Especially if it’s cultured in a mix made with flour, which also contains natural yeasts.
Oh dear. I sure hope it’s not yeast. First of all, I’d be wrong, and that’d be real big bummer.
Secondly, my prunes also have that white powder on them after sitting in my pantry for a long while and assumed I’ve been eating crystalline sugar. Or so I thought. But if memory serves me right, I had dried dates which developed the same white powdery substance…?
It’s definitely sugar on dates.
Just found this post when I searched for “white powder on dried prunes” and found I have had the exact same experience as Karl only 8 years later; scanning all the replies there did not seem to be a definitive answer. Anyone have any updated info on this topic?
Reading through, it seems that the definitive answer relies on someone who is actually experiencing the phenomena to use magnification. If you can post a photo that is looking through a magnifying lense or microscope, someone should be able to provide a definitive answer if the picture is good enough.
Though I’ll say that if the company that produces the prunes is large enough, then it would seem unlikely that all prunes in every batch could form the powder. I would expect that a range of farms would be represented, so it would be strange for all of them to be incubating their prunes with the same white critter. A fructose covering would make more sense since it would be present in the fruit regardless of where it was grown.
If you can save up enough of it, you could also see if it melts or if it just chars, if you put some in a spoon and hold a flame under it (in theory).
I’m going to suggest some version of Nitrite (use as preservatives), although I wouldn’t think they’d be packaged with organic fruit.
Are three enough? Is six too many? Man, I miss the days when Americans ate no fiber at all.
Could the dust possibly be elemental sulfur?
Here’s a cite for crystallized sugar, fwiw. Its in second paragraph. Not the best cite, but probably satisfacory. Kinda cool that the first reult of Google search on ‘plums white coating’ was this thread, but from 2012 
If the prunes Karl had bought were just being stored (or on the shelves) a bit longer before sale, that would produce his observed results.
Well, according to this site it is sugar. Do you think this is a reliable site?
I was surprised to read of the appearance of the “white powder” after a few days and not present initially–I thought the OP was referring to something which puzzled me for years, what’s known as the “blush” of (on?) fresh grapes–“ambient yeast,” which I fruitlessly (heh) and stupidly tried to wipe off.
The explanation and cites here are short and good (with a nice touch on Carravagio’s Bacchus, where the god of wine’s lips are faintly covered as well); one of them is to to Wiki Fermentation in winemaking, which has more on OP and some thread suggestions above.
Interestingly, another name for it is ‘bloom’.
On fruits like grapes, plums, blueberries, the bloom isn’t all yeast - it’s sometimes also a waxy substance that I presume the plant produces to help the developing fruits shed raindrops.