How does water diffuse in glasses of carbohydrates?

Abstract

Understanding and predicting water diffusivity in supercooled and glassy carbohydrates is of great importance in the practical applications of these mixtures, in particular to the preservation of foods and pharmaceuticals. Since the discovery more than a decade ago that water retained liquid-like heat capacity in glassy starch (Noel and Ring, 1992) several studies have shown that water is mobile in glasses of carbohydrates (van den Dries et al., 1998; van den Dries et al., 2000; Tromp et al., 1997; Hills et al., 2001; Chatakanonda et al., 2003; Kou et al., 2000; Vodovotz et al., 2000; Aldous et al., 1997) and foods (Roudaut et al., 1999). Nevertheless, the microscopic mechanism that allows water diffusion in these glasses has remained an unsolved puzzle. We present here a computer simulation study of the microscopic mechanisms of water diffusion in carbohydrate from the viscous liquid state up to the glass. To understand the nature of water diffusion in glassy food-like systems, we employed molecular dynamics (MD) simulations techniques to study in detail the structure and dynamics of a concentrated glucose solution, a simple binary glass-former that is considered to be a model for food systems (Levine and Slade, 1988). In this work, we extend a previous study for the supercooled mixture (Molinero et al., 2004), present results that unravel the mechanism of water diffusion in the glassy state (Molinero and Goddard, 2004), and compare the relaxation in supercooled glucose and a malto-oligomer (α(1→4) glucose oligomer) to assess the effect of the carbohydrate local modes on the dynamics of water.