Apparent heat capacity measurements and thermodynamic functions of d(−)-fructose by standard and temperature-modulated calorimetry

The qualitative and quantitative thermal analyses of crystalline and amorphous d (−)-fructose were studied utilising methods of standard differential scanning calorimetry (DSC), quasi-isothermal temperature-modulated differential scanning calorimetry (quasi-isothermal TMDSC), and thermogravimetric analysis (TGA). Advanced thermal analysis of fructose was performed based on heat capacity. The apparent total and apparent reversing heat capacities, as well as phase transition parameters were examined on heating and cooling. The melting temperature, T m , of crystalline d (−)-fructose shows a heating rate dependency, which increases with raising the heating rate and leads to superheating. The equilibrium melting temperatures: T m ∘ (onset) = 370 K and T m ∘ (peak) = 372 K, and the equilibrium enthalpy of fusion Δ fus H °  = 30.30 kJ · mol −1 , of crystalline d (−)-fructose were estimated on heating for the results at zero heating rate. Anomalies in the heat capacity in the liquid state of d (−)-fructose, assigned as possible tautomerisation equilibrium, were analysed by DSC and quasi-isothermal TMDSC, both on heating and cooling. Thermal stability of crystals in the region of the melting temperature was examined by TGA and quasi-isothermal TMDSC. Melting, mutarotation, and degradation processes occur simultaneously and there are differences in values of the liquid heat capacity of d (−)-fructose with varied thermal history, measured by quasi-isothermal TMDSC. Annealing of amorphous d (−)-fructose between the glass transition temperature, T g , and the melting temperature, T m , also leads to crystallization of the sample and shows changes in the total apparent heat capacity. The experimental, apparent heat capacity of fully crystalline and fully or partially amorphous d (−)-fructose was analysed with reference to the solid (vibrational) and liquid heat capacities based on the ATHAS scheme.