Caking

A jar arrives and the contents have set into a block. The customer taps it on the counter. This is a common physical complaint in the trade and it is almost always something you did in your own packing room.

What is physically happening

Humidity caking happens when a hygroscopic powder meets moist air. Water condenses at the points where particles touch and forms liquid bridges between them. Those bridges dry, leave a solid connection, and a free-flowing powder becomes one object.

It is not compression and it is not settling. It is a phase of water doing something at the contact points, which is why tapping the jar sometimes breaks it up and sometimes does nothing at all.

The humidity number

The threshold is around 55 to 60 percent relative humidity. Below it there is not enough vapor to bridge. Above it there is, and the tendency to cake rises with the powder's own water activity as well as the room's.

Two variables, then. How wet the air is, and how wet the powder already was. A dry powder in a damp room and a damp powder in a dry room both cake; the damp powder in the damp room cakes fast.

Why it usually happens at fill

Because filling is the one moment the product is open to the room in a thin layer with maximum surface area, and then immediately sealed. Whatever the air was in your packing room, you have just put a portion of it inside every jar and closed the lid on it.

Stock that caked in a sealed bag caked because the bag was opened in a humid room, or because a temperature gradient drove moisture into it. Stock that caked in the jar caked because you filled it on a wet day.

Fixing it, and not fixing it

You cannot fix a caked jar in the customer's hand and you should not try to fix caked stock by drying it. Re-drying drives off volatile oil, so you would be selling a weak product instead of a lumpy one.

What you can do is prevent it: a hygrometer in the packing room, filling on the dry part of the day, sealed bulk storage, and not leaving a bag open on the bench while you label. Anti-caking agents exist and work, and they are a formulation decision with a labeling consequence rather than a free fix.

References