Why A Grind Setting Drifts Over Time

A setting that produced a perfect brew last week can produce a noticeably different one today with nothing apparently changed. Several slow-moving factors alter what a fixed setting delivers.

Beans change as they age

Roasted coffee continues to lose carbon dioxide and to take up moisture from the air over the weeks after roasting, and both alter how it fractures.

Fresher beans are more brittle and shatter readily, while older, slightly rehydrated ones deform more before breaking and give a coarser effective grind.

The practical effect is that a brew slowly speeds up as a bag ages, requiring a finer setting to hold the same contact time.

Different lots grind differently

Density varies with growing elevation and variety, and roast level changes porosity, so a new bag can need a substantially different setting from the last.

Dark roasts are more friable and produce more fines at the same gap, while dense light roasts resist fracture and grind coarser.

Expecting a setting to transfer between coffees is the most common source of an apparently faulty grinder.

Temperature affects the machine and the beans

Burrs and their carriers expand as they warm during heavy use, which narrows the gap slightly and shifts the grind finer within a session.

Beans themselves are more brittle when cold, so a bag straight from a cold store grinds differently from one that has been on the counter.

Neither effect is large in a single dose, but both are enough to matter for espresso where small changes have visible consequences.

Residue accumulates on the burrs

Oils and fine particles build up in the grooves of the burrs and on the surfaces around them, gradually changing the effective geometry.

The build-up is worst with oily dark roasts and in humid conditions, where residue becomes sticky rather than powdery.

Cleaning the burrs restores the original behaviour, which frequently means the setting has to be moved back after a thorough clean.

Humidity works on everything at once

Ambient moisture affects bean brittleness, static behaviour and how readily grounds move through the chute, all in the same direction.

A damp day therefore produces less static, less retention and a slightly coarser fracture than a dry one with identical settings.

Recording conditions alongside settings in a brewing notebook turns these apparently random shifts into a pattern that can be anticipated.