Is chocolate with a white-ish coating safe to eat?
The white stuff you're talking about is called a "bloom" and is not mold or anything dangerous.
To understand a bloom, you have to first know a bit about what chocolate is made of. Chocolate is quite a complicated substance, and though the exact proportions vary from brand to brand, for milk chocolate you're looking at something like 10 percent cocoa, 20 percent milk powder, 50 percent sugar, and 20 percent additional cocoa butter with bits of vanilla and other odds and ends.
Mixtures like this have a tendency to separate, like cream rising to the top of milk, and there is a lot of effort that goes into chocolate fragmentation to keep all these various components homogenous.
As long as chocolate stays hard, the various bits that make it up are largely set in place (or at least can move only very, very slowly), so chocolate stays as it is.
If you warm up chocolate, some of the fatty material (especially cocoa butter) can come to the surface and form a bloom, which is why one tends to see blooms on chocolates that have been heated to the point of softening -- say, by being left in the sun or near a heater.
Even in the absence of heating, chocolate is not infinitely rigid, and the cocoa butter can separate out, which is why one often sees blooms on old chocolates even if they have been kept from heat.
Chocolates with additional fats (like those with fatty nut-containing praline centers) tend to bloom more readily than solid chocolates.
Industry spends a lot of effort trying to keep blooms from forming, and you'd be amazed at the technology that goes into studying this stuff -- all the way up to X-spectroscopy, radioactive isotope labeling, atomic force microscopy, etc., but the problem is far from solved, as you will no doubt find if you leave some holiday chocolates around until spring!
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. Include your initials and hometown. 