Phenomenological study of enthalpy relaxation of amorphous glucose, fructose, and their mixture

Abstract

Enthalpy relaxation of glucose, fructose, and their mixtures (75/25, 50/50, and 25/75 glucose/fructose) has been investigated in terms of the Tool–Narayanaswamy–Monihan (TNM) and the Adam–Gibbs model parameters using differential scanning calorimetry (DSC) data. The apparent activation energy was evaluated from the cooling rate dependence of the limiting fictive temperature and was found to decrease as the fructose content increased. The non-linear parameter x and the exponential parameter β obtained from the TNM model was found to be the same as those obtained from the Adam–Gibbs model within their standard deviations. The value of parameter x increased from 0.50 in glucose to 0.75 in fructose, while parameter β decreased from 0.64 in pure glucose to 0.50 in the 50% fructose mixture and remained at about 0.50 for the mixtures with over 50% fructose content. The parameter x which reflects the relative contributions of the temperature and structure dependence of the relaxation times was also evaluated using the peak-shift method. The values of parameter x obtained from the peak-shift method for all sugars were found in general to be higher than those obtained from curve fitting to the TNM and the Adam–Gibbs models. The fragility of the sugars was also evaluated using the enthalpy relaxation data. The addition of fructose to the glucose–fructose system produced “stronger” liquids, and less non-linear behavior.