One spice taking on another

Your coriander smells faintly of cumin. Nothing spilled, nothing was mislabeled, and both jars were closed. Aroma moves through air, and air is everywhere in a spice operation.

The barrier works both ways

The guidance on storing herbs makes a point usually stated only in one direction: airtight vapor-proof containers keep herbs from absorbing moisture from the air, and keep other foods from absorbing the fragrance of the herbs.

That second clause is the one that matters here. The container is not only protecting its contents from the room. It is protecting the room from its contents, and in a store holding forty aromatic materials every container is both.

Where it happens

Rarely in a sealed jar. Almost always in bulk storage, in the grinder, and on the bench. An open sack of cumin in a small room scents everything in that room, including the empty jars waiting to be filled.

The mill is the worst single point, because residue stays in it and the next material through picks up what the last one left.

The usual culprits

A short list does most of the damage: cumin, asafoetida, fenugreek, clove, and anything containing a high proportion of volatile sulfur or a very low odor threshold. Asafoetida in particular will find its way into products stored in the same building.

The mild, expensive materials are the victims. Saffron, cardamom and delicate herbs take on a neighbor's character and lose their own.

Separation that actually works

Sealed bulk containers rather than open bins, which is the same instruction that protects against moisture and pays twice here. Physical separation of the strongest materials, ideally a different room. A dedicated mill or a genuine cleaning routine between materials, run in order from mildest to strongest so the residue you carry is always into something that can take it.

And store empty jars closed. They are open containers sitting in your aroma.

References