Non-destructive determination of moisture content in hazelnut (Corylus avellana L.)

Abstract

There is a need in commercial hazelnut production for a rapid, nondestructive and reliable moisture content (MC) sensing in order to obtain quality nuts from the orchard to the consumer. In the presented study, the MC of in-shell hazelnuts were determined on the base of their dielectric properties, which have never been described in hazelnuts to date. The measurement system from a probe with parallel, vertically placed electrodes, a generator of sine wave signals and oscilloscope was completed for measuring voltage on the input (V1) and output (V2) of the probe, and the phase angle between V1 and V2 within a frequency range from 1kHz to 1MHz. Eight dielectric properties (impedance, admittance, resistance, capacitance, dielectric constant, dielectric loss factor, dissipation factor and phase angle) were calculated and graphically presented for hazelnuts with various MC and compared with the reference MC values determined using the standard air-oven method. All dielectric properties except the resistance and phase angles decreased with decreased hazelnut MC. Measurement frequencies were negatively correlated regarding all properties except for the phase angle. Six statistical MC-models were calculated by conducting stepwise multiple regression analysis of the dielectric properties. Model 5, which comprised conductance and capacitance of the nuts, was proved to be the more predictive for MC determination, showing the lowest standard error of estimation (0.39), the highest coefficient of determination (99.78) and the lowest mean absolute error (0.23). Good agreement was found between the MC simulated by model 5 and the observed data in both mixed hazelnut sample as well as in cultivars with nuts of different shapes and sizes. Performing conductance and capacitance measurements at frequencies between 3 and 500kHz, the moisture content of the in-shell hazelnuts could be efficiently determined, using a low-cost and non-invasive technique.