How does humidity affect coffee grinding?

Coffee beans are hygroscopic, meaning they naturally absorb and release moisture from their environment. When the ambient humidity is high, the beans take on water, which makes them slightly softer and more elastic. This change in physical structure affects how they shatter when they pass through your grinder's burrs. Instead of a clean, uniform grind, you may find that the grinder produces more 'fines'—tiny, dust-like particles—which can clog your filter and lead to over-extraction.

Static electricity is another major issue exacerbated by low humidity, but high humidity brings its own set of challenges. While very dry air causes grounds to fly everywhere due to static, high humidity causes them to clump together. These clumps can create 'channels' in your coffee bed during brewing, where water flows through the path of least resistance rather than evenly through the grounds. This results in an uneven extraction, where some coffee is over-extracted and bitter, while other parts remain under-extracted and sour.

Managing these environmental factors is part of the professional barista's craft. The best defense is to minimize the exposure of your beans to the air. Keep your coffee in a cool, dry place, and use an airtight container to protect it from fluctuations in humidity. If you live in a particularly humid climate, you might find that you need to adjust your grind setting slightly coarser to compensate for the change in bean density.

In practice, the most important takeaway is to be aware of your environment. If you notice your coffee tasting different on a rainy day compared to a dry one, it is likely due to these subtle changes in the beans. By keeping your beans protected and grinding them fresh, you can minimize the impact of humidity and maintain a consistent, high-quality brew regardless of the weather outside.