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Measurement of Moisture Content in Powdered Food Using RF Impedance

This study was conducted to measure the moisture content of powdered food using RF impedance. In frequency range of 1.0 to 30 [MHz], the impedance of parallel plate type sample holder filled with wheat flour and red-pepper powder of which moisture content range were 5.93 to 17.07 [%] at wet basis and 10.87 to 27.36 [%] at wet basis, respectively, was characterized using by Q-meter (HP4342). The reactance was a better parameter than the resistance in estimating the moisture density defined as the product of moisture content and bulk density which was used to eliminate the effect of bulk density on RF spectral data in this study. Multivariate data analyses such as principal component regression, partial least square regression and multiple linear regression were performed to develop a calibration model having moisture density and reactance spectral data as parameters for determination of moisture content of both wheat flour and red-pepper powder. Introduction The moisture content of food is not only the most important quality factor but also one of the essential parameters affecting their physical and chemical properties related to the safety of storage, capability of processing and quality control. Many techniques can be used to measure the moisture content. The method using RF impedance measures the capacitance and/or a-c (alternating current) resistance of the material between two electrodes forming a capacitor and from which the dielectric property is calculated. This method can be used with a wide range of moisture contents but has the source of measurement errors caused by changes of bulk density of wetted material within the electrodes, kinds of material and temperature of material. Other methods are infrared and near infrared spectrometry, microwave technique and nuclear magnetic resonance technique. These methods enable to measure the moisture content of materials with high accuracy. But because of high cost they are being usually used at laboratory level and it seems not to fully proper for on-line monitoring of moisture content of materials yet. Even though the electrical method has some disadvantages, it may be attractive technique because that method has merits of rapid and accurate measurement in the wider range of moisture content, simple implementation and inexpensiveness compared with other methods. Since various chemical components as well as moisture affect the electrical property, measuring only one frequency will be inadequate to measure the moisture content. In the area of moisture content measurement for agricultural products, RF impedance at two or more frequencies is being studied and hence the characteristic of electrical measuring data is multivariate and/or spectral (Chanet et al., 1999, Nelson et al., 1990). Multivariate analyses on RF impedance of grain and seed have been reported (Lawrence et al., 1998) and some good promising methodologies for moisture content Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 1020-1025 doi:10.4028/www.scientific.net/KEM.270-273.1020 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,17:37:48) Title of Publication (to be inserted by the publisher) determination are being presented. However, there are not enough research results about the measurement of moisture content of powdered food. As mentioned above, in case of RF impedance method, one of the critical parameters in moisture content measurements is the variation of bulk density (Sokhansanj and Nelson, 1988). As a bulk density compensation factor, a moisture density parameter (defined as the product of decimal moisture content and bulk density) was proposed (Noh and Nelson, 1989, Kim et al., 2002, Kim et al., 2003). When the dielectrics such as agricultural products and food are placed within the electrodes being applied by alternating electromegnatic field, the electrodes including materials are represented in terms of the parallel equivalent circuit with capacitor and resistor. Because of electrical loss of agricultural products and food the electrical phase shift, δ is occurred. The impedance is expressed as; C jX R Z − = (1) where, Z is impedance [ohm], R (ω) is resistance [ohm], XC is capacitive reactance [ohm] and j is the imaginary number, j 2 =-1. The capacitive reactance and angular velocity are expressed as; ( ) fC X C π 2 1 = (2) where, C is capacitance [pF] and f is measuring frequency [Hz]. In this study, the moisture content of powdered food was measured using RF impedance. After various multivariate statistical analyses were compared, a calibration model was presented which can estimate moisture contents of any powdered food. Materials and Methods Powdered food sample preparation. The wheat flour and red pepper powder from the market place were experimented. The initial moisture contents of wheat flour and red pepper powder were 5.93 [%w.b.] and 10.78 [%w.b.], respectively. The particle size of wheat flour was analyzed by particle size analyzer (ELZONE, 280PC, Particle Data Inc., USA). Its mean size and standard deviation were 27.54 [μm] and 6.92 [μm], respectively. The particle size of red pepper powder was analyzed by using the Tyler’s standard sieving method (ASAE Standards: S319, 1983-1984) and the mean size and standard deviation were 406.35 [μm] and 280.08 [μm], respectively. The moisture content of food sample was controlled in the constant temperature and humidity chamber. The moisture content ranges of food sample were from 5.93 [%w.b.] to 17.07 [%w.b.] for wheat flour and from 10.87 [%w.b.] to 27.36 [%w.b.] for red pepper powder. The powdered food samples were filled with various filling method in the sample holder to make three levels of bulk density. The moisture content was determined by the vacuum oven drying method (40-hours, 60 [°C], 2 [g]) and expressed as follows;