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Characterization of edible biocomposite films directly prepared from psyllium seed husk and husk flour
Food Packaging and Shelf Life 20 (2019) 100299
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Characterization of edible biocomposite films directly prepared from psyllium seed husk and husk flour Annamária Tóth, Katalin Halász
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University of Sopron, Simonyi Karoly Faculty, Institute of Wood Based Products and Technologies, 4 Bajcsy-Zsilinszky, 9400, Sopron, Hungary
A R T I C LE I N FO
A B S T R A C T
Keywords: Plantago psyllium Edible film Glycerol Poly(ethylene glycol) Biocomposite
The mucilage obtained from Plantago seeds is a promising raw material for producing edible films or coatings; however, its complete separation from the seeds is rather difficult. To avoid the difficulties of the extraction and taking advantage of the strong interfacial interactions between the seed husk and the mucilage, we examined the possibility of preparing edible biocomposite films directly from psyllium husk and husk flour. The results showed the husk and husk flour are both suitable for film formation. To reduce the brittleness of the films, two plasticizers, glycerol and poly(ethylene glycol) (Mw = 400 g/mol) were used and both plasticizers improved the flexibility of the films. According to the tensile test, the particles acted as reinforcements in the polysaccharide matrix. ATR-FTIR, solubility, water vapor absorption and water vapor permeability tests were conducted to further examine the interactions of the components and the hydrophilic nature of the films. The tests revealed that biocomposite films directly obtained from psyllium seed husk and husk flour can be novel edible films for the food and food packaging industry.
1. Introduction
standing films as well which makes them a promising material for active or active component carrier edible films or coatings. These versatile seed gum mucilages can be extracted from several kinds of seeds like chia (Dick et al., 2015), basil (Mohammad Amini, Razavi, & Zahedi, 2015), flax (Ding, Qian, Goff, Wang, & Cui, 2018; Kaewmanee et al., 2014), sage (Razavi, Amini, & Zahedi, 2015), Lepidium perfoliatum (Koocheki, Taherian, Razavi, & Bostan, 2009; Seyedi, Koocheki, Mohebbi, & Zahedi, 2014), cress (Behrouzian, Razavi, & Karazhiyan, 2013; Jouki, Khazaei, Ghasemlou, & HadiNezhad, 2013; Karazhiyan, Razavi, & Phillips, 2011), mesquite (Estévez et al., 2004) and plantago seeds (Banasaz, Hojatoleslami, Razavi, Hosseini, & Shariaty, 2013; Fischer et al., 2004; Neto, de Cássia Bergamasco, de Moraes, Neto, & Peralta, 2017). Our research focused on Plantago psyllium seed and seed husk. Plantago psyllium is the member of nearly 200 species of the Plantago genus (Yin et al., 2012). It is a common plant growing in extreme climatic conditions as well, so it can be grown all over the world, which makes its cultivation economic and the seed production for different purposes to be at low cost. Psyllium seed has been used as a herb for constipation for a long time, but, its wide range of use has been discovered only in the few past years. Recently, psyllium seed husk has been utilized in more and more products. Primarily it is used as mechanical laxative and dietary fiber source in food industries (Cui, 2000). It is used in pharmaceuticals, too, for its special
Biorenewable polymer based materials offer a number of significant advantages over the traditional synthetic materials in terms of ecofriendliness, biodegradability and easy availability. There is an increasing demand to use biobased and biodegradable films in the food packaging sector as well. Using edible films and coatings in the food and food packaging industry can decrease the amount of the synthetic polymeric materials and also extend the shelf life of the products. In the past years, much research has been devoted to investigating the characteristics and industrial applicability of various polysaccharides as edible films. Besides the most commonly used starch and non-starch carbohydrates such as cellulose derivatives, pectin, chitosan and alginate, gums extracted from plant seeds can be also efficiently used in the food and food packaging industry. In general, the seed mucilage consist of a pectinaceous and a hemicellulosic fractions (Soukoulis, Gaiani, & Hoffmann, 2018; Yu et al., 2017), which easily make hydrogen bonds with water molecules, then swells and forms hydrogels in aqueous media (Sandhu, Hudson, & Kennedy, 1981; Tripathi & Mishra, 2013). Due to the extreme water binding capacity the mucilage can be used as thickeners, structurisers, binders, stabilizers, fat substitutes during food processing (Izydorczyk, Cui, & Wang, 2005; Soukoulis et al., 2018). They are able to form self-
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Corresponding author. E-mail address: [email protected] (K. Halász).
https://doi.org/10.1016/j.fpsl.2019.01.003 Received 11 June 2018; Received in revised form 5 December 2018; Accepted 10 January 2019 2214-2894/ © 2019 Elsevier Ltd. All rights reserved.
Food Packaging and Shelf Life 20 (2019) 100299
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After determining the dry matter contents (by drying to constant weight at 103 °C for 24 h), a plasticizer, glycerol or PEG400 were added to the mixtures. The amounts of plasticizers were 50, 100 and 200% w/w dry weight. The mixtures were homogenised with magnetic stirrer for 20 min. To obtain homogeneous films bubbles were removed from the solutions prior to forming a film. The different solutions were poured onto plastic trays, and dried for 48 h at 50 °C. Finally, the obtained PG, PH and PHF films were peeled off the plates and were stored in sealed bags until use.
characteristics, especially in the treatment of colon carcinoma, irritable bowel syndrome, obesity, constipation, diabetes, high cholesterol levels, and atherosclerosis (Mishra, Sinha, Dey, & Sen, 2014). The extractable polysaccharide hydrogel from Plantago seeds is made of arabinoxylanes with a xylene backbone (1,4-linked-ß-D-xylopyranose) and side chains of arabinose, xylose and uronic acid (Fischer et al., 2004; Guo, Cui, Wang, & Young, 2008; Sandhu et al., 1981; Tripathi & Mishra, 2013; Yu et al., 2017). The psyllium seed mucilage has a high molecular weight and highly branched structure with unusual linkage (Yu et al., 2017). Although, there are only a few research studies regarding the use of psyllium seed mucilage to form film, mucilage obtained from psyllium seeds could be an attractive material. It could be used as edible films, coatings and as a carrier for active agents to increase the shelf-life of food products as shown by Ahmadi, Kalbasi-Ashtari, Oromiehie, Yarmand, and Jahandideh, (2012), who prepared films with glycerol plasticized psyllium, Banasaz et al. (2013) who developed an ascorbic acid loaded, antioxidant coating for apples and ur Rehman et al. (2015), who created an antifungal coating with garlic extract for mandarin oranges. Since it is quite difficult to separate the gel fraction from the seed the aim of our study was to investigate the possibility of creating psyllium seed based films directly from the psyllium seed husk and husk flour and to examine the effect of these particles on the mucilage matrix film properties. The aim of this research was also to study the effect of different proportions (50, 100, 200% w/w) of plasticizers of two types, glycerol and poly(ethylene glycol), on the properties of the psyllium films.
2.2.2. Surface morphology Surfce morphology of the psyllium seed, seed husk, husk flour and the different films was examined by scanning electron microscopy (SEM) (Hitachi S-3400 N). The SEM images were obtained at an accelerating voltage of 15 kV. 2.2.3. Fourier-transform infrared spectroscopy (FT-IR) The absorption spectra were conducted in the analytical infrared range (400–4000 cm¯¹) with total attenuated reflectance (ATR). After the samples were degreased, dusted and cut to the appropriate size, the samples were placed in the test area of the FT-IR spectrophotometer (JASCO FT/IT 4100). The source of light was standard, the scanning speed was 2 mm/s with a resolution of 4 cm¯¹. 2.2.4. Water vapor absorption (WVA) The water vapor absorption test took place in a climate controlled environment of a climatic chamber at 25 °C with 75% relative humidity for 72 h. The amount of the water vapor absorbed by the films was determined by the difference between the weight of the samples dried previously at 103 °C for 24 h and the weight of the samples following the test, expressed in percentages. The result includes the average weight of the samples taken from two different locations.
2. Materials and methods 2.1. Materials Plantago psyllium seeds were supplied by GreenField Sp. Z.o.o. Sp.k.(Poland); seed husk and husk flour were obtained from Naturganik Kft. (Hungary). The husk flour and the husk particle size distribution were determined by sieve analysis (Table 1). Glycerol (99.6%, Mw = 92.1 g/mol, 1.26 g/cm3) and poly(ethylene glycol) PEG400 (Mw = 380–420 g/mol, 1.15 g/cm3) were purchased from Molar Chemicals Hungary Kft.
2.2.5. Water vapor permeability (WVP) The water vapor permeability of each film was determined according to the method described by Sanyang, Sapuan, Jawaid, Ishak, and Sahari, (2015). Film were cut, adjusted and sealed in a cup containing dried silica gel. Cups were weighed and placed in climate chamber at 25 °C with 75% relative humidity for 72 h. The water vapor permeability (WVT) was calculated using the following Eq. (1):
2.2. Methods
WVP =
2.2.1. Film preparation Films were made from psyllium seed mucilage (PG), psyllium husk (PH) and psyllium husk flour (PHF). Psyllium seed mucilage for film preparation was obtained by hot water extraction. Briefly 40 g seed were swollen in 2000 ml of hot water for 2 h. The mixture was poured into and forced through a polyamide net. The obtained mucilage was collected and used for film-forming without drying. For PH and PHF films 20 g of psyllium seed husk or husk flour was dispersed in 1600 ml of distilled water and mixed at 200 rpm with a magnetic stirrer for 40 min at 70 °C. The mixtures were let to cool to room temperature.
2.2.6. Film solubility in water Water solubility of the film samples was determined according to the method described by Ahmadi et al. (2012) with slight modifications. Film samples were uniformly cut (2 cm × 2 cm), dried at 103 °C for 24 h and weighed to the nearest 0.0001 g to determine their initial dry weight. Film samples were then placed in 50 ml of distilled water (23 °C for 3 or 6 h) under continuous stirring (600 rpm) using magnetic stirrer. After that, undissolved films were filtered through Whatman No. 1 filter paper and dried at 103 ° C until they reached a constant weight to determine their final dry weight. The water solubility (WS, %), expressed as a percentage was calculated using the following Eq. (2):
psyllium seed husk
sieve hole size range (mm)
content (%)
sieve hole size range (mm)
content (%)
1.0-0.5 0.5-0.2 0.2-0.125 0.125-0.1 0.1-0.05
1% 33% 34% 14% 18%
2.0-1.0 1.0-0.5 0.5-0.2 0.2-0.125 0.125-0.1
5% 61% 31% 3% 1%
(1)
Where m (g) is the weight without test cup, d (mm) is the film thickness, A (m2 ) is the area of the film exposed, t (days) is duration for permeation, and P (Pa), the pressure difference (here 3170).
Table 1 Fraction analysis of psyllium seed husk and husk flour. psyllium seed husk flour
m×d A×t×P
WS (%) =
(W0 − WF ) * 100 W0
(2)
where W0 is the initial dry weight of the film and WF is the final weight of the dried undissolved film. All the experiments were repeated three times, and their arithmetic averages were reported. 2
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Fig. 1. Photo and SEM images of psyllium seed (a – x45, b – x500), seed husk (c– x45, d – x500) and husk flour (e– x45, f – x500) under different magnifications.
mechanism of mucilage release of chia seed coat described by de la Paz Salgado-Cruz (2013) can be presumably applicable to psyllium seed and seed husk due to their similar nature. Based on their observations the seed releases mucilage strands which interact with each other and form a thin film with a dense matrix. After the extraction and drying process, the mucilage from psyllium seeds formed a film with smooth and uniform surface structure as shown in Fig. 2a-c. The larger and smaller particles in the biocomposites films can be observed but without any cracks or openings on the film surfaces. Fig. 2d-I shows the husk particles are blended into the polysaccharide matrix they secreted. A larger husk particle covered by the mucilage is shown in Fig. 2d. Fig. 2e-f shows the flexible seed husk particles with the characteristic fibrous surface in the PH biocomposite film. The plasticizers did not alter the characteristics of the surface morphology. Similar observations was reported by several authors for plasticized seed mucilage films (Jouki, Khazaei, Ghasemlou, & HadiNezhad, 2013; Jouki, Yazdi, Mortazavi, & Koocheki, 2013; Dick et al., 2015; Zhang, Zhao, & Shi, 2016).
2.2.7. Tensile properties To determine the tensile strength, Young’ modulus and elongation at breakpoint of the films tensile test was performed by using an Instron 3345 tensile tester. Five-five samples were fixed between the grips of the device with a gauge length of 36 mm then 20 mm/min crosshead speed was applied. The width of the samples was 10 mm. The measurements were conducted at 23 ± 2 °C, with the relative humidity of 50 ± 5%. 2.2.8. Statistical analysis The differences between mean values were determined by analysis of variance (ANOVA) and Tukey’s multiple range test (with significance level of 0.05). Measurements were performed in triplicate except where stated otherwise. 3. Results and discussions 3.1. Surface morphology The photo and scanning electron microscopic images of psyllium seed, husks and husk flour are shown in Fig. 1. The elliptic cavity with round edges specific to Plantago psyllium seed (Kumar Verma & Bharti, 2017) and a wrinkled seed surface can be seen in Fig. 1a and b. A strongly fibrous structure is observed for the husk particles at higher (x500) magnification. After the film casting and drying process, all of the three mucilaginous mixtures turned into free-standing films. Although the films without plasticizers were less flexible, their structural integrity enabled them to be removed from the casting trays without any break. The photo and SEM images of these films can be seen in Fig. 2. The
3.2. Water vapor absorption As expected the hydrophilic plasticizers increased the water absorption of the films (Fig. 3), however, the poly(ethylene glycol) in 50 w/w% significantly decreased the absorption of PH and PHF films. This observation is in accordance with the change in tensile properties where PEG50 films had significantly higher strength addressed to the developed interactions. Here the same interactions prohibited the interactions of water molecules with the polymer chain or the plasticizer just as in case of water vapor transmission. Applying higher amounts of PEG caused higher WVA due to the higher concentration of free 3
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Fig. 2. Photo and SEM images of PG (a – x75, b – x200, c – x200), PH (d – x75, e – x200, f – x200)and PHF (g – x75, h – x200, i – x200) films under different magnifications.
hydroxyl groups. As glycerol contains even more hydroxyl groups along the polymer chain the glycerol plasticized samples showed more intense moisture uptake that reached the 45% when 200% w/w glycerol was added. Considering the base materials the WVA is dependent on the plasticizer type. Films prepared from husk had the lowest absorption capacity when glycerol was applied; on the other hand adding PEG in higher concentrations PG films showed the lowest absorption. In general PHF films had the highest WVA. The high water vapor absorption capacity is usually not desirable e.g. during storage, however, it can be beneficial if the film is used as the part of an active packaging where the packed good release water vapor. 3.3. Film solubility The solubility of an edible film strongly influences its use. High solubility is required if the film is consumed with the food or the film should dissolve quickly (during cooking) before consumption. Lower solubility is needed if water resistance is necessary during the storage
Fig. 3. Water vapor absorption in the function of plasticizers and film bases.
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the polymer chains due to its large molecular size, which leaves gaps (Haq et al., 2014) and creates more sites for water to interact with the film. Comparing the base materials films prepared from psyllium seed husk showed the highest solubility, with the exception of PH-PEG50, where the solubility was slightly lower after 24 h. PG films showed the lowest water sensitivity after 3 h of immersion. The behavior of plasticized PG films was changed after 24 h and psyllium seed mucilage based films had the highest (p < 0.05) solubility as Figure shows, whilst films prepared from psyllium seed husk flour had the lowest solubility. This could be because the denser structure of the plasticized PG film were disrupt and the water molecules could penetrated into the bulk material. In case of husk and husk flour films the bonding between the insoluble particles and mucilage might prevented further solvation.
3.4. Water vapor permeability (WVP) Mass transfer properties of molecules into edible films are influenced by the structure and compositon of the polymer (Miller & Krochta, 1997). Due to their hydrophilic nature polysaccharide based films show poor water barrier properties. It is known that plasticizers tend to decrease the water vapor barrier properties of polymer films (Cuq et al., 1996; Forssell, Lahtinen, Lahelin, & Myllärinen, 2002; Haq et al., 2014). Hygroscopic plasticizers such as glycerol or poly(ethylene glycol) can increase water sensitivity of the polysaccharide film and thus increase the solubility of water molecules in the polymer. The plasticizers decrease the intermolecular forces and increase the chain mobility in the macromolecules, which cause the reorganization of polymer chains and formation of interchain spaces (free volume) that enhance the diffusion of the water vapor through the film (Ahmadi et al., 2012; Jouki, Khazaei, Ghasemlou, & HadiNezhad, 2013; Jouki, Yazdi, Mortazavi, & Koocheki, 2013; Khazaei, Esmaiili, Djomeh, Ghasemlou, & Jouki, 2014; Sothornvit & Krochta, 2001). Fig. 5 shows the changes in WVP produced with different concentrations of plasticizers. As expected the plasticizers increased the water vapor permeability of the psyllium based films. The results showed that WVP depended on the type of plasticizer and the film base. In general, glycerol increased the permeability of water molecules through the film more than PEG400. A similar observation was reported by Haq et al. (2014) for gum cordia based edible films plasticized by glycerol and poly(ethylene glycol) and by Chinnan and Park (1995) and Park, Weller, Vergano, and Testin, (1993) when PEG was used as a plasticizer for methylcellulose and hydroxypropyl cellulose, respectively. The addition of 50% w/w PEG400 slightly reduced (although not
Fig. 4. Solubility in water in the function of plasticizers and film types.
(Ghasemlou, Khodaiyan, & Oromiehie, 2011; Laohakunjit & Noomhorm, 2004; Nafchi et al., 2017; Pathare, Hastak, & Bajaj, 2013). The films were partially soluble in water irrespective the type of plasticizer (Fig. 4). Although PH and PHF films contained insoluble particles PG film showed the lowest solubility, and it was able to maintain ˜90% of its integrity in water at 25 °C after 3 h, while PH and PHF films showed significantly higher (25 and 23%, respectively) solubility. It can be assumed that the dense structure of the PG film might inhibited the penetration and the attack of water molecules. After 24 h of immersion the solubility of PG film was doubled (21%) and approached the solubility of PH (28%) and PHF (30%) films, however it was still significantly lower (p < 0.05). As expected, and also reported by different authors (Ahmadi et al., 2012; Dick et al., 2015; Jouki, Khazaei, Ghasemlou, & HadiNezhad, 2013; Jouki, Yazdi, Mortazavi, & Koocheki, 2013; Nafchi et al., 2017; Tee, Wong, Tan, & Talib, 2016) for both plasticizers, the solubility of the films was found to increase with increasing concentrations (Fig. 4) due to the hydrophilic nature of plasticizers and seed mucilage. Plasticizers can also promote the solubility of the polymer matrix. The hydrophilic nature of a plasticizer can diminish interactions between biopolymer molecules and increases its solubility, resulting in more water attracted into the polymer matrix and more mobile regions with greater inter-chain distances (Cuq, Gontard, Cuq, & Guilbert, 1996; Haq, Hasnain, & Azam, 2014). Using 50% w/w PEG400, 50% w/w and 100% w/w glycerol the solubility was increased, but it remained under 30% after the 3 h of immersion. This indicates that glycerol and PEG400 could effectively bond to the polymer at these concentrations (which was shown in the mechanical properties as well especially when PEG400 was used). Over time these interactions weakened and after 24 h the degree of solubility reached ˜50%. This value is lower that Dick et al. observed (85%) when chia seed mucilage film was modified with 50% glycerol. At 50% w/w concentrations the solubility of the plasticized films were similar, the difference appeared with higher plasticizer contents, except for 100% w/w, where the solubility was still below 30% after 3 h. Films with higher plasticizer concentrations (100% and 200% w/ w), showed high solubility which can be attributed to the migration and solubility of the plasticizer from the polymer matrix. In general, samples with glycerol showed substantially lower solubility in water even if higher amounts of plasticizers were applied. This indicates that there might be stronger interactions existing between glycerol and the polymer which can hinder the leakage of the plasticizer from the film. The higher solubility of samples containing higher amounts of PEG400 can be related to the inability of PEG400 to insert effectively between
Fig. 5. Water vapor permeability in the function of plasticizers and film types. 5
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significantly) the water vapor permeability of the PH film. The polysaccharide films prepared from PHF and PG showed similar WVP, but the neat films were too brittle to handle on the permeability cups, so these samples could not be compared to the neat ones. For this reason it cannot determined if the interactions with PEG400 caused the lower WVP or the psyllium seed husk particles. According to the results, in general, plasticized PG films showed lower WVP when the plasticizer concentration was higher which could be explained by the fact that PG films contains only the extracted gel and therefore the matrix was able to interact with higher amount of plasticizer compared to PH and PHF films. Comparing the two composite films, PHF films had a lower WVP than PH films. The psyllium seed husk flour particles were presumably more effective physical barriers than the larger husk particles. Applying the large amounts of plasticizers can remarkably increase the WVP (Ahmadi et al., 2012; Ghasemlou et al., 2011; Jouki, Khazaei, Ghasemlou, & HadiNezhad, 2013; Jouki, Yazdi, Mortazavi, & Koocheki, 2013; Ladjevardi, Gharibzahedi, & Mousavi, 2015; Mali, Grossmann, Garcı́a, Martino, & Zaritzky, 2004). Their significant increase can be attributed to the clustering of the plasticizers which results in a discontinuous polymer structure that can further increase the amount of water molecules transmitted through the film. (Cerqueira, Souza, Teixeira, & Vicente, 2012; Mohammad Amini et al., 2015; Piermaria, Pinotti, Garcia, & Abraham, 2009; Yang & Paulson, 2000) 3.5. Mechanical properties Mechanical properties of a film determine its durability and usability for coatings or films. Without plasticizers, psyllium seed husk, husk flour and mucilage were more brittle and did not show a high plastical deformation. Neat film with husk had significantly (p < 0.05) lower stress at the breakpoint and a lower Young’s modulus compared to the neat PH and PG films (Fig. 6). This could be related to the larger and more rigid seed husk fraction (the darker husk parts) which might concentrate stress. Among all the samples examined, neat film prepared from husk flour required the highest load to tear. The small size, therefore the high surface area of the particles and the strong interactions between the filler and the matrix highly improved their strength compared to the PG film. The high reinforcing effect was clearly shown by the significantly higher Young’s modulus as well. Many authors reported that PEG 400 is not an appropriate choice for plasticizing seed gums and other hydrocolloid films, since the films containing PEG400 (from 20 to 100% w/w) become fragile, thick and opaque (Antoniou, Liu, Majeed, Qazi, & Zhong, 2014; Laohakunjit & Noomhorm, 2004; Olivas & Barbosa-Cánovas, 2008; Pongjanyakul & Puttipipatkhachorn, 2007; Razavi et al., 2015). However, we had different observations for films made of psyllium seed. Both glycerol and PEG400 could disrupt the initial intermolecular hydrogen bonds in the polymer and cause new hydrogen bonds to form between glycerol or PEG400 and arabinoxylan chains. The plasticizers could reduce the intermolecular hydrogen bonds between the polymer chains which resulted in increased chain mobility and thus increased deformability. The effects of glycerol and PEG 400 on the mechanical properties are shown in Fig. 6. In contrast to the conclusions of Yang and Paulson (2000), in case of psyllium seed mucilage the dipole strength of PEG400 appears to be enough to disrupt the intermolecular interactions and the thus the plasticizer can fill the area between the chains. Using PEG400 the flexibility increased until 100% w/w plasticizer content. The elongation at the breakpoint was 3.3% for PH-GLY50 and 12.1% for PH-GLY100, 3.9% for PHF-PEG50 and 25.0% for PHF-PEG100 and 1.8% for PGPEG50 and 11.6% for PG-PEG100. While 200% w/w PEG400 had a less effective plasticizing effect on the samples, the EB decreased to 4.6%, 14.8% and 9.3% for PH-PEG500, PHL-PEG200 and PG-PEG200, respectively. When 200% w/w PEG400 was used there was a large
Fig. 6. Effect of plasticizers and particles on the tensile properties.
amount of unbounded poly(ethylene glycol) between the polymer chains, which caused a decrease in flexibility compared to the a samples containing 100% w/w PEG. Although it was observed that PEG400 could reduce the brittleness of the psyllium seed mucilage films, glycerol was more effective in plasticizing as it has been shown for seed gums by different authors earlier as well (Ahmadi et al., 2012; Dick et al., 2015; Khazaei et al., 2014; Mohammad Amini et al., 2015; Razavi et al., 2015). EB increased especially when the samples contained 100 and 200% w/w. Since glycerol molecules are smaller than PEG 400 they can more easily 6
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penetrate into the polymer matrix and interact more efficiently with the arabynoxylan chains which results in more reduced intermolecular forces and more increased polymer chain mobility (Ghasemlou et al., 2011; Khazaei et al., 2014; Razavi et al., 2015; Seyedi et al., 2014). With glycerol there is a huge difference between the plasticizing effect of 50% w/w and 100% w/w glycerol. With 100% w/w glycerol the higher strain at its breakpoint with 139% was observed for samples containing psyllium seed husk. In this case the elongation was significantly higher compared to the films from psyllium seed husk particles (86%) and gel (96%). When the glycerol content was 200% w/w, a further increase in the strain at the breakpoint was observed, although with husk, the difference was not significant. Since the film prepared from the gel contains higher fraction of polymer matrix it can bind more plasticizers compared to PH and PHF films. Thus 200% w/w glycerol doubled the elongation before the breakpoint of PG film from 96% to 200%. A further increase in strain at break might be achieved by adding more glycerol; however, films containing 200% w/w of glycerol became sticky on their surface, which might reduce the usability of the film. There was a slight difference between the plasticizing effects of the plasticizers regarding the psyllium film bases. In general, samples prepared from psyllium seed husk flour became more flexible if PEG400 was used than films made of husk or gel. In contrast, glycerol showed a higher plasticizing effect on samples made from psyllium seed husk and mucilage. It is notable to mention, that although the tensile strengths were reduced due to the presence of the plasticizers (both glycerol and PEG400), 50% w/w poly(ethylene glycol) significantly enhanced the stress at break of PH films (while the strain was improved at the same time) and also a slight (but p > 0.05) improvement was observed in case of PG films. Due to the plasticizers, which enhanced the elongation, the samples showed higher tensile energy absorptions. Samples containing husk and husk particles had higher energy absorption than films prepared from psyllium seed mucilage (except when 200% w/w glycerol was used). Since the matrix and the fillers are unquestionably compatible and there is a strong adhesion between the components, the husk and the husk particles could effectively transfer the stress from the matrix to the filler phase.
Fig. 7. FTIR spectra of neat and plasticized PG, PH and PHF films.
and CeOeC stretching vibrations in glycosidic and pyranose rings (Gómez-Ordóñez & Rupérez, 2011; Kacurakova, Capek, Sasinkova, Wellner, & Ebringerova, 2000; Toğrul & Arslan, 2003). The 896 cm−1 band is the characteristic of (1–4) linkages. The difference between the spectra can be observed which was presumably caused by the different film compositions, since films prepared from husk and husk flour contained the whole mucilage not only the water extractable fraction. The great increase in the intensity of –O-H stretching vibration and CeOeH bending range (due to the presence of free hydroxyl groups) when using high amounts of glycerol is in accordance with the higher water vapor absorption capacity of the films compared to samples plasticized with PEG. The small shifts to lower frequencies, the broadened bands and the band asymmetry can be a sign of the developed Hbonds developed between the plasticizer and the polymer (SadeghiVarkani, Emam-Djomeh, & Askari, 2017; Tee et al., 2016), however, it can be the sign of the excess plasticizer fraction as well which overlaps these regions.
3.6. Fourier-transformational infrared (FT-IR) Fig. 7 shows the ATR-FTIR spectra of the neat and plasticized films. The broad band from 3690 to 3000 cm−1 corresponds to the OeH stretching vibration, resulting from the free, inter and intra-molecular bound −OH groups. This band showed remarkable increase with the glycerol and PEG concentrations for all the plasticized samples. With poly(ethylene glycol) the increment was not that intense compared to glycerol which is obviously due to the higher amounts of hydroxyle groups present in glycerol. In the -C–H stretching region (˜3000-2800 cm−1) we have had the same observation as Haq et al. (2014) regarding the intensity change of these bands for glycerol and poly(ethylene glycol) plasticized PG films. When the PG samples contained the lowest amount of PEG, the intensity of the bands was higher than for the samples of PG-PEG100 or PG-PEG200. According to Haq et al. (2014) this could be explained by the conformational changes which PEG induced in the polymer, and the decreased intensity of the C–H band when a greater amount of plasticizer was used can be due to the aligment of the modified structure along with the poly(ethylene glycol) chain. For films made of PHF and PH this phenomenon was not that prominent, but it could be observed until samples contained 100% w/w plasticizer. The absorption bands at ˜1600 cm−1 and 1416 cm−1 are assigned to the symmetric stretching of the carboxyl group (−COO−) of uronic acids (Timilsena et al. 2016). Bands in the range of ˜1200 -920 cm−1 (with the highest intensity at 1036 cm−1), the characteristic absorptions of natural polysaccharides, corresponds to the CeOeH bending
4. Conclusions The results of this study demonstrated that psyllium seed husk and husk flour are suitable materials for preparing polysaccharide based edible films for food packaging just like psyllium seed mucilage. These raw materials are sustainable, abundant and can be obtained at low cost. Usage of husk and husk flour enables a direct and easy way of filmcasting without the need of the laborious and cumbersome separation of mucilage. The biocomposite films prepared in this way have comparable properties to the pure mucilage films. Without a plasticizer, the films were rigid and brittle except the films made from husk which could be bent without fracturing. Both glycerol and PEG400 increased the elongation at break of the films. Tensile tests revealed that plasticized PH and PHF films were deformable with increased toughness due 7
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to the reinforcing effect of the husk and husk flour particles. The plasticizers increased the hydrophilicity of the films. The water vapor absorption, permeability and solubility were increased especially when the highest amounts of plasticizers were added. Using the appropriate amounts of plasticizers and choosing the appropriate raw material (pure seed mucilage, seed husk or husk flour), the hydrophilic nature of the films can be tuned to the usage of the films. It can be beneficial if the film covering the product is consumed with the product or if the film is used in active packaging systems where they absorb the moisture that develops or they release more active components when liquid forms through their increased solubility.
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Acknowledgements This article was made in frame of the, EFOP-3.6.1-16-2016-00018 – Improving the role of research + development + innovation in the higher education through institutional developments assisting intelligent specialization in Sopron and Szombathely". References Ahmadi, R., Kalbasi-Ashtari, A., Oromiehie, A., Yarmand, M.-S., & Jahandideh, F. (2012). Development and characterization of a novel biodegradable edible film obtained from psyllium seed (Plantago ovata Forsk). Journal of Food Engineering, 109(4), 745–751. Antoniou, J., Liu, F., Majeed, H., Qazi, H. J., & Zhong, F. (2014). Physicochemical and thermomechanical characterization of tara gum edible films: Effect of polyols as plasticizers. Carbohydrate Polymers, 111, 359–365. Banasaz, S., Hojatoleslami, M., Razavi, S. H., Hosseini, E., & Shariaty, M. A. (2013). The Effect of Psyllium seed gum as an edible coating and in comparison to Chitosan on the textural properties and color changes of Red Delicious Apple. International Journal of Farming and Allied Sciences, 18, 651–657. Behrouzian, F., Razavi, S., & Karazhiyan, H. (2013). The effect of pH, salts and sugars on the rheological properties of cress seed (Lepidium sativum) gum. International Journal of Food Science & Technology, 48(12), 2506–2513. Cerqueira, M. A., Souza, B. W. S., Teixeira, J. A., & Vicente, A. A. (2012). Effect of glycerol and corn oil on physicochemical properties of polysaccharide films–A comparative study. Food Hydrocolloids, 27(1), 175–184. Chinnan, M. S., & Park, H. J. (1995). Effect of plasticizer level and temperature on water vapor transmission of cellulose‐based edible films. Journal of Food Process Engineering, 18(4), 417–429. Cui, S. W. (2000). Polysaccharide gums from agricultural products: Processing, structures and functionality. CRC Press. Cuq, B., Gontard, N., Cuq, J. L., & Guilbert, S. (1996). Rheological model for the mechanical properties of myofibrillar protein-based films. Journal of Agricultural and Food Chemistry, 44(4), 1116–1122. de la Paz Salgado-Cruz, M., Calderón-Domínguez, G., Chanona-Pérez, J., Farrera-Rebollo, R. R., Méndez-Méndez, J. V., & Díaz-Ramírez, M. (2013). Chia (Salvia hispanica L.) seed mucilage release characterisation. A microstructural and image analysis study. Industrial Crops and Products, 51, 453–462. Dick, M., Costa, T. M. H., Gomaa, A., Subirade, M., de Oliveira Rios, A., & Flôres, S. H. (2015). Edible film production from chia seed mucilage: Effect of glycerol concentration on its physicochemical and mechanical properties. Carbohydrate Polymers, 130, 198–205. Ding, H. H., Qian, K., Goff, H. D., Wang, Q., & Cui, S. W. (2018). Structural and conformational characterization of arabinoxylans from flaxseed mucilage. Food Chemistry, 254, 266–271. Estévez, A. M., Saenz, C., Hurtado, M. L., Escobar, B., Espinoza, S., & Suárez, C. (2004). Extraction methods and some physical properties of mesquite (Prosopis chilensis (Mol) Stuntz) seed gum. Journal of the Science of Food and Agriculture, 84(12), 1487–1492. Fischer, M. H., Yu, N., Gray, G. R., Ralph, J., Anderson, L., & Marlett, J. A. (2004). The gel-forming polysaccharide of psyllium husk (Plantago ovata Forsk). Carbohydrate Research, 339(11), 2009–2017. Forssell, P., Lahtinen, R., Lahelin, M., & Myllärinen, P. (2002). Oxygen permeability of amylose and amylopectin films. Carbohydrate Polymers, 47(2), 125–129. Ghasemlou, M., Khodaiyan, F., & Oromiehie, A. (2011). Physical, mechanical, barrier, and thermal properties of polyol-plasticized biodegradable edible film made from kefiran. Carbohydrate Polymers, 84(1), 477–483. Gómez-Ordóñez, E., & Rupérez, P. (2011). FTIR-ATR spectroscopy as a tool for polysaccharide identification in edible brown and red seaweeds. Food Hydrocolloids, 25(6), 1514–1520. Guo, Q., Cui, S. W., Wang, Q., & Young, J. C. (2008). Fractionation and physicochemical characterization of psyllium gum. Carbohydrate Polymers, 73(1), 35–43. Haq, M. A., Hasnain, A., & Azam, M. (2014). Characterization of edible gum cordia film: Effects of plasticizers. LWT-Food Science and Technology, 55(1), 163–169. Izydorczyk, M., Cui, S. W., & Wang, Q. (2005). In S. Cui (Ed.). Polysaccharide gums: Structures, functional properties, and applications. Food carbohydrates: Chemistry, physical properties, and applications (pp. 263–307). Boca Raton, FL: Taylor & Francis.
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Yin, J.-Y., Nie, S.-P., Li, J., Li, C., Cui, S. W., & Xie, M.-Y. (2012). Mechanism of interactions between calcium and viscous polysaccharide from the seeds of Plantago asiatica l. Journal of Agricultural and Food Chemistry, 60(32), 7981–7987. Yu, L., Yakubov, G. E., Zeng, W., Xing, X., Stenson, J., Bulone, V., et al. (2017). Multilayer mucilage of Plantago ovata seeds: Rheological differences arise from variations in arabinoxylan side chains. Carbohydrate Polymers, 165, 132–141. Zhang, P., Zhao, Y., & Shi, Q. (2016). Characterization of a novel edible film based on gum ghatti: Effect of plasticizer type and concentration. Carbohydrate Polymers, 153, 345–355.
viscosity–molecular weight relationship. Carbohydrate Polymers, 54(1), 63–71. Tripathi, R., & Mishra, B. (2013). Preparation and evaluation of composite microspheres of polyacrylamide‐grafted polysaccharides. Journal of Applied Polymer Science, 130(4), 2912–2922. ur Rehman, H., Farooq, U., Akram, K., Sidhu, A. I., Shafi, A., & Sarfraz, F. (2015). Incorporation of garlic extract as antifungal agent in psyllium based edible coating for mandarin. International Journal of Food and Allied Sciences, 1(1), 11–17. Yang, L., & Paulson, A. T. (2000). Mechanical and water vapour barrier properties of edible gellan films. Food Research International, 33(7), 563–570.
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Characterization of edible biocomposite films directly prepared from psyllium seed husk and husk flour Annamária Tóth, Katalin Halász
T
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University of Sopron, Simonyi Karoly Faculty, Institute of Wood Based Products and Technologies, 4 Bajcsy-Zsilinszky, 9400, Sopron, Hungary
A R T I C LE I N FO
A B S T R A C T
Keywords: Plantago psyllium Edible film Glycerol Poly(ethylene glycol) Biocomposite
The mucilage obtained from Plantago seeds is a promising raw material for producing edible films or coatings; however, its complete separation from the seeds is rather difficult. To avoid the difficulties of the extraction and taking advantage of the strong interfacial interactions between the seed husk and the mucilage, we examined the possibility of preparing edible biocomposite films directly from psyllium husk and husk flour. The results showed the husk and husk flour are both suitable for film formation. To reduce the brittleness of the films, two plasticizers, glycerol and poly(ethylene glycol) (Mw = 400 g/mol) were used and both plasticizers improved the flexibility of the films. According to the tensile test, the particles acted as reinforcements in the polysaccharide matrix. ATR-FTIR, solubility, water vapor absorption and water vapor permeability tests were conducted to further examine the interactions of the components and the hydrophilic nature of the films. The tests revealed that biocomposite films directly obtained from psyllium seed husk and husk flour can be novel edible films for the food and food packaging industry.
1. Introduction
standing films as well which makes them a promising material for active or active component carrier edible films or coatings. These versatile seed gum mucilages can be extracted from several kinds of seeds like chia (Dick et al., 2015), basil (Mohammad Amini, Razavi, & Zahedi, 2015), flax (Ding, Qian, Goff, Wang, & Cui, 2018; Kaewmanee et al., 2014), sage (Razavi, Amini, & Zahedi, 2015), Lepidium perfoliatum (Koocheki, Taherian, Razavi, & Bostan, 2009; Seyedi, Koocheki, Mohebbi, & Zahedi, 2014), cress (Behrouzian, Razavi, & Karazhiyan, 2013; Jouki, Khazaei, Ghasemlou, & HadiNezhad, 2013; Karazhiyan, Razavi, & Phillips, 2011), mesquite (Estévez et al., 2004) and plantago seeds (Banasaz, Hojatoleslami, Razavi, Hosseini, & Shariaty, 2013; Fischer et al., 2004; Neto, de Cássia Bergamasco, de Moraes, Neto, & Peralta, 2017). Our research focused on Plantago psyllium seed and seed husk. Plantago psyllium is the member of nearly 200 species of the Plantago genus (Yin et al., 2012). It is a common plant growing in extreme climatic conditions as well, so it can be grown all over the world, which makes its cultivation economic and the seed production for different purposes to be at low cost. Psyllium seed has been used as a herb for constipation for a long time, but, its wide range of use has been discovered only in the few past years. Recently, psyllium seed husk has been utilized in more and more products. Primarily it is used as mechanical laxative and dietary fiber source in food industries (Cui, 2000). It is used in pharmaceuticals, too, for its special
Biorenewable polymer based materials offer a number of significant advantages over the traditional synthetic materials in terms of ecofriendliness, biodegradability and easy availability. There is an increasing demand to use biobased and biodegradable films in the food packaging sector as well. Using edible films and coatings in the food and food packaging industry can decrease the amount of the synthetic polymeric materials and also extend the shelf life of the products. In the past years, much research has been devoted to investigating the characteristics and industrial applicability of various polysaccharides as edible films. Besides the most commonly used starch and non-starch carbohydrates such as cellulose derivatives, pectin, chitosan and alginate, gums extracted from plant seeds can be also efficiently used in the food and food packaging industry. In general, the seed mucilage consist of a pectinaceous and a hemicellulosic fractions (Soukoulis, Gaiani, & Hoffmann, 2018; Yu et al., 2017), which easily make hydrogen bonds with water molecules, then swells and forms hydrogels in aqueous media (Sandhu, Hudson, & Kennedy, 1981; Tripathi & Mishra, 2013). Due to the extreme water binding capacity the mucilage can be used as thickeners, structurisers, binders, stabilizers, fat substitutes during food processing (Izydorczyk, Cui, & Wang, 2005; Soukoulis et al., 2018). They are able to form self-
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Corresponding author. E-mail address: [email protected] (K. Halász).
https://doi.org/10.1016/j.fpsl.2019.01.003 Received 11 June 2018; Received in revised form 5 December 2018; Accepted 10 January 2019 2214-2894/ © 2019 Elsevier Ltd. All rights reserved.
Food Packaging and Shelf Life 20 (2019) 100299
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After determining the dry matter contents (by drying to constant weight at 103 °C for 24 h), a plasticizer, glycerol or PEG400 were added to the mixtures. The amounts of plasticizers were 50, 100 and 200% w/w dry weight. The mixtures were homogenised with magnetic stirrer for 20 min. To obtain homogeneous films bubbles were removed from the solutions prior to forming a film. The different solutions were poured onto plastic trays, and dried for 48 h at 50 °C. Finally, the obtained PG, PH and PHF films were peeled off the plates and were stored in sealed bags until use.
characteristics, especially in the treatment of colon carcinoma, irritable bowel syndrome, obesity, constipation, diabetes, high cholesterol levels, and atherosclerosis (Mishra, Sinha, Dey, & Sen, 2014). The extractable polysaccharide hydrogel from Plantago seeds is made of arabinoxylanes with a xylene backbone (1,4-linked-ß-D-xylopyranose) and side chains of arabinose, xylose and uronic acid (Fischer et al., 2004; Guo, Cui, Wang, & Young, 2008; Sandhu et al., 1981; Tripathi & Mishra, 2013; Yu et al., 2017). The psyllium seed mucilage has a high molecular weight and highly branched structure with unusual linkage (Yu et al., 2017). Although, there are only a few research studies regarding the use of psyllium seed mucilage to form film, mucilage obtained from psyllium seeds could be an attractive material. It could be used as edible films, coatings and as a carrier for active agents to increase the shelf-life of food products as shown by Ahmadi, Kalbasi-Ashtari, Oromiehie, Yarmand, and Jahandideh, (2012), who prepared films with glycerol plasticized psyllium, Banasaz et al. (2013) who developed an ascorbic acid loaded, antioxidant coating for apples and ur Rehman et al. (2015), who created an antifungal coating with garlic extract for mandarin oranges. Since it is quite difficult to separate the gel fraction from the seed the aim of our study was to investigate the possibility of creating psyllium seed based films directly from the psyllium seed husk and husk flour and to examine the effect of these particles on the mucilage matrix film properties. The aim of this research was also to study the effect of different proportions (50, 100, 200% w/w) of plasticizers of two types, glycerol and poly(ethylene glycol), on the properties of the psyllium films.
2.2.2. Surface morphology Surfce morphology of the psyllium seed, seed husk, husk flour and the different films was examined by scanning electron microscopy (SEM) (Hitachi S-3400 N). The SEM images were obtained at an accelerating voltage of 15 kV. 2.2.3. Fourier-transform infrared spectroscopy (FT-IR) The absorption spectra were conducted in the analytical infrared range (400–4000 cm¯¹) with total attenuated reflectance (ATR). After the samples were degreased, dusted and cut to the appropriate size, the samples were placed in the test area of the FT-IR spectrophotometer (JASCO FT/IT 4100). The source of light was standard, the scanning speed was 2 mm/s with a resolution of 4 cm¯¹. 2.2.4. Water vapor absorption (WVA) The water vapor absorption test took place in a climate controlled environment of a climatic chamber at 25 °C with 75% relative humidity for 72 h. The amount of the water vapor absorbed by the films was determined by the difference between the weight of the samples dried previously at 103 °C for 24 h and the weight of the samples following the test, expressed in percentages. The result includes the average weight of the samples taken from two different locations.
2. Materials and methods 2.1. Materials Plantago psyllium seeds were supplied by GreenField Sp. Z.o.o. Sp.k.(Poland); seed husk and husk flour were obtained from Naturganik Kft. (Hungary). The husk flour and the husk particle size distribution were determined by sieve analysis (Table 1). Glycerol (99.6%, Mw = 92.1 g/mol, 1.26 g/cm3) and poly(ethylene glycol) PEG400 (Mw = 380–420 g/mol, 1.15 g/cm3) were purchased from Molar Chemicals Hungary Kft.
2.2.5. Water vapor permeability (WVP) The water vapor permeability of each film was determined according to the method described by Sanyang, Sapuan, Jawaid, Ishak, and Sahari, (2015). Film were cut, adjusted and sealed in a cup containing dried silica gel. Cups were weighed and placed in climate chamber at 25 °C with 75% relative humidity for 72 h. The water vapor permeability (WVT) was calculated using the following Eq. (1):
2.2. Methods
WVP =
2.2.1. Film preparation Films were made from psyllium seed mucilage (PG), psyllium husk (PH) and psyllium husk flour (PHF). Psyllium seed mucilage for film preparation was obtained by hot water extraction. Briefly 40 g seed were swollen in 2000 ml of hot water for 2 h. The mixture was poured into and forced through a polyamide net. The obtained mucilage was collected and used for film-forming without drying. For PH and PHF films 20 g of psyllium seed husk or husk flour was dispersed in 1600 ml of distilled water and mixed at 200 rpm with a magnetic stirrer for 40 min at 70 °C. The mixtures were let to cool to room temperature.
2.2.6. Film solubility in water Water solubility of the film samples was determined according to the method described by Ahmadi et al. (2012) with slight modifications. Film samples were uniformly cut (2 cm × 2 cm), dried at 103 °C for 24 h and weighed to the nearest 0.0001 g to determine their initial dry weight. Film samples were then placed in 50 ml of distilled water (23 °C for 3 or 6 h) under continuous stirring (600 rpm) using magnetic stirrer. After that, undissolved films were filtered through Whatman No. 1 filter paper and dried at 103 ° C until they reached a constant weight to determine their final dry weight. The water solubility (WS, %), expressed as a percentage was calculated using the following Eq. (2):
psyllium seed husk
sieve hole size range (mm)
content (%)
sieve hole size range (mm)
content (%)
1.0-0.5 0.5-0.2 0.2-0.125 0.125-0.1 0.1-0.05
1% 33% 34% 14% 18%
2.0-1.0 1.0-0.5 0.5-0.2 0.2-0.125 0.125-0.1
5% 61% 31% 3% 1%
(1)
Where m (g) is the weight without test cup, d (mm) is the film thickness, A (m2 ) is the area of the film exposed, t (days) is duration for permeation, and P (Pa), the pressure difference (here 3170).
Table 1 Fraction analysis of psyllium seed husk and husk flour. psyllium seed husk flour
m×d A×t×P
WS (%) =
(W0 − WF ) * 100 W0
(2)
where W0 is the initial dry weight of the film and WF is the final weight of the dried undissolved film. All the experiments were repeated three times, and their arithmetic averages were reported. 2
Food Packaging and Shelf Life 20 (2019) 100299
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Fig. 1. Photo and SEM images of psyllium seed (a – x45, b – x500), seed husk (c– x45, d – x500) and husk flour (e– x45, f – x500) under different magnifications.
mechanism of mucilage release of chia seed coat described by de la Paz Salgado-Cruz (2013) can be presumably applicable to psyllium seed and seed husk due to their similar nature. Based on their observations the seed releases mucilage strands which interact with each other and form a thin film with a dense matrix. After the extraction and drying process, the mucilage from psyllium seeds formed a film with smooth and uniform surface structure as shown in Fig. 2a-c. The larger and smaller particles in the biocomposites films can be observed but without any cracks or openings on the film surfaces. Fig. 2d-I shows the husk particles are blended into the polysaccharide matrix they secreted. A larger husk particle covered by the mucilage is shown in Fig. 2d. Fig. 2e-f shows the flexible seed husk particles with the characteristic fibrous surface in the PH biocomposite film. The plasticizers did not alter the characteristics of the surface morphology. Similar observations was reported by several authors for plasticized seed mucilage films (Jouki, Khazaei, Ghasemlou, & HadiNezhad, 2013; Jouki, Yazdi, Mortazavi, & Koocheki, 2013; Dick et al., 2015; Zhang, Zhao, & Shi, 2016).
2.2.7. Tensile properties To determine the tensile strength, Young’ modulus and elongation at breakpoint of the films tensile test was performed by using an Instron 3345 tensile tester. Five-five samples were fixed between the grips of the device with a gauge length of 36 mm then 20 mm/min crosshead speed was applied. The width of the samples was 10 mm. The measurements were conducted at 23 ± 2 °C, with the relative humidity of 50 ± 5%. 2.2.8. Statistical analysis The differences between mean values were determined by analysis of variance (ANOVA) and Tukey’s multiple range test (with significance level of 0.05). Measurements were performed in triplicate except where stated otherwise. 3. Results and discussions 3.1. Surface morphology The photo and scanning electron microscopic images of psyllium seed, husks and husk flour are shown in Fig. 1. The elliptic cavity with round edges specific to Plantago psyllium seed (Kumar Verma & Bharti, 2017) and a wrinkled seed surface can be seen in Fig. 1a and b. A strongly fibrous structure is observed for the husk particles at higher (x500) magnification. After the film casting and drying process, all of the three mucilaginous mixtures turned into free-standing films. Although the films without plasticizers were less flexible, their structural integrity enabled them to be removed from the casting trays without any break. The photo and SEM images of these films can be seen in Fig. 2. The
3.2. Water vapor absorption As expected the hydrophilic plasticizers increased the water absorption of the films (Fig. 3), however, the poly(ethylene glycol) in 50 w/w% significantly decreased the absorption of PH and PHF films. This observation is in accordance with the change in tensile properties where PEG50 films had significantly higher strength addressed to the developed interactions. Here the same interactions prohibited the interactions of water molecules with the polymer chain or the plasticizer just as in case of water vapor transmission. Applying higher amounts of PEG caused higher WVA due to the higher concentration of free 3
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Fig. 2. Photo and SEM images of PG (a – x75, b – x200, c – x200), PH (d – x75, e – x200, f – x200)and PHF (g – x75, h – x200, i – x200) films under different magnifications.
hydroxyl groups. As glycerol contains even more hydroxyl groups along the polymer chain the glycerol plasticized samples showed more intense moisture uptake that reached the 45% when 200% w/w glycerol was added. Considering the base materials the WVA is dependent on the plasticizer type. Films prepared from husk had the lowest absorption capacity when glycerol was applied; on the other hand adding PEG in higher concentrations PG films showed the lowest absorption. In general PHF films had the highest WVA. The high water vapor absorption capacity is usually not desirable e.g. during storage, however, it can be beneficial if the film is used as the part of an active packaging where the packed good release water vapor. 3.3. Film solubility The solubility of an edible film strongly influences its use. High solubility is required if the film is consumed with the food or the film should dissolve quickly (during cooking) before consumption. Lower solubility is needed if water resistance is necessary during the storage
Fig. 3. Water vapor absorption in the function of plasticizers and film bases.
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the polymer chains due to its large molecular size, which leaves gaps (Haq et al., 2014) and creates more sites for water to interact with the film. Comparing the base materials films prepared from psyllium seed husk showed the highest solubility, with the exception of PH-PEG50, where the solubility was slightly lower after 24 h. PG films showed the lowest water sensitivity after 3 h of immersion. The behavior of plasticized PG films was changed after 24 h and psyllium seed mucilage based films had the highest (p < 0.05) solubility as Figure shows, whilst films prepared from psyllium seed husk flour had the lowest solubility. This could be because the denser structure of the plasticized PG film were disrupt and the water molecules could penetrated into the bulk material. In case of husk and husk flour films the bonding between the insoluble particles and mucilage might prevented further solvation.
3.4. Water vapor permeability (WVP) Mass transfer properties of molecules into edible films are influenced by the structure and compositon of the polymer (Miller & Krochta, 1997). Due to their hydrophilic nature polysaccharide based films show poor water barrier properties. It is known that plasticizers tend to decrease the water vapor barrier properties of polymer films (Cuq et al., 1996; Forssell, Lahtinen, Lahelin, & Myllärinen, 2002; Haq et al., 2014). Hygroscopic plasticizers such as glycerol or poly(ethylene glycol) can increase water sensitivity of the polysaccharide film and thus increase the solubility of water molecules in the polymer. The plasticizers decrease the intermolecular forces and increase the chain mobility in the macromolecules, which cause the reorganization of polymer chains and formation of interchain spaces (free volume) that enhance the diffusion of the water vapor through the film (Ahmadi et al., 2012; Jouki, Khazaei, Ghasemlou, & HadiNezhad, 2013; Jouki, Yazdi, Mortazavi, & Koocheki, 2013; Khazaei, Esmaiili, Djomeh, Ghasemlou, & Jouki, 2014; Sothornvit & Krochta, 2001). Fig. 5 shows the changes in WVP produced with different concentrations of plasticizers. As expected the plasticizers increased the water vapor permeability of the psyllium based films. The results showed that WVP depended on the type of plasticizer and the film base. In general, glycerol increased the permeability of water molecules through the film more than PEG400. A similar observation was reported by Haq et al. (2014) for gum cordia based edible films plasticized by glycerol and poly(ethylene glycol) and by Chinnan and Park (1995) and Park, Weller, Vergano, and Testin, (1993) when PEG was used as a plasticizer for methylcellulose and hydroxypropyl cellulose, respectively. The addition of 50% w/w PEG400 slightly reduced (although not
Fig. 4. Solubility in water in the function of plasticizers and film types.
(Ghasemlou, Khodaiyan, & Oromiehie, 2011; Laohakunjit & Noomhorm, 2004; Nafchi et al., 2017; Pathare, Hastak, & Bajaj, 2013). The films were partially soluble in water irrespective the type of plasticizer (Fig. 4). Although PH and PHF films contained insoluble particles PG film showed the lowest solubility, and it was able to maintain ˜90% of its integrity in water at 25 °C after 3 h, while PH and PHF films showed significantly higher (25 and 23%, respectively) solubility. It can be assumed that the dense structure of the PG film might inhibited the penetration and the attack of water molecules. After 24 h of immersion the solubility of PG film was doubled (21%) and approached the solubility of PH (28%) and PHF (30%) films, however it was still significantly lower (p < 0.05). As expected, and also reported by different authors (Ahmadi et al., 2012; Dick et al., 2015; Jouki, Khazaei, Ghasemlou, & HadiNezhad, 2013; Jouki, Yazdi, Mortazavi, & Koocheki, 2013; Nafchi et al., 2017; Tee, Wong, Tan, & Talib, 2016) for both plasticizers, the solubility of the films was found to increase with increasing concentrations (Fig. 4) due to the hydrophilic nature of plasticizers and seed mucilage. Plasticizers can also promote the solubility of the polymer matrix. The hydrophilic nature of a plasticizer can diminish interactions between biopolymer molecules and increases its solubility, resulting in more water attracted into the polymer matrix and more mobile regions with greater inter-chain distances (Cuq, Gontard, Cuq, & Guilbert, 1996; Haq, Hasnain, & Azam, 2014). Using 50% w/w PEG400, 50% w/w and 100% w/w glycerol the solubility was increased, but it remained under 30% after the 3 h of immersion. This indicates that glycerol and PEG400 could effectively bond to the polymer at these concentrations (which was shown in the mechanical properties as well especially when PEG400 was used). Over time these interactions weakened and after 24 h the degree of solubility reached ˜50%. This value is lower that Dick et al. observed (85%) when chia seed mucilage film was modified with 50% glycerol. At 50% w/w concentrations the solubility of the plasticized films were similar, the difference appeared with higher plasticizer contents, except for 100% w/w, where the solubility was still below 30% after 3 h. Films with higher plasticizer concentrations (100% and 200% w/ w), showed high solubility which can be attributed to the migration and solubility of the plasticizer from the polymer matrix. In general, samples with glycerol showed substantially lower solubility in water even if higher amounts of plasticizers were applied. This indicates that there might be stronger interactions existing between glycerol and the polymer which can hinder the leakage of the plasticizer from the film. The higher solubility of samples containing higher amounts of PEG400 can be related to the inability of PEG400 to insert effectively between
Fig. 5. Water vapor permeability in the function of plasticizers and film types. 5
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significantly) the water vapor permeability of the PH film. The polysaccharide films prepared from PHF and PG showed similar WVP, but the neat films were too brittle to handle on the permeability cups, so these samples could not be compared to the neat ones. For this reason it cannot determined if the interactions with PEG400 caused the lower WVP or the psyllium seed husk particles. According to the results, in general, plasticized PG films showed lower WVP when the plasticizer concentration was higher which could be explained by the fact that PG films contains only the extracted gel and therefore the matrix was able to interact with higher amount of plasticizer compared to PH and PHF films. Comparing the two composite films, PHF films had a lower WVP than PH films. The psyllium seed husk flour particles were presumably more effective physical barriers than the larger husk particles. Applying the large amounts of plasticizers can remarkably increase the WVP (Ahmadi et al., 2012; Ghasemlou et al., 2011; Jouki, Khazaei, Ghasemlou, & HadiNezhad, 2013; Jouki, Yazdi, Mortazavi, & Koocheki, 2013; Ladjevardi, Gharibzahedi, & Mousavi, 2015; Mali, Grossmann, Garcı́a, Martino, & Zaritzky, 2004). Their significant increase can be attributed to the clustering of the plasticizers which results in a discontinuous polymer structure that can further increase the amount of water molecules transmitted through the film. (Cerqueira, Souza, Teixeira, & Vicente, 2012; Mohammad Amini et al., 2015; Piermaria, Pinotti, Garcia, & Abraham, 2009; Yang & Paulson, 2000) 3.5. Mechanical properties Mechanical properties of a film determine its durability and usability for coatings or films. Without plasticizers, psyllium seed husk, husk flour and mucilage were more brittle and did not show a high plastical deformation. Neat film with husk had significantly (p < 0.05) lower stress at the breakpoint and a lower Young’s modulus compared to the neat PH and PG films (Fig. 6). This could be related to the larger and more rigid seed husk fraction (the darker husk parts) which might concentrate stress. Among all the samples examined, neat film prepared from husk flour required the highest load to tear. The small size, therefore the high surface area of the particles and the strong interactions between the filler and the matrix highly improved their strength compared to the PG film. The high reinforcing effect was clearly shown by the significantly higher Young’s modulus as well. Many authors reported that PEG 400 is not an appropriate choice for plasticizing seed gums and other hydrocolloid films, since the films containing PEG400 (from 20 to 100% w/w) become fragile, thick and opaque (Antoniou, Liu, Majeed, Qazi, & Zhong, 2014; Laohakunjit & Noomhorm, 2004; Olivas & Barbosa-Cánovas, 2008; Pongjanyakul & Puttipipatkhachorn, 2007; Razavi et al., 2015). However, we had different observations for films made of psyllium seed. Both glycerol and PEG400 could disrupt the initial intermolecular hydrogen bonds in the polymer and cause new hydrogen bonds to form between glycerol or PEG400 and arabinoxylan chains. The plasticizers could reduce the intermolecular hydrogen bonds between the polymer chains which resulted in increased chain mobility and thus increased deformability. The effects of glycerol and PEG 400 on the mechanical properties are shown in Fig. 6. In contrast to the conclusions of Yang and Paulson (2000), in case of psyllium seed mucilage the dipole strength of PEG400 appears to be enough to disrupt the intermolecular interactions and the thus the plasticizer can fill the area between the chains. Using PEG400 the flexibility increased until 100% w/w plasticizer content. The elongation at the breakpoint was 3.3% for PH-GLY50 and 12.1% for PH-GLY100, 3.9% for PHF-PEG50 and 25.0% for PHF-PEG100 and 1.8% for PGPEG50 and 11.6% for PG-PEG100. While 200% w/w PEG400 had a less effective plasticizing effect on the samples, the EB decreased to 4.6%, 14.8% and 9.3% for PH-PEG500, PHL-PEG200 and PG-PEG200, respectively. When 200% w/w PEG400 was used there was a large
Fig. 6. Effect of plasticizers and particles on the tensile properties.
amount of unbounded poly(ethylene glycol) between the polymer chains, which caused a decrease in flexibility compared to the a samples containing 100% w/w PEG. Although it was observed that PEG400 could reduce the brittleness of the psyllium seed mucilage films, glycerol was more effective in plasticizing as it has been shown for seed gums by different authors earlier as well (Ahmadi et al., 2012; Dick et al., 2015; Khazaei et al., 2014; Mohammad Amini et al., 2015; Razavi et al., 2015). EB increased especially when the samples contained 100 and 200% w/w. Since glycerol molecules are smaller than PEG 400 they can more easily 6
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penetrate into the polymer matrix and interact more efficiently with the arabynoxylan chains which results in more reduced intermolecular forces and more increased polymer chain mobility (Ghasemlou et al., 2011; Khazaei et al., 2014; Razavi et al., 2015; Seyedi et al., 2014). With glycerol there is a huge difference between the plasticizing effect of 50% w/w and 100% w/w glycerol. With 100% w/w glycerol the higher strain at its breakpoint with 139% was observed for samples containing psyllium seed husk. In this case the elongation was significantly higher compared to the films from psyllium seed husk particles (86%) and gel (96%). When the glycerol content was 200% w/w, a further increase in the strain at the breakpoint was observed, although with husk, the difference was not significant. Since the film prepared from the gel contains higher fraction of polymer matrix it can bind more plasticizers compared to PH and PHF films. Thus 200% w/w glycerol doubled the elongation before the breakpoint of PG film from 96% to 200%. A further increase in strain at break might be achieved by adding more glycerol; however, films containing 200% w/w of glycerol became sticky on their surface, which might reduce the usability of the film. There was a slight difference between the plasticizing effects of the plasticizers regarding the psyllium film bases. In general, samples prepared from psyllium seed husk flour became more flexible if PEG400 was used than films made of husk or gel. In contrast, glycerol showed a higher plasticizing effect on samples made from psyllium seed husk and mucilage. It is notable to mention, that although the tensile strengths were reduced due to the presence of the plasticizers (both glycerol and PEG400), 50% w/w poly(ethylene glycol) significantly enhanced the stress at break of PH films (while the strain was improved at the same time) and also a slight (but p > 0.05) improvement was observed in case of PG films. Due to the plasticizers, which enhanced the elongation, the samples showed higher tensile energy absorptions. Samples containing husk and husk particles had higher energy absorption than films prepared from psyllium seed mucilage (except when 200% w/w glycerol was used). Since the matrix and the fillers are unquestionably compatible and there is a strong adhesion between the components, the husk and the husk particles could effectively transfer the stress from the matrix to the filler phase.
Fig. 7. FTIR spectra of neat and plasticized PG, PH and PHF films.
and CeOeC stretching vibrations in glycosidic and pyranose rings (Gómez-Ordóñez & Rupérez, 2011; Kacurakova, Capek, Sasinkova, Wellner, & Ebringerova, 2000; Toğrul & Arslan, 2003). The 896 cm−1 band is the characteristic of (1–4) linkages. The difference between the spectra can be observed which was presumably caused by the different film compositions, since films prepared from husk and husk flour contained the whole mucilage not only the water extractable fraction. The great increase in the intensity of –O-H stretching vibration and CeOeH bending range (due to the presence of free hydroxyl groups) when using high amounts of glycerol is in accordance with the higher water vapor absorption capacity of the films compared to samples plasticized with PEG. The small shifts to lower frequencies, the broadened bands and the band asymmetry can be a sign of the developed Hbonds developed between the plasticizer and the polymer (SadeghiVarkani, Emam-Djomeh, & Askari, 2017; Tee et al., 2016), however, it can be the sign of the excess plasticizer fraction as well which overlaps these regions.
3.6. Fourier-transformational infrared (FT-IR) Fig. 7 shows the ATR-FTIR spectra of the neat and plasticized films. The broad band from 3690 to 3000 cm−1 corresponds to the OeH stretching vibration, resulting from the free, inter and intra-molecular bound −OH groups. This band showed remarkable increase with the glycerol and PEG concentrations for all the plasticized samples. With poly(ethylene glycol) the increment was not that intense compared to glycerol which is obviously due to the higher amounts of hydroxyle groups present in glycerol. In the -C–H stretching region (˜3000-2800 cm−1) we have had the same observation as Haq et al. (2014) regarding the intensity change of these bands for glycerol and poly(ethylene glycol) plasticized PG films. When the PG samples contained the lowest amount of PEG, the intensity of the bands was higher than for the samples of PG-PEG100 or PG-PEG200. According to Haq et al. (2014) this could be explained by the conformational changes which PEG induced in the polymer, and the decreased intensity of the C–H band when a greater amount of plasticizer was used can be due to the aligment of the modified structure along with the poly(ethylene glycol) chain. For films made of PHF and PH this phenomenon was not that prominent, but it could be observed until samples contained 100% w/w plasticizer. The absorption bands at ˜1600 cm−1 and 1416 cm−1 are assigned to the symmetric stretching of the carboxyl group (−COO−) of uronic acids (Timilsena et al. 2016). Bands in the range of ˜1200 -920 cm−1 (with the highest intensity at 1036 cm−1), the characteristic absorptions of natural polysaccharides, corresponds to the CeOeH bending
4. Conclusions The results of this study demonstrated that psyllium seed husk and husk flour are suitable materials for preparing polysaccharide based edible films for food packaging just like psyllium seed mucilage. These raw materials are sustainable, abundant and can be obtained at low cost. Usage of husk and husk flour enables a direct and easy way of filmcasting without the need of the laborious and cumbersome separation of mucilage. The biocomposite films prepared in this way have comparable properties to the pure mucilage films. Without a plasticizer, the films were rigid and brittle except the films made from husk which could be bent without fracturing. Both glycerol and PEG400 increased the elongation at break of the films. Tensile tests revealed that plasticized PH and PHF films were deformable with increased toughness due 7
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to the reinforcing effect of the husk and husk flour particles. The plasticizers increased the hydrophilicity of the films. The water vapor absorption, permeability and solubility were increased especially when the highest amounts of plasticizers were added. Using the appropriate amounts of plasticizers and choosing the appropriate raw material (pure seed mucilage, seed husk or husk flour), the hydrophilic nature of the films can be tuned to the usage of the films. It can be beneficial if the film covering the product is consumed with the product or if the film is used in active packaging systems where they absorb the moisture that develops or they release more active components when liquid forms through their increased solubility.
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