I don't eat bread very often, so I have started to store it in the fridge to prevent little beasties from growing on it. In the process of restocking my fridge after being away for the holidays, I discovered that my current loaf of bread has gotten distinctly harder, which led me to ask the question: 'why, oh why, would my lovely bread go stale like that?' The answer, of course, is all about chemistry. As I learned in 'The Science of Cooking,' the starch granules that are present in flour (particularly those consisting of amylose) are crystallized. When the bread bakes, the crystals melt, and then are not able to form again quickly when the bread cools. But over time, the crystals grow, resulting in a tan brick where you used to have some yummy bagels. Two funny things which may undermine my whole fridge-strategy for storing bread are that this process is catalyzed by water and the rate of starch crystallization is fastest at 4 °C! Who knew?!?

On the other hand, it's entirely possible that both processes (both going stale and growing other life forms) are so drastically affected by preservatives that it may be a moot discussion these days, but that's a topic for another time...

Catherine (associate editor, Nature Chemical Biology)