Blend uniformity: why a correctly formulated multivitamin can still fail

The most common cause of an out-of-specification multivitamin is not a formulation error. It is segregation — a correctly made blend coming apart between the mixer and the filling line.

How blends separate

Powder mixtures are only metastable. Vibration and flow give particles the chance to sort themselves, and they do so by three mechanisms:

• Percolation — small particles filter down through the gaps between large ones. The dominant mechanism, and the reason particle size distribution matters more than average particle size.
• Trajectory segregation — larger, denser particles travel further when the powder is poured, so they concentrate at the far side of a heap.
• Elutriation — fines are carried upward by displaced air during filling and settle last, on top.

All three are made worse by exactly the thing that happens to every ingredient we ship: a long journey on a truck and a ship.

The parameters that predict trouble

• Particle size distribution, not a single mesh figure. A blend whose components span a wide size range will segregate however well it was mixed.
• Bulk and tapped density, and the Carr index derived from them. Large density differences between components are a direct segregation risk.
• Angle of repose and flow function, which describe how the material behaves when it is moved.
• Moisture, which changes cohesion in both directions — a little can bind a blend together, more can cause caking.

What this means when specifying ingredients

Matching particle size and density across the components of a blend is more effective than any amount of extra mixing. In practice that means specifying a particle size range for each ingredient rather than accepting whatever the standard grade happens to be, and it is a reasonable thing to ask a supplier for.

Granulation of the problem components is the other answer, and for micronutrients present at very low inclusion it is often the only reliable one — a premix or trituration at a manageable dilution beats trying to distribute a few grams of an active through a tonne of powder.

Test where it actually matters

Blend uniformity sampled at the mixer proves the mixer worked. Sampling at the end of the filling run, and from the top, middle and bottom of a drum that has travelled, proves the blend survived. That second test is the one worth writing into the acceptance protocol.