Thermomechanical Behavior of Wheat Gluten Films: Effect of Sucrose, Glycerin, and Sorbitol

Cereal Chem. 72(1):1-6 Glass transition temperature (Tg) and mechanical and water vapor However, the Tg failed to change with solute composition and exhibited barrier properties of wheat gluten films containing glycerin, sucrose, low correlation with barrier and mechanical properties. Initial addition glycerin-sucrose, and glycerin-sorbitol were studied. Glycerin and sucrose of glycerin increased the WVP dramatically. Sucrose decreased the WVP were immiscible; the wheat gluten film containing both solutes showed (only slightly), but resulted in a rigid and fragile film. The gluten-glycerintwo separate thermal transitions (-58"C and -51C, respectively). The sorbitol film at a ratio of 15:3:3 (w/w), 16.7% moisture (wb), showed low temperature (low-7) transition (-58°C) was due to a glycerin-rich a single effective Tg (-421C) and intermediate values for tensile strength, region. The low-T tan 8 peak height influenced the tensile strength and percent elongation, and WVP, which were between those of the 15:6:0 elongation linearly and the water vapor permeability (WVP) curvilinearly. and 15:4:2 gluten-glycerin-sucrose films. Research interest in the development of edible films and coatings for use in food packaging and preservation has recently increased. The obvious appealing characteristics of edible films include the renewable nature of their ingredients; the films' ability to function as carriers of food additives (e.g., antioxidants, flavors); and the potential use of such films in the interior of heterogeneous food systems as selective barriers to the transport of vapors, gases, and solutes. An additional anticipated benefit from the wide commercialization of edible films is the further utilization of several commodities employed as sources of film-forming materials. Comprehensive reviews on the film-forming properties of protein, polysaccharide, and lipid materials have been published (Guilbert 1986, Kester and Fennema 1986, Krochta 1992). Protein films in particular have been discussed in detail by Gennadios et al (1994). Some commercialization of protein films has been realized in collagei sausage casings (Hood 1987); gelatin pharmaceutical capsules (Rose 1987); and corn zein-based protective coatings for nutmeats and candy (Alikonis 1979, Andres 1984). The formation and property evaluation of wheat gluten films has been dealt with in several studies (Wall and Beckwith 1969; Anker et al 1972; Okamoto 1978; Park and Chinnan 1990; Aydt et al 1991; Gontard et al 1992, 1993; Gennadios et al 1993a-d). In all these studies, films were produced by drying cast aqueous ethanol solutions of wheat gluten. A plasticizer, usually glycerin, was added to reduce film brittleness and ensure the formation of free-standing films. Wheat gluten films in a dry state were very effective oxygen barriers (Gennadios et al 1993b). However, wheat gluten films, and protein films in general, are poor water vapor barriers because of the inherent hydrophilicity of the proteins (Krochta 1992). The incorporation of waxes, fatty acids, surfactants, sugars, etc., to carbohydrate and protein films, either by mixing or as a separate layer, may help in limiting the moisture migration (Kester and Fennema 1986, Kester and Fennema 1989, Rico-Pena and Torres 1990). Use of glycerin as a plasticizer not only increases flexibility, but also gives a very high water permeability. Modification of the amount of glycerin and the pH has decreased permeability, but not to a desirable level (Gontard et al 1993). Because plasticization increases diffusion through the film, it is possible that replacing glycerin with a larger molecular weight solute such as sucrose may help retard diffusion (given 'Journal series 10601, Agricultural Research Division, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln. University of Massachusetts Agricultural Experiment Station, Project MAS 000623. 2 Department of Food Science, University of Massachusetts, Amherst. 3 Department of Biological Sciences Engineering, University of Nebraska. o 1995 American Association of Cereal Chemists, Inc. that the rheological properties are acceptable). One of the objectives of this study was to investigate this possibility. Glass transition temperature (Td) has been an important parameter in the study of synthetic polymers. Above the Tg, polymeric materials exist in a soft, rubbery state, whereas below the Tg, polymers assume a glassy state (Ferry 1980). The value of Tg is governed primarily by chemical composition and the presence of added plasticizers and, secondarily, by structural features, such as chain branching, cross-linking, and crystallinity (Rogers 1985). In the case of hydrophilic synthetic and natural polymers, water functions as a plasticizer depressing the Tg (Slade and Levine 1988, Slade et al 1989). Thermal analysis techniques such as differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), are employed for measurement of Tg. The mechanical and barrier properties of polymers at any particular temperature are affected by the value of Tg (Odian 1991). For example, it is expected that the permeation of gas and vapor molecules through a film is higher above Tg where polymer chains are more mobile (Rogers 1985). The application of the glass transition concept can be a valuable aid in edible film research. In the only relevant study available, Koelsch and Labuza (1992), using DSC, determined the Tg of edible emulsion films made from methyl cellulose and fatty acids. They reported that their films, analyzed at ambient condition, were in the rubbery state since recorded Tg values were in the approximate range of-l 1 to 53° C. Thus, they reasoned the higher permeance of the film may be due to the rubbery state. Glass transition behavior of wheat gluten and its glutenin and gliadin fractions has been reported (Hoseney et al 1986, Cocero and Kokini 1991, de Graaf et al 1993). Kalichevsky et al (1992) determined the effect of sucrose and glycerin on the Tg of wheat gluten. There has been no work done, however, to relate these effects to the physical properties of edible gluten barriers, such as water vapor permeability and mechanical properties. The Tg of glycerin-plasticized wheat gluten films is expected to increase when glycerin is partially replaced with sucrose (Kalichevsky et al 1992). Most likely, such an increase in Tg will be accompanied by an improvement of the water vapor barrier characteristics of the film. Evidence for this assertion comes from a study of a starch-sucrose system where water diffusivity decreased with the addition of sucrose (Chinachoti and Stengle 1990). Diffusion is a very important (often governing) step in the whole permeation process, and substituting sucrose is expected to affect it. The objectives of the present study were to examine the effect of sucrose on the Tg, water vapor permeability, and selected mechanical properties of wheat gluten films and to investigate the effects of using sorbitol in the film formulation. Vol. 72, No. 1,1995 1 MATERIALS AND METHODS Reagents Vital wheat gluten (Whetpro-80) with -82% (db) protein content (N X 6.25; value provided by the manufacturer) was donated by Ogilvie Mills Ltd., Quebec, Canada. Glycerin and sucrose, both of ACS grade, were purchased from Fisher Scientific, Pittsburgh, PA. Ammonium hydroxide (4.96N solution in water) and D-sorbitol (>99% purity) were purchased from Aldrich Chemical Co., Milwaukee, WI. Preparation of Film-Forming Solutions Wheat gluten (15 g) was mixed and stirred with 95% ethanol (72 ml) and glycerin (6, 4, 3, or 0 g) or sorbitol (3 g) (Table I) before the obtained mixtures were homogenized (Virtishear, Virtis Co., Gardiner, NY) for 5 min at 5,000 rpm. Subsequently, mixtures were heated and stirred on a magnetic stirrer hot plate (model PC-320, Corning, Inc., New York, NY) while slowly adding 14 ml of ammonium hydroxide (4.96N) to provide an alkaline condition to disperse the proteins, and warm distilled water (48 ml) in which sucrose (0, 2, 3, or 6 g) was previously dissolved. The dispersion of protein was noted by an apparent decrease in solution viscosity, at which point the solution had a temperature of 75-770 C. After removal from the hot plate, the solutions were kept at ambient conditions (250C) for 2-3 min to allow bubbling to stop before casting. Casting and Drying Film-forming solutions were cast onto flat glass plates using a thin-layer chromatography spreader bar (Desaga, Brinkmann Co., New York, NY) and placed in an air-circulating oven TABLE I Weight Ratios of Glycerin, Sucrose, and Sorbitol Added per 15 g of Wheat Gluten in Gluten-Based Films Ratio Glycerol Sucrose Sorbitol Film (w/w) (g) (g) (g) Gluten-glycerin-sucrose 15:6:0 6 0 NAa Gluten-glycerin-sucrose 15:4:2 4 2 NA Gluten-glycerin-sucrose 15:3:3 3 3 NA Gluten-glycerin-sucrose 15:0:6 0 6 NA Gluten-glycerin-sorbitol 15:3:3 3 NA 3 'Not applicable.