Bioplastic Fibers from Gum Arabic for Greener Food Wrapping

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Bioplastic Fibers from Gum Arabic for Greener Food Wrapping Applications Vinod V. T. Padil, Chandra Senan, Stanis#aw Wac#awek, Miroslav Cernik, Seema Agarwal, and Rajender S. Varma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05896 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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ACS Sustainable Chemistry & Engineering

Bioplastic Fibers from Gum Arabic for Greener Food Wrapping Applications Vinod V. T. Padil 1,3*, Chandra Senan 2, Stanisław Wacławek 1, Miroslav Černík 1*, Seema Agarwal 3 and Rajender S Varma 4* 1 Department

of Nanomaterials in Natural Sciences, Institute for Nanomaterials, Advanced

Technologies and Innovation (CXI) , Technical University of Liberec (TUL), Studentská 1402/2, Liberec 1, Czech Republic, 461 17; Telephone: +420 723372911. 2Centre

for Water Soluble Polymers, Applied Science, Faculty of Arts, Science and Technology, Wrexham Glyndwr University, Wrexham LL11 2AW, Wales, UK

3Macromolecular

Chemistry II, University of Bayreuth, Universittsstraße 30, 95447 Bayreuth, Germany.

4Regional

Centre of Advanced Technologies and Materials, Department of Physical Chemistry,

Faculty of Science, Palacký University in Olomouc, Šlechtitelů 27, 783 71 Olomouc, Czech Republic.

*Corresponding Authors’ Email: [email protected]; [email protected]; [email protected]

1 ACS Paragon Plus Environment

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Abstract The fabrication of bioplastic fibers from Gum Arabic (GA), a natural tree gum exudate, is described via the electrospinning method. The enrichment in surface properties of this bioplastic fiber was evaluated by methane plasma and -ray irradiation treatments. The fibers with their modified forms, both treated and untreated, were investigated by various characterization techniques such as SEM, AFM, XRD, ATR-FTIR, TGA, BET surface area, water contact angle and tensile strength measurements. A switchable hydrophobic/hydrophilic functionality on GA bioplastic fibers was established through CH4 plasma and -ray irradiation treatments; higher water contact angle (130º) was observed in GA bioplastic fibers that had undergone methane plasma treatment. However, the untreated and -ray irradiated GA bioplastics exhibited hydrophilic behavior. The comparative properties such as water resistance, antioxidant potency, gas barrier attributes, antibacterial effectiveness, biodegradability and food contact migration through the GA bioplastic fibers (untreated, plasma treated and -ray irradiated) were assessed. The present work, in contrast to other existing bioplastic fibers, has the potential of becoming a viable option in greener food packaging as well as in environmental and medically related products, based on tree gums.

Keywords: Gum Arabic; Bioplastic fibers; Electrospinning; Plasma treatment; - ray irradiation; Food packaging.


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Introduction The environmental impacts of ever-growing plastic wastes are raising serious global alarms. Petroleum-based plastics cause grave pollution problems, which has dire consequences for the environment. Plastic wastes have contributed significantly to this distressing situation worldwide which leads to numerous undesirable scenarios including abiotic (e.g. inanimate iron ore and crude oil) depletion; global warming; toxicity to humans; environmental eco-toxicity; photochemical oxidation; acidification and eutrophication etc.1,2 Additionally, the constant rise in petroleum product prices, the non-biodegradability of these products and the cost associated with their waste management and disposal are the key concerns that need to be addressed promptly.3 Natural bio-based materials with their multifarious properties such as reduced costs and the possession of viable physicochemical and mechanical attributes are gearing up to replace petroleum-based plastics in a cost-effective manner. Emphasis on the development of biodegradable and biocompatible materials (based on eco-friendly sources) is crucial if we are to reduce the deleterious environmental impact of petroleum-based plastics. Bioplastic materials can be prepared from renewable polysaccharides (e.g. starch, cellulose, chitosan/chitin) and proteins like casein and gluten, wood, sugar, wheat, Sugar cane bagasse, corn, rice and potatoes; vegetable waste materials (carrot, parsley, radicchio and cauliflower); polylactic acid (produced by chemical synthesis from renewable bio-based monomers); natural oils (pine, castor, soybean and rapeseed), and polyhydroxyalkanoates (synthesized from microorganisms or genetically modified bacteria).4-8 These polymers have been developed in an attempt to meet the growing demand for food packaging applications (including environmental and medical); food ampoules; carrier bags and vegetable plant pots etc.



with the express objective to ultimately replace the use of petroleum-based polymers9,10 However, the higher cost of their production hampered marketing although these products had numerous benefits viz. biodegradability, biocompatibility, low carbon imprint and zero waste management – compared to petroleum based plastics.11,12 The production and development of bio-based materials will help to constrain the consumption of non-renewable energy and to reduce carbon dioxide emissions, both of which curtail global warming and environmental noxiousness.13 The biodegradability of the common class of polyesters viz. poly(lactic-co-glycolic acid) (PLGA); polycaprolactone (PCL); polylactic acid (PLA); poly(3-hydroxybutyrate) (PHB) and Ecoflex etc. have been comprehensively studied.14 However, the only polyester that was found to be 100% degradable was PLGA.14 In this context, newer bio-based plastic materials possessing precise biodegradable characteristics would be highly beneficial in the battle to save the environment and the planet. Currently, the potential for tree gum-based polysaccharides to serve as bioplastic fibers is minimal, owing to their high molecular masses, complex molecular structures and hydrophilic nature. A large number of bio-based materials from natural or synthetic based formulations, with the exception of tree gums, have been deliberated and deployed for applications in the environmental, food packaging, pharmaceutical, agricultural, water purification, energy and biomedical fields.15,16 Tree gums have been demonstrating exceptionally good performances in the food, pharmaceutical, adhesive, paper, textile, cosmetics and beverage industries.17 However, their fibers/films forming properties, especially their applications in food packaging and environmental bioremediation, have not yet been fully realized when compared to other natural polysaccharides. Recently, non-food applications of tree gums in the bio-nanotechnology, environmental and biomedical fields have been comprehensively reviewed.18 Gum Arabic (GA), 4 ACS Paragon Plus Environment

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a tree gum exudate, possesses highly divergent structures, harboring a core consisting of β-(1-3) galactose residues with extensive branching at the C-3 and C-6 positions (Fig. 1).17 The major components of GA are arabinose and galactose (both monosaccharide sugars); rhamnose (a deoxy sugar) and glucuronic acid, a sugar acid derived from glucose.17 Gum Arabic has been bridged for the fabrication and stabilization of metal/metal oxide nanoparticles, carbon nanotubes and quantum dot nano-colloids etc.18,19 Lately, investigation into the fabrication of GA fibers via electrospinning in aqueous media has been initiated.20

Fig.1: Molecular structure of GA main chain



In the present investigation, a comparative study of physicochemical properties (water contact angle, surface area, surface roughness, hydrophobic/hydrophilic stability); thermal and mechanical properties (tensile strength, percent elongation (%) and moduli of elasticity) of untreated and treated (by both γ-ray irradiation and methane plasma) GA bioplastic fibers have been undertaken. Furthermore, the morphological, antioxidant and antibacterial properties; oxygen permeability; biodegradability and food contact migration of the untreated and treated (both by plasma and -ray irradiation) GA bioplastic fibers were also studied in order to develop suitable biodegradable and biocompatible materials for food, environmental and medical applications. Experimental Materials GA, polyvinyl alcohol (PVA) [Mw = 88,000 g/mol; degree of deacetylation: 88%], HCl, NaOH, NH4OH, potassium persulphate and ABTS (2, 2-azinobis (3-ethylbenzoithiazoline-6-sulfonic acid) were purchased from Sigma-Aldrich, USA. Deionized water was used in all the experiments. Methods Preparation and molecular characterization of GA The preparation and molecular weight characterization of GA were conducted.20 The purification and molecular mass determination of GA is presented in supplementary information, Fig. S1.

Fabrication of bioplastic fibers of GA GA (20 wt. %) was prepared by dissolving it in deionized water. PVA (10 wt.%) solutions, for subsequent electrospinning, were then prepared by heating at 90 ºC with a magnetic stirrer for 4 6 ACS Paragon Plus Environment

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h. before mixing it with GA (20 wt. %) solutions in the ratio 50/50 [GA/PVA: 2:1 weight ratio]. Both the process and system parameters – important electrospinning characteristics – have been reported in our earlier communication.20 Electrospinning was carried out using a Nanospider electrospinning machine (Elmarco, NS IWS500U, Liberec, Czech Republic). Greener treatments for GA bioplastic fibers

-ray irradiation of GA bioplastic fibers Irradiation was performed by means of a γ-irradiator (GC-5000, BRIT’s Laboratory Irradiator, Mumbai, India) equipped with a

60Co

source, a synthetic radioactive isotope of cobalt with a

half-life of 5.2714 years, that delivered a dose of 5.9 kGy/h. The bioplastic fibers were sealed in polypropylene bags and subjected to

60Co

irradiation (- ray source) at room temperature, with

different radiation doses (10, 25, 50 & 100 kGy) directed onto the bioplastic fibers. In the present study, after screening, GA(U) untreated bioplastic fibers were irradiated with a ray radiation dose of 25 kGy to produce GA() bioplastic fibers which were used for subsequent experimental work. Of all the radiation doses (10 -100 kGy) employed, the irradiation dose spanning 10-35 kGy was shown to produce the best -ray irradiated materials in terms of properties and uses, especially for antibacterial and food packaging applications.21 Methane plasma treatment A plasma reactor (BalTec Maschinenbau AG, Pfäffikon, Switzerland) was used for the treatment of GA electrospun fibers with methane gas. The conditions adopted were as follows: 13.56 MHz – radio frequency; 300 volts – voltage; 20 watts – power; 1-5 min. – duration; 99.997% – methane gas purity; 48 cm2 – electrode area; 50 mm – inter-electrode distance and 1,000 cm3 – chamber volume. Physicochemical Characterization 7 ACS Paragon Plus Environment


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The fiber diameters and surface areas of the untreated [GA (U)], methane plasma treated [GA (P)] and γ-ray irradiated [GA (γ)] fibers were determined. The surface areas of the fibers, before and after treatments (both plasma and γ-ray irradiated) were analyzed using the BET [Brunauer– Emmett–Teller] technique (Autosorb iQ, Quantachrome Instruments, Florida, USA).

BET

theory rationalizes the physical adsorption of gas molecules onto solid surfaces and underpins an analytical method for gauging the specific areas of materials. Mechanical Characterization The mechanical properties of the GA (U), GA (P) and GA () bioplastic fibers were characterized using a Universal Mechanical Tester (computer controlled Instron ElectroPuls E1000 All -Electric Dynamic Test Instrument, Instron, MA, USA) with a 10 KN load cell under standard atmospheric conditions. The samples were cut into rectangular strips with dimensions of 10 cm × 1 cm to serve as test specimens. The thicknesses of the specimens were measured using a digital micrometer. Samples were stretched at a cross head speed of 2 mm/min which attempted to pull the specimen apart and the tests were conducted until the samples broke under the load. To ensure reliable results, the tests were performed in triplicate. SEM (Scanning electron microscopy) analysis The surface morphologies and average diameters of the bioplastic fibers were also investigated by a scanning electron microscope (ZEISS, Ultra Plus, Dresden, Germany) with ZEISS image software using 50 different points from the various SEM images. AFM (Atomic force microscopy) analysis The roughness profiles of bioplastic fiber surfaces were examined using a scanning probe atomic force microscope (Nanowizard® II, JPK Instruments AG, Germany) in tapping mode. A quartz cantilever with a tip radius of 10 nm was utilized in semi-contact mode while the surface


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roughness analysis of the bioplastic fibers was performed by means of WSxM 5.0 image browser software. ATR- FTIR (Attenuated total reflectance-Fourier transform infrared) spectrometry Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR; NICOLET iZ10, Thermo Scientific, USA) was used to identify the functional groups of PVA, GA and its corresponding bioplastic fibers namely GA (U), GA (P) and GA (). The spectrometer is equipped with a multi-reflection, variable angle, and horizontal ATR accessory. XRD Analysis An X-Ray diffraction spectrum was recorded on an X-Ray diffraction spectrometer (Bruker, AXS/8, Berlin, Germany). The diffraction spectra were obtained using Cu-Kα radiation (40 kV, 60 mA). Diffractograms were run at a scanning speed of 20/min and chart speed of 20/2 cm per 2. Thermogravimetric (TG) analysis Thermal stability and composition of the GA (U), GA (P) and GA () bioplastic fibers were determined using Mettler Toledo’s Thermogravimetric Analysis/Simultaneous Difference Thermal Analysis apparatus: TGA/SDTA851e. The experimental atmosphere was nitrogen at a flow rate of 20 mL/min and the samples were heated from 30 to 700 °C. Switchable hydrophilic/hydrophobic properties The measurement of the water contact angle () was used to determine the surface wettability of GA (U), GA (P) and GA () fibers using the sessile drop method. The water stability of GA (U), GA (P) and GA () were conducted by immersing the corresponding fibers (20 mg samples each)



in 25 mL aliquots of deionized water for various time intervals (10 min. to 48 h). The mass losses (%) of the bioplastic fibers were then calculated. Antioxidant activity The antioxidant activities of the crushed powder form of the bioplastic fibers [GA (U) and GA (γ)] were monitored by measuring the reaction between ABTS and potassium persulphate. The radical scavenging ability of ABTS.+ (viz. 2,2-Azino-bis(3-ethylbenzothiazoline)-6 sulphonic acid) was determined using the equation below.22 Percent inhibition = 100 – (A1/A2) 100

(1)

where A1 represents the absorbance of the sample at 734 nm (after 1 min. of reaction) and A2 is the absorbance of the control solution at 734 nm. Test for estimating ability of bioplastic fibers to inhibit microbial growth The antibacterial activities of the bioplastic fibers were evaluated. The bacterial strains of Gramnegative Escherichia coli (CCM 3954) and Pseudomonas aeruginosa (CCM 3955) and Grampositive Staphylococcus aureus (CCM 3953)] were obtained from the Czech Collection of Microorganisms (Masaryk University, Brno, Czech Republic). At the start, the microorganisms were cultured overnight in 5 mL of Luria-Bertani (LB) broth, held in an incubation shaker at 37 ºC and 150 rpm. The optical density (OD) was observed to be 1.0, corresponding to 8108 CFU/mL (colony forming units per mL). The bioplastic fibers [0.05 wt. % of each sample of GA (U), GA (P) and GA ()] were suspended in 10 mL of sterilized water placed in 50 mL culture tubes. Subsequently, 100 µL portions of microbial culture suspension were added to their respective tubes containing the bioplastic fibres, in order to ascertain microbial growth as well as


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the growth inhibition efficacy of the fibers. Control samples of bacterial cultures (without bioplastic fibers) and the LB medium (with bacterial cultures and bioplastic fibers) were utilized. These culture tubes, together with the negative and positive control tubes, were incubated overnight in an incubation shaker maintained at 37 °C. The following day, the OD of the cultures in each tube corresponding to CFU/mL was deduced using a UV-vis spectrophotometer. The experiments were repeated thrice and the average values (with standard deviation) are documented. The following formula was used to determine the percentage reduction rate of particular bacteria. PR (Percent Reduction) = (N1 –N0)/ N1  100

(2)

where ‘PR’ is the percent reduction rate of each bacterial colony; ‘N1’ represents the number of microorganisms recovered from the inoculated nanofibers overnight and ‘N0’ refers to the number of microorganisms recovered from the inoculated nanofibers at zero contact time. Oxygen permeability test Assessing the oxygen permeability of bioplastic fibers (untreated and treated) was accomplished, courtesy of the OX-TRAN Model 1/50 OTR test system; instrument was operated at room temperature and 0% relative humidity. The oxygen transmission rate (OTR) was calculated using the equation below, calibration of the analyser being accomplished by use of certified films. OTR= Δp/l

(3)

where ‘l ‘represents fiber thickness and ‘Δp’ signifies the partial pressure difference between the inside and outside of the chamber. Food migration Test Bioplastic fibers of GA (treated and untreated) were appraised for the migration of toxic contaminants from the packing materials (GA bioplastic) into the foodstuffs. The simulant, a



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porous polymer resin viz. poly(2,6‐diphenylphenylene oxide (Tenax®), for dry foodstuffs, was employed, in accordance to European Union Regulation N° 10/2011.23 Tenax films (rather than powder) are preferable as dry food simulants since they offer a more realistic estimation of migration of contaminants from various packing materials into food, as has been reported in detailed studies.24 The bioplastic fibers of GA [GA(U), GA(P) and GA()] all with diameters of ~3.0 cm were dispersed in a beaker containing Tenax® (80 mg) and maintained at 70 °C for 2 h. Experiments were also conducted using control samples without bioplastic fibers. Finally, the overall migration before and after heat treatment was determined by subtracting the initial mass from the final mass (of Tenax®) using a sophisticated analytical balance (Sartorius Secura11031S, Germany) Biodegradability test Biological oxygen demand (BOD), standard ISO 14851 procedure (AFNOR, 2004) and laboratory test methods were adopted to test the biodegradability of GA bioplastic fibers (both untreated and treated). The amount of oxygen consumed was determined using an OxyTop® Respirometer and average values of three independent measurements were noted. A combination of bioplastic fibers (each sample weighing 100 mg) of GA [GA () and GA (U)]; an inorganic medium (97 mL) and inoculum (3 mL) were introduced into a bottle which was kept in a dark room, maintained at 20 ± 1°C for one month. Sludge inoculum had been withdrawn from municipal wastewater treatment plants, located in Liberec, Czech Republic. The BOD values of the inorganic medium and inoculum without bioplastic fibers (BODb) were used as the blank. The following equations were utilized to determine biochemical oxygen demand for the sample (BODf) BODf (mg/ml) = BODt + BODb where BODt represents the value of sample BOD in the bottle. 12 ACS Paragon Plus Environment

(4)

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Results and discussion Electrospinning Characterization of GA (U), GA (P), GA () bioplastic fibers The electrospinning of tree gums and other biopolymers such as chitosan, chitin, cellulose, proteins, gelatin, DNA, polypeptides, collagen, silk fibroin and hyaluronic acid are complex processes due to their solution behaviors, high molecular weights and viscosities.25,26 Numerous parameters such as viscosity, surface tension, conductivity, polymer concentration, solubility and electrical voltage etc. have all directly influence the production of nanofibers obtained by electrospinning.18,25 Solution parameters such as polymer blend concentrations, viscosity, surface tension and conductivity of the electrospinning-blended solutions of GA/PVA were optimized.20 GA (up to 25 wt.%) is usually prepared as a stock solution for the electrospinning. In the present communication, a 20 wt. % of GA aqueous solution was used for blending with PVA (10 wt.%), and subsequently mixing the ratio of GA/PVA (50/50 : wt./wt.%) individually from the stock solution of GA (20 wt.%) and PVA (10 wt.%), respectively. The blended mixture of GA/PVA (2:1 ratio of GA:PVA polymer concentrations and with respective wt./wt. percentage ratios of 50:50 of GA: PVA) resulted in uniform morphology and defect-free nanofibers are shown in (Fig. 2 a). The prepared GA bioplastic fibers with average diameters of 120±50 nm were created using a blend solution of GA/PVA (50/50 wt. %) (Fig. 2 a). In the present investigation, we had increased the amount of GA in the blend solution (GA/PVA; 2:1 wt. % polymer solution ratio) – equivalent to double the amount of GA compared to the previously used blend solution of GA /PVA (1:1wt. % ratio) by the electrospinning method, as reported in our previous



communication.20 GA bioplastic fibres were cross-linked by subjecting them to heat treatment; placing the fibers in an oven for 2 h. at 80 oC. 20 The SEM images of GA (P): [methane plasma treatment time, 1.0 min.] and GA (): [-ray irradiated by radiation dose, 25 kGy] bioplastic electrospun fibers are presented in Fig. 2 (b and c). For GA (U), the same composition [GA/PVA: (50/50) wt. %] (Fig. 2 a), generated bioplastic fibers with clearly visible smooth surfaces. This is in contrast to the GA (P) and GA () bioplastic electrospun fibers which were remarkably roughened by both treatments (methane plasma & - ray irradiation), as depicted in Fig. 2 (b and c) Both plasma and -ray irradiation influence the membrane chemistry, thus affecting the hydrophilicity/hydrophobicity ratio; surface roughness; surface cleaning; etching; cross-linking; the creation of additional functional groups e.g. [hydroxyl (–OH), carbonyl (–C=O) & carboxylate (–COOH).] and polymerization/grafting. 27,28 Table 1 presents a comprehensive comparison of the physicochemical and structural properties of methane plasma [GA (P):

(treatment time; 1.0 min.)] and - ray irradiated [GA ():

(irradiation dose, 25 kGy)] and untreated bioplastics of GA [GA (U)]. The contact angles of GA (U) and GA () are 60.5o and 80o respectively, with both showing hydrophilic properties. However, for GA (P), the water contact angle was found to be 130.8o indicative of its hydrophobic properties. The effect of methane plasma treatment has thus enhanced the hydrophobicity of the bioplastic fibers while also increasing the surface area (Table 1).


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Fig. 2: Bioplastic fibers of GA obtained by electrospinning of GA/PVA (50/50 wt. %) blended solutions (a) GA (U); (b) GA (P) and (c) GA (), respectively. The BET measurements of methane GA () and GA (P) bioplastic fibers were observed to be 9.5±0.5 and 10.6±0.8 m2g-1, respectively, figures that are higher than the corresponding values for GA (U) bioplastic fibers (Table 1). Due to plasma treatment and γ-irradiation, GA (P) and GA (γ) displayed much higher surface area and mechanical properties compared to GA (U)



bioplastic fibers (Table 1). In both GA (P) and GA (γ) bioplastic fibers, the crosslinking reaction induced by esterification between carboxylic groups of GA and hydroxyl groups of PVA (after treatments) was also confirmed using ATR-FTIR analysis (Fig. 3). This can be ascribed to the hydrophobic surfaces generated by methane plasma and also to the improved surface wetting attributes and cross-linking of the bioplastic fibers.28 These results indicate that both plasma treatment and γ-ray irradiation can be effectively used for switching the hydrophilic/hydrophobic properties of GA bioplastic fibers, a feature that could be functionalized in many potential applications. ATR-FTIR spectroscopy The ATR-FTIR spectra of GA, PVA, GA (U), GA (P) and GA (γ) are illustrated in Fig. 3. The peak at 3300 cm-1 relates to the OH group while the peaks at 1159, 1082 and 1014 cm-1 (in the fingerprint region) correspond to –C-O-C- stretching vibrations of various constituent neutral and acidic molecular moieties present in the structure of GA.20 The spectral response at 2923 cm-1 represents the characteristic vibration of C–H stretching and the two peaks at 1430 cm-1 and 1326 cm-1 are indicative of the C–H deformation vibrations in PVA.20 The absorption registered between 1000 and 1100 cm-1 can be assigned to C–O stretching and O–H bending vibrations arising from the PVA chain. The emergence of a new peak at 1563 cm-1 in the GA (U) sample, represents the deformation vibration of the OH group implying the presence of hydrogen bonding between PVA and GA, forming PVA-GA, the bioplastic fiber of GA (U) (Fig. 3). The gums interact with PVA leading to nanofiber formation and the generation of hydrogen bonding between –OH groups of PVA and –COO and –OH groups of other tree gums. The additional functionality observed in GA (P) and GK (γ) is due to surface modification. Alternatively, it may


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be consequence of polymerization or grafting of hydroxyl (-OH), carbonyl (–C=O), and carboxylate (–COO-) groups via plasma treatment and -ray irradiation.27,28

Fig. 3: ATR-FTIR spectra of GA, PVA, GA (U), GA (P) and GA (). Roughness profile by AFM analysis The roughness (Ra) of the textures associated with GA (U), GA (P) and GA () determined by AFM examination are shown in Fig. 4. Average Ra values for GA (U), GA (P) and GA () were 65.85, 237.8 and 334.34 nm, respectively. The highest Ra was exhibited by GA (P) (Fig. 4 c), the inference being that GA (P) and GA () bioplastic materials, due to their high degree of nanometer surface roughness, could be utilized in various tissue engineering applications.29



Fig. 4: Surface roughness analysis by AFM for (a) GA (U); (b) GA () and (c) GA (P) electrospun fibers. Mechanical properties


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The mechanical properties of the GA (U), GA (P) and GA () electrospun bioplastic fibers were characterized. Their attributes (tensile strength, and elongation at break (%)) are summarized in Table 1 while the corresponding tensile stress-strain curves are shown in Fig. 5.

Fig. 5: Typical tensile testing plot of stress-strain curves for GA (U), GA (P) and (c) GA () bioplastic fibers. For the GA (U) sample, the highest ultimate tensile stress (16.1±1.8 MPa), and the greatest elongation at break (26.62.4 %) were observed [Table 1]. The GA () and GA (P) samples exhibited higher tensile strengths (45.4±8.8 & 52.3±9.5 MPa, respectively) compared to GA (U). However, GA () and GA (P) both displayed lower elongation at break (23.5±2.1 & 20.1±1.8 %, respectively) [Table 1]. Plasma treatment and -ray irradiation affected the mechanical properties 19 ACS Paragon Plus Environment


of GA bioplastic fibers,

as both the treated GA(P) & GA() bioplastic fibers showed

enhancement of tensile strengths and Young moduli – in comparison with GA (U) – whereas a reduction in elongation at break was recorded for GA(P) & GA(). These results corroborate what has been reported for PLLA (poly-L-lactide)/TAIC (triallyl isocyanurate) and PCL [Poly (-caprolactone)] based nanofibers.30,31 These results indicate that both plasma treatment and ray irradiation facilitate the formation of stable cross-linking networks among PVA and GK polymeric chains, thus influencing the flexibility of polymeric chains while also enhancing the tensile strengths and Young’s moduli of bioplastic fibers. XRD analysis The XRD patterns of GA, PVA, GA (U), GA (P) and GA (γ) bioplastic fibers are shown in supplementary Fig S2. The patterns were evaluated to assess the phase changes occurring during the formation of GA bioplastics and their plasma treated and γ-ray irradiated samples, GA itself being amorphous in nature.32 The semi-crystalline nature of PVA was reflected by 2θ peaks exhibited at 32o and 46o,33 2θ being the angle between the transmitted X-ray beam and reflected beam while θ is the angle between the incident beam and crystallographic reflecting plane. The GA (U) bioplastic fibers showed 2θ peaks at 20o and 30o, the slight shift indicating the presence of crystalline structures in GA (U). The GA (P) fiber clearly yielded 2θ peaks at 21º and 32º revealing crystallinity during plasma treatment of the bioplastic fibers. The GA (γ) bioplastic fibers also produced some peaks, although not prominent, in the diffractogram (Supplementary Fig. S2) signalling the phase change ensuing from increasing the crystallinity index during γirradiation. These results can be ascribed to the fact that blending PVA with GA (in the proper proportions) promotes crystallinity of the bioplastic fibers.34


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TG Analysis Fig. 6 shows the thermogravimetric analysis (TGA) curves for GA (U), GA (P) and GA () bioplastic fibers. The thermal decomposition profiles of all the samples display a one-stage decomposition pattern, illustrating the thermal stabilities of the bioplastic fibers [GA (U), GA (P) and GA ()]. It enabled their onset degradation temperatures (Tonset) [viz. 305, 311 and 310 oC,

respectively] to be found. Compared to GA (U), the GA (P) and GA () bioplastic fibers

achieved slightly improved thermal stabilities. Furthermore, the thermal stabilities of GA (U), GA (P) and GA () bioplastic fibers appeared to have been boosted considerably, as inferred by their increase in char yield viz. 8.74 % [GA (U)], 16.3 % [GA (P)] and 18.6 % [GA ()], at 700 oC,

respectively. These results suggest that both plasma treatment and -ray irradiation

facilitated the formation of cross-linking network structures (between PVA and GK) leading to higher thermal stability, compared to untreated GA fibers. However, both the plasma treatment time and -ray radiation dose are variables that affect the thermal stability of GA bioplastic fibers, reminiscent of the PLLA bioplastic.30



Fig. 6: TGA curves of GA (U), GA (P) and (c) GA () bioplastic fibers

Stability of bioplastics in the presence water The stabilities GA (U), GA (P) and GA () were tested for their water strength and the results [time versus mass loss (%)] are presented in the Fig. 7. Given that the water contact angle (WCA) of GA (U) was 60o, the bioplastic lost almost 100% of its mass within 1 h of being in water, by the process of dissolution (Fig. 7). The bioplastic GA (P) displayed a maximum WCA of 130o. However, in the case of GA (), the WCA was reduced to 80o after -ray irradiation. The GA (P) demonstrated a mass loss of merely 15% after 48 h of interaction with water, a reflection of its superior hydrophobic character. WCA decreased as a function of the irradiation doses due to formation of oxygen containing groups and the increase in surface roughness of GA () samples. This has been confirmed by SEM, AFM, ATR-FTIR and weight loss analysis (Figs. 2, 3, 4 & 7).

A higher WCA value was observed in GA (P) compared to GA (U) and GA () bioplastics. During plasma treatment, four processes occurred namely etching, activation, cross-linking and carbonization.35,36 However, as a consequence of -ray interaction, the WCA of GA () was reduced owing to surface oxidation, as well as the formation of C=O groups – thus boosting surface roughness; significant of other polysaccharides such as starch, chitosan films and bioplastic fibers.37,38


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Fig. 7: The interaction of water with modified GA bioplastic fibers of GA (U), GA () and GA (P)

Antibacterial properties of bioplastic fibers Three variations of GA bioplastics viz. GA (U), GA (P) and GA () were tested for their efficiency against common food pathogens such as Gram-positive Staphylococcus aureus (S. aureus, ATCC 12600) and Gram negative Escherichia coli (E. coli, ATCC 25922) bacteria. Whereas GA (U) does not show any antibacterial action against the common food pathogens, both GA (P) and GA () fibers exhibited potential activity in combating the tested bacteria (S. 23 ACS Paragon Plus Environment


aureus and E. coli, respectively). The effects of sterilizing bacterial colonies by means of ray irradiation on various polymeric fibers, based on chitosan, carrageenan, cellulose, CMCsodium alginate and polypyrrole-based biomaterials, has been reported.39-41 In this research, γGA (irradiation dose, 25 kGy) could be sufficient to inactivate all the tested bacteria after 4 h of radiation. As shown in Table 2, the antibacterial effectiveness of GA (γ) bioplastic fibers against S. aureus was >99% but ~ 95% when confronting E. coli. In contrast, GA (P) displayed ~ 90% & ~80% effectiveness in nullifying S. aureus and E. coli, respectively. The differences observed in the inactivation of various strains of bacteria by GA (γ) and GA (P) bioplastics could be attributed to the following causes: structural factors and alterations in physicochemical properties; the duration and dosage of both plasma treatment and γ-ray irradiation and the structural and chemical composition of the bacterial cell wall.42-44 Plasma intensity was found to correlate well with antibacterial effects under higher plasma power. The mechanism of toxicity for GA (P) is associated with the action of plasma on the bacterial cell wall, which could in turn enhance the release rate of nanofibers and plasmagenerated reactive ingredients (atoms, photons, charged ions, free radicals, excited molecules and UV photons).45,46 This is further supplemented by various aspects of the material’s surface that influence bacterial adhesion e.g. types of functional groups, electrical charge density, crystallinity, surface roughness, hydrophilicity etc.

47,48

The surface roughness after plasma

treatment which can promote bacterial adhesion resulted in altered surface chemistry, as well as changes in the topography of the GA (P) bioplastic. Surface roughness of the GA (P) was observed to have significantly increased in our present study (Fig.4), resulting in improved antiadhesion performance of the bacteria. This indicates that the selected Gram-positive bacteria (S. aureus) possessed higher sensitivity than the Gram-negative bacteria (E. coli).


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The higher antibacterial action of GA (γ) could be ascribed to factors influencing the radiation process. Our data shows that γ-ray irradiation alters the morphology of the surface, and simultaneously improves the wettability of GA (γ). Irradiation is a process utilizing ionizing radiation, namely radiant energy with the potential to penetrate and break strong chemical bonds in microorganisms and pathogens – in order to sterilize and inactivate microorganisms by damaging genetic materials (such as chromosomal DNA) which is a primary target of ionizing radiation, other targets being cytoplasmic membranes, enzymes and plasmids.49, 50 The effects of improved hydrophilicity of the GA (γ) are threefold: it enhances the adhesion strength, surface chemistry and surface roughness. Elevated antibacterial efficiency of GA (γ) may be attributed to the increased hydrophilicity and surface roughness of the membrane (a result of γ-ray irradiation) which helps to increase contact between surfaces of the fiber membrane and the bacteria. As a consequence, the properties of the ensuing products [such as polysaccharide type; degree of substitution (DS); concentration of the solution being irradiated; solvent type; absorbed dose and the prevailing atmosphere during irradiation] all play a vital role.51-53 Furthermore, the green polymer-based GA bioplastic fibers (after γ-ray irradiation and plasma treatment) could extend the shelf-life of the foodstuffs by controlling the growth of pathogenic bacteria. This has scope and potential for the development as environmentally friendly packaging material.54-56 Antioxidant activity via ABTS·+ radical scavenging capacity The antioxidant properties of GA (U), GA (P) and GA () bioplastic powders were determined and presented in Fig. 8. The GA (U) bioplastic showed >90% inhibition of the ABTS radical cation whereas GA (P) and GA () exhibited ~50% and 60 % scavenging effectiveness, respectively, resulting in strong antioxidant properties of GA bioplastic fibers.



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Fig. 8: Antioxidant activity scanning of GA powder and GA bioplastic fibers [GA (U), GA (P) and GA ()], as determined by ABTS·+ radical scavenging activity analysis.

In GA, by virtue of the amino acid (tyrosine, histidine and methionine) composition of its inherent

structure,

exhibited

potential

benefits

viz.

anticancerous;

antimalarial;

immunomodulatory effects; cytoprotectivity and the ability to act as an antioxidant agent in combating sickle cell anemia.57-60 The antioxidant attributes of natural polymers such as chitosan, alginate and carrageenan have been improved by -irradiation.61,62 Enhancement of the integral antioxidant properties of GA bioplastics (brought about by -irradiation and plasma


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treatment) would lengthen the shelf life, thus rendering GA bioplastics viable materials for use in the packaging industry. Oxygen barrier properties Gases, water and fragrance transference through packaging materials are the major factors which determine barrier performance because they affect the shelf life and the quality of food items packaged.11, 63 The oxygen permeability coefficients of GA plastic fibers/films [GA (U), GA () and GA (P)] and other common materials (synthetic and bioplastic films or fibers) are shown in Table. 3. Oxygen permeability coefficients were ascertained to be 30.5, 29.9 and 27.1 cm3m/m2 day kPa, for GA (U), GA () and GA (P), respectively. The observed oxygen permeability coefficients for GA bioplastics (Table. 3) were much lower compared to those for PVA, polyethylene, hydroxypropyl methylcellulose, chitosan and vegetable waste-PVA bioplastics.5,63-65 No statistically significant differences in oxygen permeability values were evident between GA() and GA(U). However, GA (P) showed a substantial difference in its oxygen permeability coefficient, in comparison with untreated and -ray irradiated GA bioplastic samples (Table. 3). The results perceived in our studies of GA (U) and GA () corroborated well with oxygen barrier properties of LDPE (low-density polyethylene); HDPE (high-density polyethylene); PET (polyethylene terephthalate); PVC (Polyvinyl chloride) and PLA, Poly(lactic acid) in that no significant changes have been observed for non-irradiated and irradiated (1 to 30 kGy dosage) samples during nine months of storage.66-69 Migration of components from bioplastic fibers into food The major factors governing the migration of constituents into food are the physicochemical properties (density, crystallinity, crosslinking, branching and glass transition temperature) of the packing materials; the nature of the migratory species; the quality of the simulant (size, 27 ACS Paragon Plus Environment


shape and charge) and temperature. 70,71 Bioplastic fibers [GA (U), GA (P) and GA ()] were tested with regard to migration of components into a food stimulant such as Tenax, so as to mimic the migration into dry foods e.g. cereal.72-74 The observed food contact migration for GA (U), GA () and GA (P) were determined to be 4.81.2, 4.20.9 and 3.00.8 mg/dm2, respectively. Results indicated that, for all three bioplastic variations tested, the migration intensities are under the limit (below 10 mg/dm2) set by European Union legislation.75 The current work focuses on potential materials for food packaging applications, similar to the bioplastic materials used, based on vegetable wastes;5 PHBV a.k.a. Poly(3-hydroxybutyrate-co3-hydroxyvalerate) with 8 mol% valerate76; PLA77 and chitosan-poly (butylene adipate-coterephthalate).54 Biodegradation characteristics of bioplastic fibers The impact of plastic biodegradation rates on the natural action of microorganisms (in a short time frame) has negligible eco-toxicological consequences. The major factors affecting the biodegradation process are molecular weight of the material; crystallinity; molecular configuration; temperature; humidity; pH; accessibility of oxygen and enzymatic activity.78,79 Furthermore, biodegradable packaging materials have assumed greater importance in the marketplace in order to reduce the amount of packaging waste deposited in landfill sites. The BOD values of GA bioplastic fibers are presented in Fig. 9, the results indicating that BOD assessment for GA (U) was higher (175.515.8 mgO2/L) compared to that recorded by GA () and GA (P) which, after 15 days, were 160.413.2 and 105.910.5 mgO2/L, respectively.


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Fig. 9: Representative biological oxygen demand for GA bioplastic fibers [GA (U), GA (P) and GA ()] over a period of 15days, illustrating the biodegradation characteristics.

Greater degradation occurred in GA (U) compared to GA () and GA (P) bioplastic fibers, implying that microorganisms present in the sludge could act on untreated GA. This, however, would be influenced by both acclimatization time and the greater extent of biodegradation. Our results suggest that GA bioplastic fibers biodegraded more rapidly (within 15 days) on par with cellulose-based bioplastics;5,79,80 starch-based bioplastic fibers, in contrast, required 30 days for complete degradation.81 Moreover, our investigation serves to highlight both the enhancement of 29 ACS Paragon Plus Environment


the biodegradation rate and the mechanical properties of blended bioplastic fibers (GA in conjunction with PVA) as reported for polyhydroxyalkanoate (PHA) blended distiller’s dried grains with soluble (DDGS)82 viz. the nutrient rich co-product of dry-milled ethanol production. Other examples include grafted PHB, soymeal bended PBAT (poly (butylene adipate-coterephthalate), poly (lactic acid) and poly (3-1 hydroxybutyrate-co-3-hydroxyvalerate) onto cellulose bioplastics.76, 83-85. Conclusions Gum Arabic (GA)-based electrospun bioplastic fibers were fabricated for the first time using GA blended with PVA (2: 1 wt.% of GA/PVA) via ‘green’ electrospinning process and their prospects for food packaging applications were further explored. The desirable attributes of gum arabic are its capacity to switch from being hydrophilic to hydrophobic; the enhancement of its physicochemical traits and its inherently advantageous mechanical properties. The favourable food packaging characteristics of its three bioplastic variants (untreated, plasma treated and -ray irradiated), render this natural, highly-branched, arabinogalactan polysaccharide (exuded by tapped branches of acacia senegal trees) an ideal choice for this undertaking. Both, plasma etched and γ-ray irradiated bioplastic fibers, showed significant increases in their surface chemistry, surface roughness, crystallinity, tensile strength and thermal properties, compared to their untreated counterparts. Improved hydrophobicity and low mass loss (after interacting with water) were observed for GA plasma treated fibers. However, hydrophilic character is retained in both the untreated and γ-ray irradiated GA bioplastics. The GA () bioplastic fibers demonstrated markedly discernible and significant antibacterial activity against tested pathogenic bacteria, in the assessments of plasma treated and untreated GA bioplastics. The study of low oxygen barrier properties; potent antioxidant 30 ACS Paragon Plus Environment

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activity and promising biodegradability of the GA bioplastic fibers/films, for all three variants, comply with the European Union ‘Food Contact Migration Limit’ (< 10 mg/dm3) legislation. – These aforementioned attributes make GA an interesting, and environmentally sustainable product for use as breathing food-packaging material for biodegradable bags. This is in addition to other applications in the pharmaceutical industries and agricultural arenas.

Conflict of Interest: Authors declare no conflict.

ASSOCIATED CONTENT Supporting Information The following supporting information is available online: Gum Arabic (GA) preparation, determination of molecular mass of GA using GPC-MALLS (gel permeation chromatography multi-angle laser light scattering), and XRD analysis of GA, PVA, GA (U), GA (P) and (c) GA () bioplastic fibers.

Acknowledgements The research reported in this paper was financially supported by the Ministry of Education, Youth and Sports, in the framework of the targeted support of the “National Program for Sustainability I” LO 1201 and the OPR & DI project “Extension of CxI facilities” (CZ.1.05/2.1.00/19.0386). The authors would also like to acknowledge the assistance provided by the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic in the framework of Project No. LM2015073. This work was also supported by the Ministry of Education, Youth and Sports of the Czech Republic and the



European Union – European Structural and Investment Funds in the frames of Operational Program Research, Development and Education – project Hybrid Materials for Hierarchical Structures (HyHi, Reg. No. CZ.02.1.01/0.0/0.0/16_019/0000843).

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Page 38 of 41

Table 1: Comparison of physicochemical and mechanical properties of plasma treated, γ-ray irradiated and untreated GA electrospun bioplastic fibers*

Bioplastic

Fiber

BET

Water

Tensile

Elongation

Fiber

Diameter

Variant

(nm)

Surface Area (m2g-1 )

Contact Angle (Degrees)

Strength

at Break

(MPa)

(%)

GA (U)

175±25

5.2±0.4

60.5

16.1±1.8

26.6±2.4

GA (γ)

225±50

9.5±0.5

80.0

45.4±8.8

23.5±2.1

GA (P)

250±80

10.6±0.6

130.8

52.3±9.5

20.1±1.8

*Data presented are representative of three independent experiments; Values: Mean ±S.D (n=3); Abbreviations: GA (U), GA (P) and GA (γ) represent untreated, plasma treated (1.0 minute treatment time) and γ-ray irradiated (25 kGy radiation dosage) bioplastic fibers, respectively.

Table 2: Antibacterial efficiency of GA bioplastic fibers (GA (U), GA (P) and GA () against pathogenic bacteria (E. coli, S. aureus and P. Auregenosa) after incubation at R.T for 48 h. Initial bacterial concentration: 3.0 106 CFU/mL.

Samples tested

Bacterial colony

Surviving bacteria (CFU/mL)a


Reduction rate (%)

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GA (U)

GA (P)

GA ()

E. coli

3.0106

0

S. aureus

3.0106

0

P. auregenosa

3.0106

0

E. coli

6.0105

80.00.5

S. aureus

1.3105

95.60.4

P. auregenosa

1.7105

94.20.8

E. coli

1.5106

95.00.5

S. aureus

2.7104

99.10.4

P. auregenosa

2.4104

99.20.4

Number of colonies a

Number of colony forming units (CFU/mL) = Dilution factor × volume (mL)

Table 3: A comparative analysis of oxygen permeability behavior of various natural/synthetic materials with GA bioplastic fibers/films

Bioplastic fibers or films

Oxygen permeability

Reference

(cm3m/(m2 day KPa) High-density polyethylene (HDPE)

42.7

64

Low-density polyethylene (LDPE)

1870

64

Hydroxypropyl methylcellulose

182.3

65

Chitosan

32.8

65

PVA

46.8

63



Page 40 of 41

5

Carrot/PVA

31.2

GA (U)

30.5

current study

GA (P)

27.1

current study

GA ()

29.9

current study

TABLE OF CONTENTS (TOC) GRAPHIC

Synopsis: Sustainable applications of gum Arabic bioplastics in food packaging are presented for untreated, plasma treated and -ray irradiated formulations.


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ACS Paragon Plus Environment

Bioplastic Fibers from Gum Arabic for Greener Food Wrapping

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