Effect of Different Dry-heating Time on Physicochemical Properties of Agar/β-cyclodextrin Mixture

The dry-heating treatment of agar/β-cyclodextrin mixture was studied. The effect of different dry-heating time on physicochemical properties of agar/β-cyclodextrin mixture and the changes of structures caused by dry-heating treatment were investigated by dynamic light scattering, rheometer, differential scanning calorimeter, X-ray diffraction and fourier transform infrared. The results showed that the gel strength, melting point, transparency, steady shear viscosity and particle size of agar/β-cyclodextrin mixture decreased and the yellow color of the mixture increased after dry-heating treatment. With the increasing of dry-heating time, these changes were more obvious. The analysis of critical shear rate indicated that dry-heating treatment broke the molecular chains and made the chains short and rigid. Thermal analysis showed that the transformation enthalpy of agar/β-cyclodextrin mixture decreased and the crystal structure damaged after dry-heating treatment. While the X-ray diffraction pattern also showed this characteristic. Fourier transform infrared spectra exhibited that the peaks around 3000~3800 cm^-1 of all samples shifted to higher wavelengths, which indicated that hydrogen bonds between molecules weakened after dry-heating treatment. Thus, dry-heating treatment can change the physicochemical properties of agar, especially can produce agar of low melting point and suitable gel strength, thus to satisfy different applications in the food industry.