The Kinetics and mechanism of caffeine infusion from coffee: Hydrodynamic aspects

Hydrodynamic flow conditions were controlled by means of large horizontal rotating discs. These were coated with ground and sieved Kenyan Arabica coffee, and rotated at various speeds in distilled water at 25°C. The rate of extraction of caffeine was measured in each experiment. At a constant rotation speed, the first order rate constant rose steeply with decreasing size of coffee particle and was found to be inversely proportional to the square of the particle radius. For a given particle size, however, the rate constants were independent of the rotation speed. These two findings show that the rate-determining step for extraction from coffee-coated discs was the diffusion of caffeine through the bean particle and not its diffusion through the Nernst layer. The same mechanistic conclusion can be drawn for stirred coffee suspensions by applying the hydrodynamics of turbulent flow.