Beyond Tg: optical luminescence measurements of molecular mobility in amorphous solid foods

Abstract

Current research recognizes the importance of the glassy state for stabilizing amorphous solid foods and the importance of the glass transition (Tg) as an index temperature for food stability. This reflects a realization that molecular mobility modulates both physical properties such as texture and physical and chemical processes such as reaction rate, crystallization rate, solute diffusion, and collapse; the Tg is the onset temperature for cooperative translational motion in amorphous solids. The existence of complex structural features in foods with lengths ranging from nanometers to centimeters and the recognition that complex vibrational motions are activated in the glassy state, brings about the need to go beyond macroscopic measurements of product Tgs. It is now important to determine how molecular structure and mobility as well as microscopic organization modulate the macroscopic physical properties of foods. Luminescence spectroscopy provides a powerful arsenal of biophysical tools to investigate the structure and molecular mobility of amorphous solids; measurements of emission intensity, energy (wavelength), and polarization can provide direct information about molecular structure and mobility. Luminescence measurements of the effect of hydration and temperature on amorphous solid proteins and sugars reviewed here indicate that the molecular mobility due to local vibrational and rotational motions (as opposed to global translational mobility) in glassy foods is actually quite extensive with motions occurring on time scales ranging from nanoseconds to seconds. There is thus a need to identify how the rates of specific chemical and physical processes involved in food degradation are influenced by specific modes of molecular mobility in amorphous solid foods.