Most chemical reactions go more slowly in the cold and this is of course the reason we use refrigerators to store food and slow the reactions that correspond to it going bad.

Ultimately, this is because chemical reactions need molecules to bump into one another with enough energy to have something interesting happen, and often enough for that to happen at a good rate. When things are cold, the molecules aren't moving around all that much. A good rule of thumb is that a 10-degree-Celsius drop in temperature will about halve the rate of a typical biological reaction like "going bad."

That being said, bread is special. The thing is that when bread goes stale, it doesn't so much go stale via chemical reactions as many foods do, but rather via something physical that happens to starch. Starch tends to crystallize at cool temperatures, and this process, sometimes called "retrogradation," happens about six times faster at refrigerator temperatures than at room temperature. Loss of water also speeds up this process, so you want to keep bread in a container that stops moisture from escaping.

If you want to think of an analogous process, think of a jar of honey that's been open for a while and had some water evaporate so that the sugar crystallizes. You can push it back to being runny by heating it up a bit, with the addition of a bit of water if a lot of water has been lost.

There are several ways to avoid retrogradation. One is to keep the bread at room temperature. This increases the odds of molds growing (the growth of mold involves chemical reactions responsible for the life of molds), but at least the starch doesn't crystallize as quickly. If the bread has gone stale via the retrogradation process, you can heat the bread up again (perhaps with a little moisture) and reverse the process, much as you could do with honey as mentioned above.

You can also freeze bread, which might not sound like too good an idea, but if you do it quickly, the bread doesn't spend too long at temperatures that favor retrogradation. You can heat the frozen stuff up again just before you want to eat it and it won't be bad.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.

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