*Correspondent: Fax: +301 772 3155. e-mail: maroulis@chemeng.ntua.gr Mass transfer modelling of the osmotic dehydration of some fruits Nicolaos M. Panagiotou, Vaios T. Karathanos & Zacharias B. Maroulis* Department of Chemical Engineering, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece Summary An empirical model was developed to predict the water loss and solid gain during osmot- ic dehydration of apple, banana and kiwi fruit. The model is based on a first-order kinet- ic equation, in which the rate constant is a function of the main process variables (speed of agitation, solute concentration, size of fruit and process temperature). This model was applied to a wide range of experimental data on the osmotic dehydration of apple, banana and kiwi fruit, and its parameters were estimated using non-linear regression analysis. The results showed that all of the above process variables have a significant effect on the mass transfer phenomena during osmotic dehydration. Keywords Apple, banana, kiwi fruit, solid gain, sucrose concentration, water loss. Introduction Osmotic dehydration (Raoult-Wack et al., 1991) is a process resulting in products of reduced but still relatively high moisture content (20–50%), classified as intermediate moisture foods (IMFs), but having microbiological stability due to reduced water activity (Lewicki & Lenart, 1995). Among the products usually treated by osmotic dehydration are some meat products, and espe- cially vegetables (Islam & Flink, 1982; Yang & Le Maguer, 1992) and fruits (Hough et al., 1993; Rastogi & Raghavarao, 1994). In fruit, the usual osmotic dehydration agents are aquatic solutions of low-molecular-weight pure sugars, or mixtures with corn syrup, etc. Various possibilities for the food industry were reviewed by Raoult-Wack et al. (1992) and Torreggiani (1993). The interest in introducing the osmotic dehydration process into a conventional stabilizing process has two main objectives: quality improvement (Ponting et al., 1966; Dixon & Jen, 1977; Heng et al., 1990) and energy savings (Lewicki & Lenart, 1992). When water-rich solid products are soaked in concentrated solutions, three simultaneous mass transfer phenomena arise (Raoult-Wack, 1994): 1 An important water flow from the product to the solution. 2 A solute transfer from the solution into the product; it is thus possible to introduce a desired amount of active constituent, preser- vative agent, any solute of nutritional inter- est, or sensory quality improver into the product. 3 A leaching of the product’s own solutes (sug- ars, organic acids, minerals, vitamins, etc), quantitatively negligible compared with the first two transfers, yet essential regarding the final product’s composition. Hence, this attractive technique claims to achieve simultaneously dewatering and direct for- mulation of the product (through impregnation plus leaching). Osmotic dehydration differs from conventional drying methods in two major characteristics (Raoult-Wack, 1994). Firstly, a soaking process achieves a two-way transformation of the product by both a dewatering and a formulation effect. Secondly, a soaking process does not generally produce stable products. Thus, osmotic dehydra- International Journal of Food Science and Technology 1998, 33, 267–284 © 1998 Blackwell Science Ltd 267