Analysis of Molecular or Ion Mobility in Glassy and Rubbery Foods by Electric and Proton-NMR Measurements

Abstract

The glass transition temperature Tg for many foods has been determined by thermal analysis such as differential scanning calorimetry (DSC). The thermal analysis, however, does not provide information on molecular mobility. In this review, the analysis of the glass transition of foods by electrical methods and proton-NMR was explained and compared with that obtained by DSC. The dielectric relaxation, i.e., the peak of the dielectric loss, ε″, was observed for some glassy foods and ascribed to the local motion of molecules. The relaxation time τ and the activation energy Eact seemed suitable parameters to describe the enhancement effect of water on the mobility of molecules in the glassy state. However, when the ionic conductivity dominated the electrical properties, the ε′ peak was masked. For analyzing such a system, the electric modulus, M* was an effective tool. The value of the activation energy obtained through M* formalism in the glassy state was larger than that in the rubbery state, probably due to a change in the free volume size by glass transition. For NMR research, the mobility of glucose syrup in the glassy and rubbery states has been examined. Free induction decay (FID) was measured with low-resolution NMR and was best fitted by a Gaussian line-shape multiplied by sinc function plus an exponential function. The T2 of mobile protons reached the rigid-lattice limit at a temperature lower than the midpoint Tg obtained by DSC.