ed

ino

eree othabilthecoplitya sp

strong antioxidant.

the adbeaufoor comd lipidal proir hyd

interactions and hydrophobic forces (McHugh and Krochta,1994). According to Schou et al. (2005), casein-based edible lmsare attractive for food applications due to their high nutritionalquality, excellent sensory properties and potential to providefood products with adequate protection from their surroundingenvironment. On the other hand, the water barrier propertiesof SC-glycerol lms have been described as inferior by Han

essential oils (cinnamon and ginger) in different ratios on theproperties of SC-based lms. This characterization includedmechanical behaviour, water barrier ability, optical properties,microstructure and surface morphology. The correspondinglm-forming emulsions were studied (particle size, rheology. . .)in order to gain a better understanding of lm properties. Addi-tionally, the antioxidant capacity of the essential oils was deter-mined as well as the ability of the lms to provide protectionagainst lipid oxidation. Cinnamon and ginger EO were selectedon the basis of the compatibility of their taste and avour withnuts.

* Corresponding author. Tel.: +34 96 387 7000x73625; fax: +34 96 387 7369.

Journal of Food Engineering 100 (2010) 678687

Contents lists availab

d

lsE-mail address: loathue@tal.upv.es (L. Atars).et al., 2002; Kristo et al., 2007). Lipids have the opposite charac-teristics (Greener, 1992; Perez-Gago and Krochta, 2001). There-fore, lipid addition to hydrophilic lms is an interestingalternative to obtain lms with improved attributes (Quezada-Gallo et al., 2000).

Sodium caseinate (SC) is a commercially available water-solu-ble polymer obtained by the acid precipitation of casein, themain protein in cows milk (Audic and Chaufer, 2005). Due tothe caseins structure and aminoacid sequence, it is likely thatthe lm formation is caused by hydrogen bonding, electrostatic

Like other lipids, essential oils (EO) may improve the water barrierproperties of the lms, because of their hydrophobic nature. Addi-tionally, there may be some other positive effects, such as protec-tion against microbial growth and lipid oxidation (Ahn et al., 2008;Gutirrez et al., 2008; Oussalah et al., 2004, 2006; Rojas-Gra et al.,2006, 2007; Zinoviadou et al., 2009; Hinneburg et al., 2006; Bakkaliet al., 2008). Films with EO could be used to slow down lipid oxi-dation in products with a high content in unsaturated fatty acids,such as nuts.

This work was undertaken to study the effect of adding two1. Introduction

Several reviews have pointed outapplications of edible packaging (Deet al., 1996; Krochta, 2002). The majings are proteins, polysaccharides ancharide lms show good mechanicpermeable to water because of the0260-8774/$ - see front matter 2010 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2010.05.018vantages and potentialrt et al., 1998; Guilbertponents of edible coat-s. Protein and polysac-perties, but are highlyrophilic nature (Anker

and Gennadios (2005). Fabra et al. (2008) successfully reducedthe water vapour permeability of SC lms by adding oleic acidand beeswax mixtures (protein:lipid ratio between 1:0.25 and1:0.75).

A number of lipids (fatty acids, waxes. . .) have been tested asdepressors of water vapour permeability in composite lms basedon hydrophilic proteins. In this respect, the incorporation of plantextracts to composite lms represents an interesting alternative.Cinnamon essential oilGinger essential oil 2010 Elsevier Ltd. All rights reserved.Characterization of sodium caseinate-bascinnamon or ginger essential oils

L. Atars *, J. Bonilla, A. ChiraltInstitute of Food Engineering for Development, Universidad Politcnica de Valencia, Cam

a r t i c l e i n f o

Article history:Received 19 February 2010Received in revised form 13 May 2010Accepted 22 May 2010Available online 1 June 2010

Keywords:Edible lmsSodium caseinate

a b s t r a c t

Film-forming emulsions wor ginger) and lms werratio protein:oil 1:0.100),The water vapour permethe optical properties ofScanning Electron Microsthe oils improved the abidespite the fact that, in

Journal of Foo

journal homepage: www.ell rights reserved.edible lms incorporated with

de Vera, s/n, 46022 Valencia, Spain

formulated with sodium caseinate and two essential oils (cinnamonbtained by casting. At the low oil proportion being tested (maximume lipid did not affect the mechanical behaviour of the protein lms.ity was slightly reduced by both oils. Cinnamon oil greatly affectedlms. Ginger oil resulted in lipid droplet aggregation observable by

y in dried lms, surface irregularities and gloss decrease. Neither ofof sodium caseinate lms to act as protection against lipid oxidation,ectrophotometric method, isolated cinnamon oil proved to be very a

le at ScienceDirect

Engineering

evier .com/locate / j foodeng

and ginger EO was determined using a modication of the original

Lleida, Spain), in the following protein to oil mass ratios: 1:0.025

Pnid

4i

ical curves were obtained after a stabilization time of 5 min at

Video-Based Contact Angle Meter model OCA 20 (DataPhysics

dom positions were considered for WVP tests, and four measure-

Eng(C0.025, G0.025), 1:0.050 (C0.050, G0.050), 1:0.075 (C0.075, G0.075) and1:0.100 (C0.100, G0.100). Such low oil proportions were selected onthe basis of the intense odour of cinnamon oil. Homogenizationwas carried out using a vacuum high-shear probe mixer (Ultratu-rax T25, Janke & Kunkel, Germany) at 13,500 rpm for 3 min, andthe emulsions were degasied at room temperature by means ofa vacuum pump.

The amount of emulsions that would provide 2 g of total solidswas spread evenly over each casting plate (150 mm diameter),resulting in a solid distribution of 113.2 g solids/m2. Films wereformed by drying on a level surface for approximately 24 h at45% RH and 20 C, and were peeled off from the casting surface.The lms were stored for 2 weeks in cabinets at 25 C and threelevels of relative humidity (RH), namely 33%, 53% and 75%, usingsaturated solutions of MgCl2, Mg(NO3)2 and NaCl, respectively(Panreac qumica, S.A., Castellar del Valls, Barcelona, Spain). Equi-librium moisture content of the lms at each RH level were 5, 10and 22 g water/100 g lm respectively, regardless the essentialoil content (7 % in the dried lm, in the case of maximumconcentration).

2.3. Characterization of the lm-forming dispersions

2.3.1. Lipid particle sizeParticle size analysis of the emulsions was carried out with a la-

ser diffractometer (Mastersizer 2000, Malvern Instruments,TEAC method (Re et al., 1999). Trolox (6-hydroxy-2,5,7,8-tetrame-thylchroman-2-carboxylic acid), a vitamin E analogue, was used asan antioxidant standard. ABTS (2,20-azinobis(3-ethylbenzothiazo-line-6-sulfonic acid)diammonium salt) was dissolved in water toa concentration of 7 mM and allowed to react with a 2.45 mMpotassium persulfate solution (nal concentrations) for 16 h inthe dark. All three reactants were provided by SigmaAldrich. ABTSradical cation (ABTS+), a blue chromophore, was produced duringthat period. The ABTS+ solution was diluted with ethanol to anabsorbance at 734 nm of 0.70 ( 0.02). All the determinations werecarried out in a Beckman Coulter DU 730 spectrophotometer, usingethanol as blank. Essential oils were diluted so that the addition ofa 10 ll-aliquot to 990 ll of ABTS+ dilution would produce a 1580% absorbance decrease within 6 min. Absorbance at 734 nm(A734) was registered every minute during the test. For calibration,Trolox standards of between 60 and 500 mg/l were prepared andthe same procedure was followed. The TEAC of the essential oilswas determined by comparing the corresponding percentage ofabsorbance reduction to the concentrationresponse curve. Allthe determinations were carried out ve times.

2.2. Preparation of the lm-forming emulsions and casting of the lms

SC (Llorella, Barcelona, Spain) was dispersed in distilled waterat 8% (w/w) under magnetic stirring, and glycerol (Panreac Qumi-ca, S.A., Castellar del Valls, Barcelona, Spain) was added in a pro-tein:glycerol ratio of 1:0.3. The emulsions were obtained by addingcinnamon (C) or ginger (G) essential oils, provided respectively byBiocesta S.L. (Valencia, Spain) and Esential arms (Torre-Serona,2. Materials and methods

2.1. Antioxidant capacity of the essential oils

The Trolox equivalent antioxidant capacity (TEAC) of cinnamon

L. Atars et al. / Journal of FoodWorcestershire, UK). The samples were diluted in distilled waterat 1800 rpm until an obscuration rate of 10% was obtained. TheMie theory was applied by considering refractive indexes of 1.57ments were taken along the previously cut strips for the tensiletests.

2.4.2. Tensile propertiesA universal test Machine (TA.XTplus model, Stable Micro Sys-

tems, Haslemere, England) was used to determine tensile strength(TS), lm elastic modulus (EM) and elongation (%E), according toASTM standard method D882 (ASTM, 2001). These properties weredetermined from stressstrain curves, estimated from forcedefor-mation data. Rectangular samples (25 100 mm) were cut andstored at 25 C and two RH levels, 33% and 53%, in cabinets con-taining MgCl2 or Mg(NO3)2 saturated solutions (Panreac quimica,S.A., Castellar del Valls, Barcelona). Equilibrated lm specimenswere mounted in the lm-extension grips and stretched at50 mmmin1 until breakage. The relative humidity and tempera-Instruments GmbH, Filderstadt, Germany). Image analyses werecarried out using SCA20 software. Ten replicates were made performulation.

2.4. Characterization of the lms

2.4.1. Film thickness measurementsA Palmer digital micrometer (Comecta, Barcelona, Spain) was

used to measure lm thickness to the nearest 0.001 mm. Six ran-25 C. The shear stress (c) was measured as a function of shear rate( _c) from 0 to 512 s1, in order to obtain the viscosity (g) of theformulations.

2.3.3. Density and contact angleDensity (q) was measured at 25 C by means of a pycnometer,

using water as the reference liquid. Due to the potential interestof applying these coatings to products with a high content inunsaturated fatty acids, such as nuts, the contact angle (h) wasmeasured on the surface of a peeled roasted almond. The shapeof a sessile drop (0.01 ml) was studied after 10 s by means of aD4;3 Pnid

3i

2

2.3.2. Rheological behaviourThe rheological behaviour of the formulations was analysed in

triplicate at 25 ( 1) C by means of a rotational rheometer (HAAKERheostress 1, Thermo Electric Corporation, Karlsruhe, Germany)with a type Z34DIN Ti sensor system of coaxial cylinders. Rheolog-and 1.49 for cinnamon and ginger oil, respectively. Three samplesof each lm-forming dispersion were measured in triplicate. Thedroplet size distribution, the volume-surface mean diameter(D3,2) (Eq. (1)) and the weight mean diameter (D4,3) (Eq. (2)) weredetermined. In equations 1 and 2, ni is the number of dropletswithin a determined size range and di is the droplet diameter.D3,2 is the average size based on the specic surface per unit vol-ume, whereas D4,3 represents the average size based on the unitweight of particles

D3;2 P

nid3iP

nid2i

1

ineering 100 (2010) 678687 679ture (25 C) of the testing environment were registered with athermohygrometer (testo 60-H2, Testo GmbH & Co., Lenzkirch,Germany). Nine replicates of each formulation were tested.

plot was divided by the exposed lm area to calculate the water

to a highly polished surface of black glass standard with a valuenear to 100.

Nanoscope III.a, Scanning Probe Microscope (Digital Instruments,Inc. Santa Barbara, California) with a 125 125 lm scan size and

square roughness (Rq: root-mean-square average of height devia-tions taken from the mean data plane).

Engvapour transmission rate (WVTR).The vapour pressure on the lms inner surface (p2) was ob-

tained by means of the method proposed by Mc Hugh et al.(1993) to correct the effect of concentration gradients establishedin the stagnant air gap inside the cup (Eq. (3)).

WVTR P D Ln P p2=P p1 R T Dz 3

where P: total pressure (atm); D: diffusivity of water through air at25 C (m2/s), R: gas law constant (82.057 103 m3 atmkmol1 K1); T: absolute temperature (K); Dz: mean height of stag-nant air gap (m), considering the initial and nal z value; p1: watervapour pressure on the solution surface (atm); p2: corrected watervapour pressure on the lms inner surface in the cup (atm). TheWVP was calculated as a function of p2 and the water vapour pres-sure on the outer (p3) side of the lm (Eq. (4))

WVP WVTRp2 p3 thickness: 4

2.4.4. Optical properties: K/S, colour and glossThe optical properties were determined in lms that had been

pre-equilibrated at 33% RH on the side of the lm in contact withair during drying. The surface reectance spectra of the lms wereobtained using a MINOLTA spectrocolorimeter, model CM-3600d(Minolta CO., Tokyo, Japan). The measurements were taken on bothblack and white backgrounds on an illuminated sample area 8 mmin diameter. The K/S ratio was determined by applying the Kub-elkaMunk theory (Hutchings, 1999) for multiple scattering tothe reectance spectra. As each light ux passes through the layer,it is affected by the absorption (K) and the scattering (S) coef-cients, as described by equation (Eq. (5)). In this equation, R1 isthe reectance of an innitely thick layer of the material, deter-mined through (Eq. (6)) to (Eq. (8)) in terms of the reectance Rof the sample layer backed by a known reectance Rg. R0 is thereectance of the layer with an ideal black background. Three lmsper formulation (three measurements per lm) were considered

KS 1 R1

2

2R15

R1 a b 62.4.3. Water vapour permeabilityWater vapour permeability (WVP) was determined at three

different temperatures (5, 25 and 35 C) and three RH gradients(3353%, 5375% and 53100%) using a modication of the ASTME96-95 gravimetric method (1995) for exible lms (Mc Hughet al., 1993). The formulations selected for this test were SC, SC-C0.025, SC-C0.075, SC-G0.025 and SC-G0.075. Payne permeability cupsof 3.5 cm in diameter (Elcometer SPRL, Hermelle/s Argenteau, Bel-gium) were lled with 5 ml of saturated solutions of Mg(NO3)2(HR = 53%), NaCl (HR = 75%) or distilled water (HR = 100%). Filmswere selected based on their lack of bubbles or pinholes, roundsamples were cut (three per formulation) and six random thicknessmeasurements were taken per sample. Once the lms were se-cured, each cup was placed in a pre-equilibrated cabinet tted witha fan to provide a strong driving force across the lm for water va-pour diffusion. The RH of the cabinet was held constant at 33% or53% using over-saturated solutions of MgCl2 or Mg(NO3)2. Theshiny side of the lms was exposed to the atmosphere at 53%RH, and the cabinets were stored at the three aforementioned tem-peratures. The cups were weighed periodically (0.00001 g) aftersteady state had been reached. The slope of the weight loss vs. time

680 L. Atars et al. / Journal of Fooda 12

R R0 R RgR0Rg

72.4.7. Antioxidant capacity of the lmsThe antioxidant capacity of the lms was quantied by means

of an accelerated test of oxidative rancidity, previously describedby Oss et al., 2008. Specially designed methacrylate cells werelled with 20 g of sunower oil, covered with the lms (SC, SC-C0.075, SC-G0.075) and hermetically secured. The headspace in eachcell was 15.65 cm3, and the oxygen within was displaced by bub-bling nitrogen for 15 min. Two cells were covered with aluminiumfoil (17 lm thickness) and two more were left uncovered. All thecells were placed in cabinets at 32%RH (with saturated solutionsof MgCl2), and stored at 40 C in darkness. For 50 days, samplesof 1 ml sunower oil were taken every 812 days using a syringethrough the septum placed in the cell wall. Simultaneously, 1 mlof nitrogen was introduced in the cell through a second septumto maintain the internal pressure constant. Films were tested intriplicate.

The progression of oil oxidation was monitored by analysing thea 6 lm vertical range. The formulations tested were SC, SC-C0.025,SC-C0.075, SC-G0.025 and SC-G0.075. A three-dimensional image of alm surface area (50 50 lm) was obtained in each test, and threeimages were captured per formulation. Two statistical parametersrelated with sample roughness (method ASME B46.1, 1995), werecalculated: average roughness (Ra: average of the absolute valueof the height deviations from a mean surface), and root-mean-2.4.5. Scanning Electron MicroscopyImages of the cross-sections of the lms were obtained by Scan-

ning Electron Microscopy (SEM), using a JEOL JSM-5410 (Japan)electron microscope. Film specimens were cryofractured byimmersion in liquid nitrogen and mounted on copper stubs per-pendicularly to their surface. Samples were gold coated and ob-served, using an accelerating voltage of 10 kV.

2.4.6. Surface morphologyThe surface of lms which had been previously equilibrated at

33% RH was studied by Atomic Force Microscopy (AFM), using ab a2 11=2 8Moreover, CIE-L* a* b* parameters (CIE, 1986) were obtained

from R1 data, using illuminant D65 and observer 10. Colour coor-dinates, chroma (Cab) and hue (h

ab), as well as the whiteness index

(WI) of the samples, were calculated using Eqs. (9)(11):

Cab a2 b21=2 9hab arctgb=a 10WI 100 100 L2 a2 b2

1=211

The gloss of the lms was measured at angles of 20, 60 and85 from the normal to the coating surface, according to the ASTMstandard D523 (ASTM, 1999) using a at surface gloss meter (Mul-ti.Gloss 268, Minolta, Germany). Ten replicates were taken per for-mulation and angle. Results were expressed as gloss units, relative

ineering 100 (2010) 678687peroxide value (PV) following the method described in the normUNE 55-023. The PV was expressed as mEq of oxygen per kilogramof sample.

2.5. Statistical analysis

The statistical analysis of the data was performed through ananalysis of variance (ANOVA) using Statgraphics Plus for Windows5.1 (Manugistics Corp., Rockville, MD) Fishers least signicant dif-ference (LSD) procedure was used.

3. Results and discussion

3.1. Antioxidant capacity of the essential oils

Fig. 1 represents the absorbance reduction caused by Troloxaddition (a), and the concentrationresponse curve at 6 min(R2 > 0.99) for ve sequentially and separately prepared stock stan-dards of Trolox (b). Essential oils were diluted in ethanol in differ-ent proportions (1:200 cinnamon oil, 1:20 ginger oil). At 6 minafter mixing, cinnamon oil dilution had produced a reduction inA734 of between 61% and 70%, whereas that caused by ginger oildilution ranged between 17% and 23%. Table 1 shows the TEAC re-sults at different times, i.e. the Trolox concentration (mM) whoseantioxidant activity equals that of the extract. Cinnamon oil exhib-ited a much more relevant antioxidant capacity than that of gingeroil, in agreement with that found by Mantle et al. (1998). Theseauthors determined the antioxidant capacity of diverse plant ex-

not important in terms of number, even though they are repre-sented by big peaks in volume distributions. In terms of number,

be linked to the different balance of interaction forces between sol-vent-protein and EO components as a function of the oil

ABTS .

L. Atars et al. / Journal of Food EngTable 1Trolox equivalent antioxidant capacity (TEAC) of the essential oils at specic time-points. Average values and standard deviations in brackets.

TEAC (mM)Essential oil 1 min 3 min 6 mintracts using the original TEAC method, and concluded that cinna-mon oil was amongst the extracts with the highest antioxidantpower. Such a capacity is linked to its high content in phenolic con-stituents. Shan et al. (2005) obtained a highly positive linear rela-tionship (R2 > 0.96) between TEAC values and the total phenoliccompounds of 26 spices. These authors identied the followingmajor phenolic constituents in a cinnamon extract: phenolic acids,phenolic volatile oils (2-hydroxycinnamaldehyde, cinnamyl alde-hyde, derivatives) and avan-3-ols. Similar composition was re-ported by Wang et al. (2009). As compared to cinnamon, gingeroil exhibits little antioxidant capacity, due to the low proportionof phenolic volatile oils in its composition (Golebiowski et al.,2008).

y = 0.1699xR2 = 0.9949

0

20

40

60

80

100

0 100 200 300 400 500

Trolox (mg/l)

% reduction

6 min

Fig. 1. Concentrationresponse curve at 6 min after the mixture of Trolox and+Cinnamon 263 (13) 288(16) 309 (18)Ginger 5.1 (0.8) 7.3 (1.3) 9.7 (1.5)concentration.

3.2.2. Rheological behaviourThe emulsions showed a Newtonian behaviour in every case

(Table 2), coherent with previous works on SC dispersions (813% w/w) (Barreto et al., 2003; Khwaldia et al., 2004). Viscosityranged between 22 and 26 mPa s, which could be considered asrelatively high values for lipidprotein emulsions in this concen-tration range. According to Khwaldia et al. (2004), whey proteindispersions are much less viscous than those of SC at a similar con-centration. These results could be explained by taking the presenceof aggregates into account, due to the balance of proteinproteininteractions and the afnity between the protein and the solvent,interacting by hydrogen bonding. The hydration of the protein re-sults in a high specic volume, and thus in relatively viscoussystems.

3.2.3. Density and contact angleThe density of the formulations (Table 2) ranged between 1.025

and 1.026 g/ml. Given the low proportion of oil added, no signi-cant decrease was observed (p > 0.05).

The contact angle was measured on the outer surface of roastedpeeled almonds. The slight curvature of the surface was consideredas negligible given the small size of the sessile drop. The values of hon the inner surface (contact surface of the two cotyledons) wereslightly but signicantly lower (p < 0.01) than those on the outersurface (results not shown), indicating that the latter is the leastfavourable situation for effective product coating. However, thecontact angle on the outer surface was always lower than 90 (Ta-ble 2), which indicates that all the formulations tested would wet,size distributions were monomodal. Most of our formulationsshowed maxima at 0.07 lm, and should be referred to as micro-emulsions (particles range from 0.01 to 0.2 lm, according toMcHugh, 2002). Essential oils are liquid at room temperature, thusfavouring droplet size reduction during homogenization. Further-more, the low lipid content favoured its incorporation in the pro-tein dispersion.

Average diameters, as well as size distributions, were affectedby the oil type and amount. Generally, lipid incorporation is hin-dered at high oil contents. This was not the case in the studied for-mulations, especially in those containing cinnamon oil, where theopposite effect was found: at a low oil content, the biggest averagediameters were obtained, and a more numerous population ofsmall particles formed as the oil content increased which could3.2. Characterization of the lm-forming dispersions

3.2.1. Lipid particle sizeThe size of the lipid particles in the emulsions affects important

characteristics of the lms, such as their mechanical and barrierproperties (Fabra et al., 2009a). Table 2 shows the results of D3,2(volume-surface diameter) and D4,3 (weight mean diameter) ofthe lm-forming emulsions, and g. 2 represents the particle sizedistributions in volume. These were wide and multimodal, proba-bly due to the aggregation of small particles after emulsication(Langton and Hermansson, 1993). In most cases, a population ofsmall particles (

Table 2Average diameters (D3,2, D4,3), density (q) and viscosity (g) at 25 C, and contact angle on the outer surface of a roasted peeled almond (h). Average values and standard deviations,in brackets. The same superscript means homogeneous group in LSD test.

Formulation D3,2 (lm) D4,3 (lm) g (mPa s) q25C (g/ml) h ()

SC 22.4 (0.3)b 1.02592 (0.00006)b 68.8 (1.6)de

SC-C0.025 19.4 (0.3)e 69 (4)d 22.3 (0.3)b 1.0257 (0.0006)ab 62 (6)a

SC-C0.050 17.9 (0.6) d 67 (4)d 22.5 (0.3)b 1.0258 (0.0005)b 63 (4)ab

SC-C0.075 0.72 (0.03)a 44 (8)c 22.1 (0.4)ab 1.0257 (0.0004)ab 64 (6)abc

SC-C0.100 0.629 (0.005)a 47.0 (1.4)c 21.8 (0.4)a 1.02620 (0.00019)b 65 (3)abcd

SC-G0.025 0.803 (0.009)a 42.0 (0.2)bc 23.68 (0.18)c 1.0261 (0.0009)b 67 (3)bcde

e D

682 L. Atars et al. / Journal of Food Engineering 100 (2010) 678687SC-G0.050 0.96 (0.02)a 37.2 (1.4)b

SC-G0.075 1.36 (0.15)b 31 (5)a

SC-G0.100 3.0 (0.3)c 30 (5)a

Particle Siz

5

6

)3.3. Characterization of the lms

3.3.1. Tensile propertiesTable 3 shows the results of the mechanical tests performed on

samples that had been previously equilibrated at 33% and 53% RH(nal moisture content in the lms: 5 and 10 g water/100 g lm,respectively). Moisture content had a very relevant effect on thetensile behaviour of the lms. As moisture content increased from5% to 10%, both EM and TS were reduced by a factor of between 2

0.01 0.1 1 Particle S

0

1

2

3

4

Volu

me

(%

Particle Size D

0.01 0.1 1 Particle S

0

1

2

3

4

5

Volu

me

(%)

Fig. 2. Particle size distribution in terms of volume of SC formulations with cinnam

Table 3Mechanical properties of the lms with 5% and 10% moisture content (wb). Average valuesin LSD test.

Film 5%

EM (MPa) TS (MPa) E (%)

SC 546 (82)a 21 (3)a 13 (6)a

SC-C0.025 573 (68)a 22 (3)a 13 (6)a

SC-C0.075 579 (85)a 24 (5)a 22 (17SC-G0.025 578 (66)a 22 (2)a 18 (11SC-G0.075 559 (98)a 22 (4)a 16 (1125.2 (0.2)e 1.02555 (0.00008)ab 69 (4)e

24.6 (0.3)d 1.02504 (0.00009)a 61 (3)a

25.6 (0.4)e 1.0257 (0.0002)ab 67 (5)cde

istribution and 4. Elongation was greatly affected as well, becoming 3- to 5-fold larger. These results demonstrate the plasticizing effect ofwater on the hydrophilic SC lms. According to Chen (1995), wateris the most common plasticizer and environmental RH may changethe mechanical behaviour of a hydrophilic lm. Water moleculesinteract with proteins through hydrogen bonding (Audic and Chau-fer, 2005), softening the structure and increasing molecular mobil-ity. As a result of this, at high RH the material becomes less stiff,less resistant to break and more stretchable than it was at low

10 100 1000 3000 ize (m)istribution

10 100 1000 3000 ize (m)

on oil (a) and ginger oil (b). 1:0.025 ( ), 1:0.050 ( ), 1:0.075 ( ), 1:0.100 ( ).

and standard deviations, in brackets. The same superscript means homogeneous group

10%

EM (MPa) TS (MPa) E (%)

180 (45)a 11 (2)a 67 (17)a

156 (44)a 10.2 (1.6)a 67 (19)a

)a 160 (33)a 11.4 (1.8)a 76 (21)a

)a 154 (55)a 10 (2)a 57 (24)a

)a 170 (37)a 11.6 (1.7)a 72 (28)a

RH. The plasticizing effect of water on SC lms has been reportedby several authors (Arvanitoyannis and Biliaderis, 1998; Audicand Chaufer, 2005; Fabra et al., 2009a).

The presence, type and content of EO did not have a signicanteffect on the mechanical parameters (p > 0.05), probably due to thelow content of incorporated oil. Chen (1995) stated that simpleprotein lms are stronger and more stretchable than compositelms containing lipids. However, literature provides evidence ofvery diverse effects of lipid addition on mechanical parameters.Generally, these studies refer to lms whose lipid content is muchhigher than those tested in our formulations. In this case, the incor-poration of EO to the SC protein matrix was limited by the strongodour of cinnamon oil and the intense yellow colour of SC-C lms.

In a previous study (Atars et al., 2010), cinnamon oil in thesame proportions affected the mechanical behaviour of SPI lms.In the present study, such oil proportions seem to be insufcientto have any signicant effect, which relates to the structural differ-ences between both proteins. SC lms are more rigid, resistant tobreak and extensible than those of SPI, so they are not affected

occurs (p = 0.00), as has been reported for edible hydrophilic lms

L. Atars et al. / Journal of Food Engwhere water molecules interact with the polar groups of the

Table 4Water vapour permeability of the lms tested at all combinations of RH gradients(3353%, 5375%, 53100%), and temperatures (5, 25 and 35 C). Average values andstandard deviations, in brackets. The same superscript (abc to be compared horizon-tally and xyz vertically) means homogeneous group in LSD test.

HR Film 5 C 25 C 35 C

3353% SC 0.58 (0.18)a,x 0.48 (0.08)ab,x 0.362 (0.011)b,x

SC-C0.025 0.54 (0.19)a,x 0.64 (0.14)a,y 0.407 (0.006)a,y

SC-C0.075 0.71 (0.17)a,x 0.57 (0.11)ab,xy 0.39 (0.04)b,xy

SC-G0.025 0.70 (0.18)a,x 0.57 (0.06)a,xy 0.381 (0.013)b,xy

SC-G0.075 0.65 (0.19)a,x 0.52 (0.09)ab,x 0.40 (0.02)b,y

5375% SC 3.2 (0.4)a,x 1.8 (0.2)b,x 3.1 (0.3)a,x

SC-C0.025 2.2 (0.4)a,y 2.14 (0.13)a,x 2.3 (0.3)a,y

SC-C0.075 2.6 (0.4)a,y 1.7 (0.3)b,x 2.7 (0.2)a,xy

SC-G0.025 2.29 (0.10)a,y 2.1 (0.3)a,x 2.65 (0.07)b,y

SC-G0.075 2.6 (0.3)a,y 1.8 (0.4)b,x 2.39 (0.10)ab,y

53100% SC 3.7 (0.2)a,x 3.4 (0.8)a,x 4.0 (0.5)a,x

SC-C0.025 2.9 (0.8)a,x 3.5 (0.2)a,x 3.2 (0.4)a,y

SC-C 3.2 (0.4)a,x 2.9 (0.5)a,x 3.4 (0.3)a,xyby the low proportion of incorporated oil.

3.3.2. Water vapour permeabilityThe WVP results are shown in Table 4. Of the three variables

tested (HR, T and oil type/content), HR had the highest impact onWVP (p = 0.00). Therefore, the results were statistically analysedat each HR gradient to check the effect of temperature andformulation.

It was veried that SC lms are highly permeable to water va-pour, which is coherent with the hydrophilic nature of the protein(McHugh, 2002). In their review of the most relevant features of di-verse materials, Han and Gennadios (2005) classied the waterbarrier properties of several proteins (SC, calcium caseinate, wheyprotein, soy protein and wheat gluten) as marginal or inferior. Theresults obtained are similar to those reported in literature. Kristoet al. (2007) determined that the WVP (25 C, RH 53100%) of SClms plasticized with sorbitol (1:0.33) was 4.0 0.3 g mmkPa1 h1 m2. Schou et al. (2005) obtained about 3 g mm kPa1

h1 m2 for SC:glycerol lms 1:0.32 (23 C, 50100%). Fabra et al.(2008) found that the WVP of SC:glycerol lms (1:0.3) at 5 Cand 58100% was 4.1 0.3 g mm kPa1 h1 m2.

As shown in Table 4, as RH rises, an important increase in WVP0.075

SC-G0.025 2.9 (0.7)a,x 3.6 (0.4)a,x 3.2 (0.4)a,y

SC-G0.075 3.2 (0.3)a,x 2.9 (0.8)a,x 3.2 (0.4)a,ymatrix. Gennadios et al. (1994) point out that the WVP of ediblelms, experimentally determined at a particular RH gradient,should be referred to such a gradient. At high RH, the stiffness ofthe lms is reduced by the plasticizing effect of water, and themolecular mobility in the system is favoured. Consequently, allthe properties depending on the molecular diffusion, such as masstransfer through the lm, are in turn favoured and the WVP in-creases. This positive effect of RH on the WVP has been previouslydescribed for sodium caseinate lms by Fabra et al. (2009a).

Temperature is an important factor when determining the masstransfer phenomena (Han and Gennadios, 2005). As it increases,more energy is available for the migrating substances, thuspromoting permeability. Nevertheless, in the present study, tem-perature had a variable effect, depending on the RH gradient. At3353%, the WVP reduced as temperature increased (p = 0.00). At5375%, the most effective water vapour barrier of the lms wasfound at 25 C (p = 0.00) and at 53100%, temperature was ob-served to have no signicant effect on WVP (p > 0.05).

Indeed, although a temperature increase favours molecularmobility, some other factors should be taken into account. Atlow RH, the lm structure is scarcely plasticized by water andthe molecular mobility is low. The negative correlation betweenT and WVP in these circumstances could be explained by two ef-fects: a reinforcement of the hydrophobic interactions in the pro-tein matrix, thus reducing the water transport ability (Fabraet al., 2009a), and the lower water adsorption capacity of thematrix when temperature increases (Shamblin et al., 1998). Thislower water retention of the lm will reduce the molecularmobility and so the WVP. Thus, the lowest WVP values foundin the range of experimental conditions were those obtained at3353% RH and 35 C. The negative effect of temperature onthe water barrier properties of sodium caseinate lms at lowRH gradient was previously described by Fabra et al. (2009a).On the contrary, at 53100% RH, temperature had no signicanteffect on the WVP (p > 0.05) (Table 4). This is probably due tothe strong plasticizing effect of water, which made the increasein molecular mobility promoted by the temperature increaseunappreciable.

At 5375% RH, the lms were most effective as water vapourbarriers at 25 C, which can be attributed to the balance of oppositeeffects: the positive effect of the temperature on both molecularmobility and the promotion of hydrophobic interactions and itsnegative effect on the water retention capacity of the lm. The nalWVP values would depend on the balance of such opposite effectsthat dene the ratio of water diffusion-adsorption in the polymericmatrix.

Because of the hydrophobic nature of the oils, lms with lipidswere more effective as moisture barriers than those without oil(p < 0.05). No signicant differences were found among sampleswith different types or contents of lipid phase. As commentedon above, it is likely that using only small amounts of the essen-tial oils (limited by the imparted avour) made it difcult to ob-serve a correlation between WVP and lipid content. Such acorrelation was described by Chen (1995) in SC-acetylated mono-glyceride emulsion lms, and by Snchez-Gonzlez et al. (2009)in hydroxypropylmethylcellulose lms with tea tree EO. Never-theless, the effect of the presence of oil on WVP became moreevident as the RH gradient increased. The presence of a hydro-phobic disperse phase, even at small ratios, limits water vapourtransfer since it introduces discontinuities in the hydrophilicphase that increase the tortuosity factor for mass transfer. WhenRH gradient increased and molecular mobility was promoted inthe hydrophilic matrix, the tortuosity factor controlled the mass

ineering 100 (2010) 678687 683transfer process to a greater extent. This effect has also been ob-served for SC matrices containing oleic acidbeeswax mixtures(Fabra et al., 2010).

3.3.3. Optical properties: K/S, colour and glossThe spectral distribution of K/S (400700 nm) is shown in g. 3.

K/S zincreased at low wavelengths as the cinnamon oil content in-creased, which relates to the increase in absorption caused by thecoloured compounds of cinnamon oil. High K/S values are generallyrelated to high transparency, although the effect of the selectiveabsorption of components may also be considered (Hutchings,1999). Contrarily, ginger oil had no impact on the K/S values ofSC lms. This may be related to the interactions with the proteinmatrix, and to the refraction indexes of both oils (cinnamon:1.57; ginger: 1.49), the latter differing less from those of the SPIdispersions (1.35).

Table 5 reports colour parameters, all of them signicantly af-fected by the oil type (p < 0.01). Films with cinnamon oil showedhigher Cab and lower h

ab, L* and WI than those with ginger oil.

Cab was markedly affected by the addition and amount of cinna-mon oil. The more cinnamon oil was incorporated, the higher thevalue of Cab and consequently, the lower that of WI. Both parame-ters were plotted versus the cinnamon oil ratio and the correlationcoefcients were 0.998 and 0.999 for Cab and WI, respectively. Inspite of the low proportion of oil tested, cinnamon oil notably af-fected the appearance of SC lms, which would probably limit itsapplication in light products.

Norm ASTM D523 (1999) denes gloss as the ratio of the lumi-nous ux reected from a specimen to the luminous ux reectedfrom a standard surface under the same geometric conditions. Ofall the measurements performed at different incidence angles,those at 60were selected (Table 5), given that the aforementioned

some effect on the surface of the lms, probably creating irregular-ities that reduce the specular light reection.

Similar effect of oil/lipid addition in the lm matrix on colourparameters and the surface morphology and gloss of the lmswas observed in other works (Snchez-Gonzlez et al., 2009; Fabraet al., 2009b).

3.3.4. Scanning Electron MicroscopyThe nal microstructure of the lms results from the interac-

tions between the components and the destabilizing phenomenataking place during the drying step. A microstructural study ofthe lms gives relevant information about the arrangement ofthe components, allowing for a better discussion of results. Fig. 4shows characteristic images of transversal sections of several for-mulations (SC, SC-C0.025, SC-C0.075, SC-G0.025, SC-G0.075). The micro-structure was qualitatively studied, aiming to clarify the impact ofboth oils on the structure of the protein network.

A cross-sectional analysis of the lms showed a continuous,homogeneous phase in oil-free lms, and no irregularities, suchas air bubbles or oil droplets, were detected. As compared to SClms, lms with cinnamon oil showed no apparent differences intheir microstructural aspect. It seems that the small size of the par-ticles in the emulsions was not modied as the drying process pro-gressed, and cinnamon oil stayed homogeneously distributed inthe dry protein matrix. Films with ginger oil, in contrast, had moreand more discontinuities as the ginger oil content increased(Fig. 4). The observed ginger oil droplets, which have grown en-

lm

ss p

684 L. Atars et al. / Journal of Food Engineering 100 (2010) 678687norm recommends this geometry when results are between 10 and70. All the mean values were lower than 60, taken as an averageresult, which means that all the obtained lms were only slightlyglossy. The variability of the measurements did not allow for theobservation of clear trends. In spite of this, some gloss decreasewas observed in lms with the highest proportions of ginger oil(p < 0.05). This suggests that, at such ratios, ginger oil may have

(a)

0

1

2

3

4

5

400 500 600 700 (nm)

K/S SCSC-C0.025SC-C0.050SC-C0.075SC-C0.100

Fig. 3. Spectral distributions of light absorption/dispersion coefcient K/S of

Table 5Colour coordinates L*, Cab , h

ab and WI of the lms (D65, 10). Gloss at 60 and roughne

superscript means homogeneous group in LSD test.

Film L* Cab hab

SC 79.2 (0.6)b 11.1 (0.4)c 93.1 (0.4)a

SC-C0.025 77 (4)a 22.4 (1.2)e 97.1 (3.2)bc

SC-C0.050 77.0 (1.1)a 27.3 (1.1)f 97.9 (0.8)c

SC-C0.075 76.4 (0.5)a 31.6 (0.7)g 97.0 (0.6)bc

SC-C0.100 76.1 (1.6)a 35.5 (0.5)h 96.6 (0.3)b

SC-G0.025 80.1 (0.9)bc 10.2 (0.3)a 99.8 (0.5)d

SC-G 79 (2)b 10.3 (1.5)ab 100.4 (1.5)d0.050SC-G0.075 79.7 (0.9)bc 11.0 (0.5)bc 100.1 (0.6)d

SC-G0.100 81.0 (1.7)c 12.4 (0.8)d 99.8 (0.8)dWI Gloss 60 Ra Rq

76.4 (0.7)e 39 (22)ab 55 (11)ab 119 (21)cd

68.1 (3.3)d 51 (19)bcd 40 (6)a 51 (13)a

64.3 (1.5)c 33 (15)a 60.6 (0.8)b 60 (19)d 61 (19)b 93 (19)bc

57.2 (0.9)a 43 (16)abc 77.7 (0.8)e 55 (17)cd 53 (13)ab 83 (27)ab

76.8 (1.7)e 59 (10)cd ough to be visible at the magnication level considered (about15 lm), a result of the aggregation that takes place during drying.The oil droplets appeared as either concave or convex irregulari-ties, depending on the effect of the cryofracture on the structure,and were homogeneously distributed throughout the lm thick-ness. Although the increased size of the ginger oil droplets couldpromote creaming and phase separation, the viscosity increaseassociated to the loss in lm water limits the mobility of the

(b)

0

1

2

3

4

5

400 500 600 700 (nm)

K/S SCSC-G0.025SC-G0.050SC-G0.075SC-G0.100

s with cinnamon oil (a) and ginger oil (b), as compared to those without oil.

arameters (Ra and Rq). Average values and standard deviations in brackets. The same76.9 (0.9)e 34 (17)a 73(14)b 151 (55)c

77.3 (1.8)e 34 (20)a

droplets, which remaining uniformly dispersed in the dry lm. Inconclusion, the structural differences linked to the oil type aredetermined by the different behaviour of both oil droplets duringdrying, and result from the complex interactions taking place be-tween the lipid, the protein and the solvent.

3.3.5. Surface morphologyFig. 5 shows typical surface topographies of SC, SC-C0.025, SC-

C0.075, SC-G0.025 and SC-G0.075, and the corresponding results ofroughness parameters Ra and Rq are shown in Table 5. The presenceof both oils seemed to lead to some reduction in roughness proba-bly due to the liquid state of the oils which extend over the proteinmatrix surface reducing its irregularities. However, as the oil con-tent increased in the formulation, an increase of Ra and Rq was ob-served. At both oil proportions tested (1:0.025 and 1:0.075), atrend towards higher roughness values was obtained in lms withginger oil, as compared to those with cinnamon oil. Such a trend iscoherent with the different behaviour of both oils as drying takesplace. Cinnamon oil remains homogeneously integrated in the pro-tein matrix, which results in relatively regular surfaces. In contrast,

ginger oil undergoes aggregation during the drying step, thus caus-ing irregularities on the lms surfaces (Fig. 5), coherent with thegloss reduction.

3.3.6. Antioxidant capacity of the lmsFig. 6 shows the time progression of the PV of sunower oil pro-

tected with aluminium foil and the three lms being tested (SC, SC-C0.075, SC-G0.075), and an unprotected sample. Given that the PVremained near to that of the oil protected with aluminium foil,all the formulations were seen to delay oxidative rancidity. Onthe other hand, the unprotected oil oxidised much faster. Sucheffectiveness as protectors is partly due to the low RH (32%), giventhat RH critically affects the barrier properties of the lms. Osset al. (2008) found that the protective effect of whey protein isolatelms increased as the ambient RH was reduced. Due to the proteinhydrophilicity and the plasticizing effect of water, a rise in RHwould increase the lms permeability to oxygen, thus reducingits capacity to protect against lipid oxidation.

When compared to SC lms, neither the incorporation of EO northe type of oil added had very much effect. These results suggest

L. Atars et al. / Journal of Food Engineering 100 (2010) 678687 685Fig. 4. Scanning Electron Microscopy images of transversal sections of lms.

Fig. 5. AFM images of the l

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40 45 50

PV (mEqO2/kg)

0

10

20

30

40

0 10 20 30 40 50Time (days)

////

Fig. 6. Development of the peroxide value of sunower oil protected withaluminium foil ( ), SC ( ), SC-C0.075 ( ), SC-G0.075 ( ) and unprotected ( ).

686 L. Atars et al. / Journal of Food Engineering 100 (2010) 678687that the antioxidant capacity of the isolated oils is not directly re-lated to that found in the lms.

4. Conclusions

The use of cinnamon or ginger essential oils at the low ratioused (less than 1:0.1 protein to lipid ratio) in the SC lms, limitedany impact on mechanical properties and only slightly reducedwater vapour permeability. Cinnamon oil remained homoge-neously integrated in the SC matrix, which gave rise to lms withregular surfaces, but considerably affected the optical properties ofSC lms. Due to the aggregation of lipids during drying, ginger oilcaused an increase in surface roughness and a loss of gloss. All thelms effectively protected sunower oil against oxidation, proba-bly due to their low permeability to oxygen at the low relativehumidity of the surrounding atmosphere. Combining these lipidswith others of more neutral sensory attributes could be an alterna-tive way of adjusting the functional properties of the lm.

ms surface morphology.

Acknowledgement

The authors acknowledge the nancial support from the Span-

Gutirrez, J., Barry-Ryan, C., Bourke, P., 2008. The antimicrobial efcacy of plantessential oil combinations and interactions with food ingredients. InternationalJournal of Food Microbiology 124, 9197.

Han, J.H., Gennadios, A., 2005. Edible lms and coatings: a review. Innovationsin foo

L. Atars et al. / Journal of Food Engineering 100 (2010) 678687 687ish Ministerio de Educacin y Ciencia through Project AGL2007-65503/ALI and from the Vice-rectorate for Research of UPV throughProject PAID-06-08-3242. Authors also thank Manuel Planelles andJose Luis Mova of the UPV Electronic Microscopy Service for theirassistance in the use of the AFM and SEM.

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Characterization of sodium caseinate-based edible films incorporated with cinnamon or ginger essential oilsIntroductionMaterials and methodsAntioxidant capacity of the essential oilsPreparation of the film-forming emulsions and casting of the filmsCharacterization of the film-forming dispersionsLipid particle sizeRheological behaviourDensity and contact angle

Characterization of the filmsFilm thickness measurementsTensile propertiesWater vapour permeabilityOptical properties: K/S, colour and glossScanning Electron MicroscopySurface morphologyAntioxidant capacity of the films

Statistical analysis

Results and discussionAntioxidant capacity of the essential oilsCharacterization of the film-forming dispersionsLipid particle sizeRheological behaviourDensity and contact angle

Characterization of the filmsTensile propertiesWater vapour permeabilityOptical properties: K/S, colour and glossScanning Electron MicroscopySurface morphologyAntioxidant capacity of the films

ConclusionsAcknowledgementReferences