Effect of starch retrogradation on texture of potato chips produced by low-pressure superheated steam drying Rungthip Kingcam, Sakamon Devahastin * , Naphaporn Chiewchan Department of Food Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha u-tid Road, Bangkok 10140, Thailand article info Article history: Received 19 January 2008 Received in revised form 8 April 2008 Accepted 8 April 2008 Available online 16 April 2008 Keywords: Baking Blanching Crystallinity Drying kinetics Freezing Health snack Pretreatment Texture X-ray diffraction abstract The effects of the degree of starch retrogradation, initial slice thickness and final moisture content on the texture of potato chips dried by low-pressure superheated steam drying (LPSSD) were investigated in this study. Potato slices of different initial thicknesses (1.5, 2.5 and 3.5 mm) were pretreated with three dif- ferent methods (blanching and then freezing for 24 h, blanching and then repeated freezing/thawing either for 3 or 5 cycles) to study the effects of these pretreatment methods on the degree of starch ret- rogradation. The potato slices were then dried by LPSSD at 90 °C and absolute pressure of 7 kPa to three levels of final moisture content (1.5%, 2.5% and 3.5% (d.b.)) to investigate the drying kinetics and the qual- ity of dried potato chips in terms of hardness, toughness and crispness as well as degree of crystallinity by X-ray diffraction technique. The various pretreatment methods were found to have an obvious effect on the rates of moisture reduction of the samples. Higher degrees of starch retrogradation led to an increase in the hardness and toughness of dried chips, but did not show any significant effect on the crispness of the chips. An increase in the degree of starch retrogradation led to higher degree of crystallinity of dried potato chips. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Potato chips are generally dehydrated by deep fat frying to a moisture content of 0.02 kg/kg (d.b.) or less. Potato chips have oil content that ranges from 35% to 45% (w.b.) (Garayo and Moreira, 2002). Due to the high level of oil as well as to an increased de- mand of consumers for health snack, a technique to produce potato chips without oil is required. Drying may thus be an option for the production of fat-free potato chips with the desired color and tex- ture characteristics. Most of the research efforts have focused on hot air drying of potato pieces of various shapes (e.g., Wang and Brennan, 1995; McMinn and Magee, 1996; Krokida, Tsami and Maroulis, 1998). However, there are many problems with hot air drying such as slow drying rate, poor product color, product deformation and sub- stantial degradation of nutrients. Therefore, a novel concept of low-pressure superheated steam drying (LPSSD) has been pro- posed as an alternative to dry heat-sensitive products (Devahastin et al., 2004) since it can combine the advantages of drying at re- duced temperature and pressure with those of conventional atmo- spheric-pressure superheated steam drying such as the ability to yield product with higher porosity, less shrinkage and better color (Mujumdar and Devahastin, 2000; Devahastin and Suvarnakuta, 2004). Recently, LPSSD has also been applied to produce fat-free potato chips. Leeratanarak et al. (2006) studied the drying kinetics and quality of potato chips produced by LPSSD and hot air drying. It was found that LPSSD produced potato chips of better quality than did hot air drying by lowering the chips browning index. LPSSD at 90 °C was proposed as the most favorable condition for baking po- tato chips in their work. However, the best condition proposed still led to chips of much inferior quality compared with commercially available chips, especially in terms of texture (hardness). To further improve the quality of potato chips, various pretreat- ment methods have been proposed and tested prior to LPSSD. Pim- paporn et al. (2007) studied the influences of various pretreatment methods on the low-pressure superheated steam drying kinetics and quality of dried potato chips in terms of color, texture and microstructure. The effects of blanching, combined blanching and freezing and blanching followed by immersion in chemical solu- tions, namely, glycerol and monoglyceride, at different concentra- tions and then freezing were investigated. Drying was performed at 70, 80 and 90 °C. It was found that combined blanching and freezing was the most appropriate method of pretreatment for pro- ducing good quality potato chips. This was probably due to the 0260-8774/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2008.04.008 * Corresponding author. Tel.: +66 2 470 9246; fax: +66 2 470 9240. E-mail address: sakamon.dev@kmutt.ac.th (S. Devahastin). Journal of Food Engineering 89 (2008) 72–79 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng