OPTIMIZATION OF GLYCEROL EFFECT ON THE MECHANICAL PROPERTIES AND WATER VAPOR PERMEABILITY OF WHEY PROTEIN-METHYLCELLULOSE FILMS ABSTRACT

OPTIMIZATION OF GLYCEROL EFFECT ON THE MECHANICAL PROPERTIES AND WATER VAPOR PERMEABILITY OF WHEY PROTEIN-METHYLCELLULOSE FILMS K. NAZAN TURHAN 1, Z. ÖZGE ERDOHAN SANCAK, BELGIZAR AYANA and FERRUH ERDOĞDU University of Mersin Department of Food Engineering 33343 Çiftlikköy-Mersin, Turkey Accepted for Publication January 31, 2007 ABSTRACT Biopolymer films and coatings are generally designed using biological materials such as proteins, polysaccharides, lipids and their derivatives. The use of plasticizers is also required to improve the mechanical properties (tensile strength and elongation) of the films. For application of films to food systems, it is important for the developed films to possess favorable mechanical and permeability characteristics. Therefore, knowledge of optimum conditions where the water vapor permeability (WVP) is minimized while the mechanical properties are enhanced would be significant depending on the application of the edible films. In this study, the effects of glycerol, as a plasticizer, and methylcellulose (MC) ratios on WVP and mechanical properties of the whey protein films were investigated. Optimum properties of edible films were obtained by applying the complex method optimization algorithm to this multiobjective function problem, and glycerol to total polymer ratio (MC and whey protein concentrate [WPC]) of 0.356 and 0.45 was found for the films with MC : WPC ratios of 0.3 and 0.8, respectively. With respect to the results of this study, it might be concluded that optimum conditions for different edible film-forming agents can be determined via the use of a good experimental design. PRACTICAL APPLICATIONS Edible films can improve food quality, extend shelf-life, add some beneficial functional properties to foods and reduce the use of synthetic packaging 1 Corresponding author. TEL: +90-324-3610001, ext. 7200; FAX: +90-324-3610032; EMAIL: boyaci99@yahoo.com Journal of Food Process Engineering 30 (2007) 485 500. All Rights Reserved. 2007, The Author(s) Journal compilation 2007, Blackwell Publishing 485

486 K. NAZAN TURHAN ET AL. materials. They can prevent moisture, as well as prevent oxidation, aroma or color migration by separating layers for heterogeneous foods, such as pizza, apple pie, or pouching some dry food ingredients. For application of films to food systems, it is important to develop films possessing favorable mechanical and permeability characteristics. Composite films can be formulated to combine the advantages of polysaccharides and proteins, and lessen the disadvantages of each. Incorporation of the plasticizers into the polymer matrix results in increasing film flexibility while decreasing the barrier properties of films. On the contrary, the films should possess low water vapor permeability (WVP) and high mechanical properties. In the case of an optimization study for edible film formation, minimization of WVP and maximization of mechanical properties need to be achieved at the same film-forming conditions. Published information on the optimization of film properties is rather scarce. Knowledge of the optimum point can be used to design new film formulations for specific applications. INTRODUCTION Edible films and coatings from biopolymers have been investigated for potential uses in food protection and preservation. They can improve food quality, append some beneficial functional properties such as addition of antimicrobials, antioxidants, flavors, nutrients, colorants, etc., to foods and extend shelf life because of their barrier-acting properties (Pérez-Gago and Krochta 2001). They can be applied on food surfaces or between food layers in the composite foods to prevent moisture, gas and lipid migration between foods and their surroundings, or between food layers. For choosing packaging materials for food products, one has to pay attention to their resistance to water vapor transmittance and mechanical stresses. Moisture content greatly affects the storage qualities of food, and the effectiveness of the edible films is based on retardation of moisture loss or gain and decreasing the gas exchange. On the other hand, if a material is of poor mechanical strength, transport and handling can damage the package and cause leakage during storage. Biopolymer films and coatings are generally designed using biological materials such as proteins, polysaccharides, lipids and their derivatives. The techniques for film formation, properties, and application of films and coatings from these materials have been reviewed in the literature (Gennadios et al. 1994; Hernandez 1994; Nisperos-Carriedo 1994; Pérez-Gago and Krochta 2000; Anker et al. 2002). Gas and water vapor permeation properties in edible films are affected by polymeric materials that provide the structural matrix and plasticizers which impart flexibility (Chinnan and Park 1995). Plasticizers

OPTIMIZATION OF PROPERTIES OF WP-MC FILMS 487 reduce intermolecular forces, increase mobility of biopolymer chains and thereby improve flexibility of the films, preventing them from cracking or chipping during their preparation, handling and storage. However, they also increase intermolecular spacing while reducing internal hydrogen bonding. This results in reducing the gas, vapor and solute barrier properties of the films (Gontard and Guilbert 1994). Therefore, it is necessary to improve the conditions to optimize the film strength and elongation with decreasing the barrier properties of the films. Many polysaccharides (Nisperos-Carriedo 1994) and proteins (Gennadios et al. 1994; McHugh and Krochta 1994) have good film-forming properties (Donhowe and Fennema 1993). They can be used alone or in combination. It is possible to improve film characteristics depending on target application by incorporation of film-forming agents. Some researchers have indicated that incorporation of polysaccharides into globular protein matrices may extend the functional properties of these ingredients (Coughlan et al. 2004; Erdohan and Turhan 2005). Films produced from proteins and polysaccharides show good barrier properties to oxygen and lipid migration; however, their hydrophilic nature makes them poor water vapor barriers. Whey protein-based edible films have received increased interest, partly because of attempts to exploit their functional and nutritional attributes (Banerjee and Chen 1995; Fang et al. 2002; Pérez-Gago and Krochta 2002). Whey is a major by-product of the cheese manufacture industry, with an estimated 200,000 metric tons of whey protein produced worldwide in 1999 (USDA 1999). Large volumes of fluid whey are disposed annually, creating environmental problems if not done properly (Kinsella and Whitehead 1989; Banerjee and Chen 1995). Production of value-added products such as whey proteinbased edible films and coatings is one solution to this disposal problem. Whey protein gives transparent, bland and flexible films with very good resistance to oxygen, aroma and lipid transfer at low humidities (Miller and Krochta 1997). However, the hydrophilic nature of proteins induces interactions with water, weakening the resistance to moisture transfer (McHugh and Krochta 1994). Methylcellulose (MC) is the least hydrophilic water soluble cellulose derivative and also more economical and readily available compared to other cellulose derivatives. The barrier and mechanical properties of MC-based edible films have been reported in the literature, with these films also being lower in water vapor permeability (WVP) compared to other hydrophilic edible films (Donhowe and Fennema 1993; Park and Chinnan 1995; Turhan and Şahbaz 2004). Formation of whey protein films using some polysaccharides (MC, alginate, pectin, carrageenan or konjac flour blends) has been shown to have a lower WVP and higher mechanical properties, such as tensile strength (TS) and elongation (E) (Parris et al. 1995; Coughlan et al. 2004; Erdohan and

488 K. NAZAN TURHAN ET AL. Turhan 2005), compared to the films prepared by whey proteins or polysaccharides (Debeaufort et al. 1993; Pérez-Gago and Krochta 2002). Based on these, it would be important to have films with lower WVP and higher mechanical properties. A better possible improvement in the quality characteristics of the films can be obtained by optimization of all the given parameters defining the film quality (lower WVP and higher mechanical properties). In the case of edible films, this improvement would be to determine the optimum point where the WVP is minimized while the mechanical properties (TS and E) are maximized. This problem leads to a multiobjective function constrained problem. Of course, that even may be a nonlinear problem depending on the nature of objective function and constraints. The goal in a multiobjective function optimization problem is to optimize several objective functions simultaneously. In the case of an optimization study for edible film formation, minimization of WVP and maximization of TS and E properties need to be achieved at the same film-forming conditions. This would require combination of the given objective functions for WVP, TS and E into a scalar objective function using arbitrary weight factors leading to that the optimization problem could become computationally tractable (Erdogdu and Balaban 2003). However, this scalarization might cause the results to become sensitive to the values of weighting factors, which might be difficult to preassign, and there might be a risk of losing some optimal points. There have been numerous methods suggested for these kinds of multiobjective function problems and accumulated knowledge in the literature (Miettinen 1999; Bhaskar et al. 2000; Deb 2002; Summanwar et al. 2002; Erdogdu and Balaban 2003). Biegler et al. (2002) summarized the advantages and disadvantages of the different methods. As stated by Erdogdu and Balaban (2003), the complex method among these can be accepted to be a powerful algorithm to determine the optimum of a nonlinear function within a linear or nonlinear constrained region. Therefore, the objectives of this study were to define the required objective function for the optimum properties (minimum WVP and maximum mechanical properties) of edible films and to determine the optimum point using the complex method algorithm. MATERIALS MC (MW 41 000) and whey protein concentrate, WPC (34% protein content [w/w], Avonlac 134 RW) were purchased from Sigma Aldrich Ltd (Cat. No. 274429, Poole, Dorset, U.K.) and Glanbia Foods (Richfield, ID), respectively.

OPTIMIZATION OF PROPERTIES OF WP-MC FILMS 489 FILM FORMATION MC WPC films were formulated in two different mass ratios of MC to WPC (0.3 and 0.8 MC : WPC, w/w). The glycerol (Gly) to total polymer (MC and WPC) mass ratios (Gly : TP) were between 0.25 and 0.50 for the films with MC : WPC ratio of 0.3 and between 0.25 and 1.0 for the films with MC : WPC ratio of 0.8. This difference was due to the fact that the films with an MC : WPC ratio of 0.3 did not form the film structure when the Gly to TP mass ratio was above 0.50. To form the films, the film solutions containing 5% (w/v) WPC in distilled water were stirred on a magnetic stirrer at 90 5C for 30 min and then cooled in a freezer at -15C for 15 min. Solutions were reheated to 65 5C, and MC was added to adjust the MC : WPC mass ratios of 0.3 and 0.8 (w/w), and homogenized at 9,000 rpm for 1 min in a homogenizer (DI 25, IKA-WERKE, Stanfen, Germany). Gly was then added in the given ratios and homogenized at 9,000 rpm for 1 min followed by 3 min at 20,500 rpm. After homogenizing and degassing under vacuum, the films were cast on glass plates (20 20 cm) and allowed to dry at 23 2C and 45 5% relative humidity (RH) overnight. Film thicknesses were measured at 10 different points using a digital micrometer ( 0.001 mm; Mitutoyo Co. Ltd., Tokyo, Japan), and the average film thickness was then used in determining the WVP and mechanical properties (TS and E). WVP ASTM E96 80 (ASTM 1983) was used to determine the WVP of films at 25 1C. Delrin test cups were filled with anhydrous calcium chloride (0% RH) and mounted with the film specimens. Test cups were placed in a desiccator containing MgNO 3 (52 2%RH) for 72 h. The amount of water vapor that transferred through the film and was absorbed by the desiccant was determined by weighing the test cups on an analytical balance (Sartorius BP221S, Goettingen, Germany). WVP (g mm/m 2 h kpa ) was calculated using the following equation: x WVP = C A ΔP where x is the film thickness (mm), A is the area of the exposed film (m 2 ), DP is the water vapor pressure differential across the film (kpa) and C is the slope of the mass of the test cup versus time. Slopes were calculated by linear regression, and correlation coefficients (r 2 ) for all the reported data were 0.99 or greater. WVP tests were repeated at least three times for each film.

490 K. NAZAN TURHAN ET AL. MECHANICAL TESTS A texture analyzer (Model TA-XT2, Stable Micro Systems, Surrey, U.K.) was used to determine the mechanical properties of the films according to ASTM D638M (ASTM 1993). Film specimens were cut into strips of 40 6 mm, and the strips were equilibrated in a desiccator containing MgNO 3 (52 2% RH) at 25 1C for 48 h. The crosshead speed of the texture analyzer was 0.80 mm/s. The stress strain curves were analyzed using the software provided with the texture analyzer (Texture Expert Exceed 2.3, Stable Micro Systems). The mechanical properties (TS and percent E) were then calculated from the stress strain curves. The mechanical tests were repeated at least five times for each film. OPTIMIZATION METHODOLOGY The objectives of this multiobjective constrained optimization were to minimize the WVP and to maximize the mechanical properties (TS and E) simultaneously. For this purpose, the given objective functions for WVP, TS and E were combined into a scalar objective function using arbitrary weight factors leading to a computationally tractable optimization problem. The objective function ( f ) was then generated as a maximization problem as follows: max f : 1 i + jts + ke, WVP where i, j and k are the weight factors. Inverse of the WVP was applied in the given objective function equation because the maximum of 1/WVP will result in the minimum WVP value. Combination of different weight factors was also used to determine their sensitivity for the optimal points. Complex method (Erdogdu and Balaban 2003) was then applied to determine the optimum point within the given constrained region where the ratio of glycerol to total polymers changed from 0.25 to 0.5 and from 0.25 to 1 for the films with MC : WPC ratio of 0.3 and 0.8, respectively. Step 1 The decision variable (ratio of glycerol to total polymers) was randomized within the constrained region (explicit constraints) to establish the initial point. Then, the linearized objective function was calculated with respect to the randomized variables.

OPTIMIZATION OF PROPERTIES OF WP-MC FILMS 491 Step 2 After the initial complex (consisting of three points) was constituted, the complex method was started. The optimal trial points were moved with reflection, extension and retraction based on the objective function values. Step 3 Stopping criterion for the algorithm was the last step. That the difference between the resulting objective functions of the best and worst was very small (<10 5 ) meant the algorithm could not move the optimal trial points to result in any improvement in the objective function value. Therefore, the algorithm stopped at that point giving the best trial point as the optimum point. RESULTS AND DISCUSSION Figures 1 3 and 4 6 show the changes of the WVP, TS and E versus Gly : TP ratio for the films with MC : WPC ratios of 0.3 and 0.8, respectively. The films with MC : WPC ratios of 0.3 and 0.8 contain 1.5 and 4.0-g MC in the film-forming solutions, respectively. Among the tested MC concentrations in the previous studies, 1.5 g was the lowest concentration that could form a film, 0.2 0.15 WVP 0.1 0.05 0 0 0.125 0.25 0.375 0.5 0.625 Gly/TP Ratio FIG. 1. EFFECT OF GLYCEROL TO TOTAL POLYMER RATIO (GLY : TP) ON WATER VAPOR PERMEABILITY (WVP) OF THE FILMS WITH METHYLCELLULOSE : WHEY PROTEIN CONCENTRATE RATIOS OF 0.3 The triangles depict the data obtained from the fitted equation, while the squares depict the experimental data with their standard deviation.

492 K. NAZAN TURHAN ET AL. 5 4 3 TS 2 1 0 0 0.125 0.25 0.375 0.5 0.625 Gly/TP Ratio FIG. 2. EFFECT OF GLYCEROL TO TOTAL POLYMER RATIO (GLY : TP) ON TENSILE STRENGTH (TS) OF THE FILMS WITH METHYLCELLULOSE : WHEY PROTEIN CONCENTRATE RATIOS OF 0.3 The triangles depict the data obtained from the fitted equation, while the squares depict the experimental data with their standard deviation. 30 25 20 E 15 10 5 0 0 0.125 0.25 0.375 0.5 0.625 Gly/TP Ratio FIG. 3. EFFECT OF GLYCEROL TO TOTAL POLYMER RATIO (GLY : TP) ON ELONGATION (E) OF THE FILMS WITH METHYLCELLULOSE : WHEY PROTEIN CONCENTRATE (WPC) RATIOS OF 0.3 The triangles depict the data obtained from the fitted equation, while the squares depict the experimental data with their standard deviation.

OPTIMIZATION OF PROPERTIES OF WP-MC FILMS 493 0.2 0.15 WVP 0.1 0.05 0 0 0.25 0.5 0.75 1 1.25 Gly/TP Ratio FIG. 4. EFFECT OF GLYCEROL TO TOTAL POLYMER RATIO (GLY : TP) ON WATER VAPOR PERMEABILITY (WVP) OF THE FILMS WITH METHYLCELLULOSE : WHEY PROTEIN CONCENTRATE RATIOS OF 0.8 The triangles depict the data obtained from the fitted equation, while the squares depict the experimental data with their standard deviation. 20 15 TS 10 5 0 0 0.25 0.5 0.75 1 1.25 Gly/TP Ratio FIG. 5. EFFECT OF GLYCEROL TO TOTAL POLYMER RATIO (GLY : TP) ON TENSILE STRENGTH (TS) OF THE FILMS WITH METHYLCELLULOSE : WHEY PROTEIN CONCENTRATE RATIOS OF 0.8 The triangles depict the data obtained from the fitted equation, while the squares depict the experimental data with their standard deviation.

494 K. NAZAN TURHAN ET AL. 60 50 40 E 30 20 10 0 0 0.25 0.5 0.75 1 1.25 Gly/TP Ratio FIG. 6. EFFECT OF GLYCEROL TO TOTAL POLYMER RATIO (GLY : TP) ON ELONGATION (E) OF THE FILMS WITH METHYLCELLULOSE : WHEY PROTEIN CONCENTRATE RATIOS OF 0.8 The triangles depict the data obtained from the fitted equation, while the squares depict the experimental data with their standard deviation. and MC was only partially soluble above 4.0 g. Consequently, they were selected to be the minimum and maximum MC concentrations for film production (Turhan and Şahbaz 2004). WVP of the films decreased as the MC : WPC mass ratio increased from 0.3 to 0.8 at the Gly : TP ratio of 0.5 (Figs. 1 and 4). It was observed that the incorporation of polysaccharides into whey protein films decreased WVP (Le Tien et al. 2000; Coughlan et al. 2004). The addition of MC decreased the WVP of the films by increasing the solid content and producing a smaller pore size (Gutsche 1994; Erdohan and Turhan 2005). Polymer structures significantly affect water vapor transport. Simple linear polymer chains, as in the case of MC, can be packed tightly, resulting in low permeability, but molecules with bulky side chains, as in the case of proteins, are loosely packed and have high permeability (Chen 1995). On the other hand, the film with Gly : TP ratio on 0.25 did not show the lowest WVP as we expected (Figs. 1 and 4). After drying of these films, some pinholes were observed. The presence of pinholes in an edible film greatly affected the moisture barrier properties of the films (Kamper and Fennema 1984). WVP increased significantly as Gly : TP ratio increased for the films with MC : WPC ratio of 0.3 (Fig. 1). Gly is a hydrophilic molecule of low molecular mass that could easily fit into the polymer chains, and that could establish hydrogen bonds with reactive groups of polymers. Gly incorporated to the

OPTIMIZATION OF PROPERTIES OF WP-MC FILMS 495 polymer matrix decreased the attractive forces between polymer chains, increased free volume and segmental motions; hence, water molecules diffused more easily, resulting in the higher WVP. These results are in agreement with the work reported by other researchers on edible films (Park et al. 1993; Debeaufort and Voilley 1995). However, the WVP of the films with an MC : WPC ratio of 0.8 showed a different trend compared to the films with an MC : WPC ratio of 0.3 at the same Gly : TP ratio (Fig. 4). The WVP of the films with an MC : WPC ratio of 0.8 and Gly : TP ratio of 0.5 was lower than those of films which contained 0.25, 0.75 and 1 Gly : TP ratios. It was reported that the plasticizers can retard or enhance the moisture transmission depending on their concentration (Chinnan and Park 1995). When large amounts of plasticizers were introduced into the formulation, changes in film properties were significant: increases in extensibility and flexibility and decreases in elasticity, mechanical resistance. When a plasticizer is incorporated into the polymer matrix, a competition for hydrogen bonding between polymer polymer and polymer plasticizer occurs. As a result, direct interactions between polymer chains are reduced partly because of hydrogen bond formation with plasticizer. The concentration of plasticizer also significantly increased the hydrogen bond formation. This was strongly in accordance with the present observation that increasing Gly concentration decreased TS as a result of the increase of hydrogen bonding. Similar results were obtained from the films with MC : WPC ratio of 0.3 (Fig. 2) and 0.8 (Fig. 5). The incorporation of MC to the polymer matrix and increasing Gly concentration increased elongation of the films (Figs. 3 and 6) as expected. For application of films to food systems, it is important to develop films possessing favorable mechanical and permeability characteristics. Therefore, combined analyses are crucial for predicting film behaviors and defining structure/function relationships (McHugh and Krochta 1994). To accomplish the optimization for minimizing the WVP and maximizing the TS and E values, the changes were fitted into the following equations: When MC : WPC ratio of 0.3 was used to form the films: WVP: TS: WVP = a + x b (1) b TS = a + ln( x) (2)

496 K. NAZAN TURHAN ET AL. TABLE 1. NUMERICAL COEFFICIENTS FOR EQS. 1 6 Equation number a b c 1 0.41928 1.38764 2 20.1681 11.5798 3 628.768 26.5042 4 0.63849 1.56267-0.5022 5 5.03776 0.00818-5.04336 6 0.03309 0.00526-0.00625 E: b x E = a+ ln( ) 2 x (3) When MC : WPC ratio of 0.8 was used to form the films: WVP: 3 WVP = a + b x ln( x)+ c x (4) TS: TS = a + b x c (5) E: 1 b x c E = a+ ln ( ) + 2 2 x x (6) where x is the Gly : TP ratio, and a, b and c are the numerical coefficients (Table 1). Using these equations, the complex method was applied to minimize the WVP while maximizing the TS and E, and Gly : TP ratio of 0.356 and 0.45 were found for the films with MC : WPC ratios of 0.3 and 0.8, respectively for the i, j and k values of 1/3. As seen in Figs. 4 6, the value of 0.45 was really the maximum point for E and the minimum point for WVP. Due to the method s trying to satisfy three different functions at the same time, there seemed to be some losses in the TS value. Of course, a steady decreasing change of the TS also affected this. Similar to that, the same trend was found with the films with an MC : WPC ratio of 0.3. In this case, the WVP and E

OPTIMIZATION OF PROPERTIES OF WP-MC FILMS 497 values steadily increased while TS decreased in the working range of Gly : TP ratio. Hence, the algorithm resulted in a somewhat middle value of 0.356. Erdogdu and Balaban (2003) stated that the reproducibility of an optimization algorithm to find the same result for different runs is an important issue. This would be especially significant when some additional parameters, i.e., the weight factors in this case, are applied in the algorithm. Therefore, to test the algorithm s reproducibility, different combinations of i, j and k values were applied for the case of an MC : WPC ratio of 0.8. In all different cases, the algorithm resulted in the same value of 0.45 showing the uniqueness and reproducibility of the algorithm in the given range. CONCLUSIONS Polysaccharides and proteins have good film formation properties and have been used alone and in combination to form the edible films. They actually differ in their target applications to improve the film characteristics, and the films with a lower WVP and higher mechanical properties (TS and E) are required in different applications. Therefore, an optimization study where the WVP was minimized while the TS and E values were maximized was performed. The satisfaction of these three requirements actually resulted in a nonlinear multiobjective constrained problem, and the modified complex method was applied for the optimum solution. The optimum results were found to be satisfactory, especially for the cases that the MC : WPC ratio of 0.8 was used for film formation. The results also showed the method s uniqueness and reproducibility with respect to the determined optimum conditions. With respect to the results of this study, it might be concluded that the optimum conditions for different edible film-forming agents can be determined with a good experimental design and developed mathematical relationships of the different properties of the films. REFERENCES ANKER, M., BERNTSEN, J., HERMANSSON, A.-M. and STANDING, M. 2002. Improve water vapor barrier of whey protein films by addition of an acetylated monoglyceride. Innovat. Food Sci. Emerg. Technol. 3, 81 92. ASTM. 1983. Standard test methods for water vapour transmission of materials. E96 80. In Annual Book of American Standard Testing Methods, ASTM, Philadelphia, PA.

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