Blends separating after they are mixed

You can mix a blend perfectly and still ship a jar that is wrong. Segregation is the process by which a uniform mixture comes apart, and it happens after the mixer stops.

Mixing and segregation are different problems

Mixing distributes. Segregation re-sorts. They are not opposites of the same operation, which is why more mixing does not protect against segregation and can make it worse by widening the particle size distribution.

A blend that mixes easily may segregate readily, and both facts come from the same property: how different the components are from each other.

What drives it

Segregation happens whenever particles differ in size, density, shape or surface properties and are then allowed to move relative to each other.

Two mechanisms do most of the work. Percolation: smaller particles filter down through the gaps between larger ones, which is why fines end up at the bottom. Elutriation: denser particles settle while lighter ones stay up, which is why salt sinks under dried leaf.

Powders of similar size and density blend more uniformly and are less likely to separate. That sentence is the entire design rule.

Where it happens

Everywhere the blend moves. Emptying the mixer, transferring to a holding container, scooping, funneling into jars, and the journey to the customer. A finished blend in a sack on a pallet is segregating quietly the whole way.

The result is stratified containers where the top and bottom differ, which is the technical description of the complaint you will actually receive.

Designing against it

Narrow the particle size range across components before mixing. Grinding a leafy component finer, or choosing a coarser salt, does more than any change to the mixing step.

Then minimize handling. Mix close to the filling point, fill directly from the mixer where you can, avoid free-fall into deep containers, and do not store a finished blend in bulk for weeks before jarring it. Every transfer is an opportunity you are giving it.

References