The scanners can't see powder other than it being an amorphous shape. Customs has never given a second glance at jars of powder. (Not only that, but unlabeled, not sealed and containing white powder. I carry an empty original can in case somebody questions them, but to date nobody has.)

I have had a canister of lite-salt (half NaCl, half KCl) draw scrutiny by TSA, but it was opened and they just wanted to make sure it was what it said it was. Tapped out a few grains and tested them.

In my experience customs only opens bags containing organic materials with a density approaching that of water unless they have already found something. (Note that while the powder I'm carrying is organic it's fluffy enough that it's maybe half the density of water.)