Grind consistency across a batch

Producers usually specify how fine they grind. Very few specify how consistently, and the second number causes more trouble than the first.

The spread is the specification

Two batches with the same average particle size can behave completely differently if one has a narrow distribution and the other a wide one. A wide particle size distribution is itself the cause of segregation, because it guarantees there are small particles available to percolate through the gaps between large ones.

So the target is not fine. The target is even.

Where inconsistency comes from

A mill that heats through a long run grinds differently at the end than at the start, because warm oily material smears rather than fractures. Feeding faster than the mill can process produces oversize. Feeding too slowly over-grinds and generates fines.

Different lots of the same spice arrive at different moisture contents and grind differently for that reason alone, which is a genuine surprise the first time a familiar material behaves unlike itself.

What it causes downstream

Segregation in blends, which is the expensive one. Sifter clogging, because fines fill the spaces and bridge the apertures. Fill weight drift, because bulk density moves with the spread. And inconsistent flavor release, since surface area drives how quickly the volatiles come out.

One upstream variable, four downstream symptoms, none of which look related when they arrive as separate complaints.

Holding it

Grind in consistent quantities at a consistent feed rate, let the mill cool between runs, and sift afterwards to remove the tails. Sifting is the cheapest available control on distribution and it also gives you a physical record of what your grind actually is.

Write the screen size into the product specification next to the formula. It is a product attribute, and it is the one that will change silently when a mill is replaced.

References