the higher-freezing-point fatty acids (e.g., palmitic acid) than your cheaper olive oil? That's not likely, but who knows? The amount of palmitic acid in olive oils can range from 7.5 to 20 percent.

Does your inexpensive olive oil perhaps contain more of the lower-freezing-point fatty acids (e.g., oleic acid)? Again, who knows? The oleic acid in olive oils can vary between 55 and 83 percent.

Because there are so many imponderables about the freezing points of oils, my advice is to forget about cloudiness and buy the olive oil whose flavor you like best. As long as it's extra-virgin, you won't be disappointed. If it turns cloudy in the fridge, just warm it up, and it'll become clear again.

LABELINGO: The first ingredient listed on the label of Chef Boyardee's Spaghetti and Meatballs is tomatoes. The label then informs us that the "tomatoes" consist of water and tomato puree.

Robert L. Wolke (http://www.robertwolke.com) is professor emeritus of chemistry at the University of Pittsburgh. His latest book is "What Einstein Told His Cook 2, the Sequel: Further Adventures in Kitchen Science" (W.W. Norton, 2005). He can be reached atwolke@pitt.edu.