Concentration gradient inversion via sedimentation of coffee grounds in full immersion brewing

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In full immersion brewing methods, the coffee grounds are fully submerged in water during the brew and then filtered out prior to consumption. If the brew is not agitated, most of the coffee grounds quickly float to the surface, but then eventually sediment to the bottom as they de-gas and become less buoyant. Although this behavior is readily observed, little research has investigated the impact of the transient sedimentation process on the concentration profile of total dissolved solids (TDS) within the brew. Here, we report that unagitated full immersion brews undergo a concentration gradient inversion midway through the brewing process and that the time scale of the inversion depends sensitively on the roast level of the coffee grounds. Systematic measurements of the TDS versus both depth and time reveal that the brew contains a region of nearly twice the concentration near the top air/liquid interface at the beginning of the brew, but then the higher concentration region inverts to the bottom of the container as the brew progresses. Time-lapse photography observations indicate that the time scale of the gradient inversion is consistent with the average rate of sedimentation, which occurs up to a factor of three slower for darker roasts. A key implication of our results is that sensory evaluations obtained via the traditional “cupping” method used widely in the coffee industry might be confounded by differences in TDS for different roast levels.