Retention

Retention refers to the amount of ground coffee that remains stuck inside a grinder's grinding chamber, chute, and exit pathway after a dose is ground. This retained coffee is exposed to air and goes stale quickly, and it mixes with fresh grounds in subsequent doses — effectively contaminating every dose with a small amount of stale coffee from the previous grind session.

Retention varies dramatically between grinder designs. Large commercial flat burr grinders with horizontal grinding paths and long exit chutes can retain 5 to 10 grams or more. Single-dose focused grinders with vertical chutes, bellows, and low-retention designs may retain as little as 0.1 to 0.5 grams. Hand grinders typically have very low retention because the simple, direct path from burrs to collection chamber offers few places for grounds to hide.

High retention causes two problems. First, the retained stale coffee mixes with the next fresh dose, dulling flavors. Second, when switching between grind settings (for example, from espresso to pour-over), retained coffee from the old setting exits first, producing a dose that starts at one grind size and transitions to another — a phenomenon called exchange or seasoning the grinder.

The single-dosing workflow — weighing a precise dose of beans and grinding them all with nothing left in the hopper — has become popular partly because of retention awareness. Single-dose grinders are designed with low-retention features like steep chute angles, smooth internal surfaces, bellows for air-assisted clearing, and anti-static measures to minimize grounds clinging to surfaces. Purging (running a small amount of beans through the grinder before your dose) is a workaround for grinders with higher retention.