Moisture, and what it does
Updated
On this page5
Moisture is the one variable that damages a spice in every direction at once. It dulls flavor, it cakes the powder, it invites spoilage, and it quietly changes what a jar weighs.
The number behind the grade
Spices are graded on moisture, and the reason is practical rather than aesthetic: high moisture causes clumping, shortens shelf life and opens the door to spoilage. A lot that arrives wet is not a lot you can fix by storing it well. It is a lot that starts its life closer to failing.
Where the water comes from
Almost never from the spice. Almost always from the air around it, and the mechanism is worth understanding because it is counterintuitive.
Where there is a temperature gradient across a store, moisture migrates from the warmer air into the cooler dry product. A sack against an outside wall, a bin under a roof that heats in the afternoon, a pallet near a door: each of those is a gradient, and the powder is the coldest and driest thing in the room. It acts as a sink.
This is why a warm store is a wet store, and why the advice to keep stock cool is not really about temperature. It is about removing the gradient.
The 55 to 60 percent line
Above roughly 55 to 60 percent relative humidity there is enough water vapor in the air to form liquid bridges between particles. That is the threshold where a free flowing powder starts to become a solid one, and it is a far more useful number than any instruction to keep things dry.
A cheap hygrometer in the room where you store and fill is the highest value instrument in a spice operation, and it costs less than one case of jars.
What to do about it
Keep the packing room under 55 percent if you can, and if you cannot, fill on the driest part of the day rather than the most convenient. Keep bulk stock sealed rather than open in bins, because an open bin is a device for exchanging moisture with the room. Close a bag between uses even for ten minutes.
And do not fill a jar in a humid room and seal the humidity in with it. The jar will hold that air against the powder for as long as the customer owns it.