Why does my grinder retain so much coffee?

Grinder retention is one of the most frustrating variables for home baristas. When you grind a specific dose, you expect that exact amount to end up in your portafilter. However, due to the internal geometry of the grinder—the path coffee travels from the burrs to the exit—some grounds inevitably get stuck in 'dead spots.' Depending on the design, this can range from a fraction of a gram to several grams.

Several factors influence how much coffee your grinder holds onto. Static electricity is a major culprit, particularly with light roasts, which are drier and more prone to charge accumulation. Plastic chutes often exacerbate this static compared to metal ones. Additionally, the design of the burr chamber plays a significant role; grinders with steep, direct, and short paths to the exit generally retain far less coffee than those with complex, winding paths.

For the home user, this retention creates a 'stale' problem. If you grind 18 grams but only 16 grams of fresh coffee make it out, the remaining 2 grams from your previous session are sitting in the chute. When you grind your next dose, those stale particles are pushed out first. This means your extraction profile is constantly shifting, making it nearly impossible to dial in a recipe with precision. This is why many baristas 'purge'—grinding and discarding a small amount of coffee—before pulling a shot.

To minimize this, look for 'zero-retention' grinders, which are specifically engineered with steep chutes and minimal internal volume to ensure that what goes in is exactly what comes out. If you cannot upgrade your equipment, you can mitigate the issue by weighing your beans before grinding and weighing the output after, then purging as needed. While this adds a step to your workflow, it is the only way to ensure that your carefully selected beans are the only thing ending up in your cup.