Effect of Temperature on Moisture Barrier Efficiency of Monoglyceride Edible Films in Cereal-Based Composite Foods V. Guillard,1,2 B. Broyart,3 C. Bonazzi,3 S. Guilbert,4 and N. Gontard1,5 ABSTRACT
Cereal Chem. 81(6):767–771
The effects of temperature on moisture transfer within a composite food consisting of a sponge cake (SC) separated from a high moisture content agar gel (AG) by an acetylated monoglyceride (AMG1 and AMG2) film were investigated through moisture content profile experiments. A diffusion model was successfully used to predict moisture transfer within various composite foods (AG/SC, AG/AMG1/SC, and AG/ AMG2/SC). The barrier efficiencies of the two hydrophobic films studied
were reduced by temperature increase due to activation of diffusivity and equilibrium water sorption. Despite the low melting point of highly acetylated monoglyceride films, their barrier efficiency appeared to be less sensitive to temperature than monoglyceride films with a lower degree of acetylation. Consequently, in poor storage temperature conditions, these latter monoglyceride films seemed to be more effective in enhancing the shelf-life of the composite food studied here.
The increased consumer demand for ready-to-eat foods has prompted the development in the bakery industry of composite foods consisting of a cereal-based compartment of low or intermediate aw (biscuit or sponge cake) in contact with a higher aw filling. Moisture migration is a common problem in such composite foods because of water diffusion from the upper to the lower aw compartment. This is an issue when processing biscuits or wafers with jam, caramel, jelly, or mallow, pastry tarts containing a moist savory filling, and sponge cake with a cream filling. A temperature increase during storage could markedly increase moisture transfer, thus drastically reducing the shelf-life of these products. Moisture transfer within such foods can be controlled by using an edible film with good moisture barrier properties to separate food compartments with different water activities (Fennema et al 1993; Guilbert et al 1996). Acetylated monoglyceride films, due to their low melting point and suitable barrier properties, are promising with respect to limiting moisture transfer in commercial cereal-based composite foods (Lovegreen and Feuge 1954; Guillard et al 2003a). A number of reports have been published on the effects of temperature on the permeability of edible lipid films (Lovegreen and Feuge 1954; Kester and Fennema 1989a,b) but very little data is available on water sorption and diffusion in such materials. However, knowledge of water activity and moisture diffusivity characteristics and equations that describe their relationship as a function of moisture content and temperature are important for modeling and predicting film barrier efficiency in real-use conditions (Guillard et al 2003a). Moreover, water sorption isotherm and diffusivity variations in a relevant temperature range are required to model the effects of temperature disruption during storage. Diffusion is a thermally activated process and the temperature dependence of water diffusivity is generally described by the Arrhenius equation. In contrast, the temperature dependence of water sorption equilibrium has not yet been very well described. Increasing temperature generally decreases the moisture content for a given aw, as commonly noted in cereal-based products includ-
ing wheat gluten (Bushuk and Winkler 1957), corn flour (Kumar 1974), cookies (Palou et al 1997), and biscuits (Arogba 2001). However, the inverse phenomenon has been observed in high fat content products such as oleic and peanut oil (Loncin et al 1968). Information on water transport properties at different temperatures would be very useful to gain insight into the moisture barrier properties of lipid films. Reliable diffusivity and sorption data could be used in predictive models to simulate modifications with temperature of water barrier efficiency of edible lipid films and could facilitate selection of the best materials to use with respect to storage temperature. This study was designed to evaluate the effects of temperature on the moisture barrier properties of acetylated monoglyceride films at the interface between a high aw compartment (agar gel) and an intermediate aw cereal-based compartment (sponge cake) using a predictive model that was previously developed and validated at 20°C (Guillard et al 2003a).
1 UMR
IATE (Ingénierie des Agropolymères et des Technologie Emergentes), Université Montpellier II, cc023, place Eugène Bataillon 34095 Montpellier cedex 5, France. 2 CTCPA, 11 rue Marcel Lucquet, 32000 Auch, France. 3 UMR Génie Industriel Alimentaire, ENSIA-INRA, 1 avenue des Olympiades, 91744 Massy cedex, France. 4 UMR IATE, ENSAM-INRA, 2 place Pierre Viala, 34060 Montpellier cedex 1, France. 5 Corresponding author. Phone: 33 (0)4 67 14 33 61. Fax: 33 (0)4 67 14 49 90. Email:
[email protected] Publication no. C-2004-1028-05R. © 2004 American Association of Cereal Chemists, Inc.
MATERIALS AND METHODS Materials Agar gels (AG) with 0.999 initial aw and a density of 28 kg/m3 (dry matter/bulk product) were prepared as reported in Guillard et al (2003a). Sponge-cakes (SC) with 0.84 initial aw and a density of 194 kg/m3 were produced using the recipe and procedure described in Guillard et al (2003b). Two acetylated monoglycerides (AMG) of a previous study were chosen as film-forming material (Guillard et al 2003a): ACETEM 50.00 (AMG1) and TSED619 (AMG2). The characteristics of AMG1 and AMG2 are, respectively, melting point 43 and 30°C, degree acetylation 50 and 70%, and density 908 and 890 kg/m3. For the film-forming technique, material was melted at 70°C for 10 min, laminated using a filmmaking apparatus (Braive Instruments, 41000 Chécy, France) adjusted to 0.4 mm height on a hot steel plate (70°C) that was previously covered with grease-proof paper sheets, and solidified at room temperature. After solidifying, AMG film disks 25 mm in diameter were stamped out and removed from the paper. The film thickness was measured at room temperature with a hand-held micrometer (0.001 mm accuracy, Braive Instruments). Measurements were obtained at different places (at least five) on the film. The mean thickness was 300 ± 10 µm, regardless of the film material. AMG film disks were stored for 24 hr at 5 or 35°C in a tightly sealed jar in which the relative humidity was adjusted by using NaCl saturated solution (0.75 aw) before the migration tests. AMG films 7.5 mm in diameter were prepared in the same manner for the water sorption experiments. These samples were stored for seven days at 5 or 35°C under 0% relative humidity adjusted by using a silica gel. The film thickness was assumed to be constant irrespective of the water activity. Vol. 81, No. 6, 2004
767
Moisture Content The moisture content of the food materials was determined by weighing them before and after complete desiccation (24 hr in an oven at 103°C) according to a standard procedure for cereal products (Multon and Martin 1988). Water Sorption Isotherm The water vapor sorption kinetic (adsorption type) were determined (three replicates) using a Dynamic Vapour Sorption (DVS) apparatus (Guillard et al 2003b). From the equilibrium moisture content of each relative humidity tested, the experimental isotherm curve was obtained for aw from 0 to 0.97. The Ferro-Fontan equation (1982) was used to model the sorption isotherm curves for water activity ranging between 0.70 and 1. ⎡⎛ ⎛ a X = ⎢⎜⎜ Ln ⎜⎜ ⎣⎢⎝ ⎝ a w
⎞ ⎞ ⎛ 1 ⎞⎤ ⎟ ⎟⎟ ⎟ × ⎜⎝ b ⎟⎠⎥ ⎠⎠ ⎦⎥
c
(1)
where X is the moisture content (g/g, db). The Ferro-Fontan parameters values (a, b, and c) were determined by minimizing the sum of squared error between experimentally measured and predicted moisture content values using the Levenberg-Marquardt algorithm (Gill et al 1981). An application-specific function proposed in the Matlab Optimization Toolbox was used (Anonymous 2000). The quality of the fit was determined according to the correlation coefficient (r) between the experimental and predicted values. Moisture Migration Experiments The two or three compartments of the model food were placed in contact in a hermetic glass-made diffusion cell (Guillard et al 2003c) and were kept in a constant temperature chamber at 5 or 35°C (±0.1°C). Experimental local moisture distribution profiles at a given time (1, 6, 22, and 30 days) were obtained by using an adapted microtome (Sercom, 34000 Montpellier, France) to slice the cylindrical compartments (AG and SC) into disks 2 mm thick and by determining the moisture content of each slice. The mean 95% confidence intervals determined on the basis of triplicate experimental profiles obtained at 1 and 30 days represented the
experimental error. Five different experiments were conducted for this study: three at 5°C with AG/SC, AG/AMG1/SC, and AG/ AMG2/SC systems; and two at 35°C with AG/SC and AG/AMG1/ SC systems. Because of the low melting point of AMG2 material (30°C), its water barrier properties were inefficient at 35°C and the AMG2 barrier efficiency was thus not studied at this temperature. The time-course mass mean moisture content patterns were also determined to enhance comparison of the temperature effects on sponge cake remoistening by measuring the mean mass moisture content of each compartment during another set of experiments at 5 and 35°C. Simulations and Effective Moisture Diffusivity Coefficient Moisture transfer was simulated using the model previously developed and validated for a two-compartment (Guillard et al 2003c) and for a three-compartment composite food (Guillard et al 2003a) programmed with MATLAB software (The Mathworks Natick MA). Model parameters for each compartment were initial water activity, water sorption equation, bulk density, and diffusivity or diffusivity law parameters as a function of moisture content. Parameters of the Ferro-Fontan equation for SC at 5 and 20°C are given in Guillard et al (2003b). Parameters of the Ferro-Fontan equation for SC at 35°C evaluated in this study were a = 1.0210, b = 0.0624, and c = –1/1.0732 (r = 0.999). Parameters of the FerroFontan equation for AG at 5 and 35°C were not significantly different from those obtained at 20°C (P > 0.05) (Guillard et al 2003c). For each studied temperature, diffusivity values within sponge cake and AMG films were identified from local moisture distribution profiles in two- and three-compartment systems, respectively, as described in a previous study (Guillard et al 2003c). Water Diffusivity Dependence on Temperature The temperature dependence of diffusivity was described by the Arrhenius equation: ⎛E Ln(D eff ) = Ln(D 0 ) − ⎜ a ⎝ R
⎞ 1 ⎟× ⎠ T
(2)
A straight line was fitted when plotting the experimental results of Ln(Deff) versus 1/T according to Equation 2, for SC and AMG1 samples. From the slope of the line, the activation energy (Ea) was calculated using linear parameter estimation (Levenberg-Marquardt algorithm). Presentation of Results and Statistical Analysis Although the moisture content parameter in Equation 1 and in the mathematical model was in g/g, db, all of the sorption isotherm and experimental and predicted moisture content results (local moisture distribution profile as a function of the distance from the interface and the mean mass moisture content profile as a function of time) are presented in wet basis moisture content units (g/100 g, wb) to enhance the clarity of the local moisture distribution profiles. Statistical significance of the experimental results was assessed using a single-factor analysis of variance (ANOVA). Multiple comparisons were performed by calculating the least significant difference. All tests were conducted at the 5% significance level. The root mean square error (RMSE) (g/100 g wb) for profiles at 1, 6, 22, and 30 days estimated the model prediction quality:
Fig. 1. Experimental (symbols) and modeled (curves) moisture sorption isotherms of acetylated monoglyceride (AMG1) films at 5°C (Q) and 35°C (O) and (AMG2) films at 5°C (X). Comparison with modeled sorption isotherms at 20°C for AMG1 (upper dotted curve) and AMG2 (lower dotted curve) (Guillard et al 2003c). Vertical bars are standard deviation. 768
CEREAL CHEMISTRY
⎛∧ ⎞ ⎜ y − y ⎟² ⎝ ⎠ RMSE = (N − p ) ∧
(3)
where y, and y are experimental and predicted values (g/100 g wb), N is the number of experimental moisture content measurements, and p is number of estimated model parameters.
RESULTS AND DISCUSSION Effects of Temperature on Water Sorption in Acetylated Monoglyceride Films (AMG1 and AMG2) Sorption isotherms at 5 and 35°C for AMG1 and at 5°C for AMG2 were measured and compared with those previously obtained at 20°C (Fig. 1). Because of its low melting point (30°C), AMG2 film was in the liquid state at 35°C, and the sorption curve at this temperature was thus not investigated. The equilibrium moisture data were fitted using the Ferro-Fontan model (Equation 1), as this equation was suitable for acetylated monoglyceride films at 20°C (Guillard et al 2003a). Table I shows the parameter values for the Ferro-Fontan model which was applied to describe the sorption behavior (r > 0.988 in each case). Water sorption was very low in AMG films because they have few hydrophilic sites for water sorption (HLB of 1.5, Hernandez 1994). As already observed in similar fat products, including oleic and peanut oils (Loncin et al 1968), water sorption at equilibrium significantly increased in AMG1 film between 5 and 35°C (Fig. 1). This positive effect of temperature was attributed by these authors to the increase of fat hygroscopicity with temperature. It could also be possible that the progressive uncovering of polar sites due to the disorganization of fat crystals as the temperature increases was partially responsible for the increased hygroscopicity. A similar but lower scale increase in water sorption with temperature was also observed for AMG2 between 5 and 20°C. Compared with AMG1, the lower temperature sensitivity of the AMG2 water sorption equilibrium may be attributed to the higher degree of acetylation of this material (70% vs. 50% for AMG2).
Experimental local moisture distribution profiles at 5 and 35°C were compared with the numerical model to obtain the effective moisture diffusivity (Deff) within sponge cake and acetylated monoglyceride films. As already observed at 20°C (Guillard et al 2003c), the diffusivity values obtained for sponge cake at 5 and 35°C varied as a function of moisture content (X in g/g, db) according to a power law. Thus, for moisture content ranging from 15.0 to 70.0 g/100 g wb: Deff(X) = 5.3 × 10–11 X–1.9 at 5°C
(4)
Deff(X) = 28.0 × 10–11 X–2.1 at 35°C
(5)
Contrary to sponge cake, and in accordance with results obtained at 20°C (Guillard et al 2003a), moisture diffusivity within AMG films at 5 and 35°C was constant (Table I). The close agreement between the experimental and modeling
Moisture Content Profiles and Simulations Moisture transfer at 5 and 35°C in two-compartment (AG/SC) and three-compartment composite foods (AG/AMG1/SC and AG/AMG2/SC) composite was experimentally evaluated on the basis of local moisture distribution profiles and time-course mean mass moisture content patterns. These results at 5 and 35°C supplemented those previously obtained at 20°C (Guillard et al 2003a,c). Typical experimental distribution profiles obtained in AG/AMG1/SC composite foods at 5 and 35°C are presented as examples in Fig. 2A and B. As expected, despite the presence of the barrier film at the interface, the temperature increase drastically boosted moisture transfer form the high aw compartment (agar gel) to the intermediate aw compartment (sponge cake). The timecourse mean mass moisture content patterns of SC directly in contact with AG or separated from AG via AMG1 (Fig. 3A) or AMG2 (Fig. 3B) films allowed direct comparisons between the three studied temperatures. TABLE I Effective Moisture Diffusivity Value (1011 m2/sec) and Ferro-Fontan Equation Parameter Values (a, b, c) for Acetylated Monoglyceride (AMG1 and AMG2) Films at 5, 20, and 35°C AMG1 5°C a b c Deff 20°Ca a b c Deff 35°C a b c Deff a
AMG2
2.1646 0.2602 –1/0.3246 0.90
8.2316 1.3367 –1/0.1216 1.90
1.8994 0.1982 –1/0.3686 1.90
15.8509 1.6749 –1/0.1400 2.10
1.2259 0.0218 –1/0.8109 5.70
Values at 20°C are from Guillard et al (2003a).
(–) (–) (–) (–)
Fig. 2. Moisture distribution profiles at 35°C (A) and at 5°C (B) in threecompartment food made of sponge cake (initial aw = 0.840) separated from agar gel (initial aw = 0.999) by an AMG1 film 300.0 µm thick for experimental data after 1 ("), 6 (Q), 22 (+), and 30 (O) days of contact. Respective modeled profiles (—). Vol. 81, No. 6 2004
769
data is exemplified in Fig. 2A,B for local moisture distribution profiles and in Fig. 3A,B for time-course mean mass moisture content. Good RMSE values of ±1.15 and ±2.70 g/100 g wb, lower than the experimental error (±3.00 g/100 g wb), were obtained for the three-compartment system at 5 and 35°C, respectively. A similar good fit was obtained for local moisture distribution profiles in the two-compartment system with RMSE of ±2.34 at 5°C and ±1.52 g/100 g wb at 35°C, which was consistent with the experimental error (±1.80 g/100 g wb).
attributed to the lower acetylation degree of AMG1 compared with AMG2. The effect of temperature on effective diffusivity was adequately modeled by the Arrhenius relationship (Equation 2). The Ea and the preexponential factor (D0) for sponge cake are strong functions of moisture content between 15.0 and 70.0 g/100 g wb (Equation 6), whereas they remained constant, that is, 43.7 kJ/mol and 1.32 × 10–3 m2/sec, respectively, for AMG1.
Effect of Temperature on Water Diffusivity Temperature increase significantly boosted water diffusivity in both sponge cake and AMG1 films. For example, in sponge cake at a moisture content of 45.0 g/100 g wb, diffusivity increased by ≈5 orders of magnitude between 5 and 35°C, and diffusivity within AMG1 film at 35°C was 6-fold higher than the diffusivity value at 5°C. Contrary to AMG1 film, temperature had a negligible effect on moisture diffusivity within AMG2 film (P > 0.05). The difference between the two studied fatty materials could again be
The activation energy values obtained for sponge cake (ranging from 35.5 to 47.0 kJ/mol) were in accordance with published values for similar products such as bread or muffins (50 kJ/mol for a moisture content of 10.0 to 44.4 g/100 g wb, and a temperature ranging from 20 to 80°C) (Tong and Lund 1990). For AMG1, the Ea was in accordance with the results of Higuchi and Aguiar (1959), who reported 51.8 kJ/mol for a glyceryl monostearate coating film. Ea values obtained for SC and AMG1 film were very similar, indicating that the film properties likely cannot compensate for the activating effect of temperature on diffusion within sponge cake because diffusion was influenced by temperature in the same manner in both materials. Arrhenius-type laws for AMG1 and SC were then validated by using them in the model to predict moisture content distribution profiles in AG/SC or AG/AMG1/SC systems. The resulting RMSE always remained lower than experimental error, thus indicating that the Arrhenius law is valid for representing the temperaturedependence of diffusivity. These diffusivity equations, which include both temperature and moisture content effects, are required to interpolate diffusivity values within sponge cake and acetylated monoglyceride films at temperatures other than those studied, and also to simulate effects of temperature disruption during storage.
Ea(X) = 39.2X–0.11 and D0(X) = 0.0013X–3.7
(6)
Effects of Temperature on AMG Film Efficiency and Shelf-Life of Cereal-Based Foods When comparing the remoistening of sponge cake at the same temperature in a two- and three-compartment food, the AMG films placed at the interface substantially reduced moisture transfer between compartments, provided that the temperature was below
Fig. 3. Experimental (symbols) and modeled (curves) effects of temperature on the time-course mean mass moisture content patterns of a sponge cake placed either in direct contact with an agar gel (initial aw = 0.999, grey curves and open symbols) or separated from the same agar gel by a 300.0-µm film of AMG1 (A) or AMG2 (B) (black curves and symbols). Vertical bars represent standard deviation. 770
CEREAL CHEMISTRY
Fig. 4. Predicted effects of excessive temperature (grey lines) on the time-course mean mass moisture content (black solid lines) of a sponge cake placed either (1) in direct contact with an agar gel (initial aw = 0.999) or separated of the same agar gel by a 100-µm film of (2) AMG1 or (3) AMG2. Hypothetical moisture content limit is represented by dotted line.
the melting point of the AMG material concerned (Fig. 3A,B). Considering that a critical aw value of 0.87 is required for the growth of many yeasts (Fontana 2001) and that the corresponding critical moisture content in SC was 30 g/100 g wb, the use of an AMG film increased the shelf-life of the composite food by more than 15 days at 5°C vs. one day without any film, regardless of the AMG material. At 20°C, a clear difference was noted between the two AMG films: AMG2 increased the shelf-life by nine days as compared with six days with AMG1 film and only 6 hr without any film. At 35°C, the barrier efficiency of AMG1 film was drastically reduced, with an increase in shelf-life of only 1.5 days. Contrary to AMG1, and despite its low melting point (30°C), the moisture barrier efficiency of AMG2 film in the 5–20°C temperature range seemed to be less influenced by temperature (Fig. 3B). To compare the moisture barrier efficiency of the two AMG films with temperature disruption, the predicted effects of excessive temperature variation (2 days at 20°C) during storage, at 5°C, on a SC separated from AG (initial aw = 0.999) by a 100-µm AMG1 or AMG2 film are presented in Fig. 4. Time-course mean mass moisture content patterns of a SC in direct contact with AG are compared in the same figure. The use of an AMG film at the interface between AG and SC increased the composite food shelflife by more than four days, as compared with the 10 hr obtained without any film. The activating effect of temperature on moisture transfer within AMG1 is clearly shown in Fig. 4, with a marked increase in the SC mean mass moisture content in the AG/AMG1/ SC system when the temperature increased from 5 to 20°C. Despite the lower effective diffusivity in AMG1 than in AMG2 at 5 and 20°C, the shelf-life of the composite food was reduced from 5.5 to 4.3 days when AMG1 was used instead of AMG2. The lower decrease in barrier performance of AMG2 with increasing temperature may be attributed to its high degree of acetylation (70 vs. 50% for AMG1), which could limit the increase of both water sorption and diffusion with temperature in this material. Acetylation could provide protection against structural breakdown with temperature, mainly due to the reduced water sorption at equilibrium, thus stabilizing the moisture barrier performance. Further in-depth studies should now focus on this particular behavior of monoglycerides with various degrees of acetylation. CONCLUSIONS Temperature increases have a marked impact by reducing the barrier efficiency of acetylated monoglyceride films in cerealbased foods, as investigated through a predictive model. The model was very suitable for fitting the experimental moisture content profiles in the composite food for temperatures ranging from 5 to 35°C. The results obtained with 50 and 70% acetylated monoglycerides indicated that 70% acetylated monoglyceride edible films had sensory (low melting point) and barrier properties suitable for controlling moisture transfer in a cereal-based composite food. However, this material might not be of interest for foodstuffs that could undergo marked temperature variations (>30°C) during storage because of the very low melting point of this material (30°C). In such conditions, it would be better to use a 50% acetylated monoglyceride film because of its adequate arrier properties and practical storage features, despite the poorer
performance at 20°C as compared with 70% acetylated monoglyceride films. LITERATURE CITED Anonymous. 2000. Optimization Toolbox for Use with Matlab. User’s Guide, v. 2. The Mathworks: Natick, MA Arogba, S. S. 2001. Effect of temperature on the moisture sorption isotherm of a biscuit containing processed mango (Mangifera indica) kernel flour. J. Food Eng. 48:121-125. Bushuk, W., and Winkler, C. A. 1957. Sorption of water vapour on wheat flour, starch and gluten. Cereal Chem. 34:73-86. Fennema, O., Donhowe, I. G., and Kester, J. J. 1993. Edible films: Barriers to moisture migration in frozen foods. Food Australia 45:521-525. Ferro-Fontan, C., Chirife, J., Sancho, E., and Iglesias, H. A. 1982. Analysis of a model for water sorption phenomena in foods. J. Food Sci. 47:1590-1594. Fontana, A. 2001. Water activity’s role in food safety and quality. Food Safety February-March 2001:19-21,57 Gill, E. P., Murra, W., and Wright, M. H. 1981. Practical Optimization. Academic Press: New York. Guilbert, S., Gontard, N., and Gorris, L. G. M. 1996. Prolongation of the shelf-life of perishable food products using biodegradables films and coatings. Lebens. Wiss. Technol. 29:10-17. Guillard, V., Broyart, B., Bonazzi, C., Guilbert, S., and Gontard, N. 2003a. Preventing moisture transfer in a composite food using edible lipid-based film and computer simulations. J. Food Sci. 68:2267-2277. Guillard, V., Broyart, B., Bonazzi, C., Guilbert, S., and Gontard, N. 2003b. Moisture diffusivity in sponge-cake as related to porous structure evaluation and moisture content. J. Food Sci. 68:555-562. Guillard, V., Broyart, B., Bonazzi, C., Guilbert, S., and Gontard, N. 2003c. Evolution of moisture distribution during storage in a composite food. Modelling and simulation. J. Food Sci. 68:958-966. Hernandez, E. 1994. Edible coatings from lipids and resins. Pages 279303 in: Edible coatings and films to improve food quality. J. M. Krochta, E. A. Bladwin, and M. Nisperos-Carriedo, eds. Technomic Publishing Co.: Lancaster, PA. Higughi, T., and Aguiar, A. 1959. A study of permeability to water vapour of fats, waxes, and other coating materials. J. Am. Pharm. Assoc. 48:574-583. Kester, J. J., and Fennema, O. 1989a. The influence of polymorphic form on oxygen and water-vapor transmission through lipid films. J. Am. Oil Chem. Soc. 66:1147-1153. Kester, J. J., and Fennema, O. 1989b. Resistance of lipid films to oxygen transmission. J. Am. Oil Chem. Soc. 66:1129-1138. Kumar, M. 1974. Water vapour adsorption on whole corn flour, degermed corn flour, and germ flour. J. Food Technol. 9:433-444. Loncin, M., Bimbenet, J. J., and Lenges, L. 1968. Influence of the activity of water on the spoilage of foodstuffs. J. Food Technol. 3:131-142. Lovegreen, N. V., and Feuge, R. O. 1954. Permeability of acetostearin products to water vapor. J. Agric. Food Chem. 2:558-563. Multon, J. L., and Martin, G. 1988. Standard methods for the measurement of moisture content of grains and seeds. In: Preservation and Storage of Grains, Seeds and Their By-Products. J. L. Multon, ed. Lavoisier Publishing: New York. Palou, E., Lopez-Malo, A., and Argaiz, A. 1997. Effect of temperature on the moisture sorption isotherms of some cookies and corn snacks. J. Food Eng. 31:85-93. Tong, C. H., and Lund, D. B. 1990. Effective moisture diffusivity in porous materials as a function of temperature and moisture content. Pages 482-492 in: Engineering and Food—Physical Properties and Process Control. Vol 1. W. Wiess and H. Schubert, eds. Elsevier Applied Science: London.
[Received March 23, 2004. Accepted July 26, 2004.]
Vol. 81, No. 6 2004
771
