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)
