AromaPermeability through polymer films

8/7/2019 AromaPermeability through polymer films

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PACKAGING TECHNOLOGY AND SCIENCEPackag. Technol. Sci. 2004; 17: 175185DOI:10.1002/pts.648

Development of a System for Measurement of

Permeability of Aroma Compounds through

Multilayer Polymer Films by Coupling

Dynamic Vapour Sorption with Purge-and-

Trap/Fast Gas Chromatography

By Qiaoxuan Zhou,1 Brian Guthrie2 and Keith R. Cadwallader1*1 Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, 1302 WestPennsylvania Avenue, Urbana, IL 61801, USA2 Cargill Food System Design,Wayzata, MN 55391, USA

A system for measurement of permeability of aroma compounds through laminatedpolymer films was constructed by combining dynamic vapour sorption (DVS) withpurge-and-trap/fast gas chromatography (P&TfGC). Data validation wasachieved by measuring the permeability of limonene across an HDPE film (film C)and comparing the permeation data with the literature values. The generalapplicability of this system, as well as its potential for simultaneous measurementof permeability of multiple aroma compounds, was demonstrated by measuring thepermeability of limonene and ethyl butyrate as single permeants or as co-permeants under different environmental conditions (at 25C, 30C or 35C and 0%or 75% RH) through three different multilayer polymer films (film A,HDPE/EVOH/HDPE/HDPE; film B, HDPE/nylon/HDPE/HDPE; film C,HDPE/HDPE). The results showed that the aroma barrier performance of plastic

films was determined by the polymer composition and was affected by variousfactors, such as temperature and the presence of other co-permeants. Simplicity,speed and accuracy were some of the attractive features of this system, whichindicates its potential as a useful tool that could be applied in the food industryfor screening or selection of appropriate packaging materials for specificapplications. Copyright 2004 John Wiley & Sons, Ltd.

Accepted 22 December 2003

KEY WORDS rapid measurement; permeability; aroma; co-permeant; multilayer polymer film

* Correspondence to: K. Cadwallader, Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, 1302 West Pennsylvania Avenue, Urbana, IL 61801, USAEmail: [email protected]

Copyright 2004 John Wiley & Sons, Ltd.

INTRODUCTION

With the increasing usage of polymer films as foodpackaging materials, more and more attention hasbeen paid to the aroma barrier properties of poly-meric packaging materials. This is because, unlikeglass and metal, polymers are semi-permeable to

small molecules such as flavour substances.1,2

Therefore, proper selection of packaging materialfor specific application is essential for retention ofthe desirable food flavour and hence extension ofproduct shelf-life.

Sorption, migration and permeation are com-monly used to describe the mass transfer phe-


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nomena of a permeant/polymer system, whichcan be measured by three parameters: solubilitycoefficient (S), diffusion coefficient (D) and perme-ability coefficient (P).36 When diffusion is Fickianand sorption follows Henrys law of solubility,their relationship can be expressed as: P = D S.7,8

Knowledge about these parameters is of practi-cal use for selection of suitable packaging materi-als and prediction of product shelf-life. Masstransfer of inert gases and water vapour hasbeen well investigated and standardized methodsexist.5,9 However, current knowledge on masstransfer of aroma substances is still limited and nostandard procedure is available to determine thenumerical values of these parameters. This is dueto the fact that organic substances are capable ofinteracting with the polymer matrix, leading to

polymer structural change and hence a change inpermeability. The presence of water vapour andother organic co-permeants will further complicatethe measurement. Therefore, it is a challengeto accurately determine the permeability of anaroma substance, especially the permeabilityof aroma compound mixtures, through polymerfilms.

At present, methods developed for aromapermeability measurement include sorption andpermeability measurements, with the later beingcommonly approached by isostatic or quasi-

isostatic methods.3,9

Systems utilizing the isostaticmethod have been well described.9 Basically, atwo-chamber diffusion cell is used, where perme-ant vapour at a constant concentration is generatedin one chamber with the permeated vapour samplebeing detected and quantified in the otherchamber. Generally, such systems allow for themeasurement of permeant transmission rates as afunction of temperature, relative humidity andpermeant concentration.9 However, these systems,in general, examine only one compound at a time.Tou et al.10 used a mass spectrometer system to

measure aroma compound permeation rates, andreported parts per billion sensitivity. Anothersystem with high sensitivity was developed byFranz,11 and the applicability of this system forpermeability measurement of organic vapourmixtures had been shown.12 However, thetedious experimental protocol is inefficient andlabour-intensive.

Therefore, alternative measurement systemsare still needed. Instrumentation and methodology

that provide rapid, sensitive and reliable measure-ments are especially desired. The objective of thepresent study was to develop and evaluate a newmeasurement system based on the isostatic proce-dure to meet such demands.

MATERIALS AND METHODS

Multilayer polymer film samples

Three different co-extruded blown films wereincluded in the present study. Films A and B weremultilayer films typically used as food packagingmaterials, containing both flavour and moisturebarrier layers [ethylene vinyl alcohol (EVOH) or

nylon and high-density polyethylene (HDPE),respectively]. Film C was a typical polyolefin(HDPE) film and contained no flavour barrier. Thestructures of these films are listed in Table 1.

Chemicals

Standard compounds ethyl butyrate (99% purity)and d-limonene (97% purity) and other reagent-grade chemicals (acetone and n-pentane) wereobtained commercially (Aldrich Chemical Co., St

Louis, MO). High-purity dry air used as the purgegas for the DVS system was purchased from a localgas supplier (S.J. Smith Welding Supply, Urbana,IL).

Methods

The aroma permeability measurement systemdeveloped in this study consisted of two sub-

Q. ZHOU, B. GUTHRIE AND K. R. CADWALLADERPackaging Technology

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Copyright 2004 John Wiley & Sons, Ltd. 176 Packag. Technol. Sci. 2004; 17, 175185

Table 1. Structures of the test film samples

TotalAbbreviated thicknessname Structure [mm (mil)]

EVOH barrier HDPE/HDPE/EVOH/HDPE 86.4 (3.4)Nylon barrier HDPE/HDPE/nylon/HDPE 88.9 (3.5)Non-barrier HDPE/HDPE 68.6 (2.7)


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systems: the permeation system and the detection

system (Figure 1). The permeation system wascomposed of a diffusion cell and a DVS system(Surface Measurement Systems Ltd, UK), and thedetection system consisted of a TDS G (GerstelGmbH & Co. KG, Germany) and an EZ Flash-gaschromatograph (GC) (TDX: Thermedics DetectionInc., Chelmsford, MA).

The DVS system was included as part of theinstrumentation, mainly for control of the diffu-sion cell temperature and the relative humidity ofthe purge gas conveying the penetrated permeantto the detection system. The detailed configuration

and the operating principle of this instrument canbe found at: http://www.smsuk.co.uk/products_dvs_instrument.html.

The diffusion cell used in this study was espe-cially designed for use with the DVS system andwas built by an on-campus machine shop (Depart-ment of Electrical and Computer Engineering, Uni-versity of Illinois). A schematic diagram of thediffusion cell assembly is shown in Figure 2. Thealuminium diffusion cell was composed of twomajor cylindrical parts the base and the top. Thepermeant sample was loaded directly onto the

bottom of the cell base, then the diffusion cell wasassembled and sealed with the test film through thecompression of the O-ring against the smoothsurface of the washer, to form a metal/film/O-ringseal, with an effective film exposed area of 7.07cm2.Screw-cap closure was used to provide a uniformcompression seal, with the two parts of the cellclosed and opened along the machined thread.A machined groove located on the sealing faceof the cell base was included for setting a size 024

O-ring (Kalrez 6375 O-ring from DuPont DowElastomers, Newark, DE), which was employed toaid in the sealing of the cell. An aluminium com-pression washer (1.04mm thick) was employed ontop of the test film, helping to reduce frictionbetween the test film and the rotating metal surfaceof the top part of the cell when tightening the cell,such that film wrinkling along the sealing surfacecould be minimized. After tightening the cell byhand, a specially made wrench was used to further

tighten the cell, ensuring that the cell was tightenedand sealed in a consistent manner. The cell wasanodized to strengthen the aluminium threadsand to harden the metal surface, which helped tominimize scratches and ensure a good seal.

Circular film pieces approximately 3.5cm indiameter were cut from the polymer samplesheets. Careful visual inspection was employedto avoid sampling areas containing pinholes orscratches. These film pieces were then stored in adesiccator over CaSO4 for at least 1 month prior tothe experiment. In all experiments, a sufficient

amount of permeant sample (50ml for single-compound studies and 100ml for binary mixturestudies) was loaded to ensure that constant per-meant vapour pressure was obtained throughoutthe experiment. Care was taken when assemblingthe cell to ensure that the film was located evenly.Between experiments, the O-ring and the diffusioncell were cleaned thoroughly with distilled wateror acetone, and baked in an oven at 180C for20min. Two O-rings were used in rotation.


AROMA PERMEABILITY MEASUREMENT OF POLYMER FILMS Packaging Technology

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ventflow

EZ Flash-GC

TDS Gvacuum sampling line

Detection System

DVS

Permeation System

cell

test film

cell top

cell base

compressionwasher

o-ring

Figure 1. Simplified schematic diagram of the permeabilitymeasurement system.

Figure 2. Schematic diagram of the diffusion cell assembly.


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To perform a test, the diffusion cell was loadedwith permeant sample and then placed inside theDVS sample chamber, where a stream of purge airat desired relative humidity was continuouslyflowed through at a constant rate (500scc/min)to remove and convey the penetrated permeantvapour to the detection system. At a predeter-mined time interval, a portion of the purge airventing from the sample chamber was taken andtransferred via a six-port gas sampling valve anda vacuum sampling line (1.00mm i.d., SilcosteelTM

tubing, Cat. No. 20595; Restek, Bellefonte, PA) toan absorbent trap (Cat. No. 28286-U; Supelco,Bellefonte, PA) packed with deactivated fusedSilica beads (Cat. No. 20791; Restek, Bellefonte, PA)and Tenax TATM 60/80 (Cat. No. 11982; Supelco,Bellefonte, PA). Immediately after the sampling

step, the absorbent trap was heated and back-flushed with carrier gas to transfer the trappedsubstances into a cooled injector (CIS-3, Gerstel) tocryofocus the sample. The concentrated sampleretained in the cooled injector was subsequentlytransferred into the capillary column by thermo-desorption. Both TDS and CIS-3 were operated insolvent vent mode [TDS: 1.0min dry purge; initialtemperature, 30C (1min hold); final temperature,210C (2min hold); ramp rate, 60C/min; six-portvalve temperature, 250C; transfer lines tempera-ture, 280C. CIS-3: glass wool inlet liner; vent flow,

50.0ml/min for 0.1min; splitless time, 0.5min;initial temperature, -50C (0.1min hold); ramprate, 12C/s; final temperature, 280C (2minhold)]. A Hewlett-Packard 5890 Series II gas chro-matograph (GC) equipped with an EZ Flash

accessory and a flame ionization detector (FID)was used to detect and measure the permeatedvapour sample. A RTX-5 capillary column (5m 0.25mm i.d.; 0.25mm film thickness; ThermedicsDetection Inc., Chelmsford, MA) was used for allexperiments. The column was enclosed inside aspecial resistive heating wire cage, which was

employed to facilitate fast temperature program-ming rates (up to 20C/s). The capillary columnwas ramped at 15C/min from 39C to 220C, withinitial and final hold times of 1.5 and 4.1min,respectively. The GC oven temperature was pro-grammed from 39C to 75C at a rate of 15C/min,with initial and final hold times of 1.5 and 4.1min,respectively. Both injector and detector tempera-tures were maintained at 250C. Experiments werecarried out in duplicate.

The mass of permeate that penetrated throughthe test film within a specific time period (t) wascalculated based on the response factor (a) deter-mined from an external calibration curve (y = a x),which was constructed by measuring the peakareas (y) of a series of standard solutions withknown amounts of permeant (x) in n-pentane.

RESULTS AND DISCUSSION

Instrumentation

Capability of mimicking actual applicationconditions, simplicity, speed and accuracy wereaddressed when developing this new measure-

ment system. These were attained by combiningthe desirable features of several techniques, as wellas by careful design and selection of the criticalcomponents.

The DVS system is an instrument developedto measure sorption and diffusion of water andorganic vapour using a gravimetric technique(http://www.smsuk.co.uk/products_dvs.html).Due to its capability of providing accurate andprecise temperature and relative humidity control,it was incorporated as part of the instrumentation.

Generally, tedious manual operation procedures

are required to perform a permeability testusing conventional measurement systems. This isnot only time- and labour-consuming but alsoincreases the error rate. An automatic systemcapable of providing continuous measurement isgreatly desired. The use of a TDS G device as wellas a programmable temperature vapourizer (PTV)injector (cooled inlet system) in the measurementsystem enables continuous on-line sampling withsubsequent GC analysis. In addition, by incorpo-rating an EZ FlashTM accessory into the GC system,the conventional GC was updated to a high-speed

GC with temperature programming up to 20C/s,such that a complex mixture could be separatedwithin only a few minutes. Use of high-speed GCanalysis is especially desirable when polymer filmsof poor barrier properties are tested, since moredata points can be collected within a short periodof time. The combination of the a forementionedtechniques not only makes continuous samplingpossible and practical, but also both increasesspeed and enhances precision and accuracy.


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Furthermore, caution was taken on the design ofthe diffusion cell. To ensure the reliability of thedata determined, hermetic closure of the diffusioncell is essential. This is especially true when highbarrier material is tested, since failure of obtaininga hermetic seal may enable permeant to escapethrough the seal, leading to unreliable permeationdata determination. No leakage of the permeantsample through the closure was detected for thediffusion cell used in this study. In addition, sincethe O-ring was located on the high concentrationside of the test film and was only partly com-pressed when sealed, part of the O-ring wasexposed to the permeant vapour sample beingstudied. Therefore, the material of the O-ring usedneeds to be compatible with the permeant toprevent sorption or permeation of the permeant

sample through the O-ring. Several different kindsof O-ring materials were tested and compared.Results showed that the Kalrez O-ring was mostsuitable, since it not only offers a good seal but alsoeliminates the absorption problems observed withother O-rings made of different materials. Theexcellent performance of this Kalrez O-ring wasattributed to the fact that Kalrez has the resilienceand sealing force of an elastomer with the chemi-cal inertness and thermal stability of Teflon.

The extremely accurate control of the environ-mental conditions provided by the DVS system,

and the increased accuracy and efficiency offeredby the TDS G and EZ FlashTM GC systems, impartvarious attractive features to the aroma perme-ability measurement system. This made it possibleto develop a sensitive, high-speed and reliableprocedure to simultaneously determine the masstransfer parameters (P, D and S) of multipleorganic vapours through polymeric packagingmaterials.

The permeabilities of two selected flavourprobes as single permeants and co-permeantsunder different environmental conditions were

measured using this system. Since the test filmsevaluated in the present study were laminatedmulticomponent polymers, the permeability para-meters measured were the apparent values.

Single Compound Permeability Studies

Ethyl butyrate and limonene were the two aromacompounds used in this study. They represent two

different chemical classes and are commonlyfound in various fruits and commercial flavours.Relatively high sample vapour concentrations (fullvapour pressure for individual compounds atthe corresponding temperature) were used in thepresent study, with the purpose of achievingsystem evaluation within a short period of time.The numerical values of permeant vapour pres-sure at each temperature evaluated were takenfrom the literature.13 These values were related tothe corresponding vapour concentrations throughthe ideal gas law (Table 2).

Permeability of limonene throughindividual test films

Permeation curves of the permeation rate (dm/dt)of limonene as a function of time were constructedfor individual test films. In general, the permeationcurves exhibit a nearly ideal permeation process,with an initial increasing transfer rate and a sub-sequent constant permeation rate (in equilibriumstate). Figure 3 shows the representative perme-ation profile of limonene through film A (EVOHbarrier).

Permeability coefficient P and diffusion coeffi-cient D were calculated based on the followingequations:

and

Dl

t=

2

0 57 199. .

Pdm

dt

l

A p=

D



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0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Time (min)

Permeationrate(Kg/sx10

-12)

Figure 3. Permeation profile of limonene (21mg/ml)through film A (EVOH barrier) at 25C and 0% RH.


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where dm/dt, l, A, Dp and t0.5 are equilibrium per-meant flow, film thickness, film exposure area,pressure gradient across the test film and half equi-librium time, respectively.3,9 The solubility coeffi-

cient S was calculated from the P and D valuesdetermined according to the relationship ofP = DS.7,8 The average values of the mass transfer para-meters determined for limonene through films A,B and C are shown in Table 3.

Permeation data determined for the limonene/HDPE (film C) system in the current study werecompared with those determined by other researchgroups for limonene as a single permeant throughHDPE films (Table 4).8,14 It was found that the P

and D values determined in the present studywere higher than those determined by otherresearchers (Table 4). This may be due to the factthat higher limonene vapour concentration

(approximately 21mg/ml at 25C) was used in thepresent study compared with that used by theother research groups (1.5mg/ml or 0.719mg/ml).Furthermore, it has been reported that permeationrate tends to increase at higher permeant vapourconcentration, due to polymer chain relaxationinduced by the swelling and plasticizing effectscaused by the permant molecules.8,11,15 Consideringthis, the slightly higher P and D values deter-mined here seem reasonable, but further studies


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Table 2. Permeant vapour pressures and corresponding vapour concentrations at eachcondition evaluated

25C 30C 35C

Permeant Singlea

Mixtureb

Singlea

Singlea

Ethyl butyrate Vapour pressure (Pa) 2330c 1165d 3020c 3870c

Vapour concentration (mg/ml) 109e 55e 139e 176e

Limonene Vapour pressure (Pa) 382c 191d 472c 567c

Vapour concentration (mg/ml) 21e 11e f f

a Single compound studies.b Binary mixture studies.c Value taken from Ref. 13.d Value estimated from: Pa = P0X(Pa, apparent permeant vapour pressure; P0, permeant full vapour pressure at that temperature; X, per-meant mole fraction).e Value converted from the corresponding vapour pressure through the ideal gas law: PV= nRT.fStudy not performed.

Table 3. Mass transfer parameters determined for limonene (21mg/ml) and ethyl butyrate(109mg/ml) from single compound study at 25C and 0% RH through films A (EVOH barrier),

B (nylon barrier) and C (no flavour barrier)

Permeant Film Pa (kgm/sm2 Pa) Db (m2/s) Sc (kg/m3 Pa)

A 1.94E-15 3.47E-14 5.58E-02Limonene B 2.11E-15 3.57E-14 5.91E-02

C 1.21E-13 3.78E-13 3.23E-01A 1.08E-16 4.85E-14 2.22E-03

Ethyl butyrate B 1.07E-16 5.54E-14 1.94E-03C 1.30E-14 5.58E-13 2.32E-02

a Permeability coefficient, determined from equation P= dm l/dt A Dp.b Diffusion coefficient, determined from equation D = l2/7.199t0.5.c Solubility coefficient, calculated from P= DS.


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are needed to verify the permeant concentrationeffect.

In terms of limonene, the barrier performance offilms A and B was similar and superior to film C,as indicated by the permeability coefficients (the Pvalues) determined (Table 3). By further comparingthe S and D values, it was found that both of thesevalues determined for film C were higher thanthose determined for films A and B, resulting in themuch higher P value determined for film C. Theoverall higher permeability of limonene across film

C can be attributed to its high solubility as well ashigh diffusivity in film C. In other words, both thedissolution and the diffusion processes in thebarrier layer (EVOH or nylon) are the rate-limitingprocesses for the permeation of limonene mole-cules through films A and B, leading to the overalllower permeability of limonene in these two films(Table 3). However, it should be noted that thesolubility coefficients obtained in this study couldonly be used for a rough comparison. This is dueto the indirect method through which they weredetermined. Adirect method such as sorption mea-

surement should be carried out for verification.

Permeability of ethyl butyrate throughindividual test films

Permeation curves obtained for ethyl butyrate alsoexhibited nearly ideal permeation processes. Aver-aged mass transfer parameters determined forindividual test films are listed in Table 3. Similarly,

the barrier properties of films A and B were com-parable and were superior to film C (Table 3).

When comparing the permeation data obtainedfor limonene and ethyl butyrate, it was foundthat the permeability coefficients determined forlimonene were greater than those for ethylbutyrate for each film tested. By further compar-ing the S and D values, it could be found that theoverall higher P values determined for limonenewere attributed to the higher S values, despitethe slightly lower D values determined (Table 3),

which indicates that solubility made a greater con-tribution than diffusivity on the permeability ofthese two compounds in individual test films.

A clearer comparison of the barrier performanceof these test films to ethyl butyrate and limonenewas obtained by graphically comparing the Pvalues obtained, as shown in Figure 4, from whichthe difference in barrier performance betweenfilms with or without a flavour barrier layer (filmsA and B vs. film C) is observed. This also explainswhy polyolefin films are seldom used alone aspackaging materials. Instead, packaging films with

enhanced barrier performance obtained by metal-lization or lamination with flavour barriers aregenerally used.16

Effect of relative humidity on barrierperformance of film A (EVOH barrier)

Effect of environmental moisture content onbarrier performance of film Awas evaluated, using



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Table 4. Comparison of permeation data determined forselected limonene/HDPE film systems determined by different

research groups

Film P(kgm/sm2 Pa 10-16) D (m2/s 10-16) S (kg/m3 Pa)

HDPEa 1210 3780 0.32b

HDPEc 185d 430 0.43HDPEe 160 370 0.43b

a Data from the present study, determined at limonene vapour concentration of 21 mg/mlat 25C.b S value, calculated from the directly determined Pand D values.c Data from Ref. 8, determined at limonene vapour concentration of 1.5mg/ml at 23C.d Pvalue, calculated from the directly determined D and S values.e Data from Ref. 14, determined at limonene vapour concentration of 0.719mg/ml at 25C.


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ethyl butyrate at 25C and 75% RH. The resultswere compared with those determined at 25C and0% RH, and are listed in Table 5. As can be seen,the permeability of ethyl butyrate through film A

increased with the increase of relative humidity(from 0% to 75% RH).

The results from the present study serve to showthe effect of environmental (outside) moisturecontent on the barrier performance of polymer

films containing a hydrophilic layer (such asEVOH). The observed diminished barrier perfor-mance of film A at higher relative humidity (75%)was attributed to the plasticizing effect of theabsorbed water on the hydrophilic polymer layers,leading to a decreased cohesive force betweenpolymer chains and an increased segmental mobil-ity.15,17,18 However, the effects of package interiormoisture content and initial water activity of thepolymer film were not examined in the presentstudy. Additional work is required for a betterunderstanding of the mechanisms involved.

Effect of temperature on barrierperformance of film A (EVOH barrier)

The effect of temperature on flavour barrierperformance of film A was evaluated with ethylbutyrate at three different temperatures (25C,30C and 35C) under dry conditions (0% RH).Permeation data are listed in Table 5; it was foundthat the P value increased with an increase intemperature.

In general, as temperature increases, the solubil-ity decreases while diffusivity increases; the result-ing permeability may increase or decrease,depending on whether sorption or diffusion is thepredominant factor.19 Within the temperature

range evaluated in the present study (2535C),an increase in temperature mainly benefited thediffusion process, as suggested by the increasedpermeability observed at higher temperatures(Table 5).


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AB

C

0.0E+00

2.0E-14

4.0E-14

6.0E-14

8.0E-14

1.0E-13

1.2E-13

1.4E-13

Permeabilityco

efficient

(Kgm/sm

2Pa)

Test film

ethyl butyrate

limonene

Figure 4. Comparison of permeability coefficientsdetermined for limonene (21mg/mL) and ethyl butyrate(109mg/ml) as single permeants through films A (EVOHbarrier), B (nylon barrier) and C (no flavour barrier) at

25C and 0% RH.

Table 5. Mass transfer parameters determined for ethyl butyrate through film A (EVOHbarrier) at 75% RH and 25C (conc.EB = 109mg/ml), 0% RH and 25C (conc.EB = 109mg/ml), 0%

RH and 30C (conc.EB = 139mg/ml) or 0% RH and 35C (conc.EB = 176mg/ml)

Relative humidity (%) temperature (C) Pa (kgm/sm2 Pa) Db (m2/s) Sc (kg/m3 Pa)

75 25 2.28E-16 9.00E-14 2.54E-030 25 1.08E-16 4.85E-14 2.22E-030 30 1.63E-16 9.925E-14 1.64E-030 35 1.98E-16 1.32E-13 1.50E-03

a Permeability coefficient, determined from equation P= dm l/dt A Dp.b Diffusion coefficient, determined from equation D = l2/7.199t0.5.c Solubility coefficient, calculated from P= DS.


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0.00

2.00

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6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

0 500 1000 1500 2000 2500 3000

Time (min)

Vaporpressure-normalized

permeationrate(Kg/sx10

-15)

ethyl butyrate_Mixture limonene_Mixture

ethyl butyrate_Single limonene_Single

Figure 5. Comparison of permeation profiles obtained for limonene andethyl butyrate as single permeants or as components in a binary

mixture (Conc.EB,single = 109mg/ml; Conc.EB,mixture = 55mg/ml; Conc.Limo,single= 21mg/ml; Conc.Limo,mixture = 11mg/ml) through film A (EVOH barrier)

at 25C and 0% RH.

Binary Mixture Permeability Studies

The binary mixture studies were conducted at25C and 0% RH, and the mixture was preparedby mixing pure limonene and ethyl butyrate at1:1 mole ratio.

Permeability of limonene and ethylbutyrate as co-permeants through film

A (EVOH barrier)

Permeation curves obtained for ethyl butyrate andlimonene as single permeants and as componentsof the binary mixture were compared (Figure 5).Since individual permeant vapour concentrationsin the mixture are different from those present asa single permeant, pressure-normalized perme-

ation rates were used here, which was more appro-priate for comparing data obtained at differentvapour concentrations, since the actual permeationdriving force was thereby taken into account. Masstransfer parameters determined for individualcompounds were compared with those deter-mined from single compound studies and arelisted in Table 6.

Results showed that the pressure-normalizedequilibrium permeation rate, and hence the per-

meability coefficient, of limonene increased onlyslightly in the presence of ethyl butyrate comparedwith that obtained from a single-compound study(Figure 5, Table 6). With respect to ethyl butyrate,the presence of co-permeant limonene had agreater influence on its permeability through film

A, reflected in an increase by a factor of two inthe permeability coefficient (Figure 5, Table 6). Theobserved greater impact of limonene on thepermeation process of their corresponding co-permeant may be due to the high solubility oflimonene in the HDPE layers, resulting in somestructural changes in the polymer. Consequently,the diffusivity, and hence the permeability, of theco-permeant molecules through the polymer filmwere enhanced.

However, permeation of organic vapourmixtures through multilayer polymer films is a

complex process. Both solubility and diffusivity ofa permant may be affected by the presence of otherinteracting co-permants. Further investigation isneeded to understand better the mechanismsinvolved and to describe the phenomena mathe-matically. In addition, lower permeant concentra-tions were used in the binary mixture studies, andit has been shown in the literature that permeantconcentration has an effect on the permeation datadetermined.8,11,15 Therefore, additional studies are


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Table 6. Mass transfer parameters determined for ethyl butyrate and limonene as singlepermeants (conc.EB = 109mg/ml; conc.Liom = 21mg/ml) or as components in a binary mixture

(conc.EB = 55mg/ml; conc.Limo = 11mg/ml) through films A (EVOH barrier) and C (HDPE) at 25Cand 0% RH

Pb

(kgm/sm2

Pa) Dc

(m2

/s) Sd

(kg/m3

Pa)

film compounda singlee mixturef singlee mixturef singlee mixturef

A EB 1.08E-16 2.45E-16 4.85E-14 7.16E-14 2.22E-03 3.43E-03Limo 1.94E-15 1.90E-15 3.47E-14 4.63E-14 5.58E-02 4.11E-02

C EB 1.30E-14 3.10E-14 5.58E-13 4.28E-13 2.32E-02 7.22E-02Limo 1.21E-13 1.49E-13 3.78E-13 2.81E-13 3.23E-01 5.31E-01

a EB = ethyl butyrate; Limo = limonene.b Permeability coefficient, determined from equation P= dm l/dt A Dp.c Diffusion coefficient, determined from equation D = l2/7.199t0.5.d Solubility coefficient, calculated from P= DS.e Data determined from single compound studies.fDater determined from binary mixture studies.

required to verify these findings and elucidate thephenomena observed.

Permeability of limonene and ethylbutyrate as co-permeants through film

C (no flavour barrier)

The presence of ethyl butyrate also did not affectthe permeation process of limonene through filmC, while the permeability coefficient of ethylbutyrate increased in the presence of limonene(Table 6). Again, these results might be due to thehigh solubility of limonene in the HDPE film, asexplained above.

Similar findings were reported by Hensley et al.20

The authors investigated the permeability of ethylacetate and limonene, present in form of a binary

mixture, through biaxially orientated polypropy-lene (BOPP) film, and found that only at highenough vapour activity (0.48) did the presence ofethyl acetate affect the permeation of limonene.However, at vapour activities as low as 0.1, thepresence of limonene significantly increased thepermeation rate of ethyl acetate.

Despite these limitations, results from the binarymixture studies served to show the potential ofusing this system for simultaneous measurement

of permeability of multiple volatile flavour com-pounds through polymeric materials.

CONCLUSIONS

The general applicability of this measurementsystem, as well as its potential for permeabilitymeasurement of an aroma compound mixture, wasdemonstrated in the present study. The resultsfrom this study showed that the flavour barrierperformance of a plastic film was determined bythe composition of the polymer and was affectedby various factors, such as temperature, relativehumidity and presence of a co-permeant.

Simplicity, speed and accuracy are some ofthe attractive features of this new measurement

system. The capability for providing rapid mea-surement is especially desirable to the food indus-try for selection of suitable packing materials. Inaddition, the automatic devices used in this systemallow for continuous on-line sampling and mea-suring, which is another feature that is generallynot attainable with other systems described in theliterature.

Even though relatively high permeant vapourconcentrations were used in the current study


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for the purpose of achieving system evaluationrapidly, lower permeant vapour concentrationscan be obtained by dissolving appropriateamounts of the permeant in a proper non-volatilesolvent, such as polyethylene glycol. Mixtures ofmultiple aroma compounds with desired vapourconcentrations also can be prepared in this way. Inaddition, by coupling gas chromatography withmass spectrometry (GCMS), its capability forsimultaneous measurement of the permeability ofmultiple aroma compounds can be furtherenhanced.

ACKNOWLEDGEMENTS

We acknowledge the Kellogg Company (Battlecreek,MI) for providing financial support (Award No. 01-141)for this project. We are grateful to DuPont DowElastomers for the generous donation of the Kalrez

O-rings for this study.

REFERENCES

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Hernandez R. Packag. Technol. Sci. 1996; 9: 215224.4. Hernandez RJ and Giacin JR. Factors affecting per-

meation, sorption, and migration processes inpackage-product systems. In Food Storage Stability,Taub IA, Singh RP (eds). CRC Press: Boca Raton, FL,1998; 269329.

5. Miller KS and Krochta JM. Trends Food Sci. Technol.1997; 8: 228237.

6. Risch SJ. Flavour and package interactions. InFlavour Chemistry Industrial and Academic Research,Risch SJ, Ho C-T (eds). ACS Symposium Series 756.American Chemical Society: Washington, DC, 2000;94100.

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Miltz J. J. Food Sci. 1988; 53(1): 253257.9. Hernandez RJ, Giacin JR and Baner AL. J. Plast. Film

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13. Hodgman CD, Weast RC and Selby SM (eds).Vapour pressure: vapour pressure of organic com-pounds. In Handbook of Chemistry and Physics, 41stedn. Chemical Rubber Publishing: Cleveland, OH,1959; 23632424.

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16. Brown WE. Barrier design. In Plastics in Food Packag-ing Properties, Design, and Fabrication, HughesHA (ed.). Marcel Dekker: New York, 1992; 292357.

17. Gerlowski LE. Water transport through polymers requirements and designs in food packaging. InBarrier Polymers and Structures, Koros WJ (ed.). ACSSymposium Series 423. American Chemical Society:Washington, DC, 1990; 177191.

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19. Leufven A, Johansson F and Hermansson C. Theinfluence of food and external parameters on thesorption of aroma compounds in food contact poly-

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