Of specialty coffee drinks, espresso has the most cup-to-cup variation in quality. For those who are not coffee aficionados — such as yours truly — espresso is made by forcing hot water through a packed bed of coffee grains. Many factors can affect the final output, including the amount of dry coffee used, the fineness of the grind, water temperature and pressure, and how tightly packed the granular bed is.
Conventional wisdom suggests that a fine grind is best since it increases the exposed surface area of coffee, but researchers found this is not, in fact, ideal. At very fine grinds, the bed of coffee becomes so tightly packed that water cannot pass through some sections, meaning that the coffee there is completely wasted since nothing is extracted.
Instead, a slightly coarser grind provided better and more consistent extraction because water passed through the entire bed of grains. The researchers point out that this not only produces a good, consistent cup of espresso, but it does so with less waste, something that is becoming more and more important as the climate crisis affects coffee growers. (Image credit: K. Butz; research credit: M. Cameron et al.; via Cosmos; submitted by Kam-Yung Soh)