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Dielectric Spectroscopy of Grape Juice at Microwave Frequencies

Abstract The complex permittivity of fresh juice of two cultivars of grapes, Sultania (green grapes) and Black Monukka (black grapes) was measured in terms of the dielectric constant and dielectric loss factor over the frequency range from 1 to 50 GHz and at temperatures ranging from 30 to 60°C, by using the PNA network analyzer model E8364C and open ended coaxial probe 85070E. The Cole-Cole plots and dielectric constant vs. (angular frequency) dielectric loss factor and dielectric constant vs. dielectric loss factor/(angular frequency) regression lines at different temperatures were used in Debye approximation to predict relaxation frequency of molecules for the two cultivars of grapes in the low frequency and high frequency limits, respectively. It was observed that the acidic character of green grapes is responsible for the large amplitude vibrational peaks in dielectric loss factor - frequency curves, in the high frequency region at higher temperatures. On the other hand, excess of sugar in black grapes suppresses the activity of water molecules, thereby suppressing the vibrational peaks at higher frequencies. Different relaxation frequencies found for the two cultivars of grapes suggest that they have different molecular structure.