Investigation of thermal decomposition as the cause of the loss of crystalline structure in sucrose, glucose, and fructose
Simple sugars (e.g., sucrose, glucose, and fructose) are abundantly used as basic ingredients in the food industry and as excipients in the pharmaceutical industry, due to the quality attributes they contribute to the final products, such as sweet taste, flavor, texture, color, shelf-life (e.g., flavor retention via encapsulation), and carrier function of active pharmaceutical components. These quality attributes directly depend on the processing protocol employed, in particular heating conditions, as well as the type(s) of sugar used. Thus, it is important to accurately and completely characterize the sugars prior to use in specific applications. Melting has been commonly used for sugar characterization because it is an easy and quick property to measure, yet is usually repeatable and robust. In general, melting is determined by heating a sugar at a specified scanning rate using a thermal analytical technique, such as Differential Scanning Calorimetry. The sugar loses its crystalline structure (commonly termed melting) by adsorbing heat energy, which yields the critical parameters of the resultant melting peak, i.e., the onset melting temperature (Tm onset), the peak melting temperature (Tm peak), and the enthalpy of melting (∆H). These parameters provide a good deal of information for sugar identification and characterization (e.g., purity, type, size, etc.) purposes. However, the reported melting temperatures for sugars vary widely. This variation has been attributed to a number of causes, such as differences in melting temperature determination methods, origin, impurity, polymorphs, superheating, liquefaction, and thermal decomposition and/or mutarotation in addition to melting. However, a complete explanation of the substantial variation observed in the sugar melting temperatures as a function of heating rates is not currently found in the literature. Of importance to note is that from a thermodynamic viewpoint, the heating rate dependency of the sugar melting temperatures suggests that the sugars do not experience thermodynamic melting. Because thermodynamic melting occurs at a single, time independent temperature with a constant enthalpy value (ΔH), where the crystalline solid and corresponding liquid phases are in thermodynamic equilibrium (∆G=0) at a constant pressure, the melting parameters for the sugars cannot be used for identification purpose. Therefore, the ultimate objective of this research was to elucidate the fundamental mechanism underlying the loss of crystalline structure (melting) in these sugars. With thermal and chemical analytical approaches, this research found that the kinetic process of thermal decomposition was responsible for the loss of crystalline structure in these sugars, leading to heating rate dependency in their melting parameters. This result distinguishes thermodynamic melting from the loss of crystalline structure caused by thermal decomposition (termed “apparent melting” in this research), which solves the controversy that currently exists in the literature regarding the wide variation in the melting parameters of these sugars. These results prove not only that the loss of crystalline structure in the sugars is caused by thermal decomposition, but also that apparent melting in the sugars is achieved via a time-temperature combination process. This research also attempted to determine the thermodynamic melting temperature of these sugars by suppressing thermal decomposition using fast scanning rates. In the case of fructose, its thermodynamic melting was achieved. In addition, this research explored the effect of different heating conditions on the glass transition parameters for amorphous sucrose prepared by melt-quenching (i.e., melting followed by quick cooling) because melting-quenching is one of the common methods for preparing amorphous materials. The heating conditions employed directly effected the glass transition parameters obtained, where, in general, it was found that the longer time, lower temperature heating conditions resulted in lower glass transition temperature values. Therefore, in practical applications, this research is useful for better understanding the quality and stability issues associated with heat processed sugar-containing food and pharmaceutical products and for developing new sugar based products.
Thermodynamic melting, thermal decomposition, apparent melting, sucrose, glucose, fructose, Differential Scanning Calorimetry, Modulated Differential Scanning Calorimetry, Thermogravimetric analysis, Rapid-scanning DSC
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign