Dielectric properties of parchment coffee

This paper discusses the variation of relative permittivity, dielectric los s factor and loss tangent, and the a.c. conductivity of three varieties of parchment coffee (Coffea arabica L.) in the frequency range from 75 kHz to 5 MHz, at 21.0 +/- 0.9 degreesC and 63 +/- 1% relative humidity. Included a re curves showing the variation of these four dielectric properties on mois ture (11.0 to 23.1% w.b.), bulk density (326-455 kg m(-3)), and variety (Ca tuai Vermelho, Catuai Amarelo and Mundo Novo). The importance of the determ ination of the dielectric properties of parchment coffee for the indirect m easurement of its moisture content, and the use of on-line moisture meters in automatic control of coffee dryers, are discussed briefly. Also included are definitions of the dielectric properties of interest in measuring the moisture content of granular materials. The variation of relative permittiv ity on frequency, moisture content, density and variety was more regular co mpared to the behaviour of the other dielectric properties in that it conti nuously increases with moisture and bulk density and decreases with frequen cy. Dielectric dispersion and its relation to water binding mechanisms in g rain are also discussed. The conclusion, based on experimental values and d ata found in literature, is that of all dielectric properties the relative permittivity should provide less error in parchment coffee moisture content determination. (C) 2001 Silsoe Research Institute.