Why does my grind size change during the day?

For the home barista, nothing is more frustrating than dialing in the perfect espresso shot in the morning, only to find the flow rate has changed by the afternoon. This phenomenon, known as grind drift, is a common reality in coffee preparation. It is rarely the result of a faulty machine; rather, it is a response to the physical environment and the mechanics of the grinder itself.

Temperature is perhaps the most significant factor. As a grinder runs, the motor and the burrs generate heat. Metal expands when heated, which subtly alters the gap between the burrs. Even a microscopic change in this distance can have a massive impact on your espresso extraction. If your grinder is not temperature-stable, you may find yourself constantly making micro-adjustments throughout the day to keep your shot timing consistent.

Humidity also plays a critical role. Coffee beans are hygroscopic, meaning they absorb and release moisture from the air. When humidity levels change throughout the day, the internal moisture content of the beans shifts. High humidity can make beans feel "tougher" or more pliable, while dry air makes them brittle. These changes affect how the burrs fracture the beans, directly influencing your grind size and the resulting resistance in the portafilter.

To manage grind drift, consistency is your best tool. Keep your grinder away from heat sources like the espresso machine, and if possible, keep your beans in an airtight container to mitigate the effects of humidity. Many high-end grinders are now built with thermal stability in mind to minimize this drift. Ultimately, learning to anticipate these shifts and making small, incremental adjustments is simply part of the skill set required to master espresso.