Instructions: Q Why is it that when I make split-pea soup and put the leftovers in the refrigerator, it sets up like cement? I then have to add a lot of water and beat it into submission to make it thin enough to eat with a spoon.

A Peas are seeds, and they therefore contain an abundance of starch to fuel the growth process when the seed germinates. When you simmer the peas, the starch granules are released and disperse throughout the liquid, where they take on water and swell. This process is what thickens the soup into a sort of paste.

Thats fine if you eat it while its hot. A good, thick pea soup is among lifes most comforting foods, especially in cold weather.

But when the leftovers are cooled, the swollen starch grains begin to stick together, trapping a lot of water and forming a gel. In a gel, the water is locked into a semi-solid mass and is no longer free to flow easily. To thin the soup back to its original consistency, heat and stir it thoroughly while adding more water to replace that which became trapped and immobilized in the gel.

Im told that the traditional Dutch pea soup (erwtensoep), also called snert, is deliberately made a day ahead and refrigerated, so that when it is reheated before serving, it will be thick enough to support a spoon standing straight up.

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh. His latest book is What Einstein Told His Barber: More Scientific Answers to Everyday Questions.

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