Dielectric properties of raspberry and blackberry from 0.5 to 25 GHz at 20ºC and 48ºCIntroduction

Abstract. Dielectric properties are relevant parameters for dielectric applications, such as moisture content sensors and dielectric heating. One of the easier and cheaper methods to determine dielectric properties is the open-ended coaxial probe. Berries have been recognized as functional foods, due to their high content antioxidants and are fruits of high economic importance in North America. The objective of this study was to analyze the dielectric properties of raspberry and blackberry using the open-ended coaxial probe method. The dielectric properties were measured from 0.5 to 25 GHz at 20ªC and 48ºC, important temperatures for further applications, such as determination of moisture content and disinfestation, respectively. Raspberries and blackberries were harvested from orchards in Irapuato, Guanajuato, Mexico (“El Bajio” region). Some physicochemical properties in the berries were determined: pH value for raspberries was 3.35; for blackberries 3.13; titratable acidity of raspberries was 1.4%, and for blackberries 1.05%, expressed as percentage of citric acid. The total soluble solids of the berries ranged from 8 to 8.8. Dielectric constant increased with increasing temperature, and decreasing with increasing frequency. Loss factor decreased with increasing temperature and from 1.5 to 25 GHz, increased with increasing frequency. Both berries had similar dielectric properties, for example, at 20ºC and 2.45 GHz, blackberry had a loss factor of 16.33, while raspberry had a value of 15.92. The penetration depth were also calculated. The found results are useful for further dielectric applications for these berries.