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Q. A few weeks ago, I bought an inexpensive olive oil labeled "extra-virgin." Despite storing it in my very cold refrigerator after opening it, the oil did not congeal, as it always does with more expensive brands. Does this mean it wasn't 100 percent olive oil? Would the congealing vs. the non-congealing of refrigerated olive oil then be an indicator of its quality and purity?

A. Unfortunately, it's not that simple.

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 affect exactly what temperature a given oil will freeze.

First, 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 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."

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.

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 unlikely, but who knows? The amount of palmitic acid in olive oils can range from 7.5 percent to 20 percent.

Forget about cloudiness and buy the tastiest olive oil. 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.

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