How does grind size affect extraction?

The relationship between grind size and extraction is often oversimplified as a linear progression: finer equals more extraction, coarser equals less. In reality, the mechanism is governed by the interplay between surface area and fluid dynamics. When coffee is ground, it produces a bimodal distribution of particles, including a significant fraction of 'fines'—particles smaller than 100 µm. These fines are the primary drivers of flow resistance. As the share of fines increases, the permeability of the coffee bed drops, forcing water to navigate a more tortuous path. This increases contact time but can lead to non-uniform flow, or 'channeling,' where water bypasses dense pockets of coffee, resulting in an uneven, astringent cup despite a high measured extraction yield. For the producer, this highlights that the quality of the grind is as critical as the quality of the bean. A grinder that produces a high volume of inconsistent fines can mask the inherent sweetness of a high-scoring microlot by creating a chaotic extraction environment. A common misconception is that finer is always more efficient; however, research indicates that once a certain threshold of fineness is reached, the marginal gains in extraction efficiency are offset by the physical limitations of the coffee bed, leading to a plateau or even a decline in cup quality. Understanding this allows producers and roasters to better calibrate their equipment to highlight the specific solubility profiles of different processing methods, such as the distinct cellular structures found in natural versus washed coffees.