Effects of Pretreatments on the Diffusion Kinetics and Some Quality Parameters of Osmotically Dehydrated Apple Slices Kehinde A. Taiwo, † Alexander Angersbach, Beatrice I. O. Ade-Omowaye, and Dietrich Knorr* Department of Food Biotechnology and Process Engineering, Berlin University of Technology, Ko ¨nigin-Luise Strasse 22, D-14195 Berlin, Germany This study compared mass transfer during osmotic dehydration (OD) and some quality indices of untreated apple slices to those of apple slices pretreated by either blanching, freezing, or applying high-intensity electric field pulses (HELP) or high pressure (HP). HP, HELP, and blanching increased water loss. Untreated and HELP-treated samples had comparable solids gains, which were lower (P < 0.05) than in the other samples. Apple slices turned brown after pretreatment but the L values of these samples increased with OD. The breaking force of dried samples increased with OD time, and pretreated samples had firmer dried texture than the untreated. Vitamin C content decreased with OD time, but HP- and HELP-treated apples had better retention of vitamin C. Keywords: Pretreatments; osmotic dehydration; water loss; solids gain; quality indices INTRODUCTION Fruits and vegetables are an important part of the human diet, and the apple, being the most important temperate fruit of the world, is processed into a variety of products (1, 2). Conventional dehydration of apple slices leads to a product of dark color, leathery texture, and poor flavor with a loss of nutritive values. Increas- ing consumer markets for minimally processed fruits and vegetables have prompted researchers to study “combined methods” as a preservation technique. In recent years, osmotic dehydration (OD) has received considerable attention due to the low temperature and energy requirements in addition to better retention of the initial nutritional values in the final product (3, 4). Mass transfer rates during osmotic dehydration de- pend on factors such as temperature, concentration of osmotic medium, size and geometry of the sample, sample to solution ratio, and degree of agitation of the solution (1, 3, 5-7). Recent studies have reported that mass transfer kinetics during osmotic dehydration can be enhanced by pretreating the fruit materials prior to osmotic dehydration. The influence of pretreatments such as high pressure (HP) application on pineapples, blanching and calcium infiltration on apples, high- intensity electric field pulses (HELP) application on carrots, and dehydrofreezing on apple and kiwi fruits on mass transfer kinetics have been reported (5, 8-11). During pretreatment, changes occur in the cell mem- branes which play a key role in the changes that occur within the tissue during further processing. The changes in the state of the cell membranes may vary between partial and total permeabilization depending on the treatment (5). The application of an external electrical field induces an electrical potential difference across the membrane, which leads to electrical breakdown and local structural changes of the cell membrane, thereby increasing its permeability (12). The application of high pressure has also been reported to increase the perme- ability of cell membranes of food materials, whereas freezing disrupts the cell membranes, resulting in a loss of tissue firmness (10, 13). The extent of cell membrane permeabilization can be described using the cell disin- tegration index Z p , which characterizes the proportion of damaged (permeabilized) cells within the cell system. Z p is the impedance of cells with ruptured membranes and is determined on the basis of the changes in electrical conductivity of the cell system at different frequencies. For intact cells, Z p ) 0, and for total cell disintegration, Z p ) 1. The theory and determination of Z p are described by Angersbach et al. (14). Further information on cellular integrity can be obtained by measuring the electrical conductivity of the solution in which the food materials is immersed. High conductivity is indicative of leakage of intracellular ions and, there- fore, damage to membranes (11). Texture, color, and vitamin C content are common quality indices of fruits and vegetables, and a major result of their processing is the loss of tissue firmness depending on the severity of the process. Freezing may cause severe damage to tissue, resulting in excessive softening, whereas low-temperature long-time blanching prior to freezing has been reported to improve the texture of frozen or dehydrated vegetables (13, 15). Enzymatic browning occurs in fruits and vegetables after bruising or cutting or during storage, leading to the development of unpleasant colors, flavors, and loss of nutrients. Studies on different apple cultivars have shown that susceptibility to browning may depend on polyphenol oxidase (PPO) activity or degradation of phenolic content or both (16). The browning effect in apricot cubes was significantly lower in cubes pretreated in concentrated solutions of both sucrose and maltose (2). Ascorbic acid loss occurred in the convective air- drying of steam blanched apples and dehydrofrozen kiwi fruits with higher losses occurring with longer drying period (1, 8). The purpose of this work was to compare the influence of different pretreatments [blanching, freezing, high- * Corresponding author (fax +49-30-8327663; telephone +49-30-31471250; e-mail foodtech@mailszrz.zrz.tu-berlin.de). † On sabbatical leave from Technology Planning and Devel- opment Unit, Obafemi Awolowo University, Ile-Ife, Nigeria. 2804 J. Agric. Food Chem. 2001, 49, 2804-2811 10.1021/jf0009798 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/10/2001