Water diffusion in glasses of carbohydrates

Abstract

Diffusion constants of water have been determined at various temperatures, water contents and molecular weights in glassy systems of dried glucose syrups and maltose, with water contents between 3 and 10% (w/w). The diffusion constants were determined from the rate of desorption under reduced air pressure. The initial desorption was shown to be diffusion controlled when sufficiently reduced pressures were applied. The water diffusion was found to be an activated process, with an activation energy that is quite independent of the mean molecular weight and water content of the carbohydrate matrix. The diffusion in matrices of the oligomeric chains of the glucose syrups is faster than in maltose matrices at equal water content. Using an Eyring model for interpretation, this is ascribed to longer jump distances in the activated process in the less densely packed systems containing relatively long oligomeric chains. A strong dependence of the jump distance on the water content was found. Diffusion constants of water were determined at various temperatures, contents and molecular weights in glassy systems of dried glucose syrups and maltose, with H 2O contents of 3–10% (w/w). Diffusion contants were determined from rate of desorption under reduced pressure. Initial desorption is diffusion controlled under sufficiently reduced pressures. Water diffusion was found to be an activated process, with activation energy independent of mean molecular weight and H 2O content of the carbohydrate matrix. Diffusion in oligomeric chain matrices of glucose syprups is faster than in maltose matrices of equal H 2O content. Using an Eyring model, this is due to longer jump distances in the activated process in less densely packed systems of relatively long oligomeric chains. A strong dependence of the jump distance on H 2O content was found.