Kinetics of Coffee Infusion: A Comparative Study on the Extraction Kinetics of Mineral Ions and Caffeine from Several Types of Medium Roasted Coffees

Abstract

A comparative study of the equilibrium concentrations, rates of infusion and intra-bean diffusion coefficients of caffeine, P (as H2POurn:x-wiley:00225142:media:JSFA489:tex2gif-stack-1 ), K+, Mg2+ and Mn2+ in Milli-Q-Water at 80 °C was carried out. Medium roasted coffees of particle size range 1.70–2.00 mm from six different countries—Special Kenya (Kenya), Santos (Brazil), Blue Mountain Java (Sumatra), Mocha (Ethiopia), Zimbabwe (Zimbabwe) and South African Grown (South Africa)—were used in the investigation. High-performance liquid chromatography was used for the analyses of caffeine, and inductively coupled plasma atomic emission spectrometry for the chosen elements. The equilibrium concentrations of all the species and the trend were found to be independent of the various coffee beans. The order of the rate of infusion was found to be K+ > caffeine > P (as H2POurn:x-wiley:00225142:media:JSFA489:tex2gif-stack-2) > Mg2+ > Mn2+. Examination of the rate constants clearly indicated that P (as H2POurn:x-wiley:00225142:media:JSFA489:tex2gif-stack-3), Mg2+ and Mn2+ cannot be used for identification of the coffee origin, but the values for caffeine and K+ can be used. Diffusion coefficients of caffeine and mineral ions were calculated in two separate ways using rate constants and half-lives of infusion. These were then compared with known diffusion coefficients of the same species in water for determination of hindrance factors. The hindrance factor for caffeine was found to be much smaller than the corresponding factors recorded at 25.5 °C. In general the hindrance factors in the bean were all of the order of 10, with Mn2+ being the most hindered species. This is an indication that the infusion of the various species through the coffee bean is a hindered process. This is because of the association of caffeine and mineral ions with other coffee solubles and the physical restraint within the bean matrix. © 2000 Society of Chemical Industry