Linseed Mucilage and Chitosan composite films: Preparation, physical, mechanical and microstructure properties

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1 Linseed Mucilage and Chitosan composite films: Preparation, physical, mechanical and microstructure properties Laura Esther Latorre Salamanca a, Laura Eugenia Pérez Cabrera a, Gloria Cristina Díaz Nárvaez a, Liliana Raquel Barba de Alba b a Department of Food Technology, Universidad Autónoma de Aguascalientes, Aguascalientes, Mexico. (leperez@correo.uaa.mx) b Department of Agriculture, Universidad Autónoma de Aguascalientes, Aguascalientes, Mexico. ABSTRACT Films based on biopolymers can be used to reduce water vapour, oxygen, lipid, and flavour migration between components of multi-component food products, and between food and the surroundings. Linseed mucilage contain substantial amounts (5 8%) of soluble fibre mucilaginous material and it has a high water holding capacity and shows similar functional properties to those of gum arabic, since this characteristic is its interest in its application as a coating. The objective of the present work was to develop composite films based on linseed mucilage polymer and chitosan, and to evaluate optical, barrier and microstructure properties of these films. Films were prepared using chitosan (1%) and linseed mucilage in different ratios. The concentration linseed mucilage polymer ranged from 2.0, 1.5 and 1.0% w/v. The optical properties such as transparency, opacity and colour were measured. Water vapour transmission rate of the films was also investigated. Films were evaluated for microstructure. Addition of linseed mucilage polymer in varied proportions to chitosan led to changes in transparency and opacity of films. The water vapour transmission rate change significantly upon addition of linseed mucilage polymer. Composite films obtained from linseed mucilage polymer and chitosan may reduce environmental problems associated with synthetic packaging. Keywords: linseed mucilage; chitosan; film; water vapour permeability; microestructure INTRODUCTION Shelf life of food is governed by its numerous interactions with the surroundings and can be extended using proper packaging material. The deterioration of packaged foodstuffs largely depends on the transfers that may occur between the internal environment of the packaged food and the external environment. Films based on biopolymers can be used to reduce water vapour, oxygen, lipid, and flavour migration between components of multi-component food products, and between food and the surroundings. The film properties depend on the type of material used and the process conditions employed which in turn determine their applications [1]. Biological materials used to prepare packaging material include polysaccharides, proteins, lipids and their derivatives. Specifically, within polysaccharides, cellulose derivatives, chitosan, starch, alginate, carrageenan and pectin are preferred because of their high film forming ability. Polymer blending is one of the useful methods to obtain new materials with desired functional properties and there has been great scientific and commercial progress made in the area of food applications [2]. Chitin and its deacetylated product, chitosan, have received much interest for their application in agriculture, biomedicine, biotechnology and food industry due to their biocompatibility, biodegradability and bioactivity [3]. Due to its antimicrobial activity [4], chitosan film is a promising packaging material that can be included in the active film category [5]. However, chitosan is expensive and functional properties of chitosan films can be improved by combining it with other hydrocolloids [6]. Thus importance is attached to the research on the combination of chitosan with other macromolecules [2].

2 Flaxseed gum is commercially feasible only as a by-product of the linseed oil industry therefore preparation methods of gum are based on oil meal or oil meal cake [7]. Flaxseed gum is a heterogeneous polysaccharide composed of xylose, arabinose, glucose, galactose, galacturonic acid, rhamnose and fucose [8]. Flaxseed gum is a hydrocolloid with good water-holding capacities, owing to its marked swelling capacity and high viscosity in aqueous solution [8]. Functionally, flaxseed gum resembles gum arabic more closely than the other common gums, so that it has been suggested to replace gum arabic in emulsions [7]. Flaxseed gum also exhibits weak gel -like properties that can be used to replace most of the non-gelling gums for food and non-food applications [9]. MATERIALS & METHODS Mucilage was extracted from linseed in a proportion 1:5 with deionized water at 40 C for 1h and 1400 rpm. Formulations, FA, FB and FC, were made by 100, 70 and 80 ml of mucilage and they were solubilized in distilled water to complete 100 ml. Lactic acid was added at 0.5% (v/v) and agitated for 30 minutes, chitosan at 1% (w/v) and after 24 hours, glycerol at 0.3% (v/v). ph s formulations (FA, FB and FC) were: 4.5, 4.3 and 4.4 and water activity: , and Formulations were pasteurized and filtered. Each formulation was poured in Petri dishes completing 20 g and dried in a stove at 40 C during 24 hours. Solids films were set with silica gel for a day. Optical properties Each film specimen was cut into a rectangular piece, thickness was measured in three points of the films and these were equilibrated at 50% RH for a day before placing them in a spectrophotometer test cell. Measurements were performed using air as the reference. The light transmittance of the films was scanned from wavelength of 300 to 700nm using a GBC spectrophotometer. The measurement was done in triplicate and the average of three spectra was calculated. The transparency at 600nm (T 600 ) was obtained from the following equation [10]: T 600 =(log %T)/b (1) where %T is percentage transmittance and b is the film thickness (mm). The opacity of the films was calculated by the following equation according to the method described by [11]: Opacity = absorbance at 500nm film thickness The color of the film was assessed using a colorimeter (Minolta, CR-400). A white standard color plate (L = , a = , b = ) for the instrument calibration was used as a background for color measurements of the films. The system provides the values of three color components; L* (black-white component, luminosity), and the chromaticness coordinates, a* (+red to green component) and b* (+yellow to blue component). Hunter L*, a*, and b* values were averaged from six readings across for each film, and then the total color difference ( E*) was calculated using the following equation [12]: E* = [( L*) 2 + ( a*) 2 + ( b*) 2 ] 0.5 where L*=L* L 0 *, a*=a* a 0 *, b*=b* b 0 *, where L 0 *, a 0 * and b 0 * are color values for control films and L*, a* and b* are color values for FA, FB and FC films containing chitosan. Films were equilibrated at 75% HR. A modification of the ASTM E96-95 gravimetric method for measuring water vapour permeability (WVP) of flexible films was employed, using Payne permeability cups (Elcometer SPRL). Deionised water was used inside the testing cup to achieve 100% relative humidity on one side of the film through a circular opening of 3.5 cm in diameter. Once the films were placed in the cups. The environment within the cabinets was held at constant RH using over-saturated NaCl solution. The cabinets were placed at controlled temperature of 20ºC. During WVP testing, the side of the film in contact with the PTFE plate was placed in contact with that part of the test cup having the highest relative humidity. The cups were weighed periodically after steady state was reached using an analytical balance (± g). Water vapour permeability was determined from the slope obtained from the regression analysis of weight loss data as a function of time, once the steady state was reached. The method

3 proposed by [13] to correct the effect of concentration gradients established in the stagnant air gap inside the cup was used. Microstructural analysis of cross-sections of the dry films (previously conditioned in desiccators with P 2 O 5 for at least 15 days) was carried out using SEM technique in a JEOLJFM-59LV electron microscope. Pieces of 6 x 1 mm were cut from films and mounted in copper stubs. Samples were gold coated and observed using an accelerating voltage of 10 kv. RESULTS & DISCUSSION Homogeneous, thin and flexible films were obtained from linseed mucilage and chitosan mixture solutions. Films formed could be easily removed from the petri dishes and had an average thickness of 0.05mm. The films were free standing as they did not roll over or break. Visually, all the films were colorless and translucent. Films made from linseed mucilage and chitosan transmitted 40% (Figure 1.) of incident visible light. All films showed similar transmittance. Highest transmittance in visible region was found for FA films containing 1% chitosan (w/v) and 100 ml of linseed mucilage. Figure 1. Transmittance of films as affected by the different formulation. FA, FB and FC Addition of linseed mucilage and chitosan films also led to changes in transparency and opacity of films (Table 1). It can be seen that, opacity. The films did not significantly differences for transparency, the interaction with different concentrate of linseed mucilage polymer (FA, FB, & FC), chitosan (1% for all formulations) and water molecules not modifies the refractive index of thus affecting the film transparency. The opacities of films varied with linseed mucilage polymer cocentration. FC films with 70% (v/v) of mucilage were most opaque, followed by FB (Table 2). These findings are important since film transparency or opacity are critical properties in various film applications, particularly if the film will be used as a surface food coating or for improving product appearance. Table 1. Transparency and opacity of FA, FB and, FC films FILM SAMPLE TRANSPARENCY OPACITY FA ± x10-2 ± 2.59x10-3 FB ± x10-2 ± 2.53x10-3 FC ± x10-3 ± 1.87x10-3 Color attributes are of prime importance because they directly influence consumer acceptability. The results of evaluation of color of the linseed mucilage-chitosan based films are shown in Figure 2 and Table 2. It can be seen that there was no significant change in L* value for all films, whereas b* value increased significantly upon

4 addition of less polymer (FC), these findings suggest a net increase in yellow-brown color due to incorporation of water level. Negative values of a* suggest the films had a green tint. Color b* FA FB FC Control a* Figure 2. Color parameters of FA, FB and FC films. The color index ( E*) (Table 2) is influenced by L*, a* and b* values and could describe how far apart two colors are in the color space. It was seen that E* values changed upon addition of linseed mucilage polymer due to alterations in b* and a* values of film. Table 2. Color parameters of FA, FB and FC films Film sample L* a* b* E FA 97.01±1.02 (-)1.54± ± ±1.64 FB 97.52±0.93 (-)1.71± ± ±0.81 FC 96.08±1.26 (-)2.20± ± ±2.47 The water vapor transmission rate of films plays an important role in deteriorative reactions of food; therefore, it is the most extensively studied property of films. Water vapor permeability is assumed to be independent of the water vapor pressure gradient applied across the films. However, hydrophilic materials, such as polysaccharide films, deviate from this ideal behavior due to interactions of permeating water molecules with polar groups in the film s structure [14]. The WVTR values showed in Table 3. change significantly with varying concentrations of linseed mucilage polymer. In order hand the chitosan films have relatively poor water vapor barrier characteristics, which result from their hydrophilicity. Glycerol, through its plasticizing action, changes the polymer network creating mobile regions with larger interchain distances, promoting water clustering by competing with water at active sites of the polymer matrix and the formation of micro cavities in the polymer network structure. Water sorption by biopolymers often results in swelling and conformational changes. The absorbed water plasticizes the film matrix, leading to a less dense structure where chain ends are more mobile, thus increasing transmission rate [15]. Table 3. WVT of films Film WVT (g Pa -1 s -1 m -1 ) x 10-10

5 FA FB FC 18.79± ± ±0.322 SEM micrographs of the cross-sections of the films, which show not differences, a continuous structure was observed for all films. The films microstructure is affected by the organization of the different components of the film and by how they interact during the drying process. Their microstructure provides information about the arrangement of the ingredients of the film-forming dispersions and allows us a better understanding of water vapour transmission mechanisms and mechanical properties. Fig. 3 shows the cross-sections of linseed mucilagechitosan films where the differing formutations gave rise to no observed differences. In FC film lipid particles (or their voids) that are embedded in the polymer matrix are observed. These particles (or voids) interrupt the chitosan matrix producing discontinuities in the polymer network. FA FB FC Figure 3. SEM micrographs of the cross-sections of the films CONCLUSION Composite films obtained from linseed mucilage-chitosan based may reduce environmental problems associated with synthetic packaging. Further studies would be required to determine the use of these films in commercial food systems. REFERENCES [1] Krochta J. M. & De Mulder-Johnston C Edible and Biodegradable Polymer Films: Challenge and Opportunities. Food Technology, 51,

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