Journal of Food Engineering 109 (2012) 189–201

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Journal of Food Engineering

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Comparative study of film forming behaviour of low and high amylosestarches using glycerol and xylitol as plasticizers

D. Muscat a, B. Adhikari a,⇑, R. Adhikari b, D.S. Chaudhary c

a School of Health Sciences, University of Ballarat, VIC 3353, Australiab CSIRO Materials Science and Engineering, Clayton South, VIC 3169, Australiac Department of Chemical Engineering, Curtin University of Technology, Perth, WA 6102, Australia

a r t i c l e i n f o

Article history:Received 16 February 2011Received in revised form 7 October 2011Accepted 12 October 2011Available online 19 October 2011

Keywords:High amyloseLow amyloseCorn starchGlycerolXylitolFilmMechanical propertiesWater vapour permeabilityAnti-plasticization

0260-8774/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2011.10.019

⇑ Corresponding author. Tel.: +61 3 53279249; fax:E-mail address: b.adhikari@ballarat.edu.au (B. Adh

a b s t r a c t

In this study, the film forming behaviour of low amylose (LA) and high amylose (HA) starches was stud-ied. The starch-alone and a blend of plasticizer (polyol)–starch films were developed by gelatinising atvarious temperatures and casting at 25 �C. The starch–plasticizer films contained glycerol and xylitoleither individually or in 1:1 combination. The concentration of plasticizer used was 15%, 20% and 30%for LA films while it was 20%, 30% and 40% for HA films on dry solid basis. The HA-glycerol films retainedthe highest moisture content among all the films. The HA films exhibited higher glass transition temper-ature, higher tensile strength, higher modulus of elasticity and lower elongation at break than thoseobtained from LA starch. The tensile strength and modulus of elasticity decreased and the elongationincreased with increasing plasticizer concentrations above 15% on dry solid basis regardless the starchtype. Low water vapour permeability was evident in LA and HA films plasticized by combined plasticizersat 20% plasticizer concentration. Rheological measurements showed that most of the suspensions exhib-ited Herschel–Bulkley behaviour and some of the HA suspensions exhibited Bingham plastic behaviour.At 15% (on dry solid basis) plasticizer concentration, the films obtained from both the starches were brit-tle due to the anti-plasticization behaviour.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

We are literally wrapped in petroleum based plastics. Theseplastics are used as packaging material in almost every productwe buy, most of the foods and drinks we consume. This is becausethey provide excellent protection for the product, are cheap tomanufacture and are durable. However, they are not biodegradabledue to their very large polymer molecules and stable carbon–hydrogen bond, both of which make its decomposition by organ-isms rather difficult. The non-biodegradable and non-renewablenature of these plastics is major environmental concern. Becauseof the increased awareness in preserving the natural environment,efforts have been made to develop biodegradable plastics that aremade from renewable resources, such as plants, which are compo-stable. Their characteristics include being hydrophilic, most can beprocessed directly; either plasticized, as fillers or modified bychemical reactions. Such ‘green’ changes in packaging will improvesustainability and reduce the adverse impact plastic packagingplaces on the environment because of their biodegradability(Yakimets et al., 2007).

ll rights reserved.

+61 3 53279240.ikari).

Among the biopolymers investigated as potential alternativeraw materials for plastics, starch has drawn a great deal of atten-tion. The appeal of starch is that it is biodegradable, renewable,inexpensive and for the most part easily handled (Zhang andHan, 2006). However, starch itself has poor processability, dimen-sional stability and mechanical properties for its end products.Therefore, native starch cannot be used directly as packaging mate-rial (Lu et al., 2009).

Starch is the most abundant storage glucan composed of twomain structural components, amylose and amylopectin (Sajilataet al., 2006). Amylose is a linear molecule of glucose units linkedwith a-(1 ? 4) bonds. Amylopectin is a branched molecule witha-(1 ? 6)-linked branch points and linear regions of a-(1 ? 4)-linked glucose units (Murphy, 2000). There are many hydroxylgroups on starch chains, two secondary hydroxyl groups at C-2and C-3 of each glucose residue, as well as one primary hydroxylgroup at C-6 when it is not linked because of which starch is hydro-philic material. The available hydroxyl groups on the starch chainsparticipate in the formation of hydrogen bonds (Lu et al., 2009).

In its native granular form starch has limited applications. Tofully realise the starch biopolymer’s functionality, granule disrup-tion and sometimes chemical modification are necessary. To pro-duce biodegradable packaging, starches are first gelatinised(Cooper et al., 2003). This process transforms the starch from a

190 D. Muscat et al. / Journal of Food Engineering 109 (2012) 189–201

crystalline granular into a rubbery paste with no structure at all.During gelatinisation several changes occur to the starch–watersystem and the loss of crystallinity is the most important regardingthe formation of films (Wetzel and Charalambous, 1998).

While pure starch films can be made, previous research (Zhangand Han, 2006; Lu et al., 2009) has shown them to be very brittle asa result of the strong cohesive energy density of the polymers.Plasticizers are commonly required to increase film flexibility byreduction in intramolecular hydrogen bonding along polymerchains thereby increasing the intermolecular spacing (Janjarasskuland Krochta, 2010). This results into increase in molecular mobil-ity, lowers the glass transition temperature (Tg) and decreasesthe degree of the crystallinity (Guilbert et al., 1997). The plasticiz-ers usually used are hydrophilic low molecular weight carbohy-drates, such as polyols (Zhang and Han, 2006; Laohakunjit andNoomhorm, 2004; Pareta and Edirisinghe, 2005). Polyols have atendency to adsorb water which depends on the molecular weightand number of hydroxyl groups present. Smits et al. (2003) foundthat the crystallisation tendency of granular potato starch can begreatly reduced when plasticizer containing more OH groups isused during gelatinisation. Adhikari et al. (2010) studied the dryingbehaviour of low-amylose maize starch films plasticized by glyc-erol (3 hydroxyl groups) and xylitol (5 hydroxyl groups), separatelyand in 1:1 combination. They found xylitol to be a more effectiveplasticizer due to its relatively larger molecular size and tendencyto form stronger hydrogen bond with starch molecules, comparedto glycerol. It was concluded that xylitol plasticized films had high-er moisture migration fluxes and effective moisture diffusivity val-ues. At lower plasticizer (15% (w/w) or lower) content thecombined plasticizers exhibited anti-plasticizing behaviour dueto the strong hygroscopicity and hydrophilic nature of glycerol.When more than one plasticizer (other than water) was presentin the system, strong plasticizer-plasticizer interactions were ob-served (Adhikari et al., 2010). These interactions can be coopera-tive which can enhance a particular properties or competitivewhich can retard such properties. Knowing this, a variety of plast-icizers, especially polyols, at different concentrations needed to bestudied to determine their effectiveness in preparing packagingfilms from low amylose and high amylose starches. While a signif-icant amount of work has been done on biodegradable packagingin the past, there has been minimal research on the presence ofmultiple plasticizers in starch. Furthermore, there is no compara-tive studies on high amylose (>70% amylose) and low amylose(25% or less) films developed in identical conditions.

In this context, this study had three main aims. Firstly, it aimedto study the gelatinisation and film forming behaviour of low amy-lose and high amylose starch in the presence of plasticizers con-taining different hydroxyl groups (3-OH and 5-OH). Secondly, itaimed to comprehensively study the effect of the use these twoplasticizers on the mechanical (tensile strength, modulus of elas-ticity and elongation at break) and water vapour barrier propertiesof plasticized high-amylose and low-amylose starch films.

2. Materials and methods

2.1. Materials

Low-amylose (LA) cornstarch with amylose:amylopectin ratioof (25:75) with 13.18% moisture and high amylose (HA) cornstarchwith amylose:amylopectin ratio of (80:20) and moisture content of12.60% were purchased through Tim Stock, Ballarat. Glycerol andxylitol were used as plasticizers and purchased from ConsolidatedChemical Company, Melbourne, Victoria. Xylitol was a crystallinematerial and had no noticeable moisture content. The glycerol con-tained 2% moisture in it. All the materials were used as received

and the moisture content of the raw materials was compensatedfor while preparing the solution for gelatinisation.

2.2. Methods

2.2.1. Slurry preparationSlurries of starch and starch–plasticizers in water were pre-

pared using 5% (w/w) total solids on a dry weight basis usingdeionised water. Where starch with various plasticizers (glycerol,xylitol and glycerol + xylitol) were used, the same total solid con-centration (5%, w/w) was maintained. The experimental matrixand the nomenclature of the sample are given in Table 1. In thecase of low-amylose starch, starch:plasticizer ratios of 85:15,80:20 and 70:30 were used on dry solid basis. In the case ofhigh-amylose starch, the starch–plasticizer ratios of 80:20, 70:30and 60:40 were used on dry solid basis. Where both glycerol andxylitol are used they were used in equal amount or in (50:50) basis.

2.2.2. GelatinisationGelatinisation of the starch and starch–plasticizer slurries was

carried out using a high temperature-high pressure laboratoryreactor (Amar Equipment Company, Mumbai, India). The low-amy-lose starch slurries were gelatinised at 100 �C and at 150 rpm agi-tator speed. The dispersion was held for 60 min at 100 �C beforecooling down to 90 �C. The high amylose starch was gelatinisedat 130 �C using 500 rpm agitator speed. The solution was held for120 min at 130 �C before cooling down to 90 �C. These two timesettings (60 min for low amylose starch and 120 min for high amy-lose starch) were chosen to allow complete gelatinisation as mon-itored using a polarized light microscope (Ernest Leite Wetzlar,Germany, fitted with Q-Imaging camera). These gelatinised solu-tions were vacuumed for 20 s to eliminate entrapped air bubblesbefore casting commenced.

2.2.3. Film casting and conditioningFilms were prepared by syringing 20 mL (in the case of LA

starch) and 10 mL (in the case of HA starch) fully gelatinised dis-persions, into plastic polystyrene dishes with a 90 mm diameter.Films were dried overnight at 25 ± 1 �C, in a specially designed iso-thermal drying box with gentle fan forced air circulation. Filmswere stored in a desiccator containing magnesium nitrate (52.9%RH at 25 ± 1 �C) for at least 48 h for conditioning before analysis.Triplicate experiments were carried out regarding the filmpreparation.

2.2.4. Moisture content determinationThe moisture contents of both the starch-alone and the starch–

plasticizer films were determined by a gravimetric method, where-by samples were dried at 105 ± 0.5 �C in a laboratory oven (UNE PA,Memmert, West Germany) until constant weight was achieved(Multon and Martin, 1988). Approximately 1.0 g film samples wereplaced in previously dried and cooled glass petri dish and kept inthe oven for 8 h. Weights of the samples were taken before andafter drying using a digital balance (±0.1 mg), (AE200S, Mettler,Switzerland). Tests were conducted in triplicates and the averagevalues are reported.

2.2.5. Mechanical propertiesThe mechanical properties such as tensile strength (MPa), mod-

ulus of elasticity (MPa) and elongation at break (%) of the starchand starch–plasticizer films were evaluated. The mechanical prop-erties of the films were determined in tension mode using a tex-ture analyser (TA-XT Plus™, Stable Micro Systems, UK) inaccordance with ASTM method D 882 (ASTM, 2000). Tensile gripswere used to hold the films during these tensile tests. In order topromote the failure of the films towards the centre of the film, they

Table 1A summary of description of film composition with code abbreviation for each film.

Abbreviationof film

Description of film composition

LA Low-amylose (26:74 amylose:amylopectin) starch withoutplasticizer

HA High-amylose (80:20 amylose:amylopectin) starch withoutplasticizer

LA G Low-amylose starch with different percentage of glycerol,i.e. 15%, 20% and 30%, denoted as LA G15, LA G20, LA G30

LA X Low-amylose starch with different percentage of xylitol, i.e.15%, 20% and 30%, denoted as LA X15, LA X20, LA X30

LA GX Low-amylose starch with different percentage of glyceroland xytilol (50:50), i.e. 15%, 20% and 30%, denoted as LAGX15, LA GX20, LA GX30

HA G High-amylose starch with different percentage of glycerol,i.e. 20%, 30% and 40%, denoted as HA G20, HA G30, HA G40

HA X High-amylose starch with different percentage of xylitol, i.e.20%, 30% and 40%, denoted as HA X20, HA X30, HA X40

HA GX High-amylose starch with different percentage of glyceroland xytilol (50:50), i.e. 20%, 30% and 40%, denoted as HAGX20, HA GX30, HA GX40

D. Muscat et al. / Journal of Food Engineering 109 (2012) 189–201 191

were cut in dumbbell shape. The thickness of the films was mea-sured at 10 random positions in the middle part of the dumbbellshape where the failure usually occurred. A manual micrometer(Kincrome, 0.01 ± 0.004 mm) was used for this purpose. The aver-age value of the 10 readings was used as the film thickness and theaveraged values were subsequently sued to calculate the mechan-ical and water vapour barrier properties. The tensile strength (TS inPascals) was calculated by dividing the peak force with the crosssection area of the film. Similarly the % elongation was measuredby dividing the change in length achieved at break with the film’soriginal length (0.037 m). The modulus of elasticity (EM, in Pascals)which represents the stiffness of the films was calculated from theinitial linear portion of the stress–strain curve and given by the fol-lowing equation:

EM ¼ stressstrain

¼ F=ADL=L

ð1Þ

where, F is force (N), A is the area of cross-section (m2), L and DL arethe original length and the change in length in m, respectively.

2.2.6. Water vapour permeability (WVP)The water vapour permeability of the starch and starch–plasti-

cizer films was determined using a modified ASTM E96-05 method(ASTM, 2005), as suggested by Remunan-Lopez and Bodmeier(1996). The bottles used were 22.5 mm in diameter, 42.25 mm inheight and had the mouth opening of 11.50 mm. The aluminiumlids used had aperture opening of 8.00 mm. Each bottle was filledwith approximately 7 g of calcium chloride granules. The calciumchloride granules which passed 2.36 mm sieve were used afterremoving the fines (that passed through 600 lm sieve). This desic-cant was pre-dried at 200 �C overnight before use. The film discwas then placed between two rubber liners, with hole of8.00 mm in the centre and positioned to the metal lid. The linershelped maintaining a good seal between the bottle and the lidand also helped maintaining the film integrity whilst the filmwas put in place and the lid was subsequently shut. The whole unitwas weighed and placed in the desiccator containing magnesiumnitrate (52.9% RH). The desiccator was kept in a temperature con-trolled environment (Lebec Incubator, Laboratory Equipment P/L)at 20 ± 0.5 �C. Each bottle was then periodically weighed to deter-mine extent of the water vapour transmitted through the film as52.9% RH gradient was maintained across the film. In calculatingWVP, linear regressions were performed graphically using weightgain versus time data. Triplicate tests were conducted and the

mean values are reported in ensuing sections. The WVP [g mm/(m2 h kPa)] is calculated following ASTM E96-05 protocol usingEq. (3), as follows:

WVTR ¼ GxA

ð2Þ

WVP ¼WVTRDP

¼ WVTRSðR1 � R2Þ

ð3Þ

Where, WVTR is the water vapour transmission rate [g mm/(m2 h)].G is the slope of the straight line representing change of vapourpressure over time (g/hr). A and x are the film cross-section areathrough which the vapour gets transmitted (m2) and x is the aver-age film thickness (mm). Similarly, S is the saturation vapour pres-sure at the test temperature (kPa). R1 and R2 are relative humiditymaintained by the magnesium nitrate (0.529) and calcium chloride(0.000), respectively.

2.2.7. Rheological measurementsRheological characterisation of the gelatinised starch and

starch–plasticizer dispersions was performed using a dynamic rhe-ometer (RheoStress 1, Haake, GmbH, Karlsruhe, Germany). The gel-atinised starch and starch-plasticized suspensions were kept atconstant temperatures of 25 and 60 �C for low-amylose and high-amylose starch, respectively. The temperature of the high-amylosestarch suspensions had to be maintained at 60 �C. This is becausethe dispersions having high amylose starch started forming gelsbelow this temperature. At 25 �C it became impossible to maintainit as dispersion as was used to prepare films. Silicone oil (H4160and H4000) was used to seal the outer perimeter of the sampleto prevent evaporation of the water from the edges. The tempera-ture was kept constant (±1 �C) at the desired temperaturesthroughout test using a circulating water bath (Cool Tech 320,Thermo-Haake, Germany).

Small-amplitude oscillatory tests (SAOS) were performed usinga parallel plate (PP35 Ti, 35 mm diameter) and a gap of 1.00 mmbetween the plates. The stress and the strain responses were keptwithin the linear range (Gunasekaran and Mehmet, 2000). The fre-quency sweep tests were carried out in a frequency range of0.01–10 Hz within the linear viscoelastic region at a constant stress(20 Pa). The strain sweep tests for the low-amylose and high-amy-lose starch dispersions were performed at a fixed frequency of1.592 Hz and 0.045 Hz within the linear viscoelasticity region,respectively. The viscosity of the gelatinised suspensions was mea-sured over a shear rate range of 0.0001–100 s�1. Storage modulus(G0) and loss modulus (G00) as a function of frequency (f) were mea-sured for all samples. Results reported in ensuing sections are aver-age values of at least two measurements.

From the shear stress and stress rate determined from the gel-atinised starch and starch–plasticizer suspensions strain sweepanalysis, a relationship between the two can be established. Thedata were fitted using the Herschel–Bulkley relationship as givenby the following equation (Steffe, 1996; Bourne, 2002):

r ¼ r0 þ KðcÞn ð4Þ

where, r, r0, c, K and n are shear stress (Pa), yield shear stress (Pa),consistency coefficient (Pa sn) and flow behaviour index (dimen-sionless), respectively. From the best fit values of these parametersthe nature of the fluid i.e. Herschel–Bulkey, Bingham plastic, shear-thinning and shear-thickening, etc. was determined.

2.2.8. Attenuated total reflectance-fourier transform infrared analysis(ATR-FTIR)

The conditioned starch-alone and starch-plasticized films wereanalysed on a Perkin–Elmer Spectrum 000 FTIR instrument. The

Table 2Moisture content (% wet basis) and glass transition temperature of starch alone andstarch–plasticizer films equilibrated at 52.9% RH.

Starch film Moisture content (%) Tg values (�C)

HA 14.16 ± 0.16 67.9 ± 2.4HA G20 12.33 ± 0.09 51.2 ± 1.1HA G30 24.08 ± 0.26 34.2 ± 0.9HA G40 25.91 ± 0.15 Below ambientHA X20 8.37 ± 0.44 56.1 ± 1.9HA X30 9.02 ± 0.33 34.2 ± 1.7HA X40 10.09 ± 0.15 Below ambientHA GX20 9.38 ± 0.44 55.2 ± 1.7HA GX30 17.14 ± 0.46 Below ambientHA GX40 24.75 ± 0.51 Below ambientLA 12.9 ± 0.18 64.9 ± 2.3LA G15 10.58 ± 0.34 50.3 ± 1.1

192 D. Muscat et al. / Journal of Food Engineering 109 (2012) 189–201

spectra were obtained in the range between 400 and 4000 cm�1

wave number.

2.2.9. Differential scanning calorimetry (DSC)The glass transition temperature of the films was determined by

DSC thermograms. The samples were analysed using MetterDSC-028 (Mettler Toledo, USA). The system was calibrated for tem-perature and heat flux with indium (melting point 156.6 �C, heat offusion of 3.28 kJ mol�1). The samples (6–15 mg) were weighed inaluminium pans and hermetically sealed and an empty pan wasused as the reference. Scans were carried out over the temperaturerange of 0–160 �C at heating rate of 10 �C min�1 under nitrogen.The Tg was determined at onset of the endothermic shift in heatflow.

LA G20 13.48 ± 0.17 48.7 ± 1.2LA G30 25.12 ± 0.89 30.5 ± 0.9LA X15 10.49 ± 0.52 49.2 ± 1.8LA X20 10.33 ± 0.37 48.6 ± 1.9LA X30 9.52 ± 0.25 32.7 ± 1.4LA GX15 9.83 ± 0.07 50.5 ± 1.5LA GX20 10.46 ± 0.35 47.1 ± 1.1LA GX30 16.36 ± 0.42 Below ambient

3. Results and discussion

3.1. Film cast formation

The films obtained were structurally intact, visually transpar-ent, uniform and smooth. The LA and HA starch-alone films werevery brittle and cracked upon handling. These films required extracare to retain the integrity of the films to minimise cracking oruncontrolled fracture prior to testing. The LA G15 and LA G20 filmscracked very severely upon drying. To obtain an intact film for therequired analysis these suspensions were dried in a polystyrenedish lined with a layer of aluminium sheet as suggested by Garciaet al. (1998). The films obtained from this altered casting methodwere intact upon drying. The cracking was typical of glycerol plas-ticized films due to strong hydrogen bonding between glycerol andstarch molecules at low concentrations of 15% and 20%, and it is re-ferred to as anti-plasticization (Seow et al., 1999). When anti-plas-ticization occurs it leads to increased rigidity rather than flexibilityof the plasticized film, and therefore becoming stiffer than thestarch-alone LA starch film (Chang et al., 2006). One reason for thisincreased rigidity is the glycerol molecules occupying sites initiallyoccupied by water molecules and reducing the free volume avail-able for polymer segmental mobility (Mali et al., 2008). Thestarch–plasticizer films having very high proportion of glycerol(e.g. HA G40) were hard to handle as they were highly sticky(adhesive) indicating those films were in soft rubbery state atambient temperature. In a study by Talja et al. (2007) it was re-ported that the films with sticky surfaces were obtained at highplasticizer content of glycerol. All the films were clear after drying,however, while conditioning the LA X30, HA X30 and HA X40 filmsa white residue developed on the surface, referred to as ‘‘bloom-ing’’ or ‘‘blushing’’. This occurs when the plasticizer concentrationexceeds its compatibility limit causing phase separation and phys-ical exclusion of the plasticizer (Talja et al., 2007; Bozdemir andTutas, 2003). Blooming was not observed in the films having com-bination of plasticizers. This provides further reason why two ormore plasticizers are better in forming starch-based films.

3.2. Water content

The total water content of the prepared films are shown in Table2. Table 2 shows that the HA starch-alone films had slightly higherwater content than the LA starch-alone films. This may be due tothe fact that amylose forms a network of stiff strands and porespresent in the network could possibly entrap more water and thatthe water content of films depends on the microstructure of starchnetwork. In both the LA and HA starches the addition of glycerolsignificantly increased the water content compared to the additionof xylitol due to the fact that glycerol is much more hygroscopiccompared to xylitol (Smits et al., 2003). It was observed that as

the concentration of glycerol increased, the moisture content in-creased. The moisture contents of films LA G20 and LA G30 were13.48% and 25.12%, respectively, which are much higher than themoisture contents of LA X20 and LA X30 films which were10.33% and 9.52%, respectively. The films plasticized with xylitolat various concentrations surprisingly had water contents ofapproximately 10% (w/w). The water content of all the xylitol plas-ticized films were lower than that of the starch-alone LA and HAfilms. The interesting aspect of this observation is the low moisturecontent of xylitol plasticized films which were conditioned at52.9% RH for 48 h. The capability of xylitol plasticized films toequilibrate at lower moisture content even at 52.9% RH suggeststhat xylitol plasticized or xylitol containing films will have reducedthe hygroscopicity in the films. The moisture content of the com-bined plasticizers in LA and HA films increased as the concentra-tion of plasticizer increased, but were lower than the filmsplasticized with glycerol and higher than the films plasticized byxylitol.

3.3. Mechanical properties

The mechanical properties of the starch-alone and the polyolsplasticized films that included the tensile strength (TS), modulusof elasticity (EM) and Elongation at break (E) are summarised inTable 3. From the elongation at break values shown in this table,it can be seen that the starch-alone films from both LA and HA frac-tured easily and instantly. Films with plasticizers exhibited betterpliability/flexibility and hence underwent fracture in a slower andmore sustained pace. Films plasticized with glycerol exhibitedgreater pliability/flexibility and hence their fracture was muchslower than the films plasticized with xylitol. Thus, with theincreasing concentration of plasticizers, the fracture mechanismchanged from rapid unstable brittle fracture at low strains, to elas-toplastic fracture at higher strains and finally to a slow plastic frac-ture (Chang et al., 2006).

3.3.1. Mechanical properties of LA and HA filmsAs expected the starch-alone films from both the LA and HA

starches were quite strong (44.38 and 34.32 MPa). The unplasti-cized LA and HA films also showed high elastic modulus of 1654and 1685 MPa, respectively. The high TS exhibited by theseunplasticized films is attributed to the extensive intra-molecularhydrogen bonds between amylose, amylopectin and amylose–amylopetin molecules in the absence of a plasticizer. A study by

Table 3Summary of tensile properties.

Starch film TS (MPa) EM (MPa) E (%) Thickness (mm) Moisture content (%)

HA 34.32 ± 9.75 1685.57 ± 88.40 1.41 ± 0.50 0.065 ± 0.001 14.16 ± 0.16HA G20 30.65 ± 2.49 1079.67 ± 92.64 4.60 ± 0.11 0.056 ± 0.005 12.33 ± 0.09HA G30 12.16 ± 1.37 323.06 ± 35.11 21.81 ± 4.18 0.065 ± 0.004 24.08 ± 0.26HA G40 5.05 ± 0.55 80.19 ± 24.68 23.05 ± 2.65 0.051 ± 0.011 25.91 ± 0.15HA X20 37.10 ± 3.17 1177.57 ± 84.59 4.03 ± 0.30 0.053 ± 0.002 8.37 ± 0.44HA X30 18.25 ± 0.17 546.57 ± 57.40 11.10 ± 3.93 0.057 ± 0.003 9.02 ± 0.33HA X40 13.96 ± 1.72 383.69 ± 112.44 15.12 ± 7.11 0.055 ± 0.003 10.09 ± 0.15HA GX20 37.29 ± 2.27 1127.79 ± 134.25 4.10 ± 0.95 0.059 ± 0.001 9.38 ± 0.44HA GX30 16.06 ± 0.63 453.87 ± 18.30 19.45 ± 5.49 0.057 ± 0.002 17.14 ± 0.46HA GX40 7.24 ± 0.49 123.39 ± 13.80 26.38 ± 1.50 0.059 ± 0.001 24.75 ± 0.51LA 44.38 ± 4.98 1654.52 ± 245.51 2.40 ± 0.33 0.120 ± 0.004 12.9 ± 0.18LA G15 46.08 ± 3.62 1340.85 ± 40.8 3.42 ± 0.08 0.135 ± 0.004 10.58 ± 0.34LA G20 27.59 ± 2.96 805.01 ± 77.11 13.17 ± 1.53 0.140 ± 0.003 13.48 ± 0.17LA G30 5.06 ± 0.7 63.79 ± 8.55 70.73 ± 4.95 0.132 ± 0.011 25.12 ± 0.89LA X15 44.96 ± 3.81 1343.65 ± 167.82 2.76 ± 0.32 0.116 ± 0.009 10.49 ± 0.52LA X20 32.15 ± 3.72 886.72 ± 86.15 3.65 ± 0.20 0.149 ± 0.020 10.33 ± 0.37LA X30 18.02 ± 1.17 532.9 ± 73.17 24.54 ± 5.07 0.123 ± 0.005 9.52 ± 0.25LA GX15 31.95 ± 4.56 1167.82 ± 92.63 2.79 ± 0.32 0.150 ± 0.021 9.83 ± 0.07LA GX20 35.95 ± 2.04 1009.86 ± 44.3 5.64 ± 1.66 0.139 ± 0.012 10.46 ± 0.35LA GX30 5.96 ± 1.16 108.96 ± 13.69 67.36 ± 11.75 0.147 ± 0.009 16.36 ± 0.42

D. Muscat et al. / Journal of Food Engineering 109 (2012) 189–201 193

Rindlav-Westling et al. (1998) observed that films casted fromhigh-amylose corn starch have mechanical properties in the samerange as amylose films. In general, both the LA and HA filmsshowed a decrease in TS at higher plasticizer contents (i.e. glyceroland xylitol). These plasticizers reduced the intra-molecular attrac-tion between the starch chains by favourably forming hydrogenbonds between plasticizer and starch molecules. Thus reducesthe formation of hydrogen bonds between the starch chains and al-lows greater flexibility and subsequently decreases the tensilestrength (Turhan and Sahbaz, 2004). As can be seen from Table 3,the decrease in tensile strength is greater with the addition of glyc-erol than xylitol.

3.3.2. Effect of plasticizers on tensile strengthTable 3 shows that the tensile strength of glycerol-plasticized

films is always lower than the xylitol-plasticized films at higherthan 20% plasticizer concentrations. The reduction in the tensilestrength with the increase in xylitol and glycerol concentrationsis consistent with previous researches (Laohakunjit and Noom-horm, 2004; Chang et al., 2006; Gaudin et al., 1999). The effectsof glycerol and xylitol on tensile strength in LA films appear tobe more complicated at low concentrations. The presence of boththe plasticizers individually at low concentration of 15% exhibiteda higher tensile strength value of 46.08 MPa for LA G15 and44.96 MPa for LA X15 than 44.38 MPa of the LA starch film. Thisphenomenon can be explained through the anti-plasticizationbehaviour in starch–plasticizer films when plasticizer concentra-tion is relatively low. At low concentrations plasticizer moleculesare strongly attached to the starch, restricting the starch–starchinteractions necessary to absorb the mechanical energy (Marcillaand Beltran, 2004). It appears that glycerol and xylitol can exertan anti-plasticizing effect on the tensile strength of the films whenpresent at low concentration, i.e. 15%, in this case. This range is re-ferred to as ‘‘glycerol anti-plasticizing range’’ by Chang et al.(2006). The films of the LA and HA with the combined plasticizers(glycerol + xylitol) showed tensile strength values between thefilms plasticized by individual plasticizers. For example, HA GX40had a tensile strength of 7.24 MPa, which is between the tensilestrength of HA G40 (5.05 MPa) and HA X40 (13.96 MPa).

3.3.3. Effect of plasticizers on modulus of elasticityModulus of elasticity, as an index for ‘‘stiffness’’ for both types of

plasticized films, showed a general decrease with increasing

moisture content (Table 3), a behaviour expected for a plasticizedmaterial (Chang et al., 2000). The effect of the plasticizers on themodulus of elasticity of HA and LA films could not be compared sincethe film thickness were not the same for those films (Table 3). The LAfilms had thickness ranging from 0.116 to 0.150 mm which werethicker than the HA films the thickness of which ranged from0.051 to 0.065 mm. As shown in Table 3, the modulus of elasticityof plasticized HA films were higher than plasticized LA films. Themodulus of elasticity decreased rapidly with the increase in the plas-ticizer concentration, i.e. from 1685.57 MPa for HA film to80.19 MPa for HA G40 film. This rapid decrease is further com-pounded by the combination of plasticizers (glycerol + xylitol). Theoverall reduction in the modulus of elasticity for xylitol plasticizedfilms was greater than for glycerol plasticized films, however, thexylitol plasticized films had higher stiffness, and appeared firmerthan the glycerol plasticized films.

3.3.4. Effect of plasticizers on elongation at breakThe unplasticized LA and HA films show the lowest values for

elongation at break (2.40% and 1.41%, respectively) compared tothe plasticized films. This indicates that the (residual) water–starch and starch–starch interactions in LA and HA films producegreater number of or stronger hydrogen bonds (Turhan and Sah-baz, 2004). It can be seen in both LA and HA films that increasein each plasticizer concentration result in an increase in the elon-gation. The elongation property of the films plasticized with com-bined plasticizers gets significantly improved compared to thefilms plasticized only with xylitol and gets compromised comparedto the films plasticized by only glycerol. For example, LA G20 hasan elongation of 13.17% while LA X20 had an elongation of 3.65%,whereas the elongation for LA GX20 was 5.64% (Table 3).

From the above results it can be seen that the tensile strength,modulus of elasticity and elongation properties are strongly influ-enced by the concentration of glycerol and xylitol, i.e. the concen-tration of plasticizers used. Mali et al. (2006) had reported thetensile strength and modulus of elasticity decreased and the elon-gation at break increased with increase of glycerol in various starchfilms. The tabled results agree with this observation. Except in thecases where anti-plasticization occurs, the increase in the concen-tration of these plasticizers decreases the tensile strength andmodulus of elasticity while increasing the elongation for both theLA and HA based films. The addition of these plasticizers makesLA and HA based films more ductile, which indicates that they oc-

194 D. Muscat et al. / Journal of Food Engineering 109 (2012) 189–201

cupy the space between the starch molecules and interfere withattractive intramolecular forces. Overall, the starch films derivedfrom high amylose starch showed higher tensile strength and mod-ulus of elasticity and lower elongation properties than the films de-rived from low amylose starch. This can be explained through thehigher gelatinisation and glass transition temperatures of the amy-lose fraction of starch (Lourdin et al., 1997).

3.4. Water vapour permeability (WVP)

The WVP values of the starch alone and some of the polyol-plas-ticized films are presented in Table 4. The WVP values of LA G15, LAX15 and LA GX15 films are not presented as the films showedgreater extent of cracking during WVP measurements. The crackingin these films was due to greater rigidity of starch–plasticizer ma-trix caused by anti-plasticization. The WVP values of the LA and HAfilms were 0.59 and 0.52 g mm/(m2 h kPa) respectively, as shownin Table 4. The permeability of LA films are consistent with Taljaet al. (2007), difference in our results and the authors’ results isdue to different starch used. The potato starch used by Taljaet al. (2007) would have much lower amylose content.

3.4.1. Effect of glycerol on WVPAs shown in Table 4, the films which contained glycerol as plas-

ticizer (especially in at 30% and higher) had higher permeabilityvalues compared to the LA and HA starch-alone films. This is be-cause the glycerol plasticized films tended to equilibrate at highmoisture content as shown Table 3, which is consistent with earlierfindings by Gaudin et al. (2000). This study reported that higherwater content leads to increased permeability in hydrophilic bio-polymer films. As shown in Table 4, both LA and HA, with 20% glyc-erol concentrations had a hydration capacity lower than theunplasticized films. Then at a glycerol content of 40%, the WVP va-lue increased. This trend was observed by Mali et al. (2004), statingthis behaviour could be related to structural modifications of thestarch network that occurred when glycerol was added. Accordingto Chaudhary et al. (2009), this behaviour is attributed to themolecular structure and size of the plasticizer and its interactionwith water in starch matrix. Glycerol is similar to water in sizeand can replace water at low concentrations of glycerol to createglycerol–starch interactions forming a more compact structure,resulting in lower WVP values. To reduce these strong starch–water and starch–starch interactions excess plasticizers are added(Chaudhary, 2010 ) to enhance the interactions within plasticizer-plasticizer rather than with other components. Then by increasingthe glycerol concentration to 40% increased the WVP values the

Table 4Summary of water vapour permeability and thickness.

Starch film WVTR (g mm/m2 h) WVP (g mm/m2 h kPa) Thickness (mm)

HA 0.55 ± 0.03 0.52 ± 0.03 0.065 ± 0.001HA G20 0.46 ± 0.12 0.43 ± 0.11 0.056 ± 0.005HA G30 0.56 ± 0.04 0.52 ± 0.04 0.065 ± 0.004HA G40 1.34 ± 0.07 1.26 ± 0.07 0.051 ± 0.011HA X20 0.12 ± 0.02 0.11 ± 0.02 0.053 ± 0.002HA X30 0.09 ± 0.01 0.07 ± 0.04 0.057 ± 0.003HA X40 0.07 ± 0.02 0.06 ± 0.02 0.055 ± 0.003HA GX20 0.15 ± 0.02 0.14 ± 0.02 0.059 ± 0.001HA GX30 0.26 ± 0.04 0.26 ± 0.03 0.057 ± 0.002HA GX40 0.71 ± 0.04 0.66 ± 0.04 0.059 ± 0.001LA 0.63 ± 0.14 0.59 ± 0.13 0.120 ± 0.004LA G20 0.28 ± 0.01 0.27 ± 0.01 0.140 ± 0.003LA G30 0.68 ± 0.05 0.64 ± 0.05 0.132 ± 0.011LA X20 0.11 ± 0.03 0.10 ± 0.06 0.149 ± 0.020LA X30 0.11 ± 0.01 0.10 ± 0.06 0.123 ± 0.005LA GX20 0.13 ± 0.02 0.12 ± 0.07 0.139 ± 0.012LA GX30 0.21 ± 0.02 0.20 ± 0.02 0.147 ± 0.009

film matrix became less dense, and this, added to the characterof glycerol, was flavourable to adsorption of water molecules, toexhibited plasticization ability. This looser network is formed dueto occurrence of starch–starch interaction enhancing the mobilityand flexibility of the starch chains and increasing water retentionin the starch matrix (Chang et al., 2006; Shaw et al., 2002).

3.4.2. Effects of xylitol on WVPFrom the Table 4 it can be seen that the films with the lowest

WVP values were the xylitol plasticized films (i.e. LA X20, LA X30,HA X20, HA X30 and HA X40). Although the xylitol concentrationrange (20–40%) was quite broad, the WVP values were almost con-stant at approximately 0.09 g mm/(m2 hkPa). These films alsoexhibited low moisture content (Table 3) of less than 10%. Withlow water content present a starch–water interaction formed byhydrogen bonds is more compact than the starch–starch system(Lourdin et al., 1997). It was observed that the films plasticizedwith xylitol at high concentration, i.e. LA X30, HA X30 and HAX40, exhibited blooming when they were conditioned at 52.9% RH.

3.4.3. Effect of combine plasticizers on WVPThe films plasticized with the combined plasticizers exhibited

WVP values similar to the glycerol plasticized films. At 20% com-bined plasticized films the WVP values were lower than the LAand HA stand-alone films. When the combined plasticizers concen-tration increased to 40% the WVP values increased. Although xylitolwas also present, the glycerol concentration had a greater influ-ence on the interaction created, as explained earlier. Althoughthe WVP values and moisture content of the combined plasticizersfilms seem to exhibit the same trend, they were lower than thefilms plasticizers by glycerol due to the presence of xylitol. Thesefilms at high concentrations did not show signs of blooming afterconditioning at 52.9% RH.

Overall, 20% combined plasticized films with both LA and HAexhibited the lowest WVP values and moisture content.

3.5. Rheological behaviour of the dispersions

3.5.1. Viscoelastic behaviour of the dispersionsFrequency sweep plots of storage modulus (G0) and loss modu-

lus (G00) of the starches and plasticized (i.e. glycerol and xylitol)suspensios at different concentrations are shown for LA (Fig. 1)and HA (Fig. 2). Both the LA and HA suspensions show lower stor-age modulus than the loss modulus and hence are considered to bedilute dispersions (Ikeda and Foegeding, 2005). These dispersionsare formed by heating starch granules in the presence of excesswater, swelling occurs causing the disruption of molecular orderwithin the granules. With continued heating, the hydrogen bondskeeping the granule together are weakened, and irreversible swell-ing occurs. The granules eventually burst and solution clarity in-creases. This allows for the leaching of soluble components,primarily amylose into the water (BeMiller and Whistler, 1996).The viscosity of the solution rises through a maximum and thendrops as the granules rupture and disintegrate (Orthoefer, 1987)resulting in dilute dispersions. These dispersions showed behav-iour of entangled solution at a cross-over point at frequency of0.2 Hz and G0 and G00 of 13.2 MPa for LA starch suspension. TheHA starch suspensions had a frequency of 0.4 Hz and G0 and G00 of0.2 MPa as the G0 became greater than G00 for the first time at thesefrequencies. When the concentration of plasticizers increased thecross-over point shifted to the higher frequencies. The cross-overpoint for the plasticized LA suspensions increased between 1.5 to2.2 Hz with G0 and G00 in a range of 4.1 and 12.6 MPa. The LA G15suspension exhibited a G0 and G00 value of 14.4 MPa which washigher than the LA suspension due to anti-plasticization. Thecross-over point for the plasticized HA suspension decreased

Fig. 1. Storage modulus (G’) and Loss modulus (G00) of LA suspensions at different concentrations of plasticizers as a function of frequency (Hz). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. Storage modulus (G0) and Loss modulus (G00) of HA suspensions at different concentrations of plasticizers as a function of frequency (Hz). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)

D. Muscat et al. / Journal of Food Engineering 109 (2012) 189–201 195

slightly down to a frequency of 0.23 Hz and a G0 and G00 of0.08 MPa. Below the frequency of cross-over point the dispersionsare fluid like (Ikeda and Foegeding, 2005). This cross-over point isan excellent indicator of magnitudes of average molecular weightand molecular weight distribution of the suspensions (Edwards,2008). The G0 and G00 values of HA starch suspension were lowerthan those of the LA starch suspensions by a factor of 100. A studyconducted by Singh et al. (2003) reported that amylose content sig-nificantly affects the rheological properties of the starch.

3.5.2. Structure development over time of the dispersionsTime sweep plots of G0 and G00of the starch alone and starch–

plasticizer dispersions showed LA starch suspension is time

dependent as the G0 values increased over time. This shows thatLA starch suspension is constantly undergoing molecular rear-rangements favourably to an elastic solid-like nature over time(Gunasekaran and Mehmet, 2000). All the plasticized LA suspen-sions were time independent and the elastic nature did notchange within experimental time frame used. The increase inthe concentration of the plasticizers in LA suspensions decreasedthe elastic modulus and the viscous modulus of these suspen-sions. The time sweep plots shows that HA suspensions weretime independent and increase of plasticizer concentration hadno influence on the elastic nature of the suspensions. The viscous(fluid like) nature of the all the HA suspensions increased as theplasticizers concentration increased.

196 D. Muscat et al. / Journal of Food Engineering 109 (2012) 189–201

3.5.3. Flow behaviour of the dispersionsThe flow curves of starch-alone and plasticized LA and HA sus-

pensions showed that both the LA and HA suspensions were non-Newtonian in nature. The flow curves showed that the rheologicalbehaviour can be adequately described by the Herschel–Bulkleymodel (Steffe, 1996). The shear-rate versus shear stress data ofboth the LA and HA suspensions followed the H-B model with cor-relation coefficients (R2) greater than 0.98. Most suspensionsexhibited shear-thinning behaviour (n < 1) which agrees with pre-vious reports in the literature (Bertuzzi et al., 2007; Genovese andRao, 2003) (Table 5). As shown in Table 5, the flow behaviour index(n) values for the LA suspensions ranged between 0.55 and 0.76, nvalues of the HA suspensions ranged from 0.70 to 1. All these sus-pensions showed that a finite yield shear stress (r0) was requiredfor all these dispersions to initiate the flow.

The yield stress values of LA suspensions were higher than thatof HA suspensions. From the flow behaviour index and yield stressvalue it can be concluded that the LA suspensions exhibitHerschel–Bulkley type non-Newtonian behaviour (Table 5). Itwas also found that the flow behaviour index decreased whenthe concentration of both plasticizers increased which indicatesto the tendency of moving away from Newtonian behaviour. Thesuspensions with the combined plasticizers (glycerol + xylitol)were thinner than the plasticizers added individually to the sus-pension. The HA suspensions behaved as either Herschel–Bulkleyor Bingham plastic materials. The flow behaviour indices of someof the HA suspension with higher plasticizer concentration weren = 1 (Table 5) which indicated a Newtonian behaviour. The sus-pensions exhibiting Bingham plastic behaviour are HA GX20, LAGX30, LA G30, LA G40, LA X40 and LA GX40.

3.6. Starch–plasticizer interaction

In this study, FTIR has been used to monitor microstructuralcharacterisation of composite film by evaluation of interactionsbetween the film components. The dominant functional group ofcarbohydrates are the hydroxyl groups which are involved in intra-and inter-molecule hydrogen bonding with other hydroxyl groups(Ottenhof et al., 2003). According to Wancho and Sharma (2003),hydrogen bonding or other secondary interactions between chem-ical groups of dissimilar polymers should theoretically cause a shiftin the peak position of participating groups. This kind of behaviouris exhibited by miscible blends that show extensive phase mixing.Depending whether the hydrogen bonding interaction shifts thewave number lower or higher will indicate if the composite filmcomponents are forming hydrogen bonding or competing forhydrogen bonding, respectively. The shift in peak position will de-pend on the strength of the interaction.

Table 5The rheological behaviour values for the suspensions.

LA suspensions HA suspensions

Film K n ro Film K n ro

LA 2.13 0.76 40 HA 0.037 0.86 0.055LA X15 1.80 0.68 20 HA X20 0.022 0.82 0.005LA X30 0.92 0.67 7 HA G20 0.041 0.75 0.055LA G20 1.54 0.65 15 HA X30 0.041 0.70 0.035LA X20 1.98 0.63 10 HA GX20 0.010 1 0.055LA G15 1.90 0.63 20 HA GX30 0.007 1 0.060LA GX20 1.64 0.63 10 HA G30 0.006 1 0.035LA GX15 2.27 0.60 12 HA X40 0.005 1 0.030LA G30 1.63 0.55 5 HA GX40 0.005 1 0.030LA GX30 1.57 0.55 7 HA G40 0.004 1 0.010

3.6.1. FTIR assignment of corn starchThe FTIR spectra of LA and HA films are shown in Fig. 3. These

spectra represent a typical alcohol and hydroxy compound groupfrequencies (Coates, 2000) and are similar to each other. FTIRassignment of corn starch includes the following frequencies. Thebroad band between 3000 and 3600 cm�1 is attributed by the com-plex vibrational stretches associated with the free, inter- and intra-molecular bound hydroxyl groups between neighbouringmolecules, which made up the major structure of starch (Coates,2000; Zhang and Han, 2006). Although the spectra appear to bethe similar there are differences at this peak. The LA and HA filmsexhibited wave numbers of 3323 and 3307 cm�1, respectively. Thissuggests that HA film form stronger hydrogen bonds with waterdue to higher proportion of amylose. The sharp peak at2924 cm�1 is characteristic of C–H stretching (CH2) (Park et al.,2000). The peak occurring at 1641 cm�1 is associated with thetightly bound water present in the starch (Zhang and Han, 2006)due to the hygroscopic nature of starch. The peaks at 1409 and1433 cm�1 are related to the C–H bending of CH2. Peaks at 1240,1299, and 1333 cm�1 are associated to O–H bending due to the pri-mary or secondary alcohols (Coates, 2000). Changes in structureand crystallinity are reflected in the focus area between 1200and 950 cm�1 which is the fingerprinting region unique for a mol-ecule (Cerna et al., 2003; De Giacomo et al., 2008; Yao et al., 2011).Peaks 994 and 1077 cm�1 are characteristic of the anhydroglucosering O–C stretching (Zhang and Han, 2006; Kacurakova and Math-louthi, 1996). This wave number range suggests that there is a dou-ble substitution on the central hydroxyl substituted carbon and isalso associated to C–OH bending, originated by C(4)–O in the caseof 1022 cm�1 and C(1)–H at 1047 cm�1 (De Giacomo et al., 2008).The peak at 1000 cm�1 is recognised as water sensitive and is re-lated to intramolecular hydrogen bonding of hydroxyl groups(van Soest et al., 1995) or plasticizing effect of water (Kacurakovaand Mathlouthi, 1996). According to Coates (2000) there is otherabsorption of lower importance, but often characteristic of anotherform of bending vibration of the O–H at 720–590 cm�1.

3.6.2. Effect of plasticizers with LAWhen the LA film was compared to LA G20 and LA G30 films

(Fig. 4) the O–H stretching peak at 3324 cm�1 shifted to higherwave numbers, 3344 and 3346 cm�1, respectively. This shift wascaused by a decrease in the intermolecular force of the hydroxylgroups between the glycerol–starch, starch–water and water–glyc-erol interactions. The introduction of glycerol may prevent the ref-ormation of hydrogen bonding during the drying process as theglycerol remained and solvated between the starch polymericchains (Zhang and Han, 2006). This suggests that the starch andglycerol were competing for hydrogen bonds with water as theglycerol concentrations increased causing the wave number toshift to a higher frequency. This finding was also observed by Parket al., (2000) were LA corn starch was plasticized with glycerol.When 15% of glycerol was added to LA starch the O–H stretchingpeak decreased from 3324 to 3317 cm�1. This indicates strongerand more stable hydrogen bonds were formed between the glyc-erol and starch compared with intra- and intermolecular hydrogenbonds in starch with water (Yang et al., 2006). This interaction re-duces the free volume available for polymer mobility (Mali et al.,2008) causing rigidity rather than flexibility of the film, referredto as anti-plasticization (Seow, et al., 1999).

The FTIR spectra for the LA-alone film and the xylitol plasticizedfilms are shown in Fig. 5, where the addition of xylitol had de-creased the intermolecular hydrogen bonding. This is evident atpeak 3324 cm�1 for LA film had shifted to higher wavenumber,3333 and 3324 cm�1 with the addition of 20% and 30% of xylitol,respectively. The LA X30 spectrum exhibited characteristics of xyli-tol than LA (Fig. 5) caused by phase separation and physical exclu-

Fig. 3. FTIR spectra of LA and HA starch films.

Fig. 4. FTIR spectra of LA, LA G15, LA G20 and LA G30 films.

Fig. 5. FTIR spectra of LA, LA X15, LA X20 and LA X30 films.

D. Muscat et al. / Journal of Food Engineering 109 (2012) 189–201 197

sion of xylitol from the film. It seems that 30% of xylitol had ex-ceeds the compatibility limit of these polymers causing xylitol toseparate.

The spectra of the combined plasticized LA films were com-pared to the LA film (Fig. 6) at the O–H stretching region. The addi-tion of combined plasticizers to LA affected the formation of

Fig. 6. FTIR spectra of LA, LA GX15, LA GX20 and LA GX30 films.

198 D. Muscat et al. / Journal of Food Engineering 109 (2012) 189–201

hydrogen bonds within matrix differently. At 15% there was no sig-nificant change to the hydrogen bonds formed, i.e. no wave num-ber shifting. At 20% the wave number increased from 3324 to3341 cm�1, suggesting a decrease in intermolecular force of the hy-droxyl groups in the film. When the concentration of combinedplasticizers increased to 30% there was evidence of xylitol phaseseparation as the spectrum resembled the xylitol spectrum.

3.6.3. Effect of plasticizers with HAThe FTIR spectra for HA film and glycerol plasticized films are

shown in Fig. 7. The addition of 20%, 30% and 40% of glycerol toHA starch had shifted the O–H stretching vibration maxima at peak3307 cm�1 to lower wavenumbers, i.e. 3238, 3242 and 3268 cm�1,respectively than the HA film. These shifts towards lower wave-numbers reveals that there is a favourable interaction betweenthe glycerol and HA starch through the hydroxyl groups. This indi-cates that the preparation of composites films with glycerol andHA starch made new hydrogen bonds between the hydroxylgroups in the glycerol molecules and the hydroxyl groups in theHA starch molecules. This new and strong hydrogen bond forma-tion between HA starch and glycerol replaced original interactionin HA starch. The shift to lower wave numbers was observed atall the major frequencies of these spectra suggesting a strong for-mation of hydrogen bonds within glycerol plasticizer HA films.The glycerol concentration of 20% exhibited a more favourableinteraction between the components than at higher concentrations

Fig. 7. FTIR spectra of HA, HA G2

of 30% and 40% due to the greater shift to a lower wavenumber(Coates, 2000).

The FTIR spectra for HA film and xylitol plasticized films areshown in Fig. 8. The addition of 20% and 30% of glycerol to HAstarch had shifted the O–H stretching vibration maxima at peak3307 cm�1 to higher wavenumbers, 3317 and 3315 cm�1 in com-parison with the HA film. This shift seems to indicate that the addi-tion of xylitol led to the weakening of hydrogen bonding in thefilm. Addition of xylitol may prevent the reformation of hydrogenbonding during the drying process as the xylitol remain and sol-vate between the starch polymeric chains (Park et al., 2000). TheHA X40 spectrum exhibited characteristics of xylitol than HA(Fig. 8) caused by phase separation and physical exclusion of xyli-tol from the film. It seems that 40% of xylitol had exceeds the com-patibility limit of these polymers causing xylitol to separate.

The spectra of the HA starch films plasticized by combinedplasticizers are shown in Fig. 9. The addition of 20%, 30% and 40%of glycerol + xylitol to HA starch had shifted the O–H stretchingvibration maxima at peak 3307 cm�1 to higher wavenumbers, i.e.3335, 3335 and 3343 cm�1, respectively than the HA film. Thisfilms exhibit molecules structures and interactions similar to the20% and 30% xylitol plasticized films (Fig. 8).

From the above discussions, it can be concluded that the addi-tion of plasticizers to LA starch decreased the intermolecularhydrogen bonds within the film. This was different at low concen-trations of glycerol (15%) where new hydrogen bonds were formed

0, HA G30 and HA G40 films.

Fig. 8. FTIR spectra of HA, HA X20, HA X30 and HA X40 films.

Fig. 9. FTIR spectra of HA, HA GX20, HA GX30 and HA GX40 films.

20

30

40

50

60

70

80

90

100

110

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Moisture content (fraction of total)

Gla

ss tr

ansi

tion

tem

pera

ture

(°C

) .

LA

HA

LAG15

LAG20

LAX15

LAX20

Fig. 10. Variation of glass transition temperature of starch–water, starch–glycerol,starch–xylitol films at different moisture content.

D. Muscat et al. / Journal of Food Engineering 109 (2012) 189–201 199

between the LA starch and glycerol due to anti-plasticization.Additions of high concentrations of xylitol, i.e. 30% to LA starchcaused phase separation of xylitol. HA films plasticized with glyc-erol formed new hydrogen bonds indicating a strong interactionbetween the glycerol and HA starch exists with HA G20 film exhib-iting the strongest hydrogen bond interaction within the matrix.

3.7. The glass transition behaviour

3.7.1. Low-amylose starchThe variation of Tg of both the LA and HA starch-alone films are

presented in Fig. 10. This figure also contains variation of Tg LAstarch with glycerol and xylitol as plasticizers. The Tg values ofboth the HA and LA films are very close to those reported by Luiet al., (2010) when extrapolated to the moisture content reportedby these authors. Fig. 10 shows that the effect of plasticizers inlowering the Tg of the starch–plasticizer films is very strong atlow moisture contents. At higher moisture contents, especiallyabove 20% (w/w) the starch–plasticizer and plasticizer–waterinteractions compete with the starch–water interaction. The muchlowered slopes of Tg versus moisture fraction of the glycerol andxylitol plasticized starch compared to those of the starch–waterlines indicate that the availability of water to rapidly lower theglass transition temperature is limited. Similarly, at higher mois-ture contents, the increase in the plasticizer content is unable todepress the glass transition temperature of the films very strongly.

This means that at higher moisture contents and higher plasticizercontents the three way interactions of starch–water, starch–plasti-

200 D. Muscat et al. / Journal of Food Engineering 109 (2012) 189–201

cizer and plasticizer-water are competitive, not synergistic whenthe depression in Tg of the films is concerned.

The Tg values for the LA starch films plasticized by glycerol and/or xylitol at different concentration are presented in Table 2. Thesefilms were equilibrated for at least one week at 52.9% relativehumidity using saturated magnesium nitrate proportion. One ofthe interesting features of this table is these films equilibrate at dif-ferent moisture content depending on the type and concentrationof the plasticizers. As a trend, the films with glycerol have higherequilibrium moisture content than the films plasticized with xyli-tol. Interestingly, the films plasticized with xylitol alone have equi-librium moisture contents lower than the starch-alone films. Thefilms plasticized with glycerol have much higher moisture content.This shows that the glycerol has greater affinity with water. Forthis given relative humidity (52.9%) as the equilibrium relativemoisture contents of different films are different, the resultant Tg

values of these films are affected by the presence of this water,the type and concentration of the plasticizer in the films.

As a trend, the presence of plasticizer has lowered the Tg of theplasticized films which is expected. For example the Tg of LA starch(at 12.9% moisture) is 64.9 �C (Table 2). The Tg decreased to 30.5and 32.7 �C when 30% (on dry solid basis) of glycerol and xylitolwere used as plasticizers. It has to be pointed out here that themoisture content of the LA G30 film was 25.12% (w/w) while thexylitol plasticized film was only 9.52% (w/w). The difference in Tgbetween these films is only about 2 �C. It is expected that the Tg

to be depressed at least below the ambient temperature. This sug-gests that the xylitol has greater propensity to depress the Tg of thestarch films. The glycerol probably interacts more with water andleaves less water to plasticize the starch. The lowering of Tg ofstarch by plasticizer is attributed to the reduced packing of thestarch molecules, which reduces the degree of crystallinity in thefilm (Chang et al., 2006; Mali et al., 2008).

3.7.2. High-amylose starchFrom Table 2 it can be seen that, the HA-alone and plasticized

HA films have generally higher Tg values than the LA-alone andplasticized LA films. This may be due to the fact that the amylopec-tin fraction of starch has a lower Tg than amylose fraction (Lourdinet al., 1997). This table indicates that for the same starch:plasti-cizer ratio the high amylose starch based films equilibrate at highermoisture contents than the low amylose starch based films. Fur-thermore, from Fig. 10 it can be seen that the slope of Tg versusmoisture content line is steeper than the similar slope in the caseof low amylose starch. This indicates that there is greater possibil-ity that the presence of both the plasticizer and water can depressthe Tg of high amylose starch below room temperature faster thanthe low amylose starch based films. We could not measure the Tg ofHA X40, HA G 40, HA GX30, HA GX40 film as we had only lookedinto the Tg above ambient temperature.

As was seen in the case of LA and plasticized LA films, the HAand plasticized HA films equilibrated to different moisture con-tents and lowered the Tg differently when stored at the same rela-tive humidity. At given moisture content, the xylitol plasticizedfilms had slightly higher Tg, most probably due to xylitol’s higherTg (�23.2 �C) than that of glycerol (�90 �C) Furthermore, the xylitolestablishes strong hydrogen bonds with the starch molecules(Chaudhary et al., 2011). The preference of starch to form hydrogenbonds with xylitol in the expense of water probably is the mainreason for starch–xylitol films to have higher Tg. Overall, the xylitolplasticized HA starch films had much lower equilibrium moisturecontent than the glycerol plasticized films, which is strong indica-tion of starch–xylitol interaction in the expense of water. Whenwater is unable to bond with starch or xylitol, it becomes easyfor it to evaporate out during drying.

4. Conclusions

The gelatinisation behaviour of LA and HA starch showed thatHA starch needed about 30 �C higher temperature and greater agi-tation for complete gelatinisation. Both starch-alone and plasti-cized starch films (with 15% plasticizer) were brittle and crackedupon drying. Furthermore, very high xylitol concentration of 30%and above showed phase separation when conditioned at 52.9%relative humidity. The starch films plasticized by combined plasti-cizers (glycerol + xylitol) did not exhibit anti-plasticization andblooming further providing reason for combining plasticizers.The starch–glycerol films had consistently higher moisture contentcompared to starch–xylitol films. The films plasticized by xylitoland combination of glycerol and xylitol exhibited lower moisturecontents than glycerol plasticized films mainly due to preferredstarch–xylitol hydrogen bonding which allowed easy evaporationof the moisture. The tensile strength of LA films containing low(15% dry solid basis) plasticizers was consistently higher than thetensile strength of the starch-alone films due to anti-plasticization.When the concentration of plasticizers increased the tensilestrength and the modulus of elasticity decreased, whereas theelongation at break of both the high and low amylose based filmswas increased. Overall, films with high amylose content showedhigher tensile strength and modulus of elasticity values and lowerelongation values than low amylose starch films. The starch filmsprepared by using 20% combined plasticizer provided the bestwater vapour barrier properties, and reasonable elongation, tensilestrength, modulus of elasticity without any evidence of anti-plasti-cization. The suspensions prepared from both LA and HA starchesbehaved like dilute macromolecular solutions. From the flowbehaviour index, the LA starch suspensions displayed Herschel–Bulkley type non-Newtonian behaviour, whereas the HA starchsuspensions behaved either as Herschel–Bulkley or Bingham plas-tic material. Some of the high amylose films showed signs of crys-tallinity due to high moisture content or due to extra hydroxylgroup present in its structure, as in the case of xylitol indicatingthat xylitol is able to crystallize in presence of sufficient moisturecontent, but when the plasticizers were combined, the phase sep-aration associated with the xylitol was minimised, and this wasattributed to the inter-plasticizer interaction within the hydro-philic starch–plasticizer system.

References

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