Food Chemistry 141 (2013) 3192–3200

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Food Chemistry

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Evaluation of gallic acid loaded zein sub-micron electrospun fibre matsas novel active packaging materials

0308-8146/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.foodchem.2013.06.018

⇑ Corresponding author. Address: School of Chemical Sciences, The University ofAuckland, Private Bag 92019, Auckland, New Zealand. Tel.: +64 9 3737599; fax: +649 3737422.

E-mail addresses: conradperera@gmail.com, c.perera@auckland.ac.nz(C.O. Perera).

Yun Ping Neo a, Simon Swift b, Sudip Ray a, Marija Gizdavic-Nikolaidis a, Jianyong Jin a, Conrad O. Perera a,⇑a School of Chemical Sciences, The University of Auckland, New Zealandb Molecular Medicine & Pathology, School of Medical Sciences, The University of Auckland, New Zealand

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 February 2013Received in revised form 2 May 2013Accepted 4 June 2013Available online 13 June 2013

Keywords:ElectrospinningZeinGallic acidActive packagingAntimicrobial

The applicability of gallic acid loaded zein (Ze-GA) electrospun fibre mats towards potential active foodpackaging material was evaluated. The surface chemistry of the electrospun fibre mats was determinedusing X-ray photon spectroscopy (XPS). The electrospun fibre mats showed low water activity and whit-ish colour. Thermogravimetric analysis (TGA) and Attenuated Total Reflectance-Fourier Transform Infra-red (ATR-FTIR) spectroscopy revealed the stability of the fibre mats over time. The Ze-GA fibre matsdisplayed similar rapid release profiles, with Ze-GA 20% exhibiting the fastest release rate in water ascompared to the others. Gallic acid diffuses from the electrospun fibres in a Fickian diffusion mannerand the data obtained exhibited a better fit to Higuchi model. L929 fibroblast cells were cultured onthe electrospun fibres to demonstrate the absence of cytotoxicity. Overall, the Ze-GA fibre mats demon-strated antibacterial activity and properties consistent with those considered desirable for active packag-ing material in the food industry.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The customisation of food packaging to extend shelf life, whilesustaining the quality of the product within, is a concept termedactive packaging, which has received interest in recent years in re-sponse to market changes and consumer demands (Vermeiren,Devlieghere, van Beest, de Kruijf, & Debevere, 1999). For example,a good active packaging design may incorporate antioxidant andantimicrobial agents into a biodegradable material with excellentgas and liquid barrier properties. In particular, food packaging withantimicrobial properties has received attention in recent years dueto the commercial and health problems associated with outbreaksof illness due to foodborne pathogens (Newell et al., 2010), and thequest to extend the shelf life of food products through milder pres-ervation techniques (Vermeiren et al., 1999). To date, active pack-aging approaches have been extended to biopolymer-basedpackaging involving coatings and films created from proteins, poly-saccharides and lipids (Devlieghere, Vermeulen, & Debevere, 2004;Jumaa, Furkert, & Müller, 2002; Oussalah, Caillet, Salmiéri, Saucier,& Lacroix, 2004).

Biodegradable biopolymer-based materials are able to provideinnovative solutions to environmental problems through the

reduction of waste injection into landfills, and also as alternativesto materials produced from depleted natural resources (Rhim & Ng,2007). Such biopolymer-based packagings are usually prepared viasolution casting and extrusion. Nevertheless, Vega-Lugo and Lim(2009) had suggested that the controlled release of bioactives fromconventional biopolymer films may be constrained due to the lim-ited surface area available from such packaging. As a result, re-search efforts have been focused on the fabrication ofnanostructured active biopolymer-based packaging through anelectrospinning process (Dheraprasart, Rengpipat, Supaphol, & Tat-tiyakul, 2009; Pérez-Masiá, López-Rubio, & Lagarón, 2013).

Electrospinning is a simple, versatile and low cost method forproducing fibres on the scale of few nanometres to micrometres.It involves the application of an electrical field to continuouslydraw the polymer solution from a syringe needle towards agrounded collector (Neo, Ray, Easteal, Nikolaidis, & Quek, 2012).The electrospun fibrous mats generally exhibit a large surface areato volume ratio due to the high porosity and nano to sub-micronstructure of the fibres. The advent of electrospinning has openedup new prospects towards the development of nano-architecturedmaterials with enhanced properties for applications as food con-tact materials, including the packaging of food for retail or otherapplications as well as for the storage, transport and processingof agricultural products (Bradley, Castle, & Chaudhry, 2011).

Natural phenolic compounds have been documented to possesspotent antioxidant and antimicrobial properties. Research has beenconducted to incorporate different types of phenolics into active

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packaging materials, as natural ingredients are deemed to be pre-ferred by the consumers (Alkan et al., 2011). Gallic acid was se-lected as the model phenolic compound in the present studiesdue to its simple structure, good antioxidant properties and ubiq-uity in plants. Zein is a prolamin extracted from corn with distinc-tive amino acid profile (high in proline and glutamine). Zeinportrays low solubility in aqueous solutions and has been proposedto have barrier properties for transport of gases, water vapour orsolutes (Hsu, Weng, Liao, & Chen, 2005). Zein films have been ap-plied as edible coatings on nuts to delay rancidity, and on tomatoesto delay colour changes, weight loss and to maintain firmness dur-ing storage (Cagri, Ustunol, & Ryser, 2004). Our previous studieshave demonstrated successful incorporation of gallic acid in zeinelectrospun fibres at different loading ratios with average fibrediameters ranging from 328 to 388 nm (Figure S1, Supplementaryinformation, Neo et al., 2013). In this study we have, for the firsttime, investigated the potential of gallic acid loaded zein (Ze-GA)electrospun fibres as a prospective active packaging. Our findingsprovide evidence for the efficacy and effectiveness of Ze-GA elec-trospun fibre mats for food contact applications, by evaluatingtheir surface chemistry, water activity, optical properties, storagestability, release performance, mechanism of action, cytotoxicityand antimicrobial abilities.

2. Materials and methods

2.1. Materials

Zein (Z 3625), phosphate buffered saline (PBS) tablets, resazurinsodium salt and Tween 80 were purchased from Sigma Aldrich (St.Louis, MO, USA) and used without further purification. Gallic acidstandard was obtained from Acros Organics (Geel, Belgium). Etha-nol (ACS grade) was purchased from ECP Ltd. (Auckland, New Zea-land). Milli-Q water (18 MXcm, Millipore, Bedford, MA, USA) wasused for all solution preparations. For mammalian cell cultureexperiments using murine fibroblast cell line L929 (ATCC CCL-1),Dulbecco’s Modified Eagle Medium (DMEM), Trypan blue solutionand foetal calf serum (FCS) were obtained from Life Technologies(Gaithersburg, MD, USA). Cell culture was performed in 24-welltissue culture plates (TCP) (Nunc, Denmark) and fluorescence read-ings were taken in black 96-well plates (Perkin Elmer, MA, USA),formaldehyde was obtained from Scharlau (Bacelona, Spain). Forthe culture of Candida albicans ATCC 10231, Escherichia coli ATCC2922 and Staphylococcus aureus ATCC 6838, brain heart infusionbroth (BHI) and Difco agar were purchased from Fort Richard(Auckland, New Zealand).

2.2. Electrospinning process

The zein and Ze-GA solutions were prepared and electrospun asdescribed in our previous studies (Neo et al., 2013). Briefly, zeinsolution was prepared at a concentration of 25% (w/w) by dissolv-ing zein powder in 80% ethanol aqueous solutions (etha-nol:water = 4:1 w/w) under constant stirring using a magneticstirrer at 21 �C and was designated as Ze. Zein electrospun fibrescontaining 5%, 10% and 20% (w/w) of gallic acid content in the solidfibres were prepared by first dissolving gallic acid in 80% ethanolaqueous solutions, followed by dissolving zein powder to obtain25% (w/w) zein solutions. These solutions were designated as Ze-GA 5%, Ze-GA 10% and Ze-GA 20%, respectively. The solutions wereplaced in 5 ml syringes that were driven by a syringe pump (ColeParmer, Vernon Hills, IL, USA) to give a solution feed rate of0.8 ml h�1. The positive electrode of a direct current (DC) powersupply (High voltage power supply series 230, Bertan, Hicksville,NY, USA) was connected to a 20 gauge needle of a syringe and

the negative terminal to a grounded collector covered with alumin-ium foil. The applied voltage was 16 kV with a distance betweenneedle tip and collector of 13 cm.

2.3. Surface characterisation

The surface chemistry of the Ze and Ze-GA fibres was examinedusing a Kratos Axis UltraDLD X-ray Photon spectroscopy (XPS). XPSmeasurements were carried out using monochromatic Al Ka X-rays (1486.69 eV) with the X-ray source operating at 150 W. Theanalysis area was a 300 by 700 lm spot. The core level scans werecollected with 20 eV pass energy and the analysis chamber was at10�9 Torr throughout data collection. Energy calibration was per-formed using the C 1s line at 285 eV as reference for Ze and Ze-GA fibres, but 284.7 eV for gallic acid powder. A charge neutralisersystem was used to compensate for sample charging. Data analysiswas performed using CasaXPS and the measurements were carriedout in duplicate.

2.4. Water activity

Water activity of the Ze and Ze-GA fibre mats were measuredusing a water activity meter Aqualab 4TE (Decagon, Pullman,WA, USA). The measurements were carried out in triplicate.

2.5. Colour measurement

Instrumental colour analyses of the Ze and Ze-GA fibre matswere determined using a CR300 colorimeter (Minolta, Osaka, Ja-pan) to provide background information of their optical properties.L value describes lightness, where L = 0 is black and L = 100 is closeto white; a negative a represents greenness and a positive a is red-ness; a negative b indicates blueness while a positive b is yellow-ness. Measurements were made directly on the fibre mats andthe standard values for the white calibration plate wereL = 96.86, a = �0.02 and b = 1.99. The measurements were carriedout in triplicate.

2.6. Stability test

Ze and Ze-GA fibre mats were stored individually in zip-lockplastic bags in a dark environmental chamber for different periodsof time at 21.5 �C, relative humidity (RH) of approximately 58%.After 1, 20, 40 and 60 days of storage, Thermogravimetric analyses(TGA) and Attenuated Total Reflectance-Fourier Transform Infrared(ATR-FTIR) spectroscopy were performed on the electrospun fibremats. TGA was conducted using a TGA-Q5000 (TA Instrument,New Castle, DE, USA) equipment under nitrogen (flow rate of25 ml/min) at a heating rate of 10 �C/min from 40 to 600 �C. Themeasurements were carried out in triplicate. The ATR-FTIR spectrawere recorded on a Thermo Electron NICOLET 8700 FTIR spectro-photometer using the Smart Orbit ATR accessory with diamondcrystal, single bounce at 45� (Thermo Electron Corporation, Wal-tham, MA, USA) over the wave number region of 600–3600 cm�1.Interferograms were averaged for 32 scans at 4 cm�1 resolution.The measurements were carried out in triplicate. The signals wereprocessed using the OMNIC spectroscopic software.

2.7. Release assay

The release characteristic of gallic acid from the Ze-GA electro-spun fibres were determined by a modification of total immersionmethod by Tungprapa, Jangchud, and Supaphol (2007) in Milli-Qwater. Briefly, 20 mg of the electrospun fibre mat was submergedin 40 ml of Milli-Q water at 23 �C under gentle stirring. Fifty micro-litres of the solution was withdrawn at specified time intervals,

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ranging between 0 and 125 min and diluted with 450 ll of Milli-Qwater. The sample solutions were filtered through a cellulose ace-tate 0.45 lm (Phenomenex, Torrance, CA, USA) syringe filter beforethe subsequent HPLC injection. The amount of the released gallicacid was quantified by HPLC using a Phenomenex Luna C18 col-umn (250 mm � i.d. 4.6 mm, 5 lm particle size) according to thepreviously published method by Neo, Ariffin, Tan, and Tan (2010).

2.8. Release mechanism

In order to examine the mass transport mechanisms of theincorporated gallic acid, the release profile of gallic acid from Ze-GA fibres in Mili-Q water (hydrophilic model) at 23 �C was fittedto Ritger and Peppas equation:

Mt=M1 ¼ ktn ð1Þ

where Mt is the quantity of gallic acid released at any time t; M1 isthe quantity of gallic acid at the last t of the measurements, whichin principle corresponds to the initial loading; k is the release rateconstant, and n indicates the release exponent. The release expo-nent (n) was determined by minimizing the difference betweenEq. (1) and a logarithmic plot of the experimental curves usingthe solver tool in Microsoft Excel. In addition to that, four more ki-netic release models were used to further analyse the release profileby fitting the experimental data to diffusion models as followed:

Zero-order model : Mt=M1 ¼ kt ð2Þ

First-order model : lnð1�Mt=M1Þ ¼ �kt ð3Þ

Higuchi model : Mt=M1 ¼ kt1=2 ð4Þ

Hixson� Crowell model : ð1�Mt=M1Þ1=3 ¼ �kt ð5Þ

2.9. Cytotoxicity assays

The resazurin metabolic assay was conducted to determine bio-compatibility of Ze and Ze-GA fibre mats to murine connective tis-sue (L929) fibroblast cells by following a slight modification toprevious studies (Serrano et al., 2004) as explained below.

2.9.1. Cell cultureThe L929 fibroblast cells were cultured in DMEM supplemented

with 10% (v/v) foetal calf serum with 1% (w/v) penicillin and strep-tomycin (Gibco, Invitrogen, Carlsbad, CA, USA) and incubated at37 �C in a humidified atmosphere of 5% CO2 in air. Cells were re-leased from the monolayer by 0.05% trypsin (SAFC Biosciences, Le-nexa, KS, USA) in PBS prior to the biocompatibility assay. Thenumber of viable cells was counted after staining by 0.2% solutionof Trypan blue dye using a haemocytometer and diluted to a den-sity of 105 live cells ml�1 in DMEM for seeding to each sample.

2.9.2. Determination of cell proliferationThe electrospun fibres were collected as a thin layer on different

coverslips (CS) for the assay. The samples (Ze, Ze-GA fibres), blankCS and 24-well TCP were sterilised by ultraviolet (UV) irradiationin a biosafety cabinet for 30 min on both top and bottom surfacesbefore the seeding of L929 fibroblasts cells. The L929 fibroblastcells were allowed to attach to the samples for 24 h at 37 �C in ahumidified atmosphere of 5% CO2 in air. The proliferation of thecells seeded on each sample was determined by resazurin reduc-tion assay. TCP wells without L929 fibroblast cells were used asblank and cells grown on the TCP and blank CS were used as con-trol. Briefly, the seeded fibroblast cells were washed with 1 ml ofDMEM and were treated with 500 lM resazurin in DMEM. After

4 h of incubation at 37 �C and 5% CO2 in air, the resazurin contain-ing media were collected and analysed by fluorometric measure-ment at an excitation of 530 nm and emission of 590 nm usingEnSpire 2300 multilabel reader (Perkin Elmer, MA, USA). The cellswere washed with PBS followed by the addition of 1 ml of freshDMEM to each well and the samples were incubated at 37 �C and5% CO2 in air again. The process was repeated over the course of4 days and all experiments were carried out in triplicate. The resa-zurin fluorescence from each well was plotted against time and thearea under the curve (AUC) was calculated using GraphPad Prismv5.02 (GraphPad software, San Diego, CA, USA). AUC values foreach well were normalised to the TCP well for each of the biolog-ical replicates.

2.9.3. Morphology of the proliferated L929 fibroblasts on theelectrospun fibres

The morphology of L929 fibroblast cells after 4 days of directcontact with the electrospun fibres was monitored in order tostudy the cell behaviour in the presence of Ze and Ze-GA fibresas suggested by previous studies (Gizdavic-Nikolaidis, Ray, Ben-nett, Easteal, & Cooney, 2010). First, the fibroblast cells werewashed twice with PBS to remove the remaining media constitu-ents, followed by fixing in 4% methanol free formaldehyde in PBSat room temperature for 10 min. The cells were washed twice withPBS again to remove the fixative and were permeabilised in 0.1%Triton-X 100 (PlusOne grade, GE Healthcare, Orsay, France) inPBS for 5 min. After washing the fibroblast cells twice using PBS,the cells were stained with 1 U of Phalloidin–Texas red conjugatefrom Molecular Probes (Invitrogen, Carlsbad, CA, USA) in PBS for20 min at room temperature. The cells were washed twice withPBS after the staining and were left to air-dry to complete dryness.The air-dried fibroblast cells were mounted in Prolong Gold, ananti-fading medium with 40,6-diamidino-2-phenylindole (DAPI)from Molecular Probes (Invitrogen, Carlsbad, CA, USA). The mor-phology of the fibroblast cells was examined using epifluorescencemicroscopy (Nikon Eclipse E600, Nikon, Japan) subsequent to themounting.

2.10. Antimicrobial efficacies of the electrospun fibres

The antimicrobial activities of Ze and Ze-GA fibres were testedagainst S. aureus, E. coli and C. albicans according to Japanese Indus-trial Standard (JIS Z2801:2000) assay. The electrospun fibresdeposited on aluminium foil were cut into 5 � 5 cm2 squares(approximately 20.5 ± 8 mg/sheet). A non-antimicrobial styrene-ethylene-butylene-styrene/polypropylene (SEBS/PP) blend thatserved as the cover of electrospun fibre sheets during the assaywere cut into 4 � 4 cm2 squares. The samples (electrospun fibreand SEBS/PP squares) were sterilised under UV irradiation on bothsides for 30 min before the bacterial or yeast inoculation. Stock cul-tures of the bacteria (S. aureus, E. coli) or yeast (C. albicans) weresub-cultured overnight at 37 �C in brain heart infusion (BHI) brothand diluted into saline to a concentration of approximately 107 col-ony forming units (CFU)/ml, and 100 ll of this (approximately106 CFU) was used to seed each of the sterile electrospun fibresquares. The SEBS/PP squares were aseptically transferred ontothe electrospun fibres. The bacteria or yeast suspension was spreadevenly across the surface of the fibre mat and incubated at 37 �C for24 h. After incubation, fibre mat and SEBS/PP sheet were asepti-cally separated and placed into a sterile bag with 10 ml of trypticsoy broth + 0.5% v/v Tween 20 (TSBT) (BD Bioscience, FranklinLakes, NJ, USA). Each bag was stomached for two 30 s intervalsto release the viable bacteria remaining from the fibre mat. Thenumber of CFU released were enumerated by (a) plating 0.1 ml ali-quots of serial tenfold dilutions onto BHI agar; and (b) pour platesof 9 ml stomacher liquor mixed with 11 ml molten BHI agar at

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55 �C. All experiments were carried out in triplicate on three inde-pendent occasions.

2.11. Statistical analysis

Statistical analysis was performed using MINITAB 13.0 softwareand GraphPad Prism v5.02 software for biological assays. The datafor water activity and colour measurements were compared using

Fig. 1. (a) C 1s spectroscopy and (b) O 1s spectroscopy of: (1) Ze; (2)

analysis of variance (ANOVA) at a 5% significance level. All valuesare reported as the means of three determinations, and the resultswere expressed as mean values ± standard deviation (SD). The col-lected values for the cytotoxicity assay were examined by D’Agos-tino & Pearson omnibus normality test to facilitate the distributionpattern of the data sets. Non-parametric test of Kruskal–Wallis sta-tistic was performed as the data showed a non-normaldistribution.

Ze-GA 5%, (3) Ze-GA 10%, (4) Ze-GA 20% sub-micron fibre mats.

Table 1Water activity (aw) and reflectance (Hunter L⁄ � a⁄ � b⁄) values of Ze and Ze-GAelectrospun fibres.a

Gallic acidcontent(w/w, % insolidfibres)

aw L⁄ a⁄ b⁄

0 0.446 ± 0.009a 96.49 ± 0.55b 0.034 ± 0.08d 1.368 ± 0.99f

5 0.457 ± 0.002a 94.81 ± 1.32c 0.178 ± 0.06e 0.895 ± 0.25f

10 0.447 ± 0.004a 95.68 ± 0.80bc 0.187 ± 0.12e 2.512 ± 0.59g

20 0.446 ± 0.012a 94.98 ± 1.16c 0.133 ± 0.10de 1.465 ± 0.58fg

a Data are displayed in means ± standard deviation of three replications. Meansin each column bearing different superscripts are significantly different (P < 0.05).

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3. Results and discussion

3.1. Surface chemistry of Ze and Ze-GA electrospun fibres

XPS was conducted in this study in order to determine the ex-tent of gallic acid incorporation on the surface of the electrospunfibre mats. Percentage of atomic concentration for chemical moie-ties on the near surface region (�3 nm depth) of the mats can beobtained through the XPS data, and the area under the spectra peakis proportional to the given atom intensity (Deitzel et al., 2002).The atomic surface compositions of the electrospun fibre matsare shown in Table S2 (Supplementary information). Results ob-tained showed an increase of oxygen atoms (from 12.19 to16.10%) on the surface with increased content of gallic acid withinthe fibre mats as shown in Table S2 (Supplementary information).Fig. 1a and b shows the deconvoluted curves for high resolution C1s and O 1s spectra for Ze and Ze-GA fibre mats. C 1s spectra for allthe electrospun fibre mats consist of four components, which arethe aliphatic carbons (C–C/C–H) at around 284.9 eV, C–OH groupat approximately 286.6 eV, C@O group at 287.9 eV region and C–N at near 285.9 eV. Ze-GA 20% had demonstrated higher peakintensity at 286.6 eV region (23.61%, Fig. 1a), which implied therising content of the C–OH group that was contributed by the pres-ence of gallic acid as compared to Ze (14.67%, Fig. 1a). The implica-tion was further affirmed through the intensity reduction of C–Nspectra of the Ze-GA fibres at 285.9 eV region (8.35–0.83%,Fig. 1a), which was solely attributed to zein. Three componentswere found in the O 1s spectra at 532.3 eV for C@O group,532.9 eV for C–OH group and 533.6 eV for the aromatic (ar)–OHgroup. The results obtained supported the C 1s findings by showingan increase in peak intensity at the 533.6 eV region (9.51–13.07%,Fig. 1b) with the increasing content of loaded gallic acid in the

Fig. 2. (a) Release percentage of gallic acid (%) from Ze-GA electrospun fibres in a hydrophfunction of the square root of time for Ze-GA electrospun fibre mats.

Ze-GA fibres, which was contributed by the ar–OH group. Heet al. (2006) suggested that during electrospinning, the flash sol-vent evaporation and ionic strength will tend to localise the com-ponent on or near the fibre surface. The XPS analyses confirmedthat the incorporation of gallic acid through electrospinning hadaltered the surface chemistry of zein fibre mats, which in turnmay introduce new properties to these sub-micron fibrous mats.

3.2. Water activity (aw) of the electrospun fibre mats

Water is one of the crucial factors that will determine the stor-age stability of a glassy protein matrix. Glass transition tempera-ture (Tg) of the protein will decrease with the increase of wateractivity. As a result, the glassy state of the amorphous zein proteinmatrix will turn to a rubbery state, which will affect the long termstabilisation of the incorporated gallic acid. Results obtained willprovide information on the availability of water for degradationreactions. The aw of the electrospun fibres were measured andthe results are shown in Table 1. Results suggested that the incor-poration of gallic acid did not affect the aw of Ze electrospun fibremat. The aw of the electrospun fibre mats ranged from 0.446 to0.457 and was not significantly different (P > 0.05) from each other.The low aw value of the electrospun fibre mats is desirable as apackaging material because the migration of water molecules tothe coated product is not favoured. It also implies the possible sta-bility of the incorporated gallic acid in zein electrospun fibre matsfor storage, which was further determined thermally and chemi-cally in the subsequent section (Section 3.4).

3.3. Colour measurement of the electrospun fibre mats

The appeal of a product is mainly determined visually with col-our as the most prominent visual feature. The colour measurementis crucial as it will impact the consumer’s approval of a product.The colour of Ze and Ze-GA fibres was expressed in L⁄, a⁄ and b⁄ val-ues as shown in Table 1. The electrospun fibres showed L valuesranging from 94.8 to 96.5, indicating a colour which is close towhite. The incorporation of gallic acid reduced the L value of thefibre mats significantly (P < 0.05). Ze-GA fibres exhibited higherpositive a values compared to Ze fibres indicating that they wereredder than the neat Ze fibres, while Ze-GA 10% displayed a higherpositive b value compared to the others indicating that they wereyellower than the others. Conventional single layer zein films had amean L⁄, a⁄ and b⁄ values of 93.29, �7 and 41.21 (Weller, Genna-dios, & Saraiva, 1998). The greater value of +b and �a of conven-tional single layer zein film is due to the intense yellowish colourof zein as compared to the electrospun fibre mats that exhibit a

ilic model (Mili-Q water) at 25 �C and (b) fractional release of gallic acid plotted as a

Table 2Correlation coefficients (r2) according to the different models and diffusion/releaseexponent (n) used for describing the release mechanisms of gallic acid from the Ze-GAelectrospun fibres.

Model Ze-GA5%

Ze-GA10%

Ze-GA20%

Peppas (r2) Mt/M1 = ktn 0.9526 0.9524 0.9532n 0.44 0.40 0.39Zero-order (r2) Mt/M1 = kt 0.8136 0.7846 0.7628First-order (r2) ln (1 �Mt/M1) = �kt 0.9261 0.9016 0.9361Higuchi (r2) Mt/M1 = kt1/2 0.9673 0.9570 0.9506Hixson–Crowell (r2) (1 �Mt/M1)1/3 = �kt 0.8937 0.8675 0.8887

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whitish appearance. The lighter colour of the electrospun fibremats would be more acceptable as a food packaging material thanthe darker colour films.

3.4. Time dependence stability of Ze and Ze-GA electrospun fibres

The stability of Ze and Ze-GA electrospun fibres was studiedusing TGA and ATR-FTIR. Our previous studies had revealed theweight loss curves of the electrospun fibres where the thermalevents slowly happened at temperatures greater than 160 �C, andZe-GA fibres had exhibited a degradation temperature of approxi-mately 231 �C. Our previous studies also demonstrated the pres-ence of new bands at 876, 777, 693, 1197, 1336 and 1037 cm�1

in the ATR-FTIR spectra, which was correlated to the incorporationof gallic acid (Neo et al., 2013). In the present study, TGA andATR-FTIR were performed on the electrospun fibre mats up to

Fig. 3. Zein nanofibres containing gallic acid are not cytotoxic. (a) The area under the curvfor each treatment relative to the mean tissue culture plate (TCP) value is plotted for L9electrospun fibres. (b) Epifluorescence microscopy images of L929 fibroblast cells after stZe-GA 20% sub-micron fibres. The F-actins of the cells were stained with Phalloidin–Texablue. (For interpretation of the references to colour in this figure legend, the reader is r

60 days of storage at room temperature to verify their stabilityover a long period of time. A deviation from the initial TGA weightloss curves and ATR-FTIR spectra of the Ze-GA electrospun fibremats would suggest a deterioration of gallic acid in Ze fibres overa period of time. Figure S3 (Supplementary information) showsthe weight loss curves and the ATR-FTIR spectra of the Ze-GA20% fibre mats. The curves and spectra of the Ze and Ze-GA fibresup to 60 days of storage were not extensively varied as comparedwith the initial results (Ze, Ze-GA 5%, Ze-GA 10% data not shown).Those results showed stability of the electrospun fibre mats ther-mally and chemically as a packaging material. An additional factorto be considered in future treatments will be the stability of theincorporated bioactives in electrospun fibres under exposure tolight in order to refine the study for their storage stability.

3.5. Gallic acid release profile in water

Release rate and profile were investigated in the present studyto gain insight on the delivery performance of gallic acid from zeinelectrospun fibre matrices to the surrounding environment. Ourprevious studies had suggested a loading efficiency of 99–105%,indicating almost 100% of the gallic acid was loaded within theelectrospun fibre mats (Neo et al., 2013). These values were usedto calculate the percentage release of gallic acid from the Ze-GAelectrospun fibre mats in the present study. The release perfor-mance of gallic acid from the Ze-GA fibres was conducted in ahydrophilic media (water) as a model food simulating solvent.Fig. 2a illustrates that the electrospun fibres with different gallicacid loadings displayed similar release trends. It was found thatthe release rate of the gallic acid increased with the increasing

e (AUC; inset) for metabolic activity measured using a resazurin assay. Time courses29 fibroblast cells grown on glass cover slips (CS) and CS coated with Ze and Ze-GAaining using Phalloidin–Texas red and DAPI: (1) Ze; (2) Ze-GA 5%; (3) Ze-GA 10%; (4)s red and are shown in red and the nuclei were stained with DAPI and are shown in

eferred to the web version of this article.)

Fig. 4. Effects of Ze and Ze-GA electrospun fibre mats on the growth of (a) S. aureus;(b) E. coli; (c) C. albicans.

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content of loaded gallic acid. Ze-GA 5%, Ze-GA 10% and Ze-GA 20%demonstrated a release percentage of 58%, 60% and 78%, respec-tively during the same release time (10 min of immersion inwater). The release performance was characterised by a gradual,but rapid, release in the first 20 min corresponding to 67–88% ofthe total loaded gallic acid and approximately 90% release was de-tected beyond that time. There are various factors that contributeto the rapid release of the loaded gallic acid from the electrospunfibres, the major reason being the high porosity (large surface area)of the electrospun fibre mats that increases the mass transfer rateof gallic acid and diffusion through the electrospun fibres (Xiao,Davidson, & Zhong, 2011). Secondly, variation in the gallic acid re-lease rate between the electrospun fibres might be due to differentamounts of incorporated gallic acid presents on or near the fibresurface, and the findings from the XPS studies supported thishypothesis. Lastly, it could be due to the fact that water is a goodsolvent for gallic acid (solubility equals to 1.1 g/100 ml of water).

3.6. Determination of the release mechanism

Elucidation of the possible release mechanism of gallic acidfrom the zein electrospun fibre matrices is of special importancein order to engineer or modify the current system for differentapplications. Mathematical models provide information on thetransportation of gallic acid from zein fibre mats that are criticalto the development of sustained-controlled delivery systems. Byincorporating the first 65% of the gallic acid release data, the diffu-sion or release exponent (n) were determined through the Ritgerand Peppas model. Ritger and Peppas (1987) proposed the criteriafor release kinetics from swellable systems, which indicatedn = 0.45 as Fickian diffusion; while 0.45 < n < 0.89 indicates ananomalous non-Fickian diffusion; and n P 0.89 indicates superCase II diffusion. The Ze-GA fibres showed n values of 0.44, 0.40and 0.39 for Ze-GA 5%, Ze-GA 10% and Ze-GA 20%, respectively (Ta-ble 2). Therefore, Fickian diffusion was the major release mecha-nism, which suggested that the release of gallic acid waspartially attributed to the diffusion or permeation through theswollen zein sub-micron fibrous matrix, and partly through thewater filled pores and channels in the matrix structure (SinhaRoy & Rohera, 2002). Four empirical diffusion models were appliedto the collected data for the explanation of possible release mech-anism. Modelling analysis was carried out by fitting the releasedata until 80% of the gallic acid was released. Analysis of correla-tion coefficient (r2) of linear relationship between the amount ofgallic acid released and time were conducted for the 4 differentmodels. The release kinetics for gallic acid from the Ze-GA electro-spun fibres showed the best correlation with the Higuchi model(Table 2) with r2 ranging from 0.9506–0.9673. The Higuchi equa-tion is commonly used to model diffusion controlled release froma porous matrix (Macri, Sheihet, Singer, Kohn, & Clark, 2012). Thedissolution data was further characterised by evaluating the linear-ity of the fractional amount of gallic acid released in the Higuchi’ssquare root of time model (Fig. 2b). The linear relationship sug-gests that gallic acid is released primarily by a diffusion-controlledprocess.

3.7. Cytotoxicity of the electrospun fibres

Candidate materials for food packaging should not be cytotoxic.To demonstrate that Ze-GA electrospun fibres were not cytotoxic,L929 fibroblast cells were seeded onto the electrospun fibres andproliferation was determined by a resazurin metabolic assay.Fig. 3a shows a box plot of L929 fibroblast cell proliferation foreach sample. Median values for the samples are represented bythe line within each box and the box represents 50% of thecollected experimental data. A comparison of Ze and Ze-GA

electrospun fibres demonstrated that metabolic activity over timefor L929 cells growing upon them showed no significant negative(cytotoxic) effect (ANOVA test, P > 0.05). Overall, the results indi-cated that the Ze and Ze-GA electrospun fibres did not induce anadverse effect on the metabolic activity of L929 fibroblast cells.Fig. 3b shows the morphology of L929 fibroblast cells after 4 daysof direct contact to the Ze and Ze-GA electrospun fibres. The cellswere able to proliferate and adhere to the surface of electrospun fi-bres comparable to that on TCP. The fibroblast cells had developed

Y.P. Neo et al. / Food Chemistry 141 (2013) 3192–3200 3199

a spindle like, dendritic morphology which is indicative of ahealthy state. Images obtained confirmed the biocompatibility ofZe and Ze-GA electrospun fibres with L929 fibroblast cells that pre-sented normal morphological attachment and growth.

3.8. Antimicrobial efficacies of Ze and Ze-GA electrospun fibres

In the present study, it was found that the Ze-GA electrospun fi-bres exhibited antimicrobial efficacy towards both S. aureus and E.coli through a significant reduction of bacteria enumerated as com-pared to the Ze fibres. Fig. 4 summarises the dynamic range ofreduction for approximately 21 mg of Ze-GA electrospun fibrescoated on a 5 � 5 cm2 of aluminium foil separately towards 106–7

CFU of the Gram positive, Gram negative and yeast cells tested.Fig. 4a and b depict the average reduction of the growth of S. aureusand E. coli, respectively, by Ze-GA fibre mats as compared to Ze fi-bre mat. Results suggested an approximately 6-log reduction onthe growth of S. aureus and E. coli by the Ze-GA electrospun fibremats. Generally, a 5-log reduction is considered adequate for mostfood products on their microbial stability (Holah, Taylor, Dawson,& Hall, 2002). The effectiveness of Ze-GA electrospun fibres to-wards both S. aureus and E. coli is suggested to be due to the abilityof gallic acid to disrupt the cell peptidoglycan and/or disintegratethe outer membrane of the bacteria through the chelation of diva-lent cations (Nohynek et al., 2006; Tranter, Tassou, & Nychas,1993). On the other hand, the Ze-GA fibre mats had only a moder-ate effect against C. albicans with a log reduction of 1–2 (Fig. 4c).This result agrees with several other studies that suggested lowerinhibitory activities of gallic acid towards C. albicans (Binutu & Cor-dell, 2000). The present study clearly demonstrates that the incor-porated gallic acid is highly active against the bacteria andeffective against yeast cells after the electrospinning process.

4. Conclusion

A consumer desirable food contact packaging material has to benatural, effective, easily handled and must not produce harmful ortoxic substances with undesirable appearance and smell. Electro-spun fibre mats present an opportunity to produce an easy andeffective active material due to its large surface area. In this studywe revealed the potential benefits of loading natural phenolic com-pounds to zein biopolymer through the electrospinning process.Ze-GA fibre mats displayed a whitish appearance, low water activ-ity and are thermally and chemically stable after 60 days of storageat 21.5 �C, with a relative humidity of approximately 58%. The fastrelease profile of gallic acid from the electrospun fibres is due tothe large surface area and also the localisation of gallic acid onthe fibre surface, which suggested that the electrospun fibre matswould be good candidates for use on dry foods, as edible coatingsor as part of multi-layer structures. The incorporated gallic acid isan effective inhibitor of model Gram-positive and Gram-negativebacteria (S. aureus and E. coli) and a modest inhibitor of model fun-gi (C. albicans). Gallic acid retained its antimicrobial abilities afterthe electrospinning process, and greatly enhanced the antimicro-bial properties of zein for application as an active packaging mate-rial. The Ze-GA electrospun fibres are not cytotoxic and exhibitedantimicrobial properties. Hence, the antimicrobial properties ofgallic acid are successfully captured and utilised by encapsulatingthis valuable natural functional material in a biopolymer throughelectrospinning. These attributes have rendered them as potentialnovel and safe food contact materials, which showed promisingmaterial combination desirable for active packaging in food indus-try. Further studies will involve improvement on the gallic acid re-lease performance of the zein fibre mats by retarding its fastrelease and their applications on actual food products.

Acknowledgements

The authors would like to express their sincere thanks to ColinDoyle from the Research Centre for Surface and Materials Science(RCSMS), Geoff Waterhouse, Benedict Uy, Adeline Le Cocq and ShinTien Hoh for their helpful discussions on XPS, biocompatibility,antimicrobial assay and release mechanism. Yun Ping Neo is sup-ported by a University of Auckland Doctoral Scholarship.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.foodchem.2013.06.018.

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