Physical Properties of Foods and Effect of Water on Them III Analysis of Molecular Mobility in Glassy and Rubbery Foods by Electric and Pulse-NMR Measurements

Abstract

The analysis of the glass transition of foods by electrical methods and proton-NMR was reviewed and compared with that obtained by differential scanning calorimetry (DSC) . The glass transition temperature T_g for many foods are usually determined by DSC, which does not provide direct information on molecular mobility. 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 z and the activation energy E_act 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 due to glass transition. For NMR research, the mobility of maltose 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 lineshape multiplied by sinc function plus an exponential function. The T₂ of mobile protons obtained from the FID spectra for maltose started to increase at a temperature lower than the midpoint T_g obtained by DSC. The dependency of secondary moment M₂ on temperature for maltose showed typically one or two transition temperatures where the M₂ decrease became steeper, reflecting motional changes in the immobile protons.