Molecular Weight Effects on Enthalpy Relaxation and Fragility of Amorphous Carbohydrates

Abstract

Many food processing techniques such as dehydration, concentration, extrusion, and freezing produce amorphous foods that can be stored below their glass transition temperature (Wungtanagorn and Schmidt 2001). Below glass transition temperature (Tg), amorphous food constituents exist in a thermodynamically unstable nonequilibrium and disordered state. Isothermal storage/aging of glassy amorphous food components results in structural relaxations that achieve a more stable equilibrium state over extended time periods (Struik 1978). Since the equilibrium state is a low energy state, some of the energy is lost/relaxed in the nonequilibrium glassy amorphous state during isothermal storage of food components. This energy can be recovered in the form of enthalpy during the reheating of the glassy system by using a differential scanning calorimeter, since physical aging is a reversible process. The enthalpy recovered during reheating of the aged material system is a measure of the system’s molecular mobility at the selected aging temperature (Gupta et al. 2004). Structural relaxation in the glassy state of amorphous food components during isothermal storage/aging is also known as enthalpy relaxation/physical aging. Many macroscopic properties of glassy materials, such as volume, enthalpy, refractive index, electrical conductivity, and viscosity, change during physical aging (Struik 1978). The changes in macroscopic properties may adversely affect the physicochemical stability during the isothermal storage of low water amorphous foods and food constituents (Farahnaky et al. 2008).