Fines – Fine grain. Fine particles are the smallest particles produced when coffee is ground; they are described as beans with a diameter of fewer than 100 micrometers. Acceptable also refers to any grain smaller than the bulk of milled grains or consists solely of cell fragments.
On the other hand, the fine grain is vital to the espresso extraction process by any definition. Espresso extraction is dependent on surface erosion from the beans (M Petracco, 2005); hence a sizeable fine-grain surface area is required for optimal espresso extraction.
The term “fine migration” refers to the assumption that fine particles travel beneath the coffee during the espresso-making process. Fine particles pass through the coffee mass in one shot, following the flow of the water until they reach the basket, where they tend to congregate and form a dense layer. During the first several seconds of extraction, this layer retains tiny particles and constitutes a considerable flow barrier.
The fine particles only migrate downhill during extraction (when water is injected into the puck), according to the Baristahustle tests, and are unaffected by puck preparation activities. Several additional articles go into greater detail about the impact of this procedure.