It set solid in the tub

A tub that has turned into a brick is the most visible failure in this property and it starts, like most things here, with moisture.

The mechanism

Water condenses at the points where particles touch and forms bridges between them. Those bridges set, and a free-flowing powder becomes one object.

It is not compression and it is not settling. It is water doing something at the contact points, which is why tapping a caked tub sometimes helps and sometimes does nothing at all.

Why this category is worse than most

Because supplement powders are mainly hygroscopic by nature: they pull moisture from their environment rather than merely tolerating it.

And because they are opened repeatedly. A tub is opened daily, in a kitchen, with a scoop that may be damp, over two or three months. Each opening is an exchange of humid room air for the dry air inside.

What it breaks besides appearance

This is the part people underestimate. Moisture uptake wets the solids and affects stability, flow, compactibility, dosing accuracy and hardness.

So a tub on its way to caking is already delivering inconsistent doses well before it is visibly solid. The caking is the late symptom of a process that has been changing the product for weeks.

And the dryness that kept it microbiologically safe is gone with it.

Preventing it

An anticaking or flow agent at a fraction of a percent, which is what they exist for.

A moisture barrier: an induction seal or a foil-lined pouch rather than a friction-fit lid, and a desiccant for the most hygroscopic formulas.

Dry filling conditions, since whatever air you seal in stays with the product.

And an instruction on the label to keep it closed and dry, because the customer's kitchen is where most of the exposure happens and it is the only part you cannot control.

References