Sealing

The closure is the part of the package doing the actual preservation work, and it is the part most often chosen on appearance and price.

What the seal is holding out

Moisture first. A jar's contents will equilibrate with whatever air can reach them over the months the product sits in a cupboard, and moisture is what cakes it, dulls it and shortens it.

Then oxygen, which oxidizes pigment and volatile oil. Then aroma, in both directions: keeping the spice's own smell in and the kitchen's out.

Also worth remembering that the seal and the headspace act together. The air you sealed in is a fixed quantity of moisture the product will absorb regardless of how good the barrier is.

Liner, not lid

A cap that threads on is not a seal. The barrier is the liner inside it, and liner material is a specification most suppliers publish and most buyers ignore.

A foam liner is a gasket and very little else. A lined closure with an appropriate barrier material is a different product at a similar price. If a jar is failing on moisture uptake, the liner is the first thing to look at, not the jar.

Induction and when it earns its cost

An induction seal bonds a foil membrane to the jar land, giving a genuine hermetic barrier plus visible evidence that it has not been opened. It needs a sealing unit and a compatible closure and jar, so it is a real capital step.

It earns that cost where the product is slow moving, sold online into a cupboard, or priced high enough that a fading jar is a reputational problem. It is harder to justify on a fast-turning market stall line.

Tamper evidence

Separate from preservation, though induction gives you both. A shrink band or a breakaway closure tells a buyer the jar has not been opened, and wholesale buyers will ask for it by name even where retail customers do not.

Decide it before you order jars rather than after, because it constrains the closure and therefore the finish, and changing it later means changing the jar.

References