Water content measurement of thin sheet starch products using a conductance technique N.A. Vlachos, T.D. Karapantsios * Food Process Engineering Laboratory, Department of Food Technology, Technological Educational Institution of Thessaloniki, P.O. Box 14561, GR 541 01, Thessaloniki, Greece Received 15 October 1999; accepted 3 April 2000 Abstract An electrical conductance technique for water content measurements of drum-drying and extrusion cooking starch products is described in this paper. It is based on the conductance measurement of a thin sheet of material between two electrically isolated metallic plates, acting as electrodes. Water content measurements are calibrated against the conventional loss-in-weight method. Once calibrated the conductance technique yields satisfactory accuracy, repeatability and stability compared to the loss-in-weight method. Yet, it oers the distinct advantage of a major reduction in measurement time (less than a minute). The latter is of critical importance, as clearly demonstrated by moisture absorption/desorption experiments. The method is easy to use and calibrate, readily installed and can be implemented at relatively low cost. The conductometric technique employed in this work, enabled rapid and reliable water content measurements of maize starch sheets (thickness of 0.085, 0.5 and 1 mm) ranging from around 0% to 20%. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Electrical conductance; Maize starch; Water content; Moisture; Drum drying; Extrusion cooking 1. Introduction Drying plays an essential role in food technology and is subject to a fast growing interest due to increasing requirements in product quality. In order to better control and optimize the drying processes, moisture content of a food article is considered to be a key factor (Bonazzi et al., 1996). In principle, moisture determi- nation can be regarded as a simple analytical procedure. However, such a procedure is actually beset with many complexities and the methods proposed for the moisture estimation are rarely absolute (Keey, 1972). From an economic point of view, the need for a reliable and rapid moisture content estimation is of major importance. The most widely recognized methods for moisture determination are the oven methods, based on calcu- lating the moisture content of the sample by the loss of its weight upon drying at a controlled temperature. These methods, though simple and direct, are time consuming, since the heating period can last several hours (e.g., Henderson, Perry & Young, 1997; Keey, 1972). Therefore, the loss-in-weight techniques are practically unsuitable for automatic control purposes and as underlined by Carr-Brion (1986) there are no industrial applications of these methods. Nevertheless, the loss-in-weight techniques usually constitute the basic method against which other faster and more practical indirect techniques are calibrated. Quite a few indirect techniques, i.e., electrical conduc- tance and capacitance, infrared thermometry, optical absorption, microwave attenuation, have been em- ployed in the past with considerable success for mea- suring moisture in various food articles (e.g., Troller, 1983; Goldstein, Bein, Gersh, Lee & Zakin, 1991; Brzezinski & Palka, 1994; Rodriguez, Vasseur & Cour- tois, 1996; Zanoni, Peri, Giovanelli & Pagliarini, 1999; etc.). Several comprehensive reviews, e.g. Henderson et al. (1997), Keey (1972) and Carr-Brion (1986), outline such methods with emphasis on their advantages and disadvantages. In addition to the fact that these tech- niques often require a rather tedious calibration proce- dure, most of them are quite expensive. Speci®cally for the on-line moisture determination of solid products, Carr-Brion (1986) considers electrical measurements as the most attractive option for it combines eectiveness with low cost. However, it is recognized that in these Journal of Food Engineering 46 (2000) 91±98 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +30-31-791373; fax: +30-31-791360. E-mail address: karapant@alexandros.cperi.forth.gr (T.D. Kara- pantsios). 0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 7 3 - X