116—JOURNAL OF FOOD SCIENCE—Volume 61, No. 1, 1996 Determination of Sorbic Acid Diffusivity in Edible Wheat Gluten and Lipid Based Films A. REDL, N. GONTARD, and S. GUILBERT ABSTRACT Edible films using wheat gluten as the structural matrix with or without different lipid components and pure lipid films were tested for sorbic acid retention properties. A diffusion mechanism following Fick’s first law was identified. An experimental procedure was developed for deter- mining the diffusivity coefficient of sorbic acid in edible films immersed in an aqueous medium. The effect of film composition and temperature was investigated. The diffusion coefficient of sorbic acid in a gluten based film was 7.6  10 -12 m 2 /sec. The addition of a lipid component such as acetylated monoglyceride led to a 50% reduction in diffusivity. In a pure lipid film such as beeswax the diffusion coefficient of sorbic acid was 2.7  10 -16 m 2 /sec. The effect of temperature could be de- scribed by an Arrhenius type model, with activation energy ranging from 30.0 to 39.8 kJ/mole. Key Words: edible films, wheat gluten, sorbic acid, lipid, diffusivity INTRODUCTION FOOD QUALITY may be affected by diffusion. The progress of chemical reactions depends on whether reactants have sufficient mobility to move to the reaction site. Control of moisture, gas and other solutes diffusion on the food surface, as well as exchange between different regions in a heterogeneous food, can be a main stability factor for a food product. Control of surface microbial growth, often the main cause of spoilage for refrig- erated food products (Vitkov, 1973, Guilbert, 1988; Torres and Karel 1985), is also important. Food processors have used pre- servatives in surface treatments to control surface microbial problems. The use of potassium sorbate dips reduced the total number of viable bacteria during refrigeration and under tem- perature abuse (Robach, 1979; Robach and Sofos, 1982; Lueck, 1984). However, there is very little extension of shelflife fol- lowing such surface treatments. Microorganisms eventually overcome sorbate induced bacteriostasis due to preservative dif- fusion into the food. Diffusion results in a reduction of preser- vative concentrations on the surface where microbial spoilage can occur (Greer, 1981; Torres and Karel, 1985). The use of edible films as a gas moisture or solute barrier has been proposed to protect food products. Polysaccharide based films (e.g. cellulose, modified cellulose and starch), proteins (e.g. zein, gluten, collagen, gelatin, ovalbumin and myofibrillar proteins), plant and microbial polysacharides (e.g. agar, carra- geenan, alginate, pectin, dextran, pullulan, curdlan), waxes and lipid derivates have been considered for food applications (Guil- bert, 1986; Cuq et al., 1995; Kester and Fennema, 1986). Casein and carnauba wax coatings have been used by Guilbert (1988) in conjunction with sorbic acid to control surface microbial growth. The diffusion barrier properties of these coatings were confirmed in microbial tests using intermediate moisture papaya cubes and Staphylococcus rouxii or Aspergillus niger as chal- lenge microorganisms. Reducing preservative diffusion due to barrier properties of the coatings was identified as a potential means of stability improvement. Torres et al. (1985a) investi- Authors Redl and Gontard are with CIRAD-SAR/ENSIA-SIARC, B.P. 5035-73 rue J.F. Breton 34090 Montpellier Cedex 01-France. Author Guilbert is with ENSAM, 2, Place Pierre Viala, 34060 Mont- pellier Cedex 01-France. gated intermediate moisture food models, coated or noncoated, and reported that diffusion of sorbic acid into the model food could be decreased 150- to 300-fold when coated with zein films. Further studies were conducted by Vojdany and Torres (1989a,b; 1990) to evaluate the apparent potassium sorbate per- meability of polysacharide-based films (methylcellulose, hydroxymethylcellulose) alone and combined with various fatty acids. By this testing method, sorbic acid, or its corresponding salt (potassium sorbate), diffused through a film placed between two compartments with a different diffusant concentration. The addition of fatty acids decreased the apparent permeability with a methylcellulose-palmitic acid film (weight ratio 45:20), thus giving the best results (10-fold decrease). Pure fatty acid films or coatings were estimated as having the lowest permeability but could not be tested properly because of their fragility, which was not compatible with the experimental procedure used. Our objective was to develop an easy and rapid method to determine the diffusion coefficient of sorbic acid incorporated in edible films where samples of films containing sorbic acid were immersed in distilled water. The tests involved gluten based films, with or without different lipid components, as pre- viously reported by Gontard et al. (1992, 1993), and pure lipid films. THEORETICAL CONSIDERATIONS THE DIFFUSION of sorbic acid incorporated in thin layer films placed in aqueous medium was assumed to follow Fick’s law, expressed in one dimension, with a non-concentration-dependent diffusion coefficient (Crank, 1975): 2 C  C = D (1) 2 t x with: C, concentration of sorbic acid in film (kg/m 3 ); D, diffusion co- efficient (m 2 /sec); coordinate dimension in the direction of transport (m); t, time (sec). For an initially uniform sorbic acid distribution in the film and zero sorbic acid concentration in the liquid, a solution of (1) as follows can be obtained: (Crank, 1975)  4C 1 0 C = (2) Σ n=0  2n + 1 2 2 D(2n + 1)  t (2n + 1) x exp - sin 2   h h with: C 0 , initial concentration of sorbic acid in film (kg/m 3 ); h, height of film (m); n, even number. Equation (2) upon integration over the total height of the film gives the fractional mass release (Crank, 1975):  M 8 t = 1 - (3) Σ 2 2 n=0 M (2n + 1)  t= 2 2 D(2n + 1)  t exp - 2   h with: M t , sorbic acid diffused at time t (mg/cm 2 ); M t= , sorbic acid dif- fused at infinite time (mg/cm 2 ). This is the exact solution of (1) for the given boundary and initial conditions. It has been used by Britton et al. (1988), Shantamurthy (1990), Messadi and Vergnaud (1981, 1982) and Messadi et al. (1981)