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Q & A: Soggy Donuts

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Most recent answer: 10/22/2007
Q:
I bought a donut yesterday and put it in a zip-lock baggy. The donut was coated with granulated sugar (aka "table sugar"). When I opened it this morning, almost all of the sugar had turned to a sticky, semi-liquid form. This never happens when I leave a donut "exposed" on my desk overnight. Why did the sugar liquify in the plastic bag?
- Dave Graham (age 31)
Hansen Information Technologies, Sacramento
A:
Dave -

Donuts contain a lot of water. Normally, if you just let a donut sit out, the water will evaporate into the air and the donut will get dried out (stale). But when you sealed the donut in the plastic bag, you sealed the moisture in too. When some of the moisture left your donut and evaporated into the air, the easiest place for it to stick was to the sugar, converting it to sugar water.
This is part of a general problem that thermodynamics makes for food packaging. Water doesn't flow downhill only in ordinary physical space but also in chemical space. Like any chemical, it flows to the state of lowest 'chemical potential'. For water, that means something like 'to whatever is driest'. Initially the donut is much moister than the sugar, but it can't stay that way.
A more serious version of the same problem arises for raisin bran. You can buy nice crispy cereal and you can buy nice moist raisins. But you can't buy nice crispy cereal with nice moist raisins mixed in. The reason is that the water will flow (as vapor) from the moist fruit to the dry cereal. The standard solution is to use very dry raisins, which almost no one likes. Another solution is to try to make a chemical barrier to slow down the flow of water from the raisins to the cereal by coating the raisins with hydrogenated vegetable oil. That's not a very wholesome ingredient, and it doesn't seem to work very well anyway.

-Tamara and Mike

(published on 10/22/2007)

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