Estimation of effective diffusivities and glass transition temperature of polydextrose as a function of moisture content C. Ribeiro 1 , J.E. Zimeri, E. Yildiz, J.L. Kokini * Center for Advanced Food Technology, Department of Food Science, Cook College, Rutgers University, 65 Dudley Road, New Brunswick, NJ 08901, USA Received 26 January 2002; revised 5 June 2002; accepted 10 June 2002 Abstract The glass transition temperature (T g ) of polydextrose was determined by differential scanning calorimetry as a function of moisture content, and was fitted to the Gordon – Taylor equation. T g decreased with an increase in moisture content, confirming the plasticizing effect of water on polydextrose. The effective diffusivity of water in polydextrose (D ), which increased with an increase in storage water activity (a w ), was estimated at room temperature both above and below the glass transition by applying the method of slopes [Effective water diffusivity in starch materials, MS Thesis, Rutgers University (1987); Effect of gelatinization and sugars on the effective moisture diffusivity in starch materials, MS Thesis, Rutgers University (1988)] to the experimental data. D showed a sharp increase in regions close to the glass transition, where polydextrose changed from the glassy to the rubbery state due to increased mobility. D was also correlated to a model [The effect of water activity on moisture diffusivity in soy flour as determined from a free volume based approach (2002)] based on the free volume theory [J. Appl. Polym. Sci. 22 (1978) 2325; AIChE J. 38 (1992) 405], which captured the change in diffusion mechanism as a function of glass transition. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polydextrose; Glass transition; Diffusivity; Free volume; Gordon– Taylor 1. Introduction Polydextrose is a bulking agent used to provide body and texture in reduced-calorie foods. It is a non-crystalline powder that can be used to stabilize foods by preventing sugar and polyol crystallization (Staley Mfg. Co., 1997). Polydextrose is a water soluble, randomly bonded bulk polymer with an average degree of polymerization (d.p.) of about 10 glucose residues, obtained by thermal polymerization of D-glucose in the presence of sorbitol and phosphoric acid. Because of this random polym- erization process, it can contain almost any possible combination of a and b linkages in its structure. Heating above the glass transition temperature (T g ) leads to a flowable melt that, after cooling, produces a clear glass with a brittle texture (Cultor Food Science, 1998). Other polymers of D-glucose have structures similar to that of polydextrose. For example, maltodex- trin, obtained by the hydrolysis of starch, can have the same d.p. and yet have very different physical properties because of its a(1–4) and a(1–6) bonds, as opposed to random bonds in polydextrose. Several studies related to the phase transitions of small carbohydrate – water systems have been reported in the literature. For example, Slade and Levine (1988) studied the non-equilibrium behavior of small carbohydrate–water systems and reported values of T g and T m (temperature of melting) of various polyol sugars. Orford, Parker, and Ring (1990) found that the T g of a carbohydrate depended strongly on molecular weight and less on its structure. Roos and Karel (1991) studied the effect of molecular weight, water plasticization, and composition on T g of maltodex- trins, maltose, and sucrose. All maltodextrins of different molecular weights were plasticized by water in a similar manner, and the decrease in T g was linear with respect to water activity (a w ) over the range of 0.11–0.85. Limited data has been published on the phase transitions of polydextrose and its effect on the stability of foods. Bell and Touma (1996) studied the T g of polydextrose at two moisture contents using a temperature-cycling differential scanning calorimeter (DSC). Kim, Hansen, and Setser (1986) determined that equivalent concentrations of poly- dextrose and sucrose had similar effects of increasing the 0144-8617/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. PII: S0144-8617(02)00182-0 Carbohydrate Polymers 51 (2003) 273–280 www.elsevier.com/locate/carbpol 1 Present address: University of Virginia, Clark Hall, Charlottesville, VA 22903, USA. * Corresponding author. Tel.: þ 1-732-932-8306; fax: þ1-732-932-8690. E-mail address: kokini@aesop.rutgers.edu (J.L. Kokini).