The relative importance of internal and external physical resistances to mass transfer for caffeine release from apple pectin tablets

The relative importance of the physical resistances to mass transfer have been explored by using halved 13 mm diameter apple-pectin tablets containing caffeine, in different external stirring environments within a beaker containing simulated gastric fluid. The effects of different external (outside of the tablets) mass-transfer resistances to the tablets created through two different stirrer types and stirrer speeds, and different internal (inside of the tablets) mass-transfer resistances created through different tablet concentrations and thicknesses, have been studied. These studies enable internal diffusion coefficients of caffeine through the apple pectin matrix to be estimated, as well as estimating the external mass-transfer coefficients from benzoic acid dissolution, which are in the range of 6.5 × 10^-6 m/s - 2.4 × 10^-5 m/s for the 0.6 mm thick tablets and 4.0 × 10^-6 m/s - 1.6 × 10^-5 m/s for the 7 mm thick tablets. The diffusion coefficients for different caffeine concentrations in the apple pectin half-tablets have also been calculated in this study. The diffusivity of caffeine in the 7 mm half-tablets with 1% caffeine through 99% pectin was around (1.8 ± 0.5) × 10^-10 m²/s. This study points towards the development of multifilm mass-transfer theory for food digestion to create a more fundamentally based understanding of in-vitro digestion systems as an addition to the use of realistic in-vitro food digestion apparatus and give a better correlation between in-vitro and in-vivo digestion tests.