*Correspondent: The Institute of Biochemistry, Food Science and Nutrition, Faculty of Agricultural, Food and Environmental Sciences, The Hebrew University of Jerusalem, P.O.Box 12, Rehovot 76100, Israel. Fax: 972 8947 6189. e-mail: ssaguy@agri.huji.ac.il Effective water diffusivity in deep-fat fried restructured potato product Eli J. Pinthus 1 , R. Paul Singh 2 , Moshe Rubnov 1 & I. Sam Saguy 1 * 1 The Institute of Biochemistry Food Science and Nutrition Faculty of Agricultural, Food and Environmental Sciences, The Hebrew University of Jerusalem, Israel 2 Department of Biological and Agricultural Engineering UC Davis, CA, US Summary Water diffusivity was determined for a restructured potato product undergoing deep-fat frying. Pre-fried product gel-strength, expressed by its deformability modulus, varied from 37 to 127 kPa and markedly affected the effective diffusion coefficients, ranging from 3.31 to 1.58  10 -9 m 2 s -1 , respectively. Pre- and post-fried effective water diffusivity decreased with higher deformability modulus. Frying time reduced the effective water diffusivity only when the initial deformability modulus was higher than 52 and 79 kPa for 1 and 5 min, respectively. Oil uptake criterion was found to increase with the effective water diffusivity for frying durations of 1 and 5 min. Keywords Alginate gel, crust, deformability, diffusion, frying. Introduction Deep-fat frying is a complicated process in which both heat and mass transfer take place simultane- ously (Saguy & Pinthus, 1995). It is characterized by both movement of oil into the product, and water in the form of vapour from the product into the oil. Various semi-empirical models describing the rate of water loss and oil absorption have been suggested (Ashkenazi et al., 1984; Gamble et al., 1987; Keller & Escher, 1989; Kozempel et al., 1991; Rice & Gamble, 1989). Additional model- ling of the frying process was recently reported (Farkas et al., 1996a,b). Physical properties such as gel-strength affect both water loss and oil uptake. Water loss during deep-fat frying of restructured potato products was significantly lower at higher gel-strength (Pinthus et al., 1992). Crust is formed during most deep-fat frying processes, and is one of the most palatable characteristics of fried foods (Varela, 1988). Its development influences heat and mass transfer processes, and oil uptake. Recent studies devoted to the crust phenomena (Ateba & Mittal, 1994), developed a mathematical model describing the dynamics of crust formation and the kinetics of crust colour and firmness changes for the frying of meatballs. Crust physical properties of deep-fat fried restructured potato product showed an effect on both oil uptake and water loss (Pinthus et al., 1995a). The main objective of this research was to study the relationship between water diffusivity and the mechanical properties of a restructured potato product and its crust. Materials and methods Frying of restructured potato product A restructured potato product based on an algi- nate system was prepared according to the proce- dure outlined previously (Pinthus et al., 1992). It included the following basic ingredients: potato flakes (Idaho), 230 g; water, 753 g; NaCl, 5 g. To International Journal of Food Science and Technology 1997, 32, 235–240 © 1997 Blackwell Science Ltd 235