Comparing the thermal decomposition kinetics of cane and beet sucrose to examine thermal behavior differences
Sucrose from cane and beet sources is greater than 99.8% pure. However, sucrose from both sources displays different thermal behavior. In their DSC thermal profiles, cane sucrose displays a small endothermic peak (small peak) before the main endothermic peak (large peak), which is not present in beet. The presence of the small peak results in a lower onset temperature for thermal decomposition in cane sucrose, compared to beet. To compare the thermal behavior of these sucrose sources, the kinetic parameters for the thermal decomposition of crystalline cane and beet sucrose were determined herein. Since sucrose thermal decomposition is a complicated process, causing the formation of decomposition products, loss of crystalline structure, and, at sufficiently high temperatures, these events can overlap with true melting, a variety of kinetic methods were used to characterize the thermal behavior of the system. Initially, a nonisothermal kinetic method was used to obtain the kinetic parameters for cane and beet sucrose thermal decomposition. Commercial beet sucrose (US beet) exhibited a higher activation energy (Ea) than either analytical grade (Sigma cane) or commercial cane sucrose (US cane), which displayed similar Ea values. The higher Ea for US beet suggested that thermal decomposition is inhibited in beet sucrose, compared to cane. The nonisothermal method was also used to explore the effect of lot-to-lot variation on the kinetic parameters of Sigma cane to fully characterize the thermal behavior of the material. While there were differences in the thermal behavior parameters for each lot, the kinetic parameters for the small peak were similar for all lots, suggesting that lot-to-lot variation does not lead to differences in the kinetic parameters. Although there were not differences in the small peak kinetic parameters, the use of several lots does provide a better predictor of the variability that can occur when different lots of sucrose are used in a product. Next, isothermal experiments were performed to assess the accuracy of the kinetic parameters obtained from nonisothermal experiments. To compare these experimental conditions, the predicted rate constant (k) and half-life (t1/2) values determined from nonisothermal experiments were compared to those obtained from isothermal experiments at 130°C. Based on the results of the isothermal experiments, the nonisothermal kinetic parameters overestimate k for cane-sourced sucrose, and underestimate k for beet sucrose. To further explore the differences between nonisothermal and isothermal methods, the Ea for sucrose thermal decomposition was determined using the isothermal isoconversional kinetic method, which allows for the Ea to be determined as a function of the extent of the reaction (α). Additionally, the use of isothermal methods allows for the kinetic parameters to be determined without the interference of the overlap of true melting. Under isothermal conditions, US cane displayed the largest Ea value at 2% α, while the Ea values displayed by Sigma cane and US beet at 2% α were similar. Additionally, all sucrose sources exhibited a decrease in Ea as the extent of the reaction increased, suggesting autocatalytic behavior. The larger Ea displayed by US cane compared to the other sucrose sources may be due to the lower purity of US cane or due to the α at which these values were compared, as all sources have similar Ea at 50% conversion. Once the kinetic parameters had been determined using established kinetic methods, novel methods to extract the kinetic parameters using data from the reversing heat capacity (RevCp) signal from quasi-isothermal (QI) and nonisothermal MDSC experiments were examined. For QI-MDSC experiments, the Ea values determined for all sucrose sources from t1/2 values matched those determined using the isothermal isoconversional method at 50% α. The agreement of the Ea values indicates that the t1/2 from the QI-MDSC RevCp signal can be used to model the kinetic parameters of a reaction where loss of crystalline structure occurs with thermal decomposition. In comparison, the Ea values determined using the nonisothermal MDSC RevCp signal were not equivalent to those obtained from the MDSC total heat flow signal over the same range of heating rates. The difference in these values appears to be caused by the onset temperature of the MDSC RevCp signal occurring at a higher temperature than that of the total heat flow signal, suggesting that the obtained kinetic parameters are for a higher extent of the reaction than the values from the total heat flow signal. Although the nonisothermal MDSC RevCp signal does not provide equivalent kinetic parameters to the total heat flow signal, both the QI and nonisothermal MDSC RevCp signals can be used to explore the mechanism of the reaction. Based on the shape of the RevCp signal it is possible to determine if the entire event is kinetic (step change) or thermodynamic (peak). If the reaction displays some thermodynamic behavior (peak in the RevCp signal), the contribution of the thermodynamic event to the total heat flow signal can be semi-quantified based on the ratio of the RevCp and total heat flow peak enthalpies at a given heating rate. Finally, the impact of the kinetics of sucrose thermal decomposition on the thermal behavior of melt quenched amorphous sucrose was examined by determining the heating rate dependence of the glass transition temperature. The Tg of sucrose was lowest at low heating rates (0.5 and 1°C/min), increase with increasing heating rate to a heating rate of 17.5°C/min, then decrease as the heating rate continued to increase. The decrease at high heating rates is due to the higher final temperatures required for complete loss of crystalline structure. In addition to examining the heating rate dependence of sucrose, modified forms of the Gordon-Taylor equation for ternary and quaternary systems were applied to determine if they could predict the T¬g of melt quenched amorphous sucrose. None of the modified forms of the Gordon-Taylor equation accurately described the experimentally determined Tg at all of the examined heating rates, which may be due to the complicated nature of melt quenched amorphous sucrose. Overall, this research provides a detailed understanding of the kinetics of crystalline cane and beet sucrose thermal decomposition, which are important when considering the thermal processing of sucrose in food products.
Sucrose, thermal decomposition, kinetics
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign