Water diffusivity in starch-based systems.
The objective of this study was to investigate the influence of structure, and component interactions, on the sorption and transport properties of water in starch-based systems. We compared the effective diffusivity (Deff) of water in two starches, with differing amylose-amylopectin ratios, using either kinetics of water adsorption or analysis of drying curves (water desorption) to estimate Deff. The effect of incorporating small sugar molecules into the granular or gelatinized starch matrices on Deff was measured by drying curve analysis. To investigate the possible mechanisms of water transport, the porosity and microscopic appearance of the samples at different stages of drying were determined. In a complementary study, sorption isotherms and the number of accessible "binding" sites in the starch and starch-sugar systems were determined using gravimetric analysis and inverse gas chromatography (IGC) 'probe analysis'. In the case of the starch-sugar systems, the measurements were made after the components had been 'mechanically mixed', or after more intimate mixing had been achieved by a co-freeze-drying process. The Deff of the starches was found to depend, in a complex way, on the moisture content of the samples. At relatively high moisture contents, the predominant mode of water transport was by liquid diffusion. As the samples became drier, their porosity increased, and the predominant mode of moisture transport was by vapor phase diffusion. As the samples became very dry (less than 10% water content), Deff fell significantly. Incorporation of sugars, in general, led to a reduction of Deff, which was correlated with a corresponding fall in porosity. In agreement with the findings of other workers, for the starches studied, the value of Deff determined from water adsorption measurements was significantly less than Deff determined from water desorption (drying curve analysis). The form of the Deff versus moisture content relationship was, however, independent of the method of measurement (adsorption or desorption). The water sorption and IGC probe analysis results indicated that some physicochemical interaction was expedited by the freeze-drying process. This interaction was manifested by a reduction in water sorption at a given relative vapor pressure, and by major changes in the accessibility of the co-freeze-dried samples to organic probe molecules. Taken together, the results indicate that water transport (diffusion) in starches and in starch-sugar mixtures is dependent significantly on gross structural features (development of porosity during drying), but that specific molecular, physico-chemical interactions must also be considered.