om

im

, Fin

Reinforcement

Ionic cross-linking

onMFCelluallyluloitios-lina. T

reduced water vapour permeability (WVP) after the addition of cellulose bres, except when unmodied

biobaars (Haof biocateria, suchds of cw toxi

micro- and nanobres have been studied as reinforcement agentsin synthetic (Zoppe, Peresin, Habibi, Venditti, & Rojas, 2009) andnatural polymers (Johnson, Zink-Sharp, Renneckar, & Glasser,2009).

Alginate is another type of linear polysaccharide, consisting of(14)-linked b-D-mannuronate and a-L-guluronate units. The rela-tive proportions of mannuronate and gulunorate vary within thesource of the alginate (Johnson, Craig, & Mercer, 1997). The most

res. In thisths rangin

19.0 lm to 25 nm, were used as reinforcement agents for abiocomposites lms to be utilised in application such as pacEffects of the different bres on the lm properties andcompatibility between cellulose bres and alginate were studied.Moreover, an ionic cross-linking of the biocomposite lms withCa2+ ion, and grease and water vapour barrier properties were alsostudied. Properties of lms, and the compatibility between cellu-lose bres and alginate were studied by Fourier transfer infraredspectroscopy (FTIR), eld emission scanning electron microscopy(FESEM), energy-dispersive X-ray spectroscopy (EDS) and wide-angle X-ray diffraction (WAXD).

Corresponding author. Tel.: +358 294 48 1631; fax: +358 8 553 1593.

Food Chemistry 151 (2014) 343351

Contents lists availab

he

lseE-mail address: osmo.hormi@oulu. (O.E.O. Hormi).and nanobres in composite materials has been a scientic interestin recent years (Abdul Khalil, Bhat, & Ireana Yusra, 2012). Cellulose

forced with different types of wood cellulose bfour different wood cellulose bres, with wid0308-8146/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.foodchem.2013.11.037paper,g fromlginatekaging.on theCellulose is a linear polysaccharide of (14)-linked b-D-glucose,produced by a variety of plants and algae, along with some bacte-ria. In nature, cellulose is produced as brous material which haslong been used as a main component in paper, and as a startingmaterial e.g. for edible packages (Ayranci & Tunc, 2001; Cheng,Abd Karim, & Seow, 2008). Additionally, the use of cellulose micro-

the desired mechanical properties, such as high strength. Toincrease the mechanical properties of alginate based biocompos-ites, cellulose nanocrystals have been studied as reinforcementagents in alginate lms (Cheng, Lu, Zhang, Shi, & Cao, 2012). How-ever, there is only a scarcity of information on the mechanicalproperties of biocomposite materials with an alginate matrix rein-1. Introduction

Composite materials derived fromlots of attention during the recent yeOne of themost studied applicationsmentally friendly food packaging mZampraka, 2007). Natural polymersideal starting materials for these kinbiodegradability, biocompatibility, losed sources have drawnnsen & Plackett, 2008).omposites is in environ-ls (Kristo, Biliaderis, &as polysaccharides, areomposites due to theircity and renewability.

common source of alginate is the cell wall of brown algae, andfor commercial purposes alginate is extracted from seaweed. Algi-nate and alginate-based biocomposites have been used as, forexample, adsorbents for ionic dyes (Sui et al., 2012), antibacteriallms (Benavides, Villalobos-Carvajal, & Reyes, 2012), wound heal-ing materials (Sikareepaisan, Ruktanonchai, & Supaphol, 2011), andstimulus response drug releasing materials (Mourio et al., 2011).

Although alginate based biocomposites have strong potential tobe used for example in packaging applications, they usually lackAlginateBiocomposite

birch pulp was used. 2013 Elsevier Ltd. All rights reserved.Biocomposite cellulose-alginate lms: Pr

Juho Antti Sirvi a, Aleksi Kolehmainen a, Henrikki LiaDepartment of Chemistry, University of Oulu, P.O. Box 3000, FI-90014, Finlandb Fibre and Particle Engineering Laboratory, University of Oulu, P.O. Box 4300, FI-90014

a r t i c l e i n f o

Article history:Received 19 April 2013Received in revised form 19 September 2013Accepted 6 November 2013Available online 16 November 2013

Keywords:CelluloseNanocellulose

a b s t r a c t

Biocomposite lms basedmicrobrillated cellulose (anionic dicarboxylic acid clulose bre materials. Ioniction of micro- and nanocellms remarkably, e.g. add97.97 MPa. After ionic crosfrom 69.63 to 125.31 MP

Food C

journal homepage: www.eising packaging materials

atainen b, Jouko Niinimki b, Osmo E.O. Hormi a,

land

cellulose and alginate were produced using unmodied birch pulp,), nanobrillated cellulose (NFC) and birch pulp derivate, nanobrillatedlose (DCC), having widths of bres ranging from 19.0 lm to 25 nm as cel-cross-linked biocomposites were produced using Ca2+ cross-linking. Addi-ses as a reinforcement increased the mechanical properties of the alginaten of 15% of NFC increased a tensile strength of the lm from 70.02 toking, the tensile strength of the lm containing 10% of DCC was increasedhe biocomposite lms showed excellent grease barrier properties and

le at ScienceDirect

mistry

vier .com/locate / foodchem

2.1. Materials

xylyl]azo)-2-naphthol), used in grease barrier tests, was obtained

ICMFC50, ICMFC10, ICDCC10, respectively.

misfrom SigmaAldrich (Germany) and Ottawa-grade sand (particlesize 2030 mesh) from Fisher Scientic (UK) and Turpentine (Oulu,A1 Wood turpentine, Finland) from Kiilto Ltd. All of otherchemicals used in the periodate and chlorite oxidations (NaIO4,NaClO2 and CH3COOH), aldehyde and carboxyl content analyses(NH2OHHCl, CH3COONa2H2O, NaCl, and NaOH) and ionicalcrosslinking (CaCl22H2O) were obtained as p.a. grade fromSigmaAldrich (Germany) and were used without further purica-tion. Deionised water was used throughout the experiments.

2.2. Preparation of biocomposite lms

The biocomposite lms based on cellulose and alginate wereprepared using the solvent-casting method. At rst, alginate wasslowly dissolved in deionised water. Then cellulose (unmodiedbirch pulp, MFC, NFC or DCC) dispersion was added to this solution,whilst varying the amount of cellulose from 0% to 50% of the lmdry weight. Total volume of the alginate and cellulose dispersionswas 135 ml and a dry weight of lms was 1.515 g, resulting in lmswith a grammage of 30 g/m2. After addition of the cellulose disper-sion, the resulting alginate/cellulose mixture was dispersed usingan UltraTurrax disperser (Germany) (11,000 rpm) for ten minutesto obtain a homogeneous dispersion after which a water circula-tion vacuum was used to remove air bubbles from the dispersion.Finally, the dispersion was cast in a polystyrene tray of 500 cm2

area. Films were allowed to dry from 3 to 5 days at room temper-ature to obtain 13 biocomposite lms with 1%, 10%, 15%, 20% and50% of MFC, 1%, 5%, 10% and 15% of NFC, 1% and 10% of DCC, and50% of birch pulp of the lm dry weight were, designated asMFC1, MFC5, MFC10, MFC15, MFC20, MFC50, NFC1, NFC5, NFC10,NFC15, DCC1, DCC10 and BIRCH50, respectively. Pristine alginatelm was also produced as a reference.

2.3. Preparation of ionically cross-linked biocomposite lmsBleached birch pulp (Betula verrucosa and pendula) was ob-tained as dry sheets and used as a bre material after being disin-tegrated in deionised water. The properties of cellulose arepresented elsewhere (Liimatainen, Sirvi, Haapala, Hormi, & Nii-nimki, 2011). The commercial grade microbrillated wood cellu-lose (MFC, KY-100G) was obtained from Daicel Industries Ltd.(Japan).

The combined treatment of Valley beating and high-pressurehomogenisation was used to prepare nanobrillated celluloses(NFC) from the birch pulp. Prior to homogenising, the bleachedbirch was ground in a Valley beater to a dry weight of 2.0% over50 h, yielding 5503 g of weight. The samples where then dilutedto a dry weight of 0.7% and homogenised with a homogeniser(Invensys APV 2000, Denmark) at a pressure of 3501200 bar tocreate NFC. NFC had same charge density as original pulp(0.08 mmol/g). Charge density is mainly due to the presence ofhemicelluloses.

Nanobrillated dicarboxylic acid cellulose (DCC) with a chargedensity of 0.75 mmol/g was prepared as reported earlier(Liimatainen, Visanko, Sirvio, Hormi, & Niinimaki, 2012).

Sodium alginate (Scogin XL, low viscosity) was kindly donatedby FMC Biopolymer (Ireland). Oil Red O (1-([4-(xylylazo)2. Experimental section

344 J.A. Sirvi et al. / Food CheIn order to ionically cross-link a lm, it was immersed in 400 mlof 10% CaCl22H2O solution for 30 min. The lm was then the care-fully removed from the solution and rinsed thoroughly with2.4. Mechanical properties

The lms were cut into 15 150 mm stripes and conditioned at23 C and 50% humidity for 48 h before testing. Specic tensilestrength (r/q), elongation at break and specic modulus (E/q)were determined according to ISO Standard, 1995 and Lorentzen& Wettre tensile strength tester (Sweden). A test speed of50 mm/min was used. Initial distance between grips was set to100 mm. Each test was replicated ve times and the results arepresented as averages of these measurements. Tensile strength(r) and modulus (E) was calculated using the density (q) from r/q and E/q, respectively.

Film thicknesses were determined as averages of ve randompositions of the lm using Lorentzen & Wettre thickness metre(Sweden).

2.5. Field emission scanning electron microscope and energy-dispersive X-ray spectroscopy

FESEM (Zeiss Ultra Plus, Carl Zeiss SMT AG, Germany) was usedto examine morphologies of used cellulosics and the surface of thelms. Fibres were sputter-coated with Pd, and an accelerating volt-age of 5 kV was used whilst the lms were sputter-coated with car-bon, and an accelerating voltage of 15 kV was used. EDS was usedto obtain surface elemental analysis of the lms.

2.6. Fourier transform infrared spectrum

FTIR spectra of lms were recorded using a Bruker FT-IR spec-trometer (USA) using spectral width ranging from 4000 to400 cm1 with 2 cm1 resolution and an accumulation of 32 scans.

2.7. X-ray diffraction

For X-ray diffraction (XRD) analyses, samples were analyzed be-tween 2h = 5 and 114 with angle step size 2h = 0.02 in a D8 Dis-cover X-ray Diffractometer (Bruker AXS Inc., Madison, MI, USA)equipped with a Co Ka (40 kV/35 mA) source. Pure birch pulp,MFC, NFC and DCC lms were prepared by casting a 1% solutionof cellulose dispersion on a Teon plate to obtain a lm with agrammage of 30 g/m2. The degree of crystallinity of pure celluloselms in terms of the crystallinity index (CrI) was calculated fromthe peak intensity of the main crystalline plane (002) diffraction(I002) at 22.8 and from the peak intensity at 18.0 associated withthe amorphous fraction of cellulose (Iam) according to followingequation (Segal, Creely, Martin, & Conrad, 1959)

CrI I002 IamI002

100%

2.8. Grease barrier properties

The grease resistance of the lms was determined according toTAPPI Standard, 2010. Each test was replicated four times: twotimes for the bottom layers and two times for the upper layers of

2deionised water. The lm was placed on a polycarbonate plateand small weights were placed on the edges of the lm to preventthe shrinkage of the lm during air-drying (24 h). Ionic cross-linking was conducted with a pristine alginate, BIRCH50, MFC50,NFC10 and DCC10 lms and they were designated as ICBICH50,

try 151 (2014) 343351the 50 50 mm lm specimens.The method involves the use of turpentine colored with an oil-

soluble red dye to measure the penetration of the grease through a

sure a tight seal. The test assemblywasweighed and placed in a con-

emisdition-controlled chamber (23 C and 50% humidity, equipped witha fan to ensure good circulation of air inside the chamber) and theweight lostwas recorded in one hour intervals for 8 h. A steady statewas reached with all samples during the rst hour. The slope of thestrain line of weight lost vs. time was calculated, and the water va-pour transmission rate (WVTR) was determined by dividing theslope by the exposed lm area. WVP was calculated as follows:

WVP WVTRSR1 R2

where S is the saturation vapour pressure at the test temperature(28 102 Pa at 23 C), R1 is the relative humidity in the dish ex-pressed as a fraction (1), and R2 is the relative humidity at the va-pour sink expressed as a fraction (0.5).

3. Results and discussion

3.1. Cellulose and alginate biocomposite lms

3.1.1. Preparation of biocomposite lmsFESEM images of the cellulose bres used to obtain biocompos-

ite lms are presented in Supplementary data (Fig. S1). The un-treated birch pulp consisted mainly of cellulose bres (width19.0 lm) with 3.4% of nes, according to FibreMaster image ana-lyzer. MFC consisted of long brils, but also included larger mi-cro-sized particles such as pieces of cell walls. NFC consistedmainly of nanobril bundles with widths smaller than 100 nm,whilst DCC contained single nanobrils and nanobril bundleswith widths of below 25 nm. However, it is possible that the brilsexisting in the suspension are actually thinner compared to the -brils observed in FESEM images due to the agglomeration of nano-bres during the sample preparation (Liimatainen et al., 2012).

The appearance of dried lms depended on the amount and thetype of cellulose used in the lm. Alginate lms with no added cel-lulose were fully transparent, and the transparency decreased asthe amount of cellulose increased: MFC50 and BIRCH50 were tur-bid with low transparency. Due to the small bre sizes, lms withNFC and DCC were only slightly less transparent compare to pris-greaseproof material. Moisture-free colored turpentine wasprepared by mixing 100 ml of turpentine with 1.0 g of Oil Red Ored dye and 5 g of anhydrous CaCl2 in a ask. The ask was closedwith stopper, and the mixture was shaken well and allowed tostand for 10 h. Finally, the mixture was ltrated and stored in anairtight bottle.

For grease barrier tests, a lm was placed on a white printingpaper, and 5 g of sand was placed on top of the lm. A volume of1.1 ml of colored turpentine was added to the sand pile. The lmwas carefully slid to a new unexposed portion of the paper, andthe time when colored-turpentine-stained paper was observedwas recorded. During the rst minute, the lm was slid every15 s, then at 1 min intervals for the next 5 min and nally at5 min intervals until a maximum time of 30 min was reached.

2.9. Water vapour permeability

Water vapour permeability (WVP) was determined according toASTM Standard, 1995. All lms were conditioned at 23 C and 50%humidity for 48 h before being tested. Schott Duran laboratoryglass bottles with a hole drilled in their caps were lled to one-fourth their volume with deionised water. The test specimen wasplaced between the bottle and cap, and a rubber sealwas used to en-

J.A. Sirvi et al. / Food Chtine alginate lm. All the lms were exible and could be easilyhandled. Films with cellulose had higher apparent roughness thanthe pure alginate lm. The surface facing towards the tray duringthe drying was signicantly smoother compared to the surfacefacing towards the air during the drying. Nano-sized celluloses(NFC and DCC) provided smoothest surfaces, whereas MFC andespecially unmodied birch pulp reinforced lms were signi-cantly rougher. The visual appearance of the lms obtained fromFESEM is shown in a later chapter.

3.1.2. Preparation of ionically cross-linked biocomposite lmsCross-linking can be used to increase the strength of materials

(Bhattacharya, Rawlins, & Ray, 2009). It can also be used todecrease water solubility of polymers, such as polysaccharides(Ramesh, Hasegawa, Sugimoto, Maki, & Ueda, 2008). To obtaincross-linked biocomposite lms, Ca2+ ions were used as an ioniccrosslinker. The mechanism of the ionic cross-linking of alginateis well described in literature (Sikorski, Mo, Skjk-Brk, & Stokke,2007). During the drying, fractures were observed in the pristinealginate lm and intact ionically cross-linked alginate lm couldnot be produced. Fractures may be due to the high tension causedby lm shrinking. However, intact ionically cross-linked lms fromBIRCH50, MFC50, NFC10 and DCC10 were obtained.

The appearances of ICBIRCH50 and ICMFC50 were smooth andthey were only slightly wrinkled on the edges. ICNFC10 in turnwas more wrinkled and bent, whilst ICDCC10 was smooth and onlyslightly bent on the edges of the lm. Minor fractures were ob-served at the edges of ICDCC10. Due to the higher amount of algi-nate, ICNFC10 and ICDCC10 were more prone to shrinking, andhence they were more wrinkled compared to ICBIRCH50 andICMFC50.

All lms dissolved or disintegrated in water without cross-link-ing treatment. After cross-linking the lms still wetted easily, butthey did not dissolve in water. It is known that cross-linking ofalginate with multivalent ions decreases dramatically the watersolubility of the alginate (Nakamura, Nishimura, Hatakeyama, &Hatakeyama, 1995).

3.2. Fourier transform infrared spectrum

FTIR spectra of the pristine alginate lm, MFC50, NFC10, DCC10and BIRCH50 before and after crosslinking are presented in Fig. 1and assignments of the most characteristic vibration modes of allof the biocomposite lms determined by literature (Fuks, Filipiuk,& Majdan, 2006; Papageorgiou et al., 2010) are presented inSupplementary data (Table S1). In the spectrum of the pristine algi-nate lm, characteristic vibration bands of m(COO)asym andm(COO)sym are observed at 1613.66 and 1414.54 cm1, respectively,and the band of CO stretching vibrations is observed at1038.02 cm1.

In the FTIR spectra of the biocomposite lms m(COO)asym peak isshifted to a lower wavenumber as the amount of cellulose is in-creased. The band of m(COO)asym of MFC1 is at 1614.42 cm1 andit is shifted to 1607.97 cm1 when the amount of MFC is increasedto 50%. The lowest m(COO)asym wavenumber is observed withBIRCH50 (1603.19 cm1). This may be due to the presence of largeamount of hemicelluloses in birch pulp, which may have betterinteraction with alginate than cellulose. The CO stretching vibra-tion bands in MFC50, BIRCH50, and NFC10 are also shifted toslightly lower wavenumber and are at 1031.51, 1027.27 and1026.18 cm1, respectively. Wavenumber of the peak of CO ofDCC10 is close to pristine alginate lm, 1035.50 cm1, which indi-cates that, compared to other cellulose materials, anionic DCC haslowest interaction with alginate.

After the crosslinking, the m(COO)asym band of the pristine algi-nate lm is shifted to a lower wavenumber (1602.78 cm1),

try 151 (2014) 343351 345whereas the m(COO)sym band has a slightly higher wavenumber(1420.94 cm1). This is due to the different interactions betweenmonovalent sodium and divalent calcium ions with alginate

osite

346 J.A. Sirvi et al. / Food Chemis(Papageorgiou et al., 2010). From the Table 1 it can be observedthat Dm(COO) (m(COO)asym m(COO)sym) of the pristine alginate isdecreased from 199.12 to 181.84 cm1 after ionic cross-linking,which indicates that a more uniform, structured, carboxylategroups are formed during the cross-linking (Fuks et al., 2006). Ithas been proposed that there is pseudo bridged unidentatecomplex between alginate and multivalent metal cations(Papageorgiou et al., 2010). However, recent density functionalcalculations have shown that there is only an ionic interactionbetween alkaline metals and alginate (Agulhon, Markova, Robitzer,Quinard, & Mineva, 2012).

Values of Dm(COO)Ca of the ICMFC50 and ICBIRCH50 have alsobeen shifted from 194.12 to 181.01 cm1 and from 188.90 to175.12 cm1, respectively. However, values of Dm(COO) of lmscontaining nano-sized cellulose are virtually the same before andafter cross-linking. Nano-sized celluloses may interact with cal-cium alginate which leads to the change of the bonding betweenalginate and the calcium ion.

It has been previously proposed that anionically chargednanocelluloses can take part in the formation of highly cross-linkedalginate after calcium cross-linking (Lin, Bruzzese, & Dufresne,2012). Nanocelluloses used in the present study had anioniccharges of 0.08 mmol/g (NFC) and 0.75 mmol/g (DCC) and thusNFC and DCC may form ionic cross-linking network with calciumalginate. Even though birch pulp has same anionic charge as NFC,it has signicantly larger bre size and lower surface area, due towhich birch pulp has fewer ionic groups available to interact withcalcium alginate than NFC.

Fig. 1. FTIR spectra of alginate and biocomp3.3. Field emission scanning electron microscope and energy-dispersive X-ray spectroscopy

FESEM images of biocomposite lms are presented in the Fig. 2.Images are from the surface faced towards the tray during the dry-ing. The lms were observed to be very sensitive and especially the

Table 1Asymmetric and symmetric carboxyl vibration frequencies of alginate.

Sample m(COO)asym m(COO)sym Dm(COO)

Alg 1613.66 1414.54 199.12MFC50 1607.97 1413.85 194.12NFC10 1609.31 1420.79 188.52DCC10 1609.13 1414.78 194.35BIRCH50 1603.19 1414.29 188.9ICAlg 1602.78 1420.94 181.84ICMFC50 1606.32 1425.31 181.01ICNFC10 1609.31 1420.79 188.52ICDCC10 1614.54 1421.09 193.45ICBIRCH50 1598.18 1422.48 175.7lms with high amount of alginate were easily destroyed duringthe zooming and thus only large scale images were obtained. Thepristine alginate lm (Fig. 2a) has a smooth and even surface(white spots on the surface are holes formed on the surface dueto the electron beams of the microscope). The surface of MFC50(Fig. 2b) was also relative smooth and even, however, some cellu-lose brils are buried under alginate matrix. Compared to MFC50,the surface of BIRCH50 (Fig. 2c) is signicantly less even and largepores can be observed on the surface. This is due to the much largerbre size of birch pulp compared to MFC. Due to the nano-sized -bres, the surfaces of NFC10 (Fig. 2d) and DCC10 (Fig. 2e) aresmooth and no individual bres can be observed in the FESEMimages.

After ionic cross-linking, the surfaces of pristine alginate, NFC10and DCC10 lms were smooth and identical to non-cross-linkedlms (images not shown). However, the surface of ICBIRCH50(Fig. 2f) was much more even and fewer pores were observed com-pared to BIRCH50. It is possible that shrinkage of alginate duringthe cross-linking compresses the BIRCH50 lm, which in turn re-duces the pore sizes of the lm. ICBIRCH50 (Fig. 2g) surface facedtowards the air during the drying was also examined and it is ob-served to be rougher having individual cellulose bres clearly vis-ible. In a comparison, the surface of ICMFC50 (Fig. 2h) was alsoobserved to be rough, but due to the smaller bre size, no individ-ual bres or visible pores can be observed on its surface.

Results from EDS-elemental analysis are presented in Supple-mentary data (Table S2). All surface sodium ions were changedto calcium ions during the cross-linking. In case of ICNFC10, 0.6%of chlorine is still present in the surface, which indicates that smallamount of unreacted CaCl2 remains on the surface of lm after

lms (a) before and (b) after cross-linking.

try 151 (2014) 343351washing. Interestingly, all lms except ICMFC50 have similaramounts of calcium ions on their surfaces (17.020.0%). ICMFC50,which has only 3.9% of calcium in its surface, may have higheramount of cellulose bres on the surface compared to otherbiocomposite lms, which reduces the amount of calcium boundto the surface of lm.

3.4. Wide-angle X-ray diffraction

X-ray diffractograms of the pristine alginate, BIRCH50 MFC50,NFC10 and DCC10 lms before and after ionic cross-linking arepresented in Fig. 3, along with the diffractograms of the pure birchpulp, MFC, NFC, DCC lms. All celluloses showed characteristicpeaks of cellulose I with the main 2h diffraction angles close to14.5, 16.0, and 22.2 associated with the 101, 101 and 002 crystal-line planes, respectively. CrIs of the birch pulp, MFC, NFC and DCCwere 56.6%, 63.1%, 48.8% and 44.9%, respectively, which are consis-tent with the literature (Iwamoto, Nakagaito, & Yano, 2007). On theother hand, the diffractogram of the pristine alginate lm shows

J.A. Sirvi et al. / Food Chemistry 151 (2014) 343351 347only a broad halo which indicates amorphous nature (Huq et al.,2012).

The characteristic cellulose peaks are undetectable in the dif-fractograms of NFC10 and DCC10 due to the large amorphous peakof alginate, but they are easily observed in the diffractograms ofMFC50 and BIRCH50. The CrI of cellulose in BIRCH50 is observedto decrease to 48.3%, making it 17% less crystalline than the purebirch pulp lm. The CrI of MFC50 is decreased even more, and itwas observed to be 47.2%, making it 34% less crystalline than thepure MFC lm. This is an unusual phenomenon; however, it hasbeen proposed that the hydrogen bonding interaction between cel-lulose crystals and additives can lead to decrease of the crystallin-ity of cellulose (Ul-Islam, Khan, & Park, 2012). Additives, such as

Fig. 2. FESEM images of the surface of the lms faced towards the tray during the drICBIRCH50 and images of the surfaces of the biocomposite lms faced towards the air d

20 40

Intens

ity

2 (degree)

DCC10

DCC

NFC10

NFC

BRICH50

BIRCH

MFC50

MFC

Alg

(a) (b

Fig. 3. XRD patterns for the biocomposite lms (a) before and (b) after the cross-linking.are also shown.chitosan, can penetrate cellulose bres and disrupt the hydrogenbonding of cellulose leading to decrease in crystallinity (Ul-Islam,Shah, Ha, & Park, 2011). In this context, alginate may penetrate cel-lulose bres during the lm preparation and interact with crystal-line parts of the cellulose, which leads to the decrease of thecrystalline order of cellulose. Furthermore, it is possible the largersurface area of MFC bres than birch pulp bres allows more ef-cient penetration of alginate into MFC than birch pulp. This isshown in the higher decrease of the crystallinity of cellulose inMFC50 than in BIRCH50.

After ionic cross-linking, there is no change in the X-raydiffractograms of the pristine alginate lms, NFC10 and DCC10.However, the crystallinity of the lms containing birch pulp and

ying: (a) pristine alginate lm, (b) MFC50, (c) BIRCH50, (d) NFC10, (e) DCC10, (f)uring the drying: (g) ICBIRCH50 and (h) ICMFC50.

20 40

2 (degree)

Intens

ity

ICDCC10

ICNFC10

ICBRICH50

ICMFC50

ICAlg

)

XRD patterns pristine alginate and pure birch pulp (BIRCH), MFC, NFC and DCC lms

Values of r showed slightly different pattern compared to r/qvalues due to the large variations on the densities of lms.

On the other hand, NFC1 has a 15% higher r value compared tothe pristine alginate lm and by increasing NFC content in the lmto 15%, the r value is increased 22%. NFC15 had an r value of97.97 MPa, which is 40% greater that of the pristine alginate lm.Compared to the MFC lms, NFC lms have signicantly higher rvalues, e.g. NFC10 had 63% greater r value than MFC10. It has pre-viously been reported that 8% nanosized cellulose, i.e. nanocrystal-line cellulose (NCC) increases the r of alginate lm from 57 to75.24 MPa (Huq et al., 2012). This and our present studies suggestthat NCC and NFC have quite similar reinforcement effect on algi-nate lms.

The r value of DCC1 was 13% greater than that of the pristinealginate lm. However, when the amount of DCC is increased from1% to 10%, the r value of the lm is decreased from 79.37 to69.63 MPa. The observation further indicates poor interaction be-tween alginate and DCC.

Mechanical properties of the biocomposite lms were also stud-ied after ionic cross-linking. The slides of ICNFC10 were wrinkledand uneven which affected on their properties as minor fractureswere formed during the preparation of the slides, and the accurate

4

6

8

Spec

ific GPa

1

1.25

1.5

1.75

2

2.25

2.5

2.75

3

3.25

3.5

Stra

ins

(%)

(c)

Fig. 4. (a) Values of r/q (kN m/kg) and r (MPa) b) E/q (MNm/kg) and E (GPa) and(c) strains (%) of lms (error bar represents standard deviations).

misFig. 4a shows that when the amount of MFC in the compositewas increased, r values of the lms decreased compared to pristinealginate lms, and MFC10 has a 29% lower r value than pristinealginate lm. The r value of BRICH50 was also signicantly lowerthan the r values of the other lms. As previously noted, based onMFC are increased, ICMFC50 and ICBIRCH50 having CrI of 55.2%and 54.2%, respectively. This phenomenon has previously been ob-served with the blend cotton linter cellulose and alginate mem-brane after ionic cross-linking with Ca2+ (Yang, Zhang, Peng, &Zhong, 2000).

3.5. Mechanical properties

3.5.1. Specic tensile strength and tensile strengthThicknesses of lms are presented in Supplementary data

(Fig. S2). The thicknesses of the lms increased with the amountof added cellulose due to the brous structure of cellulose. The dif-ference between the thicknesses of the lms containing the nano-and microbrillated and unbrillated cellulose was enormous, e.g.the BIRCH50 was 88% thicker than MFC50. The addition of a smallamount of NFC to alginate does not have a signicant impact onthe thickness of lms; however, NFC15 has a 23% higher thicknesscompared to the pristine alginate lm. Thicknesses of lms con-taining DCC were slightly higher compared to the pristine alginatelms and the lms containing NFC. This suggests that some DCCagglomerates are formed during the preparation of lms contain-ing DCC and alginate.

After the cross-linking, the thickness of the alginate lms withbirch pulp and MFC is decreased whilst thickness of NFC10 is in-creased 17% during ionic cross-linking. On the other hand, thereis no signicant change in the thickness of the DCC10 after cross-linking.

An increase of the thickness in the alginate lms during thecross-linking has been reported previously (Rhim, 2004). The in-crease is due to the changes in the morphology of the alginate dur-ing the cross-linking in the aqueous calcium chloride solution(Russo, Malinconico, & Santagate, 2007). However, in the case ofICMFC50 and ICBIRCH50, the decreases in the thicknesses areprobably due to the shrinkage of the alginate which in turn com-presses the voids in the biocomposites and makes these lmsdenser.

Tensile strengths of the lms are presented in Fig. 4a. To takeinto account high differences in the densities of lms, r/q valueshave also been reported. The pristine alginate lm had an r/q va-lue of 49.0 kN m/kg and lms with 1% to 15% of MFC have statisti-cally the same r/q values, whilst MFC20 has a 17% and MFC50 a38% greater r/q values than the pristine alginate lm. On the otherhand, BIRCH50 had practically the same r/q value as the pristinealginate lm.

Compared to MFC, nano-sized NFC has a clearly better rein-forcement effect on the alginate lm. An r/q value of80.10 kN m/kg was obtained when 10% of NFC was used, whichis 63% and 19% greater than those of MFC10 and MFC50. However,the increase of NFC content from 10% to 15% did not further en-hance r/q of the lms.

Interestingly, DCC1 and DCC10 have practically the same r/qvalues as pristine alginate lm. This could be explained by the elec-trostatic repulsion between the anionic carboxyl groups of alginateand DCC, which prevents effective interaction between alginateand DCC, previously observed in FTIR study.

348 J.A. Sirvi et al. / Food Chethe FESEM images, surface of BIRCH50 was uneven, and the poreson the surface may serve as the breaking points in the lm duringtension.10

30

50

70

90

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130

Spec

ific

stre

ngth

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try 151 (2014) 343351results of ICNFC10 was not obtained. On the other hand, ICMFC50,BIRCH50 and ICDCC10 were smooth and easily processed and theyshowed higher r/q values compared to their non-cross-linked

linking, respectively.

be observed when amount of NFC is increased from 10% to 15%.

emisFurthermore, the addition of DCC to alginate lm does not increasethe E/q of lm.

E values of the lms shows slight decrease when the amount ofMFC was increased. However, statistically all of the MFC lmsshowed almost same E that of the pristine alginate lm. Similarlyto the r values, the E value of BIRCH50 is also signicantly lowerthan the E values of the other lms.

Addition of the NFC to the alginate lm increased E values of thelm and NFC1 had 13% and NFC5 27% higher E values than the pris-tine alginate lm. In addition, NFC10 had 51% higher E value thanMFC10. Statistically there was no difference in E values betweenthe pristine alginate lm and lms containing DCC bres.

The E/q value of MFC50 was not affected by ionic crosslinking;however, the E value of ICMFC50 was 30% lower than the E value ofMFC50. The E/q value of ICBIRCH50 was also almost same as theE/q value of BIRCH50 and the E value of BIRCH50 was actuallyincreased 69% after cross-linking. In the biocomposite lms with10% of DCC, the E/q and the E values were increased 21% and41%, respectively, after ionic cross-linking.

3.5.3. Elongation at breakValues of the strain of the lms are presented in Fig. 4c. It is

known that incorporation of nanocellulose bres to other polysac-The r values of lms were also remarkably increased duringcross-linking and ICMFC50 had a 49% greater r value thanMFC50, and ICDCC10 has a r value of 125.31 MPa which is 80%greater r compared to DCC10. To the best of our knowledge thisis the highest r of alginate biocomposite lms reported so farand comparable with cellophane, a commonly used cellulose-based packaging material as r value of cellophane is reported tobe as high as 120 MPa (Spence, Venditti, Habibi, Rojas, & Pawlak,2010). As previously noted, based on the FTIR studies, the anionicgroups can take part in the formation of cross-linked network dur-ing the ionic cross-linking, which results in enhancement of themechanical properties of the lm.

Interestingly, the r value of BIRCH50 increased 93% during theionically cross-linking. As previously noted, based on the FESEMimages, cross-linking made lm containing birch pulp more evenand reduced the amount of pores on the surface, which reducedthe amount of breaking points in the lm. However, r value ofICBIRCH50 is still 67% lower that of pristine alginate lm and threefold lower than the r value of ICDCC10.

3.5.2. Specic modulus and modulusThe E/q values of the lms are presented in Fig. 4b. The pristine

alginate lm had an E/q value of 9.38 M Nm/kg. An increase of theMFC amount from 1% to 15% does not increase the E/q value of thebiocomposite lm; however, MFC20 and MFC50 had 25% and 47%greater E/q value than the pristine alginate lm, respectively.Interestingly, BIRCH50 showed 19% greater E/q value that of thepristine alginate lm. However, MFC50 had a 24% greater E/q valuethan BIRCH50.

The E/q values of the lms containing NFC are consistent withthe r/q values. When amount of NFC was increased, the E/q valueof the biocomposite lm was also increased, and NFC10 had 55%and 31% higher E/q value than the pristine alginate lm andMFC10, respectively. No signicant increase in the E/q values cancounterparts. Highest improvement in r/q was observed withICDCC10 which had 54% greater r/q value than DCC10. The r/q va-lue of MFC50 and BIRCH50 increased 13% and 10% after ionic cross-

J.A. Sirvi et al. / Food Chcharide matrixs decreases strain of the lms (Fernandes et al.,2010). Films with 10% and 15% NFC showed signicant decreasein strain and they had 66% and 61% lower strains than the pristinealginate lm, respectively. Statistically, other lms showed similarstrain values than the pristine alginate lm.

The strain values of the biocomposite lms were slightly de-creased during the cross-linking, and strains of the ICMFC50,ICBIRCH50 and ICDCC10 were 28%, 42% and 7% lower comparedto their non-cross-linked counterparts.

Results from mechanical studies showed that addition of cellu-lose, especially NFC to the alginate lms increased their strengthsand moduli values, but decreased strain and exibility of the lm.The improvements in the mechanical properties of compositescontaining cellulose bres are due to the formation of tightlyhydrogen bonded networks between bres and matrix.

It was also observed that the chemical nature of the cellulose -bre plays important part in the formation of strong biocomposites.Before cross-linking, the anionic DCC did not have reinforcementeffect on the alginate, due to the electrostatic repulsion between -bres and alginate. Even though there are anionic groups in NFC, thecharge density is signicantly lower that of DCC and consequentlyNFC had good compatibility with alginate. On the other hand, an-ionic groups of the DCC were observed to take part in the ioniccross-linking network after Ca2+ cross-linking, resulting in remark-able increase in the mechanical properties of the lm.

3.6. Barrier properties

3.6.1. Grease barrier propertiesAll of the studied cellulose-containing alginate lms except

BIRCH50 had excellent grease barrier properties, i.e. the dyed tur-pentine did not penetrate lms within the 30 min testing period.However, the dyed turpentine penetrated BIRCH50 immediatelywhich was probably due to porous structure of the lm previouslynoted, based on the FESEM images. On the other hand, surfaces ofthe other lms were smooth and visible pores were not observedbased on FESEM images. Moreover, the good grease barrier proper-ties of alginate explain partly the excellent grease barrier proper-ties of the biocomposite lm (Ham-Pichavant, Sbe, Pardon, &Coma, 2005).

After ionic cross-linking, lms retained their grease barrierproperties and no penetration of the dyed turpentine was observedin ICNFC10, ICDCC10 and in ICMFC50. However, penetration of thedyed turpentine occurred immediately with ICBIRCH50.

3.6.2. Water vapour permeabilityThe WVP of the pristine alginate lm, BIRCH50, MFC50, NFC10

and DCC10 was measured before and after ionic crosslinking(Fig. 5a). The WVP of pristine alginate lm was 8.4 107 g/(Pa s m2). Due to the high hydrophilicity, alginate has low watervapour barrier properties (Olivas & Barbosa-Cnovas, 2008). Theaddition of micro- and nanocellulose bres to the alginate lm de-creased the WVP, e.g. the WVP of the MFC50 was a 23% lower thanthat of the pristine alginate lm. BIRCH50 had in turn a 6% higherWVP that of the pristine alginate lm.

It is known that barrier properties of lms are improved whenadditives have lower permeability and materials are well dispersed(Khan et al., 2010). Furthermore, addition of micro- and nano-sizedcellulose bres to the alginate matrix makes alginate lms moretortuous, and thus decreases the pathways to water molecules topass through the lm (Huq et al., 2012). This could explain the im-proved water vapour barrier properties of alginate lms after addi-tion of MFC, DCC and NFC bres. The large pores, observed inFESEM images, may create direct pathways for water vapour topenetrate through BIRCH50, resulting in decreased water vapourbarrier properties compared to the pristine alginate lm.

try 151 (2014) 343351 349It is known that ionic cross-linking improves water vapour bar-rier properties of alginate and composites containing alginate(Rhim, 2004). Here water vapour barrier properties were also

misobserved to improve after ionic cross-linking and both ICNFC10and ICDDC10 have a 13% lowerWVP than the pristine alginate lm.Furthermore, ICNFC10 has a 6% lower WVP than NFC10 andICDD10 has a 9% lower WVP than DCC10.

WVP of MFC50 was increased after cross-linking, and ICMFC50has a 14% higher WVP than MFC50. However, the WVP of ICMFC50

Fig. 5. (a) WVP (g/(Pa s m2)) and (b) SWVP (kg m/(Pa s m2)) of lms (error barrepresents standard deviations).

350 J.A. Sirvi et al. / Food Cheis still a 6% lower than the pristine alginate lm. On the other hand,the water vapour barrier properties of BIRCH50 was decreasedafter cross-linking and ICBIRCH50 has 11% and 5% lowerWVP com-pared to BIRCH50 and the pristine alginate lm, respectively. Aspreviously noted, based on the FESEM images, the number of poresin the surface of BIRCH50 was reduced during the cross-linking.This in turn reduced the direct pathways of water vapour to pene-trate through the lm.

To take into account the differences in the thicknesses of lms,SWVPs are presented in Fig. 5b. There were no signicant differ-ences in the SWVPs of the lms, expect in the lms containingbirch pulp, i.e. SWVP of BRICH50 is more than threefoldhigher than that of the pristine alginate lm. After cross-linking,SWVP of the BIRCH50 decreased 32%, but was still 91% highercompared to pristine alginate lms. Compared to commonpackaging materials, such as cellophane, all of the biocompositelms had lower SWVP than cellophane whose SWVP is reportedto be 6.9 1014 kg m/(Pa s m2) (Proval, Debeaufort, Despre, &Voilley, 2002).

4. Conclusion

In this work, biocomposite lms based on cellulose and alginatewere produced using four different wood cellulose bres withdifferent bre sizes, chemical nature and proportions. Films werealso further cross-linked using Ca2+. Celluloses showed good inter-action with alginate matrix and the mechanical properties of lmswere increased by increasing the amount of the bre in thebiocomposite and by decreasing the bre size of cellulose. How-ever, despite the nanosized bre structure, anionic charges pre-vented an efcient interaction between DCC and alginate without

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Cheng, Y., Lu, L., Zhang, W., Shi, J., & Cao, Y. (2012). Reinforced low density alginate-a cross-linking. On the other hand, anionic carboxylate groupscreated a highly intense cross-linked network between alginateand DCC, when the lm was ionically cross-linked with Ca2+. A bio-composite lm with remarkably high mechanical strength wasproduced by ionically cross-linking the alginate lm containing10% of DCC. All of the biocomposite lms except the lm withunmodied birch pulp, showed excellent grease barrier properties.Furthermore, the WVP of alginate lm was decreased by the addi-tion of MFC and nano-sized cellulose bres, and by ionic cross-link-ing. This work demonstrates that biocomposite lms based oncellulose bres and alginate have high potential to be used as highstrength packaging materials.

Acknowledgments

The authors acknowledge the support from the FutureBiorenery Programme of Forestcluster Ltd. V.T.T. is acknowledgedfor the pulp analysis.

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.2013.11.037.

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J.A. Sirvi et al. / Food Chemistry 151 (2014) 343351 351

Biocomposite cellulose-alginate films: Promising packaging materials1 Introduction2 Experimental section2.1 Materials2.2 Preparation of biocomposite films2.3 Preparation of ionically cross-linked biocomposite films2.4 Mechanical properties2.5 Field emission scanning electron microscope and energy-dispersive X-ray spectroscopy2.6 Fourier transform infrared spectrum2.7 X-ray diffraction2.8 Grease barrier properties2.9 Water vapour permeability

3 Results and discussion3.1 Cellulose and alginate biocomposite films3.1.1 Preparation of biocomposite films3.1.2 Preparation of ionically cross-linked biocomposite films

3.2 Fourier transform infrared spectrum3.3 Field emission scanning electron microscope and energy-dispersive X-ray spectroscopy3.4 Wide-angle X-ray diffraction3.5 Mechanical properties3.5.1 Specific tensile strength and tensile strength3.5.2 Specific modulus and modulus3.5.3 Elongation at break

3.6 Barrier properties3.6.1 Grease barrier properties3.6.2 Water vapour permeability

4 ConclusionAcknowledgmentsAppendix A Supplementary dataReferences