pubs.acs.org/JAFC Published on Web 12/22/2010 © 2010 American Chemical Society 684 J. Agric. Food Chem. 2011, 59, 684–701 DOI:10.1021/jf1042344 Investigation of the Heating Rate Dependency Associated with the Loss of Crystalline Structure in Sucrose, Glucose, and Fructose Using a Thermal Analysis Approach (Part I) JOO WON LEE Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, 399A Bevier Hall, 905 South Goodwin Avenue, Urbana, Illinois 61801, United States LEONARD C. THOMAS DSC Solutions LLC, 27 East Braeburn Drive, Smyrna, Delaware 19977, United States SHELLY J. SCHMIDT* Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, 367 Bevier Hall, 905 South Goodwin Avenue, Urbana, Illinois 61801, United States Thermodynamic melting occurs at a single, time-independent temperature with a constant enthalpy value. However, substantial variation in the melting parameters (T m onset , T m peak , and ΔH) for sucrose, glucose, and fructose has been reported in the literature. Although a number of explanations have been put forth, they do not completely account for the observed variation. Thus, this research was performed to elucidate the fundamental mechanism underlying the loss of crystalline structure in the sugars using both thermal (Part I) and chemical (Part II) analysis approaches. A strong heating rate dependency observed in the melting parameters for the sugars implies the occurrence of a kinetic process during the loss of crystalline structure. The difference in heat capacity and modulated heat flow amplitude in the stepwise quasi-isothermal modulated differential scanning calorimetry experiments for the sugars compared to indium and mannitol (thermodynamic melting comparison materials) strongly suggests thermal decomposition as the kinetic process responsible for the loss of crystalline structure, which is the critical difference between our conclusion and others. We propose the term “apparent melting” to distinguish the loss of crystalline structure due to a kinetic process, such as thermal decomposition, from thermodynamic melting. KEYWORDS: Thermodynamic melting; apparent melting; kinetic processes; thermal decomposition; sucrose; glucose; fructose; (modulated) differential scanning calorimetry; thermogravimetric analysis INTRODUCTION Melting is a first-order phase transition from the crystalline solid phase to the liquid phase ( 1 , 2 ), with no change in chemical composition. The parameters associated with the melting process (onset melting temperature, T m onset ; peak melting temperature, T m peak ; and enthalpy of melting, ΔH) are usually measured by heating a crystalline material at a specified rate to a temperature where the melting endothermic peak is complete, using a thermal analysis technique, such as differential scanning calorimetry (DSC) or differential thermal analysis (DTA). The melting parameters provide a good deal of information about the char- acteristics of the crystalline material (e.g., purity, type, size, etc.), thus, melting parameters have been used as unique material properties for identification and characterization of crystalline materials. However, for some crystalline sugars a wide range of melting parameters has been reported in the literature (Table 1). An important observation contrary to the definition of thermo- dynamic melting based on Table 1 is that the melting parameters tend to increase strongly with increasing heating rate. Thermo- dynamic melting occurs at a single, time-independent (i.e., heat- ing rate independent) temperature (often reported as T m onset ), where the crystalline solid and corresponding liquid phases are in thermodynamic equilibrium (same Gibbs energy, ΔG = 0) at a constant pressure (Figure 1). Thus, widely varying sugar melting parameters are not consistent with the definition of thermo- dynamic melting and thus necessitate further investigation. DSC, one of the thermal analysis techniques used in this research, measures the heat flow difference between a sample and inert reference (typically an empty pan) as a function of temperature and time. Integration of the heat flow signal provides enthalpy (H), which is a function of the material’s specific heat (C p ) and energy absorbed or released by the material due to phase *Corresponding author. Phone: 217-333-6963. Fax: 217-265-0925. E-mail: sjs@illinois.edu.