How do I adjust grind for different roast levels?

The relationship between roast level and grind size is rooted in the physical transformation of the coffee bean during the roasting process. As coffee beans are exposed to heat, they undergo pyrolysis, which breaks down cellular structures and causes the beans to expand. A light roast retains much of its original density and hardness, requiring more surface area—achieved through a finer grind—to allow water to penetrate and extract the desired compounds effectively.

In contrast, dark roasts are subjected to longer roasting times, which makes the bean structure significantly more porous and brittle. When these beans hit the burrs of your grinder, they shatter more easily, creating a higher volume of fine particles. If you use the same setting for a dark roast that you used for a light roast, you will likely experience over-extraction, resulting in a cup that tastes harsh, ashy, or excessively bitter.

To manage this in practice, treat your grinder as a tool for balancing solubility. When switching from a light roast to a dark roast, move your grinder setting one or two steps coarser. This adjustment compensates for the increased solubility of the darker bean, ensuring that the water flows through the coffee bed at an appropriate rate rather than stalling or over-extracting the delicate sugars and acids.

It is also important to note that roast level affects how your coffee ages. Freshly roasted light beans often contain more trapped CO2, which can impact the initial extraction. As beans age, they lose moisture and gas, which may require you to tighten your grind slightly over the life of the bag to maintain a consistent extraction yield. Always dial in your coffee by taste, using the roast level as your starting point for these necessary adjustments.