I was about to suggest this not realising people did it already. I don't understand though why it would make them wet as the liquid nitrogen would be dry (water long being removed before you get down to temperatures that Nitrogen would liquify at), therefore, as long as you grind it in a dry area or maybe have a grinding machine that vents (as the air will expand as the spices are ground) but doesn't let in air, I can't see why they'd get wet? Also if you're really cunning about it, you may be able to store the spices under the nitrogen gas which is liberated and not let them back into an oxidative environment (which will presumably prevent oxidation of some of the delicate oils.)
Just be aware that use of liquid nitrogen does bring H&S risks but having tipped some over my feet several times and having kept all of my toes it's not as scary as most people think (don't try it at home though)!
I think I wasn't clear on my post. When I mentioned fresh herb/spice, I mean really fresh, before any kind of dehydration process. Thus, there would bound to be some moisture content in them. The addition of LN helps to make them brittle and easier to break into small pieces. LN then evaporates, as per your explaination, but leaving the moisture still in the "powder" to thaw. In the end, you get a moist/wet powder if they are not used before they thaw.
Attached some pics from Cooking Issues
Powdered fresh herbs. This technique gives you a powder as fine as any dried herb, but with fresh. It looks great, smells great, and tastes great. It is the best way to evenly apply a mixture of herbs. 1) blanch your herbs in boiling water or they will turn black. Squeeze out extra water. 2) Fill a Vita-Prep pitcher at least halfway with LN. The large pitchers with the wet blades don’t work well for this because product sprays everywhere. The smaller pitcher with the dry blade never causes problems. Put your herbs in the pitcher and let them freeze. 3) Cover and blend, slowly working your way up to high speed. Don’t wait too long to turn on the blender after you’ve added LN or the bearings will make an awful noise. 4) Pour the mix through a chinois into a bain. 5) The stuff that is left in the chinois is too coarse for us. Put it back in the pitcher for the next round. 6) Pour the mixture in the bain through a coffee filter. This is the good stuff. 7) Put the powder in a chinois and tap it onto your dish—do it while the powder is still frozen. You can also fold this stuff into whipped cream, mashed potatoes, etc. Remember the flavor and aroma will increase dramatically as the herbs thaw. Don’t overdo it. 8 ) Allow to thaw.
1) Frozen olive oil. 2) Frozen maple syrup. 3) Frozen honey. 4) Frozen honey as it thaws.
And yes, I do agree that LN should be used with caution! You are lucky to still have your toes. I assume you had your safety shoes on?
I think the cryogenic systems are by no means cheap. But if the customers request/expects such high quality herbs/spices, they should be able to pay for it.