Glass Transition and Reaction Rates: Nonenzymatic Browning in Glassy and Liquid Systems Satu M. Lievonen,* Tommi J. Laaksonen, and Yrjo ¨ H. Roos Department of Food Technology, P.O. Box 27 (Viikki B), FIN-00014 University of Helsinki, Helsinki, Finland Effects of physical state and glass transition on nonenzymatic browning rate in water, glycerol, poly(vinylpyrrolidone) (PVP), and maltodextrin (MD) systems were studied. All systems had the same concentration of reactants, glucose and lysine, in the water phase. The systems, except water, had also comparable water activities (0.33). Sorption isotherms and glass transition temperatures (T g ) at various water contents for the freeze-dried PVP and MD systems were determined. Nonenzymatic browning rate was determined at several temperatures from optical density at 280 and 420 nm. The PVP and MD systems contained 12.9 and 8.2 g of H 2 O/100 g dry matter and had T g values of 67 and 62 °C at 33% relative humidity and 24 °C, respectively. The liquids exhibited significantly higher browning rates than the concentrated systems, especially below the T g values. The browning rate was higher in the PVP than in the MD system, suggesting that a possible phase separation may affect nonenzymatic browning in foods. Keywords: Glass transition; nonenzymatic browning; physical state; reaction rate INTRODUCTION The significance of glass transition on the stability of amorphous food materials has been intensively studied since the 1980s (Levine and Slade, 1986; Slade and Levine, 1988). The premise of these studies has been suggestions that the physical state of a food system has important implications in food processing and storage stability. There is a general agreement about the influence of glass transition on physical changes, such as collapse and crystallization (Roos et al., 1996), although the use of a single glass transition temperature has been criticized (Peleg, 1996, 1997). It has also been proposed that glass transition affects rates of diffusion- controlled chemical and enzymatic reactions through changes in molecular mobility, which is likely to be extremely slow below the glass transition because of the high viscosity of the matrix (Slade and Levine, 1991). However, experimental results have not confirmed that the glass transition, as such, results in a change in the reaction rates. For instance, in a study of aspartame degradation, the effect of the glass transition was found to be insignificant, although the reaction pathway was dependent on molecular mobility (Bell and Hageman, 1994). On the other hand, rotational mobility of a tempol probe increased 100-fold within the glass transi- tion temperature range in an ESR study (Roozen et al., 1991). However, most studies have given evidence of the independent effects of glass transition on the reac- tion rates together with other factors, such as water content, water activity, temperature, and reactants concentration (Shimada et al., 1991; Karmas et al., 1992; Roos and Himberg, 1994; Buera and Karel, 1995; Bell, 1996). Nonenzymatic browning is probably the most studied example of a possibly diffusion-controlled chemical reaction in connection with glass transition. It is a good model for studying the effects of the glass transition on reaction kinetics, because the reaction mechanism contains several condensation steps that require the diffusion and collision of reactants [e.g., Namiki (1988)]. Moreover, nonenzymatic browning is an important chemical reaction in foods, as it produces flavors and colors, but it may also decrease quality during process- ing and storage. Therefore, control of the reaction rate has been given much attention. Probably the first study relating glass transition temperature with nonenzy- matic browning kinetics was published by Karmas et al. (1992). They found that even if the browning rate in different food models was strongly dependent on moisture and temperature, it was also affected by the glass transition. A corresponding conclusion was made by Roos and Himberg (1994), who studied nonenzymatic browning as a function of water content, water activity, and glass transition temperature at chilling tempera- tures. However, many studies have stressed that the effects of the physical changes of the matrix materials, such as crystallization and collapse, coinciding with the glass transition should also be taken into consideration (Karmas et al., 1992; Karmas and Karel, 1994; Buera and Karel, 1995). It has been difficult to distinguish directly the effects of glass transition on the nonenzymatic browning rate from the effects of other factors, mainly temperature and moisture content. The glass transition temperature range in model systems is often controlled by changing the water content of the system. The problem is that water activity and a reactant concentration will be changed at the same time. Bell (1996) solved the problem by using PVP polymers with different molec- ular weights as matrix materials. The models had different glass transition temperatures, but the same water activities and moisture contents, and the effect of the physical state and water activity could be more clearly separated. The state of the system was observed * Author to whom correspondence should be addressed (telephone +358 9 708 5206; fax +358 9 708 5212; e-mail satu.lievonen@helsinki.fi). 2778 J. Agric. Food Chem. 1998, 46, 2778-2784 S0021-8561(98)00064-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/01/1998