Water ingress into starch and sucrose:starch systems

The ingress of water from various solutions into dense starch and sucrose:starch plaques has been observed over a range of temperatures using stray field nuclear magnetic resonance imaging. Profiles of water content as a function of depth from the surface and sorption time have been analysed to obtain mutual diffusion coefficients. This has been done using the Boltzmann transform and by measuring the front position as a function of time which produces an average mutual diffusion coefficient. This average diffusion coefficient can be fit with an Arrenhius expression with an activation energy of 56±4 kJ mol−1. It was found that the average diffusion coefficients for 30:70 and 50:50 sucrose:starch mixtures could be calculated using a weighted mean of the average diffusion coefficients for pure sucrose and pure starch. Although the diffusion coefficients for amorphous and crystalline sucrose are quite different, this has a minor effect on the calculated diffusion coefficients for the range of crystallinities observed in the sucrose:starch mixtures.