Departamento de Ciência e Tecnologia Agroindustrial, Universidade Federal de Pelotas, Pelotas 96010-900, RS, Brazil
b
Centro de Desenvolvimento Tecnológico, Engenharia de Materiais, Universidade federal de Pelotas, Pelotas 96010-900, RS, Brazil
Received 27 January 2015, Revised 30 June 2015, Accepted 1 July 2015, Available online 13 July 2015
Highlights
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Films produced with oxidized starch are more homogeneous than native starch.
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Films of starch oxidized with 1.5% active chlorine had the highest tensile strength.
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The presence of cellulose fibers in the films increased their tensile strength.
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Celullose fibers in films increases its thermal stability.
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The addition of cellulose fiber in the films of oxidized starches reduced their solubility.
Abstract
Starch and cellulose fibers were isolated from grains and the husk from barley, respectively. Biodegradable films of native starch or oxidized starches and glycerol with different concentrations of cellulose fibers (0%, 10% and 20%) were prepared. The films were characterized by morphological, mechanical, barrier, and thermal properties. Cellulose fibers isolated from the barley husk were obtained with 75% purity and high crystallinity. The morphology of the films of the oxidized starches, regardless of the fiber addition, was more homogeneous as compared to the film of the native starch. The addition of cellulose fibers in the films increased the tensile strength and decreased elongation. The water vapor permeability of the film of oxidized starch with 20% of cellulose fibers was lower than the without fibers. However the films with cellulose fibers had the highest decomposition with the initial temperature and thermal stability. The oxidized starch and cellulose fibers from barley have a good potential for use in packaging. The addition of cellulose fibers in starch films can contribute to the development of films more resistant that can be applied in food systems to maintain its integrity.
Keywords
Barley husk
Oxidation
Tensile strength
Thermal stability
Water vapor permeability
1. Introduction
The use of starch for the production of biodegradable films has been studied since it is a natural source, renewable, abundant, and at a low cost. Although the native starch has been intensively studied for the preparation of biodegradable films, this does not present suitable properties for development of packaging, since it has a high solubility in water, a high hygroscopicity, a poor melting point, a high retrogradation, and lower mechanical properties in comparison with the materials based on synthetic polymers (Lomelí-Ramírez et al., 2014).
Oxidized starches have been suggested for use in biodegradable materials in food packaging, because they present better mechanical and barrier properties compared to the native starch films (Fonseca et al., 2015; García-Tejeda et al., 2013; Hu, Chen, & Gao, 2009; Zavareze et al., 2012). Starch oxidation is mainly performed through the reaction of starch with an oxidizing agent under a controlled pH and temperature. In commercial conversions, sodium hypochlorite is usually used as the oxidizing agent. The reactions of the hypochlorite oxidation of starch includes the cleavage of polymer chains and the oxidation of hydroxyl groups to the carbonyl and carboxyl groups, altering the molecular structure of the starch (Halal et al., 2015).
Barley (Hordeum vulgare) is an important starch source, presenting approximately 65% starch. Barley husk is composed of lignocellulosic agroindustrial residues, which is about 20% barley. Bledzki, Mamun, and Volk (2010) evaluated the chemical composition of the barley husk and found 39% cellulose. Despite the wide availability of starch and cellulose fiber in barley there is little research about this cereal compared to other cereals such as maize, wheat and rice.
Fonseca et al. (2015) studied the effect of the different sodium hypochlorite concentrations on the film forming capacity from potato starch. However, there are no reports on films developed from oxidized barley starches with different degrees of oxidation. A previous study was conducted by our group (Halal et al., 2015) to characterize the barley starches oxidized at different sodium hypochlorite concentrations (1.0%, 1.5%, and 2.0% active chlorine). These oxidized starches were used to produce the films of this study. There are also no studies on the isolation of cellulose fiber obtained from the husk of barley, as well as its applicability, as reinforcement in films of barley starch. In this context, the aim of the study was to develop and characterize films based on oxidized starch and cellulose fiber from barley, proposing the utilization of full barley grains. The films were characterized by morphological, mechanical, barrier, and thermal properties.
2. Materials and methods
2.1. Materials
Barley grains (Hordeum sativum) from cultivar BRS 195 were provided by the University of Passo Fundo, Brazil. The barley grains were dehusked using a Zaccaria machine (model PAZ-1-DTA, Industrias Machina Zaccaria S/A, São Paulo, Brazil). The grains of dehusked barley were used for starch extraction and the husk was used to isolate the cellulose fibers. All the chemical reagents used in this work were of an analytical grade.
2.2. Starch isolation
Barley starch was isolated by the method described by Bello-Pérez, Agama-Acevedo, Zamudio-Flores, Mendez-Montealvo, and Rodriguez-Ambriz (2010), with some modifications. The grains were soaked in a 0.02 mol L−1 sodium acetate buffer containing 0.01 mol L−1 of mercury chloride (1:1 v/v) and then adjusted to pH 6.5 with a 2 mol L−1sodium acetate buffer (2:1 (v/w) solution/grains ratio). This dispersion was kept at room temperature and stirred occasionally for 24 h. Thereafter, the steep water was drained off and more distilled water was filled to the remaining solids followed by a vigorous stirring in a domestic blender for 5 min. The resulting material was screened through a 200-mesh sieve and centrifuged at 7000 g for 10 min at room temperature (25 ± 2 °C). The supernatant water was discarded and the solids were resuspended using a 0.1 mol L−1 aqueous solution of sodium chloride and toluene (7:1). The mixture was kept under 50 rpm stirring (IKA, RW20, German) for 15 h at room temperature (25 ± 2 °C) followed by centrifugation at 7000 × g for 10 min. The supernatant containing the toluene with proteins and fat was discarded, being this procedure repeated twice. After, the starch slurry was adjusted to pH 6.5 with a 1 mol L−1 NaOH solution and centrifuged at 7000 × g for 10 min and the supernatant was discarded. The resulting starch was dried at 40 °C for 16 h until approximately a 9% moisture content and stored at 17 ± 2 °C in a sealed container. The starch isolated from barley showed approximately 99% purity (0.2% protein, 0.6% fat and 0.1% ash).
2.3. Starch oxidation
Starch oxidation was performed according to the method described by Halal et al. (2015). The barley starches were oxidized at different sodium hypochlorite concentrations (1.0%, 1.5%, and 2.0% active chlorine). The levels of carbonyl (CO) and carboxyl (COOH) for 100 glucose units (GU) in the starches were: native (0.01 CO/100 GU; 0.00 COOH/100 GU), oxidized with 1.0% active chlorine (0.09 CO/100 GU; 0.17 COOH/100 GU), oxidized with 1.5% active chlorine (0.11 CO/100 GU; 0.21 COOH/100 GU), and oxidized with 2.0% active chlorine (0.15 CO/100 GU; 0.22 COOH/100 GU).
2.4. Cellulose fibers isolation
The cellulose fibers were isolated according to Johar and Ahmad (2012), with some modifications. The barley husks were washed, dried, milled and subsequently subjected to a mixture of toluene and ethanol (2:1, v/v) for 16 h in order to remove lipids, followed by a drying process at 50 °C for 24 h. The removal of lignin and hemicellulose was performed using an alkali treatment. The barley husks were dispersed in a 4% (v/w) solution of NaOH in a glass reactor with mechanical stirring (IKA, RW20, German) at 80 °C for 4 h. At the end of the treatment, the solids were filtered and washed with distilled water. This alkali treatment was carried out three times. After alkali treatment, a bleaching step was performed to remove the remaining lignin from the barley husks. The bleaching was carried out by adding the husks in a mixture of equal parts of buffer solution of sodium acetate (27 g of NaOH and 75 mL of glacial acetic acid for 1 L of water) and aqueous solution of sodium chlorite (1.7%). This material was placed in a jacketed glass reactor with controlled temperature conditions at 95 °C for 4 h and with mechanical stirring (IKA, RW20, German). Subsequently, the material was filtered using a 200-mesh sieve and washed with distilled water. The bleaching process was carried out four times. The cellulose fibers were dried in an oven with air circulation (Nova Ética, 400-6ND, São Paulo, Brazil) at 50 °C for 24 h and stored in a sealed container.
2.5. Characterization of fibers
The milled barley husk fiber, fiber treated with alkali and bleached fiber, were photographed using a digital camera (Sony, DSC-W510, Brazil) to observe the color and overall appearance, what can be an indicative of the cellulose purification.
The lignin content of the milled barley husk fiber and bleached fiber was determined according to the TAPPI T13m-54 method (TAPPI, 1991). The contents of holocellulose (cellulose + hemicellulose) and cellulose were determined by the TAPPI T19m-54 method (TAPPI, 1954).
The milled barley husk fiber, fiber treated with alkali and bleached fiber, was characterized by using a FTIR spectrometer (IRPrestige21, Shimadzu, Kyoto, Japan), and equipped with an attenuated total reflection (ATR) accessory (Pike Tech, Madison, WI.). An average of 30 scans with a resolution of 2 cm−1 was taken for each sample, within a frequency of 4000–700 cm−1.
The relative crystallinity (RC%) of the milled barley husk fiber, fiber treated with alkali and bleached fiber, was evaluated in an X-ray diffractometer (XRD-6000, Shimadzu, Kyoto, Japan), using a CuKα radiation (λ = 1.54 Å) with a scan region (2θ) from 5° to 40°. The calculation of the relative crystallinity of the fibers was according the method described by Segal, Creely, Martin, and Conrad (1959).
The microstructure visualization of the milled barley husk fiber, fiber treated with alkali and bleached fiber, was performed by scanning electron microscopy (SEM) (JEOL JSM-6610LV, Japan). The images were captured at magnifications of 50×, 200× and 2000×.
2.6. Preparation of films
The films were prepared by a casting technique, using the methodology of Müller et al. (2009a), with some modifications. Preliminary tests were performed to define the fiber concentrations added in the films. Concentrations of 10, 20, 25 and 30 g fiber/100 g dry starch were used in the tests preliminaries. The films with concentrations of 25 and 30 g fiber/100 g dry starch presented high thickness (higher than 0.220 mm), thus reducing the malleability of the films. Furthermore, there was not change in the tensile strength of the films containing 25 g fiber/100 g dry starch and a small increase for the films with 30 g fiber/100 g dry starch as compared to the film with 20 g fiber/100 g dry starch. Therefore, films were prepared with concentrations of 10 and 20 g fiber/100 g dry starch.
For preparation of films, a solution was prepared with 3% starch in 100 g of distilled water, 0.30 g glycerol/g dry starch, 0.01 g guar gum/g of dry starch (to prevent sedimentation of fibers), and 0 g, 10 g and 20 g fiber/100 g dry starch. The cellulose fiber and the guar gum were suspended in water with subsequent stirring in an Ultraturrax homogenizer (IKA, T18B, Werke, Germany) at 14,000 rpm for 10 min, to which the starch and glycerol were added after this. The solution was heated at 90 °C for 10 min. Then, 20 g of each film solution was spread on acrylic plates of 9 cm diameter and dried in an oven with air circulation at 30 °C for 16 h. The film samples were stored in a hermetic container at 16 °C and approximately 65% relative humidity (RH) through the use of a saturated solution of ammonium nitrate (NH4NO3) for 4 days. For the analysis of the mechanical properties, the films were also evaluated with 85% relative humidity using a saturated solution of potassium chloride (KCl). After this storage period the films were analyzed.
2.7. Morphology of the films
The surface and cross-section morphology of films were visualized by scanning electron microscope (JEOL, JSM-6610LV, New Jersey, USA) with accelerating voltage of 10 kV. For cross-sections, the samples were fractured under liquid nitrogen prior to visualization. Samples were then placed in a stub and coated with gold using a sputter Desk V (JEOL, New Jersey, USA) and examined using 50× and 500× magnification.
2.8. Thickness and mechanical properties of the films
The film thickness was determined by a micrometer to the nearest 0.001 mm, at 8 random positions around the film, where average rates were used in the calculations.
The tensile strength and percentage of elongation at the break of the films were evaluated by a tensile test using a Texture Analyser (TA.XTplus, Stable Micro Systems) based on the ASTM D-882-91 method ASTM (1995a).
2.9. Moisture, solubility in water and WVP of the films
Moisture content was determined by measuring the weight loss of the film, after drying it in an oven at 105 °C until the weight was constant. The results were expressed as a percentage of the moisture content of the samples.
Solubility in water was calculated as the percentage of dry matter of the solubilized film after immersion for 24 h in water at 25 °C according to the method described by Gontard, Duchez, Cuq, and Guilbert (1994).
The water vapor permeability (WVP) tests of the films were performed following the E96-95 ASTM standard method (ASTM, 1995b). Each sample was placed and sealed over the circular opening of a permeation cell containing anhydrous calcium chloride (0% RH). The cells were then conditioned into desiccators with a saturated sodium chloride solution (75% RH) at 25 °C until the samples reached steady-state conditions and then the cells weight were measured at 48 h.
2.10. Thermal analysis of the films
Thermogravimetric analysis was performed to study the degradation characteristics of the films. The thermal stability of each sample was determined using a thermogravimetric analyzer (TGA) (TA-60WS, Shimadzu, Kyoto, Japan) based on the Zainuddin, Ahmad, Kargarzadeh, Abdullah, and Dufresne (2013). Samples (8–10 mg) were heated from 30 °C to 600 °C at a heating rate of 10 °C/min. A flow of 50 mL min−1 of nitrogen was used.
2.11. Statistical analysis
Analytical determinations for the samples were performed in triplicate and standard deviations were reported, except for thermal analysis. Means were compared by Tukey's test at a 5% level of significance by analysis of the variance (ANOVA).
3. Results and discussion
3.1. Characterization of fibers
Fig. 1 shows the photographs of the milled barley husk fiber (Fig. 1A), fiber treated with alkali (Fig. 1B), and bleached fiber (Fig. 1C). The milled barley husk fiber presented a brown colour (Fig. 1A); after the alkali treatment there was a reduction in tone, presenting a brown-orange colour (Fig. 1B). After the treatment of bleaching, the material presented a completely white colour (Fig. 1C). Johar and Ahmad (2012) applied alkali and bleaching treatments on rice husk and also found a reduction in the tonality of the fiber after these treatments. These authors reported that the color changes were due to removal of the lignin and hemicellulose. The white color observed in the final product was an indication of a high purity of cellulosic material; however, further analysis is needed, such as chemical composition, morphology, crystallinity, and functional groups for their characterization.
The milled barley husk fiber presented 40.8% cellulose, 22.0% hemicelluloses and 25.0% lignin. With the bleaching treatment there was an increase in cellulose content (75.0%), with a reduction of the hemicellulose content (13.0%), and lignin (10.0%) in the bleached fiber composition compared to the milled husk fiber; therefore the treatment used for the purification of the cellulose was effective.
The morphology of the milled barley husk fiber, fiber treated with alkali and bleached fiber, in a magnitude of 50×, 200× and 2000× is shown in Fig. 2. The milled barley husk fiber showed a more compact structure with an irregular surface and protuberances (Fig. 2A–C) when compared to the fiber surface treated with alkali (Fig. 2D–F). In the fiber treated with alkali there was a structural disintegration and reduction in the compacted material. This disintegration of the structure promoted by alkaline treatment is due mainly to the partial removal of hemicellulose and lignin (Alemdar & Sain, 2008). The removal of hemicellulose and lignin is consistent with the results of the colour and chemical composition of the fibers. The bleached fibers showed an individualized and fibrous structure, with a rod cell and an elongated shape (Fig. 2G–I), with average diameter of 8 μm.
Fig. 3 shows the relative crystallinity and the X-ray diffraction pattern of the milled barley husk fiber, fiber treated with alkali and bleached fiber, which showed three peaks (15.4°, 22.7° and 34.5°). Zainuddin et al. (2013) studied bleached hibiscus fibers and found similar peaks to this work (16.0°, 22.5° and 34.5°), which are characteristic peaks of lignocellulosic materials. According to Rosa et al. (2010) the diffraction peaks near 2θ = 16° and 2θ = 22° were typical of cellulose I and indicate a high crystallinity after bleaching step.
The milled barley husk fiber presented a relative crystallinity of 38.9% and an increase of the relative crystallinity to 56.4% and 73.2% when the alkali treatment and bleaching were applied, respectively (Fig. 3). Zainuddin et al. (2013) also reported an increase in the relative crystallinity of hibiscus fibers after alkali and bleaching treatments, of 60.8% fiber without treatment to 72.8% in the bleached fiber. According to Li, Fei, Cai, Feng, and Yao (2009) an increase in the relative crystallinity is due to the removal of non-cellulosic materials.
The spectra of the milled barley husk fiber, fiber treated with alkali and bleached fiber analyzed by FTIR, are shown in Fig. 4a and b. Fig. 4b shows the magnification of the region between 1700 cm−1 and 700 cm−1. The spectra showed bands characteristic of the functional groups of the components of lignocellulosic fibers (cellulose, hemicellulose and lignin). These components mainly presented in their groups of alkanesaromatic structures and different functional groups such as ester, ketone and alcohol. The regions observed at 3330 and 2896 cm−1 were related to the OH and CH groups. The OH reflect the hydrophilic tendency of the fibers. The band at 2896 cm−1 is typical of the stretching vibrations of the CH bonds in hemicelluloses and cellulose (Tibolla, Pelissari, & Menegalli, 2014). It was noted that the fibers after alkali and bleaching treatments had higher intensity of the bands at 3330 cm−1 and 2896 cm−1 than milled barley husk fiber spectra (Fig. 4a), as a result of the hemicellulose and lignin removal (Rosa et al., 2010).
The milled barley husk fiber presented a band at 1240 cm−1 associated with the guaiacyl ring breathing with stretching CO (Zuluaga et al., 2009). It was also observed that this band showed a lower intensity in the fiber treated with alkali (Fig. 4a) and bleached fiber, as compared to milled husk fiber (Fig. 4a). This result suggests that the lignin had partially removed the cellulose fibers in the alkali and bleaching treatments, which was confirmed with the chemical composition, micrographs (Fig. 2), and the increase of the relative crystallinity of the fibers (Fig. 3).
The bleached fiber spectrum also showed bands at 1160 cm−1 and 1105 cm−1 with intensities greater than the spectra of the milled husk fiber and fiber treated with alkali (Fig. 4b). The band at 1160 cm−1 was related to the C3 carbon vibrations of cellulose. The band at 1105 cm−1 refers to the vibration of the cellulose glycosidic bonds COC.
The presence of the cellulose can also be detected from bands at 1051 cm−1, 1022 cm−1and 896 cm−1. The band at 1051 cm−1 is attributed to CO stretching of cellulose, hemicelluloses, and lignin (Rosa et al., 2010). The band 1022 cm−1 is associated with the COC pyranose ring skeletal vibration gives. The bleached fiber spectrum showed increased intensity of these bands when compared to the spectra of milled barley husk fiber and fiber treated with alkali (Fig. 4b). The greater intensity in this region shows higher cellulose content. The peak in region 896 cm−1 of the milled barley husk fiber, fiber treated with alkali and bleached fiber (Fig. 4b) refer to typical cellulose structures, as reported by Alemdar and Sain (2008).
In the characterizations of the fibers it was observed that the bleached fiber showed high purity of cellulose and therefore was used as a reinforcement of films, and called as cellulose fibers throughout the text.
3.2. Microstructure of the films
Surface and cross section morphology of native and oxidized (2.0% active chlorine) starches films, with and without cellulose fibers, are shown in Fig. 5. Comparing the starch films without fibers, it was observed that the films with native starch showed more pores in its external surface and small cracks in the internal cross-section (Fig. 5A and B), while the oxidized starches films presented surface and cross section with greater homogeneity and continuity (Fig. 5E and F).
The homogeneity of the oxidized starch film was attributed to the effect of depolymerization of the starch molecules which occurs due to oxidation. The depolymerization of the starch molecules allows greater interaction between the plasticizer and the starch. In addition, the oxidized starch with 2.0% active chlorine had a low retrogradation, as verified in a previous study (Halal et al., 2015); thus the chains have difficulty to re-associate, increasing the free space between the glucose molecules, which facilitate the interaction between constituents of the films.
The film of oxidized starch with 2.0% active chlorine reinforced with 20% cellulose fibers (Fig. 5G and H) showed a more homogeneous surface and a more aleatory distribution of fibers when compared to the film of native starch and 20% fiber (Fig. 5C and D). The homogeneity of films with oxidized starch and cellulose fibers may be due to the depolymerization and low retrogradation of oxidized starch, which gives a good dispersion of the cellulose fiber in the starch matrix, avoiding agglomeration. The distribution of fibers in the cross section of film was aleatory, uniform and parallel to the surface (Fig. 5D and H).
3.3. Thickness and mechanical properties of films
Table 1 shows the thickness values of films. The values of films thicknesses ranged from 0.076 mm to 0.151 mm (Table 1). Comparing the starch films without addition of fibers, it was observed that the films with oxidized starches had a higher thickness when compared to native starch film (Table 1). In oxidized starches, the formation of more carbonyl or carboxyl depends on the reaction conditions. In this study, the oxidation reaction took place under alkaline pH conditions, which favors the formation of carboxyl groups (Wurzburg, 1986). This suggests that interactions occurred between the water molecules and the hydroxyl groups of starch molecules and the carboxyl (COOH) groups. Thereby it makes the film less hydrophobic than if there were a predominant presence of carbonyl groups (CO) or hydroxyl groups (OH). These interactions can retain more water during the drying process, and thus to increase the thickness of the films prepared with oxidized starches.
The results are the means of three determinations. Values with different letters in the same column are significantly different (p < 0.05).
b
Active Cl: Concentration of active chlorine (g Cl/100 g barley starch, d.b.).
The addition of fibers increased the thicknesses due to a higher presence of dry solids after the drying process when compared to the film without cellulose fibers; furthermore, the presence of fibers promoted small protuberances on the surface of the matrix increasing the heterogeneity.
Table 2 shows results of the tensile strength, elongation at break and Young's Modulus of the native and oxidized starches films with and without cellulose fibers conditioned at two different relative humidity (RH = 65% and RH = 85%). The desired properties of a package depend on the application. In general, packages that do not require high elongation need a higher tensile strength to provide structural integrity to packaged products. In others situations, a package with high flexibility are desirable to wrap and protect from the environment (Gontard et al., 1994).
Table 2
Filmsa
Fiber (%)
Tensile strength (MPa)
Elongation (%)
Young's modulus (Mpa)
65% RH
85% RH
65% RH
85% RH
65% RH
85% RH
Native starch
0
4.68 ± 0.12de,ns
4.85 ± 0.32hi
71.7 ± 6.8a,*
34.4 ± 3.4ª
25.81 ± 2.96g,*
63.33 ± 5.30e,
10
8.32 ± 0.32b*
5.30 ± 0.01gh
26.7 ± 0.2c*
17.8 ± 0.6bc
155.38 ± 2.14d,*
67.41 ± 1.64e
20
8.33 ± 1.04b,ns
8.85 ± 0.12cd
22.7 ± 1.7cd,*
11.9 ± 0.1d
175.40 ± 22.66d,ns
130.88 ± 14.93cd
Oxidized starch (1.0% active Clb)
0
4.36 ± 0.48de,*
3.76 ± 0.04i
24.2 ± 3.9cd,ns
18.6 ± 1.0b
70.99 ± 0.57f,*
95.22 ± 1.18de
10
5.99 ± 0.71cd,*
7.93 ± 0.20cde
8.6 ± 0.9e,*
17.6 ± 0.9bc
158.08 ± 0.59d,*
163.53 ± 2.52c
20
7.23 ± 0.47bc,ns
7.77 ± 0.94cde
9.6 ± 0.3e,*
15.6 ± 0.7bcd
114.19 ± 2.88e,*
132.99 ± 1.58cd
Oxidized starch (1.5% active Cl)
0
11.08 ± 0.30a,*
7.69 ± 0.58de
24.2 ± 1.5cd,*
15.0 ± 0.9bcd
288.63 ± 4.59b,*
168.66 ± 14.85c
10
10.37 ± 0.27a,*
11.47 ± 0.04b
10.2 ± 1.7e,*
12.9 ± 0.8cd
208.77 ± 7.21c,*
144.59 ± 1.57c
20
11.76 ± 0.49a,*
14.49 ± 0.29a
7.5 ± 0.9e,*
10.6 ± 0.9d
447.42 ± 25.78ª,*
267.16 ± 15.69b
Oxidized starch (2.0% active Cl)
0
4.23 ± 0.17e,*
6.38 ± 0.06fg
47.1 ± 2.8b,*
13.8 ± 0.3bcd
45.52 ± 2.23fg,*
85.98 ± 10.35e
10
4.79 ± 0.14de,*
7.20 ± 0.58ef
23.9 ± 0.0cd,*
11.2 ± 0.6d
57.28 ± 0.38fg,*
297.52 ± 22.61a
20
8.60 ± 0.50bns
8.87 ± 0.21c
18.5 ± 1.8d,*
11.3 ± 0.4d
144.14 ± 3.39de,*
169.75 ± 6.63c
a The results are the means of three determinations. Values with different letters in the same column are significantly different (p < 0.05).
* and ns, significant and not significant, respectively, by t test (p ≤ 0.05) between 65% RH and 85% RH.
b Active Cl: Concentration of active chlorine (g Cl/100 g barley starch. d.b.).
When comparing the films without fibers, conditioned at relative humidity (RH) of 65% or 85%, only the oxidized starch film with 1.5% active chlorine showed a higher tensile strength (Table 2). This may be due to the formation of carboxyl and carbonyl groups, as well as the partial depolymerization of the oxidized starch.
According to Zamudio-Flores, Vargas-Torres, Pérez-González, Bosquez-Molina, and Bello-Pérez (2006), the presence of the carbonyl and carboxyl groups in the oxidized starch can produce hydrogen bonds with the hydroxyl groups of the amylose and amylopectin molecules. These bonds may provide greater structural integrity in the polymer matrix and thus enhance the tensile strength of the films. However, the oxidation of starch at high levels results in a higher depolymerization of starch molecules. With this, probably the tensile strength of the oxidized starch film with 1.0% active chlorine did not differ from the film of native starch due to the insertion of carbonyl and carboxyl groups. These have not been sufficient for a formation of hydrogen bonds with the hydroxyl groups of amylose and amylopectin molecules. On the other hand, the oxidized starch with 2.0% active chlorine, although it had more presence of these groups, probably had a high depolymerization, resulting in a reduction in tensile strength of the film. The oxidized starch with 1.5% active chlorine had an intermediate formation of carbonyl and carboxyl groups and a lower depolymerization than the oxidized starch with 2.0% active chlorine. Thus, the oxidized starch with 1.5% active chlorine had most suitable characteristics for application in films, where it is a desired superior tensile strength.
The films of native starch and oxidized starches with 1.0% and 2.0% active chlorine, reinforced with 20% cellulose fibers (conditioned at 65% RH), exhibited a higher tensile strength when compared to films with 0% to 10% cellulose fibers (Table 2). However, the addition of fibers in the oxidized starch film with 1.5% active chlorine did not affect this mechanical property. The native and oxidized starch films (1.0%, 1.5% and 2.0% active chlorine) with 20% cellulose fibers and conditioned at 85% RH had higher tensile strength when compared to films without cellulose fibers (Table 2). Müller et al. (2009b) developed films of native cassava starch with 0, 10, 30 or 50% of cellulose fibers, and found that the tensile strength of films progressively increased with the addition of these fibers. These results reflect the chemical and structural compatibility between starch and cellulose fibers, allowing a strong adhesion between the polymer matrix and the fiber (Ma et al., 2005; Müller et al., 2009a).
The film of oxidized starch with 1.5% of active chlorine and filled by cellulose fibers and conditioned at 85% RH had higher tensile strengths than films conditioned at 65% RH (Table 2). Thus, it suggests that the addition of cellulose fibers in these oxidized starch films is an effective way to stabilize its structure in greater relative humidity. The results showed that the relative humidity can influence the mechanical properties of the starch films.
The films of native and oxidized starches made with fibers, despite the fiber concentration and relative humidity, showed lower elongation when compared to the films without fibers (Table 2). Analyzing the results of tensile strength and elongation may suggest that the cellulose fibers interact strongly with the starch matrix which restricts the movement of the chain of the polymer matrix (Lu, Weng, & Cao, 2005). The same behavior was found in films of cassava and maize native starches reinforced with cellulose fibers (Ma et al., 2005; Müller et al., 2009b).
Analyzing the films conditioned at 65% RH, it was observed that the addition of 20% cellulose fibers as reinforcement in starch films provided an increase in Young's module (Table 2). The maximum value Young's module was observed in the film with starch 1.5% of active chlorine and 20% cellulose fiber, an increase of 155% on the starch film with 1.5% active chlorine without fibers. The films added with 20% cellulose fibers and conditioned in the 85% RH showed higher Young's module when compared to films without the addition of cellulose fibers (Table 2). This behavior was also observed in studies on cassava starch films reinforced with fibers (Müller et al., 2009a). This significant increasing of films rigidity has been attributed to the similarity between the chemical structures of cellulose and starch (Ma et al., 2005).
3.4. Moisture, water solubility, and water vapor permeability (WVP) of the films
The moisture, solubility in water and WVP of films of native and oxidized starches with and without cellulose fibers are shown in Table 1. The moisture of films without the addition of fibers varied from 20.2% to 24.3%, and the highest values were found in films of oxidized starches with higher levels of active chlorine (1.5% and 2.0% active chlorine) (Table 1).
The films remained intact after being immersed in water for 24 h under constant stirring. There also was an increase in the water solubility of the films made with starch oxidized in higher level of active chlorine and without fibers, ranging from 16.0% (native starch) to 24.4% (2.0% active chlorine oxidized starch) (Table 1). This increase in the moisture and water solubility of the oxidized starches films can be a result of the inclusion of carboxyl groups in the starch chain, which produces repulsion forces between polymer chains (Vanier et al., 2012) and allows greater mobility of water, giving to the film higher moisture content and solubility in water than native starch films.
The cellulose in the starch films promoted water solubility decreasing (Table 1). Curvelo et al. (2001) and Müller et al. (2009a) also observed that the addition of cellulose fibers in the films of maize and cassava starches decreased their solubility in water. These authors attributed these results to lower hygroscopicity of the fibers in relation to starch. Moreover, the fibers interact with the hydrophilic sites of the starch, replacing the starch with water linkages (Averous, Fringant, & Moro, 2001).
The water vapor permeability of films with no fibers ranged from 2.29 to 3.38 g mm/m2 day kPa, and the films of oxidized starches showed a higher WVP when compared to the film of native starch (Table 1). According to Talja, Helén, Roos, and Jouppila (2007), the WVP depends of the water solubility coefficient of the film, the water diffusion rate and the partial pressure of the water vapor. The oxidation of starch included carbonyl and carboxyl groups in the barley starch resulting in films with higher moisture and water solubility. As mentioned previously, the oxidation promotes repulsion forces between polymer chains (Vanier et al., 2012), what increases the inter chain spacing allowing to migrate more water vapor through the film. The results of WVP are consistent with the moisture and water solubility of the starch films without cellulose fibers (Table 1).
There was no linear relation between the thickness and the WVP of the films without fibers. However, Mali, Grossmann, García, Martino, and Zaritzky (2004) reported that the WVP of yam starch films linearly increase as the film thickness raise. Cuq, Gontard, Cuq, and Guilbert (1996) also reported an increased in WVP with the increase of film thickness in hydrophilic films.
The addition of cellulose fibers in the native starch films increased the WVP as the concentration of cellulose fibers increased (Table 1). However, the addition of 20% cellulose fibers in films with oxidized starches with 2.0% active chlorine decreased the WVP as compared to the films with 0% and 10% cellulose fibers (Table 1). Therefore, the addition of 20% cellulose fibers was sufficient to provide a physical barrier through the interaction of the fiber with the polymeric matrix of oxidized starch and plasticizer, and thus difficult for the water permeation.
The addition of fibers had a positive effect in reducing the water vapor permeability only in films made with oxidized starches with 2.0% of active chlorine. Some authors attribute the reduction in the water vapor permeability of the starch and cellulose films to the lower hydrophilicity of cellulose in comparison to the starch due to a high crystallinity of the cellulose, since the moisture transfer occurs mainly through non-crystalline areas (Bilbao-Sainz, Avena-Bustillos, Wood, Williams, & Mchugh, 2010).
3.5. Thermal properties of the films
Thermogravimetric analysis was used to characterize the films, since it provides the thermal degradation temperatures of them as well as the effect of the cellulose on their thermal stabilities. The onset decomposition temperature, thermal differential analysis (DTA) peaks and residues percentage at 200 °C, 400 °C and 600 °C of the films are shown in Table 3.
Active Cl: concentration of active chlorine (g Cl/100 g barley starch, d.b.).
The films had an initial mass loss of up to 100 °C, which is due to evaporation of water. The films presented an onset decomposition temperature above 135 °C (Table 3). TGA studies were further supported by DTA peak values as shown in Table 3. Five DTA peaks ranging from 202 °C to 417 °C were observed in native starch films without addition of fibers (Table 3), which are related to the decomposition of glycerol (peak 202 °C and 242 °C) and starch (peaks 314 °C, 327 °C, 417 °C). According to Garcia, Ribba, Dufresne, Aranguren, and Goyanes (2009), the addition of glycerol in the starch composite films decreases their thermal stability, and according to Lawal et al. (2005), the glycerol degradation temperatures are in the range of 120 °C to 300 °C. The total degradation of the starch occurs in temperatures above 300 °C (Machado et al., 2014). For oxidized starches film without the addition of cellulose fiber, the DTA peaks were obtained also related to glycerol (ranging from 166 °C to 266 °C) and starch (ranging from 310 °C to 469 °C) (Table 3).
The starch films without the addition of fibers presented higher residues percentage in the oxidized films at temperatures of 200 °C, 400 °C and 600 °C (Table 3), which shows a low thermal stability of oxidized starches films as compared to native starch films (Table 3). It can be attributed to the reduced crystallinity and the lower enthalpy (ΔH) of the oxidized barley starches when compared to that of native barley starch, as verified in our previous study (Halal et al., 2015). The oxidation caused weakening of starch granules structure due to the partial degradation of starch molecules in the crystalline lamellae. Consequently, less energy was required to decompose the oxidized starches and the films made with oxidized starches.
Starches films with fibers had higher initial decomposition temperature and one additional DTA peak, when compared to films without addition of fibers (Table 3). The higher initial decomposition temperature of the starch film with fibers suggests that the presence of fibers increases the thermal stability of the film, probably due to greater complexation of glycerol with cellulose. The cellulose and hemicellulose degrade at temperatures around 250 °C and 370 °C; and the degradation of lignin occurs above these temperatures (Zainuddin et al., 2013). The addition of fibers to the films of starch showed a positive effect on their thermal stability (Table 3). Ma et al. (2005) studied starch packaging reinforced with cellulose fibers and also found a better thermal stability of the package when compared to those without fibers. The increased in the thermal stability of the starch films with cellulose can be attributed to the high relative crystallinity of the cellulose (Fig. 3), as it presents high chain packing organization. Kim, Eom, and Wada (2010) evaluated the thermal decomposition of three types of cellulose and found that the thermal stability of the pulp increased with the increase of the crystallinity of the cellulose. Reddy and Rhim (2014) developed agar-based films reinforced with nanocellulose isolated from mulberry pulp and reported that the agar and nanocellulose films exhibited slightly higher thermal stability than the agar films without the addition of nanocellulose. In study performed by Khan et al. (2012) on chitosan-based biodegradable films reinforced with nanocellulose, reported that the addition of fibers did not change the thermal stability of chitosan films.
4. Conclusion
Cellulose fibers from barley husk were obtained with 75% purity and high crystallinity. The properties of films are dependent on the degree of oxidation of the starch and concentration of fibers used as reinforcement. The morphology of the films of oxidized starches, without the addition of fibers, was more homogeneous as compared to the films of native starch. The use of oxidized starch with 1.5% active chlorine increased the tensile strength of films. The addition of cellulose fibers in native and oxidized starches films increased the tensile strength, and decreased the elongation and moisture. In the films of oxidized starches, the cellulose fiber reduced their solubility. The addition of fibers increased the complexation of glycerol with cellulose and increased the thermal stability of the films.
Depending on the desirable industrial application, the packing requires different mechanical and water solubility properties. The results indicate that the oxidized starch and cellulose fibers from barley have a good potential for use in packaging. However, more studies are needed to assess their barrier action against oxygen and their performance in different types of packaging and food systems.
Acknowledgments
We would like to thank CAPES, CNPq, CsF, FAPERGS, SCT-RS and Pólo de Inovação Tecnológica em Alimentos da Região Sul.
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OH and C
O (Zuluaga et al., 2009). It was also observed that this band showed a lower intensity in the fiber treated with alkali (Fig. 4a) and bleached fiber, as compared to milled husk fiber (Fig. 4a). This result suggests that the lignin had partially removed the cellulose fibers in the alkali and bleaching treatments, which was confirmed with the chemical composition, micrographs (Fig. 2), and the increase of the relative crystallinity of the fibers (Fig. 3).
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