Influence of the Storage Time on the Dielectric Properties of mangoes

Abstract

In an attempt to develop microwave or radio frequency based pest control strategy, dielectric properties (DPs) of mangoes were determined with an open-ended coaxial-line probe connected to an impedance analyzer. The influence of frequency (1-1800 MHz), temperature (20-60°C) and maturity (16 days storage at 21°C) on dielectric constant (e’) and loss factor (e”) were evaluated. Additionally, moisture content, soluble solids, acidity, pH, maturity index and electrical conductivity of the mango fruit after 0, 4, 8 and 16 days of storage were determined. Loss factor increased but dielectric constant decreased between 10 and 1800 MHz when the temperature increased. The values for e’ and e” decreased with storage time, caused mainly by reduced moisture content and increased pH. DPs values decreased with increased frequency but this reduction was larger for the loss factor than for the dielectric constant. Larger penetration was achieved using radio frequencies compared with microwave frequencies. Maturity index increased from 10.59 to 208.42 during 16 days of storage, while electrical conductivity decreased from 0.339 to 0.208 S/m. Predicted e” values based on the electrical conductivity measurements matched well with the experimental e” values at frequencies lower than 300 MHz, which can be interpreted as a predominant ionic conduction at these frequencies. DPs measurements throughout storage time might be used to select the optimal maturity to develop dielectric heating for postharvest thermal treatments of mangoes and establish a correlation with the maturity.