characterization of moulded-fibre packaging with respect to water vapour sorption and permeation at...

PACKAGING TECHNOLOGY AND SCIENCEPackag. Technol. Sci. 2005; 18: 59–69Published online 29 November 2004 in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/pts.673

Characterization of Moulded-fibre Packagingwith Respect to Water Vapour Sorption andPermeation at Different Combinations ofInternal and External Humidity

Gitte Sørensen1,2 and Jens Risbo2

1 Hartmann A/S, Klampenborgvej 203, DK-2800 Lyngby, Denmark2 The Royal Veterinary and Agricultural University, Department of Dairy and Food Science, Rolighedsvej 30,DK-1958 Frederiksberg C, Denmark

A moulded-fibre packaging system was characterized under conditions simulatingreal-life packaging of food. A steady-state moisture flux through the moulded-fibrepackaging was generated by subjecting the system to different combinations ofinternal humidity [33–97% r.h. (0.33–0.97aw of contents), RH(i)] and surroundinghumidity [33–97% r.h., RH(e)]. The objective was to resolve whether a hygroscopicfibre material absorbs moisture proportional to the rate of moisture transport, andthe moulded-fibre material was thus characterized with respect to accumulation ofmoisture in the fibre material, water vapour transmission rate (WVTR) andpermeability (k/x). These steady-state properties showed significant asymmetrydepending on direction of moisture transport. When moisture was transported outof the system [RH(i) > RH(e)] the fibre material adsorbed moisture to aconsiderable lesser extent compared to when moisture was transported into thesystem [RH(i) < RH(e)], just as (k/x) increased by 15–20%. Taking both directions ofmoisture transport into account, the moisture content of the fibre materialdepended largely on surrounding humidity, even at high internal humidity.Moisture contents ranged from 5.5g/100g dry fibre at RH(e) 33% r.h. to 16.4–25.1g/100g dry fibre at RH(e) 97% r.h. The observed asymmetry was shown toderive from the experimental set-up and not from the material itself. A minimaltheory based on the various transport steps in the experimental set-up wasproposed in order to qualitatively explain this asymmetry. The rate of moistureadsorption in moulded-fibre was described by the normalized response functionH(t). Response times to reach equilibrium moisture contents were 6 and 8h forRH(e) 33 and 53% r.h., and 40 and 41h for RH(e) 75 and 97% r.h. Copyright © 2004John Wiley & Sons, Ltd.Received 5 January 2004; Revised 13 May 2004; Accepted 1 October 2004

KEY WORDS: paper-fibre packaging; moulded fibre; moisture sorption; water vapourtransmission rate; water vapour permeability; food packaging

* Correspondence to: G. Sørensen, Hartmann A/S, Klampenborgvej 203, DK-2800 Lyngby, Denmark.Email: [email protected]/grant sponsor: Danish Academy of Technical Sciences

Copyright © 2004 John Wiley & Sons, Ltd.

INTRODUCTIONPaper-fibre materials have a long history as pack-aging for food, e.g. paper bags for dry foods(cereal, sugar), greaseproof/glassine paper forfatty foods (butter, margarine), Kraft paper used incarton board for ready meals, etc. Paper fibres arevery absorptive to liquids such as water, greaseand oil, etc., and usually the fibre materials havebeen improved with regard to wet-strength, greaseand oil resistance in order to function convincinglyas packaging for food. Another parameter affect-ing packaging performance significantly is watervapour adsorption and permeation in the fibrematerial when considering the entire lifespan of afood packaged in a paper-fibre material, i.e. frommanufacture of, for example, a paper-fibre tray,packaging of food at the food producer, transportand storage at the retail market, and final usage bythe consumer. Food products often exhibit a dif-ferent water activity (aw) compared to humidity of the surrounding air. As food aw is related to percentage relative humidity (RH) by aw =%RH/100%, the difference in RH causes a gradi-ent in partial pressure of water vapour across thefibre packaging material, and consequently a mois-ture flux. The combined effect of humidity insidea fibre packaging [internal (RH(i) determined byfood aw = RH(i)/100] and humidity of surroundingair [external RH(e)] can be mapped into a two-dimensional RH(i)/RH(e) plane, as illustrated inFigure 1. The plane is divided into two halves bythe equilibrium diagonal, where no driving forceexits for moisture permeation. At this line thepackaging materials adsorb moisture according tothe equilibrium moisture–sorption isotherm. Oneither side of the diagonal the material will be subjected to non-equilibrium conditions and thesteady-state properties are functions of both vari-ables, RH(i) and RH(e). Moisture loss (watervapour is transported out of the package) is theprevalent transport process when semi-moist andmoist foods are subjected to normal conditions of environmental humidity (Figure 1 lower left).Only for dry foods or semi-moist foods at humidconditions will the moisture flux favour moisturegain in the food (water vapour is transported intothe package, as seen in Figure 1, upper right).

It is generally recognized that loss or gain ofmoisture plays a critical role in deciding thequality and shelf-life of food with regard to micro-

biological, chemical and physical stability.1 In thesame manner the various combinations of food aw,in the following termed ‘internal humidity’ andsurrounding ‘external humidity’, can influencepackaging performance through different degreesof moisture adsorption into the fibre materials, asboth the permeability and mechanical propertiesof hygroscopic materials are substantially affectedby moisture. The moisture sorption-inducedeffects on mechanical properties include increasedcreep, hygroexpansion and decreased tensile,burst, bending and compression strengths.2–6 It isalmost impossible to hinder the transport of watervapour in a porous, hygroscopic medium such aspaper fibre except, for example, by introducing afunctional vapour barrier between the paper-fibrematerial and the source of moisture. This has ledto the well-known marriage between paper-fibrepackaging materials and plastic coatings or filmswith satisfactory water vapour permeability. Infood packaging the barrier material is usuallyintended to control the transfer of moisturebetween surrounding air and headspace inside thepackaging. The main focus for the food scientist is to optimize the barrier permeability in order toavoid undesirable quality changes in food. If thecritical decrease of mechanical strength due toincreased moisture content of paper-fibre packag-ing is considered, it is equally important to

G. SØRENSEN AND J. RISBOPackaging Technologyand Science

Copyright © 2004 John Wiley & Sons, Ltd. 60 Packag. Technol. Sci. 2005; 18, 59–69

33%

Relative humidity of surroundings [RH(e)]

Water activity, aw,

of fooda [RH(i)]

Moisture gain in food

Moisture loss from food

< 0.6 Dry foods(nuts, dehydrated cereals,milk powder,pasta, cookies, crackers)

0.6–0.92 Semi-moistfoods (cereal grains, raisin, syrups, jams, jellies, dried fruits, flour,coffee, salami)

> 0.92 Moist foods (meat, fish, fruits,vegetables, cheese,milk, bread, eggs)

53% 75% 97%

Figure 1. Moisture transport in food packaging atenvironmental relative humidities [RH(e)] and food water

activities, aw, referring to internal humidity [RH(i)],common in food packaging.a From Fennema.1

examine the effects on mechanical functionality ofthe packaging, if a large moisture flux is allowedacross the packaging material. The objective wouldbe to resolve whether the hygroscopic fibre mate-rial absorbs moisture proportional to the rate ofwater vapour transport, therefore making it neces-sary to recommend a specific vapour barrier inorder to prevent softening of the fibre packaging.

The aim of the present investigation was tostudy moisture uptake into, and moisture perme-ation through, a moulded-fibre packaging systemsubjected to a range of external and internalhumidities, thereby simulating real-life packagingof various kinds of food under different conditionswith respect to surrounding humidity and internalhumidity relating to food aw inside the tray. Amoulded-fibre tray containing food simulants withdifferent aw, sealed inside the tray, was placed insurroundings with varying relative humidities,thus creating a flux of water vapour across the fibre tray. The average moisture content of themoulded-fibre tray and the rate of water vapourtransport were determined during both unsteadyand steady-state moisture flux, just as sorptionkinetics were examined yielding the rate of watervapour adsorption in moulded-fibre packagingfollowing step changes in relative humidity of thesurroundings.

MATERIALS AND METHODS

Moulded-fibre trays

Moulded-fibre trays (rectangular shape 4 ¥ 11 ¥14cm, surface area 258cm2, volume 462ml) madefrom 50% unused, unprinted newspaper wasteand 50% new fibres (chemi-thermomechanicalpulp, CTMP) were used. Grammage was 347g/m2;all samples were afterpressed in hot tools to anaverage thickness of 1mm, giving an oven-drydensity of 0.35g/cm3.

Experimental design

The moulded-fibre trays were preconditioned at23°C and 30% r.h. for at least 48h. In that waychanges in moisture content of the moulded-fibretrays could be attributed exclusively to adsorptionof moisture.

The experimental set-up was constructed to simulate real-life packaging of food in moulded-fibre trays. Appropriate salt solutions were used tocontrol the humidity of the air surrounding the moulded-fibre tray as well as acting as foodsimulants with different aw. The aw of the food simulant would determine relative humidity of the headspace inside the moulded-fibre tray, as aw is related to percentage relative humidity (% r.h.) by aw = % r.h./100. In the following, internal humidity, RH(i), is therefore used insteadof food water activity, aw, for investigations of flux. The food simulant (salt solution, see below)was placed inside the moulded-fibre tray in a rectangular plastic tray (surface area of simulant60.5cm2). The moulded-fibre tray was then heat-sealed with an aluminium-based lid film with alow water vapour transmission rate (WVTR) (0.6g/m2/day, determined by the cup method,8 withdemineralized water at external conditions 23°Cand 30% r.h.) to ensure that vapour transportthrough the fibre material was the predominanttransport process. For investigations of sorptionrate, empty moulded-fibre trays without lids wereused.

All experiments were performed at ambienttemperature [25°C ± 0.8°C in a glass climaticcabinet (0.4 ¥ 0.4 ¥ 0.6m) (Figure 2), for differentcombinations of humidity of the surrounding air(RH(e))] and internal package humidity/wateractivity of the food simulant [RH(i)] (triplicatedeterminations). The humidity of the surroundingair was controlled by saturated salt solutions (see below), agitated by magnetic stirrers. Themoulded-fibre tray was placed inside the cabinetand weight changes of the moulded-fibre packaging system monitored continuously (every5min) for a minimum of 70h with a balance (0.001g) connected to a personal computer forautomatic data acquisition. During the measure-ments, the temperature and humidity of the surrounding air were monitored by a Testo 175-H2temperature (± 0.5°C) and humidity (± 3.0%)logger (Testo GmbH, Germany). In the climaticcabinet, the moulded-fibre tray was assumed to be exposed to a constant humidity immediatelyafter transfer from preconditioning at 30% r.h. The small decrease in surrounding air humidity from opening of the cabinet quickly restored itselfand was expected not to influence moistureadsorption.


WATER VAPOUR SORPTION/PERMEATION OF MOULDED-FIBRE Packaging Technologyand Science

Salt solutions

All salt solutions were made from reagents of analytical reagent grade. Demineralized water wasused throughout. The salt solutions were preparedas supersaturated salt slurries in order to ensurethat the solutions would maintain the expectedconstant water activity throughout the experi-ments. The salt solutions were prepared corre-sponding to aw 0.33 (MgCl2·6H20), 0.53[Mg(NO3)2·6H2O], 0.75 (NaCl), and 0.97 (K2SO4) at25°C.9

Moisture content

The moisture content of moulded-fibre trays wasdetermined as average moisture content (AMC)when a steady-state flux was established across thefibre material. Immediately after suspension of thecontinuous data acquisition, the aluminium filmlid and inner plastic tray were removed. Moisturecontent was then determined by oven drying at105°C for 24h in a laboratory oven.

Theory and data analyses

Water-vapour transmission rate (WVTR). A fluxof water vapour across a packaging material is

created when a gradient in partial pressure ofwater vapour or relative humidity exists. Thesteady-state flux of moisture, J (g/m2/day), andthe partial pressure gradient (p1 - p2), is relatedthrough:

(1)

where k/x is the permeance (g/m2/day/mmHg) of the packaging material, W is the steady-stateweight loss rate (g moisture/day) of water vapourthrough the paper material at specified conditionsof humidity and temperature, A is surface area ofthe fibre tray (m2) and x is thickness of the mater-ial. The total flux, J, or WVTR was found as slopeof the steady-state (linear) part of the weight loss(or weight gain) vs. time curve, divided by traysurface area, A. The water vapour permeance, k/x,was determined as water vapour transmission rate(WVTR) corrected for the vapour pressure differ-ence (mmHg), (p1 - p2) according to equation (1).For simple materials, where moisture transport iswell described by Fickian diffusion, such as manypolymer materials, k/x is expected to be constantand to depend only on temperature. For stronglyhygroscopic materials, such as paper-fibre materi-als, k/x may depend on the steady-state moisturecontent and thus on both the internal and externalhumidity. k/x can in this case be considered as aneffective permeance.

Sorption kinetics. When a sample of paper-fibrematerial preconditioned at given humidity is sub-jected to a step-change in humidity, the materialwill adsorb or desorp moisture until a new equi-librium situation can be attained. The equilibriummoisture content corresponds to the informationcontained in the moisture–sorption isotherm.6 Thetime development of this equilibration process canbe described through the step-response function,H(t), which is defined through the relation:

(2)

where m(ti) - m(t0) is the change in moisturecontent (dry basis) at time, ti, and m(t•) - m(t0) isthe total change in moisture content, when themoulded fibre has reached equilibrium at a givenrelative humidity of the surroundings.10 If theadsorption process is governed by linear dynam-ics, such as Fick’s law of diffusion, the step-

H tm t m tm t m t

i( ) =( ) - ( )( ) - ( )•

0

0

JWA

kx

p p= = ÈÎÍ

˘˚̇

-( )1 2 ,



Weight balance 0.001 g

Surrounding air

Moulded fibre tray

Internal saturatedsalt solution, RH(i)

Headspace humidity

Aluminium lid film

•Magnetic stirrer

Glass cabinet

External saturatedsalt solution, RH(e)

Figure 2. Experimental set-up used for investigation ofmoisture transport in moulded-fibre trays simulating real-life packaging of food. Appropriate salt solutions acting asfood simulants determining internal humidity, RH(i), were

sealed inside the tray.

response function is independent of the step sizein humidity. A possible change in adsorptionmechanism will show up as change in the appear-ance and time-scale of H(t).

RESULTS

Steady-state moisture content

When moulded-fibre trays are subjected to differ-ent humidities on the inner and outer sides, mois-ture contents of the trays are not given by themoisture–sorption isotherm, as the material is notin equilibrium. One of the main tasks of this studywas to describe the accumulation of moisture inmoulded-fibre trays in such non-equilibrium situations, when a steady-state vapour flux wascreated across the moulded-fibre material. Theresults of these measurements are given in Table 1.The equilibrium moisture content (EMC) constitut-ing the moisture isotherm for the moulded-fibretray is found in the diagonal, where humidity is thesame on both sides of the packaging (enhancednumbers) and the off-diagonal numbers corre-spond to steady-state average moisture contents(AMC) under non-equilibrium situations. The‘moisture matrix’ shown in Table 1 is highly asym-

metric, as the material adsorbed moisture to aminor extent when moisture was transported out ofthe package as compared to when moisture wastransported into the package. When moisture wastransported out of the package, the moisturecontent was independent of internal humidity andinstead controlled by external humidity, accordingto the isotherm values. Likewise, when moisturewas transported into the package, the moisturecontent was predominantly dependent on externalhumidity but also on the internal humidity, whichwas able to lower the moisture content somewhatwhen compared to the isotherm values. Taking bothdirections of transport into account, it was foundthat relative humidity of the surroundings (RH(e))was decisive for the steady-state average moisturecontent and accounted for more than 95% of the significant effect of humidity (p < 0.05). IncreasingRH(e) from 33% to 97% r.h. significantly increasedAMC (p < 0.05), from average 5.5 ± 0.3g/100g dryfibre at 33% r.h. to values in the range 16.4 ± 0.3 -25.1 ± 0.5g/100g dry fibre at 97% r.h.

For moulded fibre, it was investigated whetherthe observed asymmetry originated from the fibrematerial itself, as moulded fibre during productionhas a build-in asymmetry in fibre orientation.Paper fibres are sucked onto a vacuum-tool in athree-dimensional geometry, and the fibres willnaturally orientate themselves around the suction-



Table 1.Average moisture content (AMC) in g/100g dry fibre ofmoulded fibre trays subjected to a moisture flux across the fibrematrix created by relative humidity of the surroundings [RH(e)]

and relative humidity inside the tray [RH(i)]

% Relative humidity% Relative humidity of surrounding air [RH(e)]

in headspace [RH(i)] 33 53 75 97

Moisture flux from surroundings to package¨——————

33 5.2 (0.3)a 5.8 (0.2)ab 8.8 (0.6)c 16.4 (0.3)f

53 5.5 (0.1)a 6.5 (0.3)b 9.9 (0.3)d 18.7 (0.7)g

75 5.6 (0.1)a 6.5 (0.1)b 10.7 (0.2)e 21.3 (0.5)h

97 5.7 (0.2)a 6.8 (0.4)b 11.1 (0.3)e 25.1 (0.5)i

——————ÆMoisture flux from package to surroundings

Number in bold type correspond to the moisture sorption isotherm. Numbers in brack-ets are standard deviation. AMC with same lettera–i are not significantly different (p < 0.05).

holes, but otherwise quite randomly in the fibrematrix. Likewise, the surface of the moulded-fibretray is smoother on the side that is in contact withthe suction-tool (the net-side). In a diffusion cupexperiment (data not shown) with a given largehumidity gradient across the fibre material, mois-ture content of the samples did not differ signifi-cantly (p < 0.05) with regard to effect of orientationof the fibre matrix (surfaces of the fibre sampleswere interchanged between inside and outside ofthe cup), but showed the same dependency ofdirection of flux, and thus external humidity, asdescribed above. The observed asymmetry ofmoisture sorption must hence be accounted for inthe experimental set-up and will be discussedfurther below.

Steady-state moisture transport

Another parameter describing the steady-stateproperties of moulded-fibre materials under non-equilibrium situations is the moisture flux, J, orwater vapour transmission rate (WVTR), which isshown for different combinations of external andinternal humidities in Table 2. As expected fromequation (1), the moisture transport is governed bythe internal and external humidity of the packag-

ing system. The vapour flux in Table 2 is symbol-ized by 0, when no driving force existed, by posi-tive WVTR values for moisture flux fromsurroundings into the package, and by negativeWVTR values for moisture flux from the packageto the surroundings. As for average moisturecontent of the fibre tray, WVTR was also influencedby the direction of the flux and behaved asym-metrically when mirrored around the diagonal‘zero flux’ axes. Generally, moisture gain in thepackaging system generated significantly largerwater vapour transmission rates than loss of mois-ture (p < 0.05), when comparing WVTR of packag-ing systems with opposite internal and externalhumidities, e.g. WVTR was 58.9 ± 4.5g/m2/day atRH(i) 75% r.h. and RH(e) 33% r.h., whereas WVTRat RH(i) 33% r.h. and RH(e) 75% r.h. increased to83.7 ± 4.3g/m2/day. In order to investigate thisasymmetry further, and also to separate the trivialeffect of the driving force (p1 - p2) from a potentialnon-trivial effect originating from the paper mate-rial, the effective permeability, (k/x) was calculatedaccording to equation (1) (Table 3). k/x formoulded fibre was found to behave somewhatirregularly but with the same asymmetry as forWVTR when mirrored around the diagonal. Thepermeability was found to increase overall by15–20% when moisture was transported into the



Table 2.Water vapour transmission rate,WVTR, in g/m2/day formoulded fibre trays with a moisture flux across the fibre matrix

created by relative humidity of the surroundings [RH(e)] andrelative humidity inside the tray [RH(i)]




33 0 25.4 (0.1)a 83.7 (4.3)d 119.0 (7.2)f

53 -32.7 (1.5)ab 0 39.2 (1.7)be 71.4 (7.6)g

75 -58.9 (4.5)c -47.9 (0.6)e 0 41.6 (1.9)e

97 -84.8 (1.1)d -62.1 (2.1)c -25.9 (3.8)a 0——————Æ

Moisture flux from package to surroundings

Negative WVTR denotes moisture loss from the packaging system, and positive WVTRmoisture gain by the system. Numbers in brackets are standard deviation. AMC with samelettera–g are not significantly different (p < 0.05).

package compared to moisture transport out of thepackage. However, variation of the permeabilitywas much smaller compared to variation of thesteady-state moisture content.

Sorption kinetics

The dynamic uptake of moisture at unsteady-stateconditions was examined by the time dependenceof moisture adsorption in a situation where inter-nal and external humidity were the same. The normalized moisture gain curve, H(t), was constructed for moulded-fibre trays experiencing astep change in RH(e) from 30% r.h. to humidities33%, 53%, 75% and 97% r.h., respectively (Figure 3).The rate of adsorption [slope of H(t)] was largestimmediately after the step change in humidity,after which the rate of moisture gain decreased andthe fibre tray reached equilibrium moisture content(EMC). This is a consequence of the decrease indriving force when the humidity of the fibre trayand the surrounding air equilibrate. The experi-mental set-up is similar to the construction ofisotherms, and H(t) is thus the dynamic informa-tion derived from isotherm experiments.10

H(t) for the different step-changes in humidityshowed very different response times, i.e. the timeto reach EMC. A larger step in humidity resulted

in a slower adsorption rate and a longer time toreach equilibrium, e.g. from 30% to 97% r.h., thana small step in humidity, e.g. from 30% to 33% r.h.,i.e. the more moisture the paper fibres are able toabsorb, the longer it takes. Consequently, theresponse times were 6 and 8h for 33% and 53% r.h.(EMC 5.2 and 6.5g/100g dry fibre), respectively,but 40 and 41h for 75% and 97% (EMC 10.7 and25.1g/100g/dry fibre), respectively.

Comparison of the sorption curves in Figure 3with the unsteady part of the moisture gain/losscurves from determination of WVTR showed analmost similar unsteady-state progress, except thatfor WVTR a moisture flux was generated acrossthe fibre material from the beginning. Allmoulded-fibre samples were preconditioned at30% r.h. and therefore susceptible to adsorption of moisture at the selected external humidities[RH(e)]. From WVTR determination it wasobserved that if steady-state AMC was close to theinitial AMC of the fibre tray after preconditioning,steady-state flux would set in almost immediately(within 1h); see e.g. Figure 4a for loss of moisturefrom the packaging system at RH(e) 33% r.h. Incontrast, if steady-state AMC increased greatlycompared to initial AMC, steady-state flux wasachieved much later (after >15h), e.g. see Figure 4bfor moisture gain in the packaging system at RH(e)97% r.h. This could suggest that the dominating



Table 3.The material permeability,WVP or k/x, in g/m2/d/mmHgfor moulded-fibre trays with a moisture flux across the fibre

matrix created by relative humidity of the surroundings [RH(e)],and relative humidity inside the tray [RH(i)]




33 – 7.2 (0.1)bcd 8.5 (0.4)a 8.1 (0.5)ac

53 8.0 (0.3)ac – 7.2 (0.3)bcd 7.2 (1.0)cd

75 6.4 (0.4)de 8.2 (0.1)ab – 8.4 (0.7)a

97 6.0 (0.1)e 6.0 (0.3)e 5.7 (0.8)e –——————Æ

Moisture flux from package to surroundings

Numbers in brackets are standard deviation. k/x with the same lettera–e are not significantlydifferent (p < 0.05).

transport process from the start was moisture sorp-tion in the very hygroscopic moulded-fibre mater-ial relative to both the initial AMC of the tray[RH(e)] and RH(i), before the water vapourdriving force was allowed to take over and aregular moisture flux was established. In Figure 4cthis is implied for moisture loss of the packagingsystem at RH(e) 75% r.h and RH(i) 97% r.h. Atunsteady state the packaging system (dotted line)actually started by gaining moisture for 3.5h, andthereby followed the moisture adsorption curvefor RH(e) 75% r.h. (solid line), even though avapour gradient existed between RH(e) and RH(i)favouring moisture loss. Only after 3.5h did thepackaging system begin to lose moisture. AMC ofthe moulded-fibre tray subjected to moisture fluxin Figure 4c was not significantly different fromisotherm moisture content (Table 1). The peak inmoisture change, however, did not reach the mois-ture change of the tray approaching equilibriummoisture content. It is therefore still uncertainwhether the tray at moisture flux adsorbed mois-ture fully until it attained steady-state AMC beforemoisture flux took over, or whether AMC wasattained only during the given period of moistureflux. With the experimental set-up used in thisstudy, it was not possible to distinguish betweenmoisture change in the entire packaging system(moulded-fibre tray + food simulant) and moisturechange in the moulded-fibre tray. If response times

for adsorption from H(t) were assumed, the traywould adsorb moisture for approximately 40hbefore reaching AMC.

DISCUSSION

When moulded-fibre trays are used for food pack-aging in real life, it is optimal that humidity of thepackaged food and the surrounding environment



Time in hours

0 10 20 30 40

H(t

)

0.0

0.2

0.4

0.6

0.8

1.0

RH(e) 33% r.h.RH(e) 53% r.h.RH(e) 75% r.h.RH(e) 97% r.h.

Figure 3. Normalized moisture gain curve [H(t)], formoulded-fibre trays experiencing a step change in

humidity from preconditioning at 30% r.h. to differentsurrounding humidities [RH(e)].

Time in hours

0 10 20 30 40 50 60 70

–6

–4

–2

0

2

Adsorption at RH(e) 33% r.h.Moisture flux at RH(e) 33% r.h. and RH(i) 53% rhMoisture flux at RH(e) 33% r.h. and RH(i) 75% rhMoisture flux at RH(e) 33% r.h. and RH(i) 97% rh

*5.2

*5.5

*5.6

*5.7

a.

Time in hours

0 10 20 30 40 50 60 70

0

2

4

6

8

10Adsorption at RH(e) 97% r.h.Moisture flux at RH(e) 97% r.h. and RH(i) 33% r.h.Moisture flux at RH(e) 97% r.h. and RH(i) 53% r.h.Moisture flux at RH(e) 97% r.h. and RH(i) 75% r.h.

*16.4

*18.7

*21.3

*25.1

b.

Time in hours

0 10 20 30 40 50 60 70

Moi

stur

e ch

ange

in g

Moi

stur

e ch

ange

in g

Moi

stur

e ch

ange

in g

–1.5

–1.0

–0.5

0.0

0.5

1.0

Adsorption at RH(e) 75% r.h.Moisture flux at RH(e) 75% r.h. and RH(i) 97% r.h.

*10.7

*11.1

c.

Figure 4. Comparison of moisture gain/loss (in g) withtime (h) for moulded-fibre packaging with no flux

[adsorption at RH(e)] or a moisture flux across the fibrematrix due to difference between RH(e) and RH(i). (a)Surrounding humidity [RH(e)], 33% r.h; (b) RH(e) 97%r.h; and (c) RH(e) 75% r.h. *AMC in g/100g dry fibre.



do not induce loss of mechanical functionality ofthe tray as result of moisture sorption in the paper-fibre material. The experimental results presentedin this paper showed that, even for moist foods, thesurrounding humidity is the major factor deter-mining moisture content of the fibre tray. Humid-ity of the surroundings is known to varyconsiderably with time of day, time of year, geo-graphically and, more importantly, when the pack-aging is shifted from one environment to another,e.g. ambient conditions at the food producer, in cold storage, during transportation, and atdisplay in the supermarket. In previous studies by Sørensen and Hoffmann,6,7 the mechanicalstrength of moulded-fibre trays was found to besignificantly affected both by increasing moisturecontent at constant humidity conditions and thecontinuing increase and decrease in moisturecontent at varying humidities in particular. Thevariations in environmental temperature andhumidity are therefore important considerations in design of the tray, in order to compensate for expected loss of mechanical strength. In thiscontext it is worth reflecting on the response timesfor moisture adsorption presented in Figure 3. Thefibre material responded rapidly to a given changein surrounding humidity [no delay/lag time inH(t)], but the time to reach EMC was rather long(almost 2 days) for large changes in humidity. Thisis slower than found in other studies of steady-state moisture sorption in plane paper-fibre mate-rials, in which equilibrium moisture content wasreached within a few hours,11–13 except in Leisen14

where moisture content only stabilized after 10h inresponse to a step change from 0% to 95% r.h. Thedifferent time scales of H(t) depending on the finalhumidity may be related to changes in the mois-ture transport mechanism. At high humidities thiswill involve an increased degree of moisture dif-fusion in the fibres and capillary condensation,which is generally considered a slower processthan moisture diffusion in the pores, which pre-vails at low humidities.11,12,15 A practical implica-tion of the large response times is that the tray thenneeds to be exposed to a high humidity environ-ment for a longer period before the weakening ofthe fibre tray by moisture adsorption will take fulleffect.

The results of the steady-state moisture contentpresented in this study showed an asymmetrywith respect to the directions of moisture trans-

port. Additional experiments showed that thepaper material itself behaved symmetrically, andthe asymmetry must therefore derive from theexperimental set-up. In order to explain this asym-metry qualitatively, a minimal theory is proposed.The theory is inspired by the permeation theoryfrom multilayer films.16 In the proposed theory the moisture transport process is divided into three sub-processes. For moisture transport intothe packaging system, these are: (1) external trans-port from the external salt solution to the paper-fibre material; (2) transport through the paper-fibrematerial; and (3) transport from the paper-fibrematerial to the internal salt solution. For moisturetransport out of the packaging system, theprocesses are reversed. The three separate trans-port processes are driven by local gradients inhumidity [RH(1–4) in Figure 5a] and each isassigned a resistance to moisture transport, R, cor-responding to reciprocal permeability (R = x/k), asdepicted in Figure 5b. The paper-fibre materialpossessed a very low vapour barrier property,which means that R2 is in the same order of mag-nitude as R1 and R3, and the three sub-processestherefore all contributed to the overall observedresistance. In the particular experimental set-upused in this study, the transport processes (1) and(3) are not equivalent, as the space outside thepackage is much greater than the space inside thepackage. It is therefore likely that R3 is greater thanR1. At steady-state transport (flux J is constant) thedifference in humidity (the driving force) is pro-portional to the resistance, i.e. a large resistancewill result in a large humidity gradient.16 As theouter resistance, R1, is expected to be smaller thanthe internal resistance, R3, the average humidity ofthe paper-fibre material [determined by RH(2) andRH(3)] is always closer to the external humidity[equal to RH(1)] than to the internal humidity[equal to RH(4)], thus giving rise to the observedasymmetry. It is worth noticing that some degreeof asymmetry is naturally found in real-life storageof food, as air in most cases circulates to a higherdegree externally than inside the food tray. Theeffective external resistance to water vapour trans-port may be fairly small, as convection may alsocontribute to the transport mechanism externally.

The typical water vapour transmission ratefound for a moulded-fibre tray was in the range25–119g/m2/day, which per tray equals a loss or gain of moisture by the food of 0.7–3.1g

water/day. This is unacceptable, especially withregard to high moisture loss, if the trays are usedfor other than disposable/throwaway packaging,where no barrier properties are needed, as storagefor just a few days would render the cereals toosoft from adsorption of moisture, or the beefwould dry out from loss of moisture.

Evidently, from the viewpoint of both food andmoulded-fibre product quality it may be necessaryto enhance the barrier properties by applying anadditional vapour barrier layer to one of the sidesof the paper-fibre tray. In this situation the perme-ation process will be characterized by an addi-tional resistance, R4, corresponding to permeationthrough the barrier layer. For a correctly chosenbarrier, this resistance will be much greater thanR1, R2 and R3 and thus the dominating differencein humidity will be over the vapour barrier layer(R4). The permeabilities of polymeric films (high-or low-density polyethylene) often used as watervapour barriers in food packaging are typically in



Surrounding air

Headspace humidity

RH(3)RH(2)

RH(1)

RH(4)

Surrounding air Headspace

Fibre matrix

RH(e) > RH(i)

Arb

itrar

y un

it

Surrounding air Headspace

Fibre matrix

RH(i) < RH(e)

R2

R3

Overall flux

••

•

••

•

R1

R2

R3

RH(e)RH(1)RH(2)RH(3)

RH(1) RH(e)

RH(i) RH(4)

RH(3)RH(2)RH(1)

R1(a)

(b)A

rbitrary unit

RH(i)

Figure 5. (a) Local humidities (RH) in the moulded-fibre packagingsystem. RH(1) is humidity corresponding to water activity of the

external salt solution simulating surrounding humidity; RH(2) and RH(3)are humidities on the outer and inner surface of the fibre tray,

respectively, and RH(4) is internal humidity corresponding to wateractivity of the food simulant inside the fibre tray. (b) Schematic

illustration showing effect of moisture transport resistances (R) on localhumidity levels (RH) (arbitrary unit). R1 is resistance to moisture

transport in surrounding air, R2 is resistance to moisture transport in the moulded-fibre matrix, R3 is resistance to moisture

transport in headspace.

the range 0.1–0.5g/mm/m2/day (at 38°C, 90%RH). This, for 25–50mm polyethylene barrier films,will give WVTRs that are up to 30 times lower thanmoulded-fibre WVTR. Consequently, the addi-tional barrier film will contribute further to asym-metry of the moisture–sorption matrix of thepackaging system and the steady-state humidity ofthe moulded-fibre tray will be very close to thehumidity of the unprotected side of the packaging.Hence, for storage of moist food in dry environ-ments, the additional barrier layer should beapplied on the internal (humid) side of the pack-aging, in order to avoid moisture loss of theproduct and at the same time secure mechanicalfunctionality of the paper-fibre material. In theopposite case, where dry food is stored in a humidenvironment, the barrier layer must be located onthe external (humid) side of the packaging. In both cases, the steady-state moisture content andmechanical functionality of the package will, to agood approximation, be governed by equilibrium

moisture sorption according to humidity of theunprotected side of the package.6

ACKNOWLEDGEMENTS

The Danish Academy of Technical Sciences supportedthis work.

REFERENCES

1. Fennema OR. Food Chemistry, 2nd edn. MarcelDekker: New York, 1985.

2. Johnson JA, Bennett KA and Montrey HM. Failurephenomena. In Handbook of Physical and MechanicalTesting of Paper and Paperboard, vol. 1. Mark RE (ed.).Marcel Dekker: New York, 1983; Ch. 5.

3. Salmen L. Responses of paper properties to changesin moisture content and temperature. In Products ofPapermaking, vol 1. Baker CF (ed.). Pira Interna-tional: Leatherhead, 1993; 369–430.

4. Kajanto I and Niskanen K. Dimensional stability. InPaper physics, Book 16 of Paper Science and Tech-nology Series, Niskanen K (ed.). Fapet: Finland,1998; Ch. 7.

5. Haslach HW Jr. The moisture and rate-dependentmechanical properties of paper: a review. Mechan.Time-depend. Mat. 2000; 4: 169–210.

6. Sørensen G and Hoffmann J. Moisture sorption inmoulded-fibre trays and effect on static compressionstrength. Packag. Technol. Sci. 2003; 16: 159–169.

7. Sørensen G and Hoffmann J. Moisture-inducedeffects on stacking strength of moulded-fibre pack-aging at varying environmental conditions. Packag.Technol. Sci. (in press).

8. ASTM E96-95. Standard Test Methods for Water VaporTransmission of Materials.American Society for Testingand Materials: Philidelphia, PA, retrieved 1999.

9. Greenspan L. Humidity fixed point of binary satu-rated aqueous solution. J. Res. Natl Bureau Stand. APhys. Chem. 1977; 81A(1): 89–96.

10. Risbo J. The dynamics of moisture migration inpackaged multi-component food systems I: shelf-life predictions for a cereal–raisin system. J. FoodEng. 2003; 58: 239–246.

11. Foss WR, Bronkhorst CA and Bennett KA. Simulta-neous Heat and Mass Transport in Paper Sheets duringMoisture Sorption from Humid Air. AIChE Sympo-sium Series, vol. 95, No. 322, 80–88.

12. Hägglund R, Westerlind B, Gulliksson M and Nordstrand T. Diffusion of Water Vapor in Paper.AIChE Symposium Series, vol. 95, No. 322, 71–79.

13. Bandyopadhyay A, Radhakrishnan H, Ramarao BVand Chatterjee SG. Moisture sorption response ofpaper subjected to ramp humidity changes: model-ing and experiments. Indust. Eng. Chem. Res. 2000;39(1): 219–226, 244.

14. Leisen J, Hojjatie B, Coffin DW et al. Through-planediffusion of moisture in paper detected by magneticresonance imaging. Indust. Eng. Chem. Res. 2002;41(25): 6555–6565.

15. Ahlen A. Diffusion of sorbed water vapour throughpaper and cellulose film. Tappi J. 1970; 40(7):1320–1326.

16. Robertson GL. Permeability of thermoplastic poly-mers. In Food Packaging: Principles and Practice.Marcel Dekker: New Yo0rk 1993; Ch. 4.



characterization of moulded-fibre packaging with respect to water vapour sorption and permeation at...

Documents