Effect of drying conditions on the quality of vacuum-microwave dried potato cubes Joanna Bondaruk a , Marek Markowski a, * , Wioletta Błaszczak b a Department of Agri-Food Process Engineering, University of Warmia and Mazury in Olsztyn, Heweliusza 14, 10-718 Olsztyn, Poland b Division of Food Science, Institute of Animal Reproduction and Food Research, Polish Academy of Sciences, Tuwima 10, 10-747 Olsztyn, Poland Received 1 August 2006; received in revised form 29 October 2006; accepted 29 October 2006 Available online 19 December 2006 Abstract The influence of drying conditions on the color, starch content, sugar content, mechanical properties and microstructure of dried potatoes was studied. Statistically significant differences between the color of raw and dried materials were observed for every drying methods. It was observed that the vacuum-microwave drying technique prevents color damage during drying. Potato cubes dried in a vacuum-microwave oven had lower starch and total sugar losses than those dried under forced convection conditions. The maximum force values obtained from compression tests indicated statistical differences between samples dried in a microwave-vacuum drier and a convection drier. The averaged force and energy required to cause 3 and 5.5 mm deformation were the highest for blanched and hot-air dried (70 °C) potato cubes, and the lowest for vacuum-microwave dried material (24 kPa). The application of microwave energy led to different physical changes in product microstructure, compared to those observed during hot-air drying. The extent of changes was depended on the method and parameters of drying. In both cases the drying process caused deformation and disintegration of cell walls and starch granules. Experiments show that in the case of hot-air drying the intensity of structural changes depends on drying temper- ature. A higher temperature causes greater damage to the microstructure of potato cubes. Microwave drying at 24 kPa ensured the shortest drying time and the best overall quality of dried potato cubes, and thus was chosen as the most appropriate technique for potato drying. Ó 2006 Published by Elsevier Ltd. Keywords: Vacuum-microwave drying; Potatoes; Quality 1. Introduction Hot-air drying has been to date the most common drying method employed for food materials. However, this method has many disadvantages, including poor quality of dried products, low energy efficiency and a long drying time. It has been reported that hot-air drying of food materials, involving their prolonged exposure to elevated drying temperatures, results in substantial deteri- oration of such quality attributes as color, nutrient con- centration, flavor and texture (Bouraout, Richard, & Durance, 1994; Maskan, 2000; Yongsawatdigul & Gun- asekaran, 1996). Because of the low thermal conductivity of food mate- rials in the falling rate drying period, heat transfer to the inner sections of foods during conventional heating is limited. The desire to eliminate this problem, to prevent significant quality deterioration, as well as to achieve fast and effective thermal processing has resulted in the increasing use of microwaves for food drying. Micro- waves have been used as a heat source since the 1940s (Mermelstein, 1997). This technique has been extensively employed in the food and chemical engineering industries (Ayappa, Davis, Davis, & Gordon, 1991; Chatterjee, 0260-8774/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.jfoodeng.2006.10.028 * Corresponding author. E-mail address: marek@uwm.edu.pl (M. Markowski). www.elsevier.com/locate/jfoodeng Journal of Food Engineering 81 (2007) 306–312