Accelerated drying of button mushrooms, Brussels sprouts and cauliflower by applying power ultrasound and its rehydration properties Anet Rez ˇek Jambrak a, * , Timothy J. Mason b , Larysa Paniwnyk b , Vesna Lelas a a Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia b Sonochemistry Centre, Faculty of Health and Life Sciences, Coventry University, Priory Street, Coventry CV1 5FB, UK Received 25 April 2006; received in revised form 9 October 2006; accepted 17 October 2006 Available online 28 November 2006 Abstract The aim of this work was to use ultrasound as a pre-treatment method prior drying of mushrooms, Brussels sprouts and cauliflower in order to achieve reduction in drying time and to understand the effect of the ultrasound in mass transfer process, where diffusivity is the limiting step in the process. Pre-treatment with 20 kHz probe and 40 kHz bath for 3 and 10 min have been compared with blanched (80 °C/3 min) and untreated samples. The procedures used were either freeze drying or conventional drying at a temperature of 60 °C and air velocity (v = 0.3 m/s) for sonicated, blanched and untreated samples. The effect of ultrasound and blanching pre-treatments on weight and moisture loss/gain, upon drying and rehydration were investigated. The drying time after ultrasound treatment was shortened for all samples, as compared to untreated. The rehydration properties (weight gain, %) were found to be the best for freeze-dried samples which showed weight gains for mushrooms (45.3%), Brussels sprouts (21.4%) and cauliflower (51%). The rehydration properties for ultrasound treated samples were higher than those for untreated samples. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Ultrasonic pre-treatment; Ultrasound probe; Ultrasound bath; Vegetable; Drying; Rehydration 1. Introduction Power ultrasound represents a novel tool in food indus- try processing and is becoming increasingly appreciated. Ultrasonic energy has been used in drying processes because high amplitude vibrations are capable of increas- ing heat and mass transfer processes in materials by pro- ducing changes in concentration gradients, diffusion coefficients or boundary layer. In this way it is possible to remove moisture without significantly heating the prod- uct. This is one of the main reasons for use of ultrasonic energy in many applications in food technology (Ensmin- ger, 1988; Riera et al., 2003). Some of non-thermal pro- cesses (high pressure – HP, high intensity electric field pulses – HELP, ultrasound – US, supercritical carbon dioxide, etc.) have been applied in the processing of food materials. Results showed enhanced mass transfer during osmotic dehydration of high pressure treated pineapples, HELP treated carrots and coconuts, etc. (Ade-Omowaye, Angersbach, Eshtiaghi, & Knorr, 2000; Rastogi, Eshtiaghi, & Knorr, 1999; Rastogi & Niranjan, 1998). Ultrasonic is a rapidly growing field of research and development in the food industry, which can mainly be classified into two fields: high frequency-low energy diag- nostic ultrasound in the MHz range, and low frequency- high energy power ultrasound. High frequency-low energy ultrasound is usually used as an analytical technique for quality assurance, process control and non-destructive inspection, which has been applied to determine food prop- erties, to measure flow rate, to inspect food packages, etc. (Floros & Liang, 1994; Mason, 1998; Mason, Paniwnyk, & Lorimer, 1996; McClements, 1995). High energy power 0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.10.009 * Corresponding author. Tel.: +385 1 4605 035; fax: +385 1 4605 072. E-mail address: arezek@pbf.hr (A.R. Jambrak). www.elsevier.com/locate/jfoodeng Journal of Food Engineering 81 (2007) 88–97