Pour-over coffee: Mixing by a water jet impinging on a granular bed with avalanche dynamics

Coffee is one of the most consumed beverages in the world. However, issues such as climate change threaten the growth of the temperature-sensitive Coffea arabica plant, more commonly known as Arabica coffee. Therefore, it is crucial to make beverages more efficient by using less coffee while still meeting the high demand for the beverage. Here, we explore pour-over filter coffees, in which a water jet impinges on a water layer above a granular bed. To reveal its internal dynamics, we first substitute opaque coffee grounds with silica gel particles in a glass cone, imaged with a laser sheet and a high-speed camera. We discover an avalanche effect that leads to strong mixing at various pour heights, even with a gentle pour-over jet. We also find that this mixing is not significantly impacted by a layer of floating grains, which is often present in pour-overs. Next, we perform experiments with real coffee grounds to measure the extraction yield of total dissolved solids. Together, these results indicate that the extraction of the coffee can be tuned by prolonging the mixing time with slower but more effective pours using avalanche dynamics. This suggests that instead of increasing the amount of beans, the sensory profile and the strength of the beverage can be adjusted by varying the flow rate and the pour height. In this way, the extraction efficiency could be better controlled to help alleviate the demand on coffee beans worldwide.