What is the difference between wet and dry milling?

To understand the journey from farm to cup, it is vital to distinguish between the two primary milling stages: wet milling and dry milling. While the terms are sometimes confused with processing methods like 'washed' or 'natural,' they actually refer to distinct phases in the post-harvest timeline. Wet milling is the primary processing stage that occurs shortly after harvest. Its goal is to remove the fruit from the seed. In a wet mill, cherries are sorted, depulped, and often fermented to remove the sticky mucilage before the beans are dried to a stable moisture content.

Dry milling, on the other hand, is the final stage of preparation before coffee is exported. By the time coffee reaches the dry mill, it has already been dried—either as parchment coffee (from a washed process) or as a whole dried cherry (from a natural process). The dry mill’s primary function is to remove the protective parchment layer or the dried fruit skin, a process known as hulling. Once the green coffee bean is exposed, the dry mill uses specialized machinery to sort the beans by size, density, and color to remove defects.

It is a common misconception that a dry mill can 'fix' poor quality. While a dry mill is highly effective at removing physical defects like broken beans, insect damage, or foreign objects, it cannot improve the inherent flavor profile of the coffee. If the initial wet milling or drying process was flawed—resulting in fermentation defects or mold—the dry mill can only mitigate the damage by sorting out the worst offenders. The quality of the final cup is largely determined by the care taken during the initial harvest and wet milling stages.

In practice, these two stages often happen in different locations. Many farmers operate small-scale wet mills on their own land to maintain control over the initial processing. However, dry milling is typically performed at larger, centralized facilities operated by cooperatives or exporters. These facilities house the heavy-duty equipment required for precise grading and final quality control. For the consumer, this means that the consistency of your morning brew is the result of a long chain of custody, where the wet mill sets the flavor potential and the dry mill ensures the physical quality and uniformity of the beans you purchase.