Effect of relative humidity on carvacrol release and permeation

Effect of relative humidity on carvacrol release and
permeation properties of chitosan based films and
coating
Mia KUREK, Alain GUINAULT, Andr´ee VOILEY, Kata GALIC, Fr´ed´eric
Debeaufort
To cite this version:
Mia KUREK, Alain GUINAULT, Andr´ee VOILEY, Kata GALIC, Fr´ed´eric Debeaufort. Effect
of relative humidity on carvacrol release and permeation properties of chitosan based films and
coating. Food Chemistry, Elsevier, 2014, 144, pp.9-17. <10.1016/j.foodchem.2012.11.132>.
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Submitted on 16 May 2014
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Mia KUREK, Alain GUINAULT, Andrée VOILEY, Kata GALIC, Frédéric DEBEAUFORT - Effect of
relative humidity on carvacrol release and permeation properties of chitosan based films and
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Effect of relative humidity on carvacrol release and permeation
properties of chitosan based films and coatings
Mia Kurek a,b, Alain Guinault c, Andrée Voilley a, Kata Galic´ b, Frédéric Debeaufort a,d,⇑
a
PAM-PAPC, 1 esplanade Erasme, Université de Bourgogne – Agrosup Dijon, F-21000 Dijon, France
Laboratory for Food Packaging, Faculty of Food Technology and Biotechnology, University of Zagreb, HR-10000 Zagreb, Croatia
c
PIMM – CNAM, 151 boulevard de l’hôpital, 75013 Paris, France
d
IUT-Dijon Departement Genie Biologique, 7 Boulevard Dr. Petitjean, B.P. 17867, F-21078 Dijon Cedex, France
b
a b s t r a c t
Keywords:
Biopolymer
Relative humidity
Permeability
Contact angle
Active compound release
Swelling
The influence of water vapour conditions on mass transport and barrier properties of chitosan based films
and coatings were studied in relation to surface and structural properties. Water contact angles, material
swelling, polymer degradation temperature, barrier properties (PO2, PCO2, WVP) and aroma diffusion
coefficients were determined. The solvent nature and the presence of carvacrol influenced the surface
and structural properties and then the barrier performance of activated chitosan films. Increasing RH
from 0% to 100% led to a significant increase in material swelling. The plasticization effect of water
was more pronounced at high humid environment, while at low RH the matrix plasticization was induced
by carvacrol. The deposit of a thin chitosan layer on polyethylene decreased PO2 and PCO2 both in dry and
humid conditions. The carvacrol release from the chitosan matrix was strongly influenced by RH. A temperature increase from 4 to 37 °C also had an impact on carvacrol diffusivity but to a lesser extent than
RH.
1. Introduction
Active packaging and more specifically antimicrobial packaging
films are innovative concepts in food packaging. They have been
developed to meet consumer demand for greater food and microbiological safety. Indeed, these systems provide greater efficiency
in food protection because they allow the stability of the antimicrobial agent, and ensure the control of its release to the food over
time. For example, a too slow release might cause insufficient
microbial inhibition, while a too fast release might be the reason
why the inhibition is not sustained (Li, Kennedy, Peng, Yie, & Xie,
2006). The release rate depends on the polymer type, on the film
preparation method, on the interactions between polymeric and
antimicrobial materials (Cha, Cooksey, Chinnan, & Park, 2003)
and environmental conditions (Cagri, Ustunol, & Ryser, 2004).
Therefore, developing new antimicrobial delivery technologies
and utilising them in product development is crucial for food
industries to compete and survive.
Much research in material sciences is focused on the structureproperties relationship to predict and to control the functional film
properties. Currently, special attention is given to chitosan, N-acet⇑ Corresponding author at: PAM-PAPC, 1 esplanade Erasme, Université de
Bourgogne – Agrosup Dijon, F-21000 Dijon, France. Tel.: +33 380 39 6547; fax:
+33 380 39 6469.
E-mail address: frederic.[email protected] (F. Debeaufort).
yl-D-glucosamine, due to its low toxicity, biodegradability, stability
and relatively low cost as it is a by-product and a renewable material from some industries. Chitosan is a water sensible material
that naturally interacts with water. Yakimets et al. (2007) determined the water content of biopolymer films as a critical variable
that leads to water-induced transformations (for example, amorphous-crystalline transition) that have a strong impact on the
molecular mobility and functional properties. The crystalline structure of hydrated chitosan is a twofold helix. This structure can be
converted to a dehydrated form, very similar to the hydrated
one, but with molecular packing and water content quite different
(Ogawa, Yui, & Okuyama, 2004). Moisture has a plasticizing or
swelling effect on polymers, so it increases gas permeability (Ashley, 1985). Water increases the polymer-free volume, allowing the
segments of polymeric chains to be mobile (McHugh, Aujard, &
Krochta, 1994). Moreover, in order to satisfy adequate functional
properties, the film must be designed according to some surface
properties. Contact angle measurements enable investigation of
the wetting behaviour of the biopolymer surface and can be a good
indicator for the determination of their hydrophilic nature (Peroval, Debeaufort, Despre, & Voilley, 2002). Then it represents a
good way for the development of hydrophilic biodegradable and
well-characterised delivery systems and for understanding the
mechanisms of polymer surface degradation and active compound
release, which could be of importance both for food packaging and
for pharmaceutical applications. However, poor water resistance
and mechanical performance are limiting factors for use of biopolymer materials manufactured only from natural polymers. That
is why there is growing demand in development and characterisation of bio-based/synthetic polymer systems.
Today, it is very fashionable to use essential oils and their constituents as they show a great potential to be used as antimicrobial
compounds both in direct food contact and via vapour. Carvacrol is
a phenolic compound extracted from oregano and thyme oil. Its
inhibitory effect on the growth of various microorganisms is well
documented and described extensively (Ben Arfa, Combes, Preziosi-Belloy, Gontard, & Chalier, 2006; Burt, 2004; Nostro & Papalia,
2012).Carvacrol might be incorporated within biopolymer material
that can be used as a self standing film or it can be coated onto the
synthetic packaging materials. The efficiency of this system is
determined by the controlled diffusion and release and is maintained in concentrations high enough for antimicrobial impact
where necessary. The use of volatile antimicrobial agents has many
advantages. This system can be used effectively for fresh products
such as meat, cheese, fruits, vegetables, or dry products, highly
porous food etc. Because spices and extracts provide the majority
of volatile antimicrobial agents and are commonly GRAS classified
(Generally Recognised As Safe), this system is linked to the food
and pharmaceutical research and development area and is easily
accepted by consumers and regulatory bodies.
During the storage and the use of packaging material, the properties of chitosan films may be changed after the incorporation of
active compound. The loss of active volatile compounds from
bio-based matrix at specified temperature and relative humidity
(RH) (that will simulate the timeline in the ‘real food product’ shelf
life), requires the knowledge of the polymer water sensitivity, the
diffusion coefficient of active compounds, release rates and migration amounts according to moisture levels. Mass transfers through
food packaging films exist whatever the type of material used, even
if several of them are associated. The transfer mechanism of molecules like water vapour and gases through the film are different
according to the film structure that might be changed during processing and storage. Increased storage temperature and humidity
can accelerate the migration of the active agents in the film. Thus,
the protective action of antimicrobial films will be minimized, due
to the high diffusion rates in the polymer and in the food.
The aim of this study was to investigate the influence of RH on
the surface and thermal properties of bio-based polymer self
standing films and coatings applied onto polyethylene films. For
this purpose we used chitosan and carvacrol as models of biopolymer matrices and active compounds with an antimicrobial potential. The effect of incorporated carvacrol on the structure changes
was studied. Water vapour, oxygen and carbon dioxide permeability has also been studied in order to monitor the film behaviour
according to the temperature and RH. Furthermore, to better
understand the influence of both temperature and RH on the carvacrol release, kinetics were studied at 4, 20 and 37 °C and 0%, 75%
and >96% RH during more than 2 months. Chosen temperatures
represent the storage conditions of most fresh food products, of
ambient conditions or conditions of use, and those for optimal
microbial growth.
as a polyolefin material. Carvacrol (CVC) (purity >97%, Fluka) was
used as the aroma compound in order to improve the functional
film properties. Acetic acid (glacial 100%, Merck, Darmstadt, Germany) and pure ethanol (absolute, Sigma–Aldrich) were used as
solvents in the preparation of the film forming solutions (FFS). Silica gel, magnesium chloride (MgCl2, Sigma), sodium chloride (NaCl,
Sigma) were used to prepare saturated salt solutions to fix the RH
at <2%, 33% and 75% and for water vapour permeability measurements, aroma release measurements and for sample conditioning
prior to thermal analysis. Deionised water was used for surface
analysis, aroma release determination and to fix RH at >96% for
permeability measurements. No further purification of chemicals
has been done and freshly prepared solutions were always used.
2.2. Film preparation
2.2.1. Self standing chitosan films
A chitosan solution was prepared by dissolving the chitosan
powder in a 1% (v/v) aqueous acetic acid, to obtain 2% (w/v) film
forming solutions (FFS). To achieve a complete dispersion of the
chitosan, the solution was stirred for 2 h at room temperature. To
prepare the aqueous hydroalcoholic acid media, ethanol was
mixed in a ratio ethanol:aqueous acetic acid of 30:70. Carvacrol
(0.5%, w/v) was homogenised either in aqueous acid CS solution
or hydroalcoholic acid CS solution at 24000 rpm for 10 min with
an Ultra Turrax (T25 IKA) to obtain film forming solutions with
an incorporated aroma compound. In order to obtain films, solvents were removed by drying in a ventilated climatic chamber
(KBF 240 Binder, ODIL, France) at 20 °C and 30% RH. After drying,
the films were peeled off and stored in the same ventilated climatic
chamber at 25 °C and 30% RH before measurements. The films prepared in the acetic acid solution were coded as CSA, those in hydroalcoholic acid solution as CSE and those containing carvacrol as
CVC.
2.2.2. Chitosan coated polyethylene films
The hydroalcoholic acid chitosan solution with/or without carvacrol was prepared as described in Section 2.2.1. The coating
was carried out according to Sollogoub et al. (2009), at room temperature, using a Nordson slot die (ChameleonTM), appropriate to
fluids of viscosity ranging between 0.5 and 2 Pa/s. The die is fed
continuously by a gear pump, the flow rate of which is adjustable
from 5 to 500 mL/min. A roll winding device creates the movement
of the support material at a speed ranging between 0.2 and 4 m/
min. The deposit width is set to 100 mm and the die opening to
150 m. Films were dried in a flow of a dry air at 50 °C and RH
<10%. After drying, the films were stored in a ventilated climatic
chamber (KBF 240 Binder, ODIL, France) before measurements at
25 °C and 30% RH.
2.3. Film characterisation
2.3.1. Film thickness
The film thickness was measured with an electronic gauge (PosiTector 6000, DeFelsko Corporation, USA). The average value of five
thickness measurements per type of film was used in all
calculations.
2. Materials and methods
2.1. Materials and reagents
Commercial grade chitosan (CS) (France Chitine, Marseille,
France, powder 652, having a molecular mass of 165 kDa, low viscosity, food grade, degree of deacetylation of >85%) was used as the
film-forming matrix. A commercial, low density polyethylene film
(LDPE) (Wipak, Lille, LD-PE45 UFP; thickness of 45 lm) was used
2.3.2. Contact angle and wettability
The contact angle, surface hydrophobicity and wettability of
films were measured by the sessile drop method, in which a droplet of the tested liquid was placed on a horizontal film surface
using a DGD-DX goniometer (GBX, Romans-sur-Isere, France),
equipped with the DIGIDROP image analysis software (GBX, Romans-sur-Isere, France) according to Karbowiak, Debeaufort, and
Voilley (2006). Water droplets (1.5 lL approx.) were deposited
on the film surface (‘air side or support side’) with a precision syringe. The experimentally acquired data were: contact angle (h),
droplet surface area exposed to air, droplet base area in contact
with the film and droplet volume (V) as a function of time (t).
The effect of evaporation was analysed on an aluminium foil which
is considered to be totally impermeable to water and aqueous solutions. The wetting kinetics lasted for 200 s. All the films were
stored in a climatic chamber (KBF 240 Binder, ODIL, France) at
30% RH and 25 °C prior to measurements. At least six measurements per film were carried out.
2.3.3. Aroma compound release
In order to follow the carvacrol release, an extraction technique
and a gas chromatography analysis were performed. Samples were
conditioned at <2%, 75% and >96% RH and 4, 20 and 37 °C for more
than 60 days. At each sampling time, the dosage of the carvacrol
residues was tested according to Kurek, Descours, Galic´, Voilley,
and Debeaufort (2012a). For each sample three repetitions were
performed. To determine diffusion coefficient of carvacrol, Fick’s
second law was used which describes the change in the concentration of diffusing molecules in the films with respect to time and position. To be able to use Fick’s second law, it was assumed that
there is no chemical reaction between carvacrol and film matrix.
Thus the mass transfer in the film takes place only by diffusion
coefficient of carvacrol in the film, D, considered as constant. A
solution of Fick’s second law is given by Crank (1975). Carvacrol
apparent diffusivity was estimated by fitting Eq. (1) to the experimental kinetic data using a pre-estimation of D using Excel.
M t X1
8
ð2n þ 1Þ2 p2
¼
exp
Dt
n¼0
M0
4L2
ð2n þ 1Þ2 p2
!
ð1Þ
where t is the time (s) and Mt the amount of carvacrol released from
the film at time t (g/g). The equation was fit with n = 6.
2.3.4. Water vapour permeability measurement (WVP)
The WVP of films was determined gravimetrically using a modified ASTM E96-80 (1980) standard method, adapted to edible
materials by Debeaufort, Martin-Polo, and Voilley (1993), using
the RH differentials of 33–0, 75–30 and 100–30% and the temperature of 25 ± 1 °C. Prior to the WVP measurements, all the film
samples were equilibrated at 25 ± 1 °C and 30% RH for 72 h. The
film samples were then placed between two Teflon rings on the
top of the glass cell containing a silica gel (<2%), salt solutions of
MgCl2 (RH 33%), NaCl (RH 75%) or distilled water (RH 100%).
These permeation cells were introduced into a ventilated chamber
(KBF 240 Binder, ODIL, France) maintained at 30% RH and 25 ± 1 °C.
WVP (g/m s Pa) was calculated using the following equation, from
the change in the cell weight versus time at the steady state:
WVP ¼
Mm
e
Mt Mp A
ð2Þ
where Dm/Dt is the weight of moisture loss per unit of time (g/s), A
is the film area exposed to the moisture transfer (9.08 104 m2), e
is the film thickness (m), and Dp is the water vapour pressure differential between the two sides of the film (Pa). Three replicates for
each film type and RH gradient were made.
2.3.5. Gas permeability measurements
The gas permeability determination was performed using a
manometric method, on a permeability testing appliance, Brugger,
Type GDP-C (Brugger Feinmechanik GmbH, Germany). The increase in pressure during the test period was assessed and displayed by an external computer. Data were recorded and
permeance was calculated by a GDP-C Software (with temperature
compensation connection). The sample temperature (25 °C) was
adjusted using an external Thermostat (HAAKE F3 with Waterbath
K). The desired RH (96%) was regulated in external saturation
system. Then, humidified gas circulated in the permeation cell.
2.3.6. Differential scanning calorimetry (DSC)
A differential scanning calorimetry was performed using a DSC
Q1000-O506 (TA Instruments). An empty capsule was used as an
inert reference and the calibration was performed using the indium standard. The accuracy of the measurements was estimated
at ±0.1 °C. The heating and the cooling rates under nitrogen atmosphere were fixed at 10 °C/min. Since chitosan is a hydrophilic
polymer which tends to retain moisture, the experiment consisted
of the 2 runs in order to eliminate the moisture effect. The following temperature program that ranged between 80 and 350 °C was
used for all the samples: (a) equilibrating at 25 °C, cooling from 25
to 80 °C at a rate of 10 °C/min, isothermal for 10 min and heating
to 220 °C, isothermal for 5 min, (b) cooling down to 80 °C, isothermal for 10 min, (c) reheating to 350 °C, isothermal for 5 min,
(d) and finally cooling down to 25 °C. Prior to the experiment samples were conditioned at 25 °C and <2%, 75% and >96% RH for at
least 7 days. Reproducibility was tested by carrying out the two
measurements for each sample. The mass of all the samples was
around 10 mg.
2.3.7. Statistical analysis
The statistical analysis of the data was performed through variance analysis (ANOVA) using Xlstat-Pro (win) 7.5.3. (Addinsoft,
New York). The data were ranked and statistical differences were
evaluated on the ranks with a one-way analysis of variance (ANOVA) and Tukey’s multiple comparison tests. In all cases, a value of
p < 0.05 was considered to be significant.
3. Results
3.1. Surface hydrophobicity and wettability
First of all, material wettability was tested in order to better
understand how relative humidity (RH) influences the surface
properties of chitosan/carvacrol based films and coatings. The contact angle is dependent on the relative magnitude of cohesive and
adhesive molecular forces that exist respectively within the liquid
and between the liquid and the solid. To estimate the resistance of
films to liquid moisture, contact angle (that indicates surface
hydrophobicity) at the time of deposit (0 s) and at metastable equilibrium (30 s), water absorption rate (wettability), swelling and delay of swelling of chitosan based films and chitosan coated
polyethylene (PE) films were determined. Results are given in Table 1. Practically, a large (h >65°) and small (h <65°) contact angles
represent the quantitative definition of a hydrophobic and hydrophilic surfaces, respectively (Vogler, 1998). Moreover, in a parallel
study surface free energy and critical surface tension were determined (Kurek et al., 2012b). These parameters allow the estimation
of materials hydrophobicity. From Table 1 and Fig. 1 three types of
behaviour can be observed. Changes in behaviour were depended
on the polymer nature, the nature of the solvents used in chitosan
film preparation and the carvacrol addition.
First of all, during the experiment, the contact angle of water
droplet deposited on the PE film linearly decreased. This phenomenon was mainly due to evaporation and spreading. Indeed, the
water cannot be absorbed on the PE, because it is a dense material
with a maximum capacity of water absorption less than 0.2% of its
mass. It can neither be spread because in this case the volume of
drop would be kept constant. The adsorption rate is affected when
the excess water evaporates during the measurement. Thus, the
‘‘adsorption’’ rate on aluminium foil was determined to estimate
Table 1
Contact angle at time 0 s (ht0), and at 30 s (ht30), absorption flux (Fabs), time delay before swelling and swelling percentage data for the analysed films. CSA – films prepared in the
aqueous acetic acid solution; CSE – films prepared in hydroalcoholic acid solution; CSACVC, CSECVC – carvacrol containing films; PE – polyethylene; PECSE – chitosan coated PE
film; PECSECVC – chitosan film containing carvacrol coated PE film.
Film
CSA
CSACVC
CSE
CSECVC
PE
PECSE
PECSECVC
Side
ht0 (°)
ht30 (°)
b,c
Air
Support
Air
Support
Air
Support
Air
Support
93.64 ± 0.23
88.8 ± 0.24c
42.17 ± 0.75e
98.71 ± 1.78a,b
87.26 ± 0.51c
88.19 ± 2.44c
47.28 ± 5.86d
100.64 ± 9.52a
90.18 ± 2.07c
88.70 ± 3.77c
93.58 ± 0.52c
93.26 ± 1.04c
89.63 ± 0.11c
PE
CSE
PE
CSECVC
b,c
89.88 ± 0.18
94.32 ± 0.76a,b
34.05 ± 0.95e
91.33 ± 1.11b,c
89.90 ± 1.57b,c
86.50 ± 1.25a
38.27 ± 3.12a
92.27 ± 10.39a,b,c
85.48 ± 4.28d
87.99 ± 3.78c,d
90.70 ± 0.59b,c
91.95 ± 1.46a,b,c
88.45 ± 0.28b,c,d
Fabs (103 lL/mm2s)
Delay before swelling (s)
Swelling (%)
Direct swelling
1.15 ± 0.06b,c
1.50 ± 0.37a,b
1.65 ± 0.12a
0.32 ± 0.02d
0.95 ± 0.04c
1.48 ± 0.19a,b
1.40 ± 0.26a,b,c
Evaporation
Evaporation
0.40 ± 0.03d
Evaporation
1.08 ± 0.25b,c
/
2.9 ± 0.42d,e
72.4 ± 11.03a
/
10.8 ± 1.13c,d
15.8 ± 3.39c
28.67 ± 6.31b
/
/
/
/
/
2.93 ± 0.11d,e
53.51 ± 5.46a
41.26 ± 0.99a,b,c
31.89 ± 0.72b,c
/
22.89 ± 1.79c
51.82 ± 5.94a
41.51 ± 15.92a,b
/
/
/
/
/
39.25 ± 1.05a,b,c
Mean of at least five measurements ± standard deviation. Values followed by the same letter (a–e) are not significantly different at the p-level of 5%.
(a)
CSA air
t120
t0
CSACVC air
t120
t0
(b)
CSE air
t0
t0
(c)
t120
CSECVC air
t120
PECSE side PE
t120
t0
PECSE side CSE
t120
t0
(d) PECSECVC side PE
t120
t0
PECSECVCsideCSECVC
t120
t0
Fig. 1. Shape and volume changes of water drop deposit on different chitosan based films (a and b), chitosan coated polyethylene (c) and activated chitosan coated
polyethylene (d).
the evaporation rate of the measurement, considering that aluminium does not significantly absorb water during the measurement.
The data obtained on aluminium were similar to those on PE, confirming that evaporation is predominant. On the coated PE the
water droplet profile began to change immediately upon deposition
onto the chitosan-coated surface (Fig. 1). Close values of surface
free energies (35.76 and 37.32 mN/m for PE and coated PE, respectively), explain why no significant changes were observed in the
contact angle values of PE and coated PE. Still, the presence of the
thin chitosan layer (6 lm) caused the swelling of the coated side.
Contrarily, films based on chitosan have non-linear behaviour
where changes such as solvatation, hydration and/or swelling occurred (Fig. 1 and Table 1). This behaviour differed depending on
the solvent and the presence of carvacrol (CVC). Moreover, a different response was observed from the ‘‘air’’ to the ‘‘support’’ side.
Thus, films prepared in the aqueous acidic solution (CSA) tend to
be swollen (with a surface deformation) on the air side, since after
submission an immediate increase in the drop volume was observed. Contrarily, films prepared in the hydroalcoholic acid solution (CSE) also swelled. The swelling occurred after the first
absorption that was characterised by a decrease in both volume
and contact angle of water droplet. The presence of water at the surface of chitosan results in low frictional surface forces, which is a
desirable property in developing biocompatible materials. Moreover during contact angle measurement the swelling phenomenon
was induced probably due to the plasticization by water and partial
solubilisation of chitosan macromolecular chains. This behaviour
seems to be contradictory to that observed for the water vapour
permeability, which showed a lower resistance of CSE films to
water transfer (results will be discussed later). Farris et al. (2011)
stated that reduction in the solid/liquid contact area is accompanied by a steep decrease in volume. Taking this into account, the
sensitivity of films to the liquid moisture transfer was evaluated
by the determination of the water droplet adsorption rate. The
water adsorption rate of the air side of CSA film was not determined
because the film started to swell directly as the water droplet was
deposited. This phenomenon was followed by a decreased hydrophobicity which favoured the wetting of the surface and thus the
decrease of the contact angle. The absorption period before swelling
was longer for air sides of CSACVC and CSECVC films. Moreover, for
CSECVC films Fabs was doubled (Table 1). The support sides of carvacrol containing films were the most hydrophobic surfaces (highest contact angle). This might be explained by the different
orientation of carvacrol droplets in the support side compared to
the air side because of the evaporation phenomenon of carvacrol
during film drying. Moreover, this was supported by the lower Fabs
(0.32–1.15 lL/mm2 s1) for air sides of CSA and CSE films than for
CSACVC and CSECVC (1.40–1.65 lL/mm2 s1). To deepen analysis,
the Owens and Wendt method was used to determine the film
interfacial tension. The addition of carvacrol led to a sharp increase
of the interfacial tension which should result in a decrease in its
wettability and thus its affinity for water. But this was not observed,
then the films containing carvacrol absorbed water faster and
swelled. That’s why this behaviour could be explained by the film
structure. Kurek et al. (2012b) pointed out that carvacrol droplets
caused irregularities in the polymer matrix. In this case, the surface
roughness was increased. Thus, the side effect could be attributed
to a reorientation of the molecules during film drying as previously
reported (Karbowiak et al., 2006; Ogawa et al., 2004). Additionally,
nonpolar impurities in samples might increase surface heterogeneity (Cunha et al., 2008). Consequently, it favoured water penetration
because of the capillary forces. Still, the PE coated samples did not
exhibit these changes to a large extent, probably due to different
drying procedures and lower coating thicknesses.
CSECVC film had higher contact angle values than CSACVC
measured at time 0 or 30 s. Hydroalcoholic solvent favoured the
solubility of carvacrol and thus a more homogeneous droplet/matrix layer was formed. Here, the side effect was even more pronounced. This behaviour in the presence of carvacrol perfectly
supports the results obtained for the water vapour permeability,
in particular for the highest RH differential (DRH 100–30%). The
swelling of material is desirable from an application point of view
that might be the main guideline in the controlled release of active
compounds. Moreover, it turns out that the surface phenomena
play an important role in the mechanism of permeation.
3.2. Thermal properties of chitosan based films influenced by relative
humidity
A thermal analysis was performed in order to confirm the
hypothesis about plasticization influenced by water and/or carvacrol. Before measurements, all the samples were conditioned to
three RH (0, 75 and 100%). Tg is a value associated with the system mobility and is defined as a physical change from the glassy to
the rubbery state in amorphous materials promoted by heat. The
determination of Tg in chitosan based films was a difficult task because changes in the inclination of the baseline in the DSC thermograms were very weak. That is why we are generally speaking
about the dehydration temperature (Td) related to the evaporation
process (characteristic phenomenon of hydrophilic polymers), the
dissociation temperature related to the dissociation process of
the interchain hydrogen bonding of the chitosan (TDS) and the degradation temperature related to the decomposition of the polysaccharide backbone (TD) (Kurek et al., 2012c). Small endothermic
dehydration peaks around 60 and 96 °C in dry (RH 0%) samples
indicated that even after conditioning there was still a small quantity of water in the system, probably due to solvent traces after
drying. The observed TDS values for chitosan films with and without carvacrol changed with the RH. It was attributed to the different intensity of the plasticisation effect of water and carvacrol.
When the RH was close to zero, the plasticisation effect of carvacrol was more pronounced. It was attributed to the fact that in
the system, there was not enough free water and then, carvacrol
had the plasticizing role increasing the mobility of chitosan polymer chains. Thus, in dry systems, TDS values were consequently
lower for CSECVC (131 °C) than for CSE samples (170 °C). Additional, less energy was needed to establish the film network. Furthermore, DHd, DHDS, DHD of CSECVC (around 0.7, 40 and 90 J/g
respectively) were lower compared to CSE films (around 2, 70
and 112 J/g, respectively). When RH increased, the plasticization
effect of water became more pronounced than that of carvacrol.
Thus for 75–100% RH range the dissociation temperature were
lower for samples without carvacrol (133 °C for CSE and 159 °C
for CSECVC films). When increasing RH, the number of water molecules in the system increases. So the possibility for the chitosan
chains to interact with water molecules increases too. The hydrophilic character of chitosan and thus water binding capacity tends
to draw additional water into the matrix limiting the interactions
of carvacrol with the polymer chains and determine the final distribution of water in the system. The quantity of absorbed water increases and more energy was required (higher enthalpies at higher
RH). The remarking effect was also in significant increase in carvacrol release at humidities above 75% that will be later discussed.
For the chitosan coated samples, only Td and TD temperatures
were detected for chitosan part and Tm for PE. No significant
changes were observed due to the low amount of the chitosan
when applied as a thin coating layer (3–6 lm).
3.3. Water vapour permeability
Water vapour permeability (WVP) is the most important and
extensive properties of biopolymer films because of its direct
influence on the deteriorative reactions in packed food. The permeability of films is not a constant (or intrinsic characteristic of the
material) because it increases with RH gradient, contrary to the
predicted sorption–diffusion model that describes permeability.
This non-ideal behaviour is generally attributed to a structural
modification of the film such as swelling due to the sorption of
water vapour. This swelling was also observed by others for biopolymers based materials (Karbowiak et al., 2006; Kokoszka,
Debeaufort, Hambleton, Lenart, & Voilley, 2010).
Polyethylene films are known to be very hydrophobic, low
moisture sensitive and relatively not permeable to water vapour.
Moreover, its WVP does not change with RH differential. In the previous study it was reported (Kurek, Šcetar, Voilley, Galic, & Debeaufort, 2012d) that the presence of the hydrophilic chitosan layer
presented a reservoir of water on the PE surface. It is likely that
high water sorption by chitosan resulted in a much higher concentration of water on the surface, even in a liquid state, which favoured the sorption of water by PE, and therefore increased its
permeability. At higher RH differentials, the addition of carvacrol
in the coating layer decreased WVP, probably because of the
hydrophobicity of the aroma compound while at lower RH the effect was opposite.
From Fig. 2 it is evident that all chitosan based films are not
good water barrier materials compared to polyethylene. It is probably because of the inherent hygroscopic character of chitosan. Its
WVP was nearly 2–3 orders of magnitude greater than that of PE
film. Increased moisture pressure gradient significantly affected
WVP.
At lower RH differentials (33–0%), films without aroma compound (CSA and CSE) had lower permeability, from 10% to 15%,
than that containing carvacrol (CSACVC and CSECVC) as shown in
Fig 2. This, once again, suggests a plasticizing effect of carvacrol,
only noticeable for the lowest RH and less marked plasticizing effect of water. Despite the increase in the hydrophobic character of
the film when carvacrol was added, its incorporation might have
negatively influenced the attractive forces between chitosan molecules and it might have increased segmental movements between
them (Bonilla, Atarés, Vargas, & Chiralt, 2012). This hypothesis was
reinforced with the DSC results that displayed a modification in TDS
and TD.
The addition of hydrophobic substances could decrease the
hydrophilic portion of the film decreasing its affinity for water at
high RH (Hernández-Muñoz, López-Rubio, Del-Valle, Almenar, &
Gavara, 2004). But in our study, in self supported chitosan films,
the WVP tends to reach a plateau at high moisture environments
(Fig. 2). This is due to water impact on the molecular mobility that
masked the hydrophobic effect of carvacrol. Then at 75–30% and at
100–30% RH differentials WVP were not significantly different. Indeed, adsorbed moisture has a plasticizing effect on the biopolymer network and leads to changes from glassy to rubbery
material state (Phan, Debeaufort, Luu, & Voilley, 2005). At higher
RH film swelled and the swollen network of chitosan, together
with the decreased density and the local viscosity, facilitated the
diffusion of water molecules that was more rapid than through
the glassy material.
3.4. Oxygen and carbon dioxide permeability
When a chitosan film is applied with a food product, the gas
concentration in the packaging may change during the storage.
The chemical composition of the food surface is dynamic and biochemical changes in food, microbial respiration, gas solubility and
permeability through packaging film will influence product quality. The efficiency of chitosan films and coatings strongly depends
on its water vapour and gas barrier properties. These parameters
are also depended on the chemical composition and the structure
of the film forming polymers, the product characteristics and the
storage conditions. To approach the ‘real’ product conditions, influence of RH on the oxygen (PO2) and carbon dioxide (PCO2) barrier
properties were examined (Fig. 3). Generally, biopolymers are
aimed to be a good barrier to gases at low RH. Thinking of chitosan
as a new kind of oxygen-barrier coating material, it is possible to
obtain a high oxygen barrier property on common plastic films
and also to improve the biodegradability of the produced material.
The PO2 and PCO2 of chitosan coated PE and self standing chitosan films are given in Fig. 3a and b. In dry conditions PCO2
(1 1017 g/m s Pa) was lower than PO2 (6 1017 g/m s Pa) for
chitosan coated PE films. The gas permeability of PE films alone
was not significantly affected by RH. At the temperatures of study,
PE was probably in the rubbery state and as it is apolar material, it
should not interact with water molecules. On the contrary, for
chitosan coated samples both PO2 and PCO2 were significantly
higher at >96% RH, precisely 10 times for PO2 and 1000 times for
PCO2. The effect of RH was less pronounced for carvacrol containing coated PE. Already in dry conditions these films were less performing than PECSE. PO2 and PCO2 of PECSECVC at >96% increased
for 4 and 19 times respectively, but still remained 1.6 and 1.5 times
lower than PE film itself. Hagenmeier and Shaw (1991) also reported an exponential increase in PO2 of shellac coatings with
increasing RH.
PO2 and PCO2 changes with RH and composition of self standing
chitosan films are given in Fig. 3b. In dry conditions, the permeability of both gases increased with the incorporation of carvacrol. This
was mostly due to microstructural changes in the chitosan
Fig. 2. Water vapour permeability (WVP) at 20 °C and three humidity differentials
100–30% RH,
75–30% RH and 33–0% RH of chitosan films and carvacrol activated
chitosan films compared to chitosan coated PE films. a–fDifferent superscripts indicate significant differences between formulations (p < 0.05). PE – polyethylene; PECSE –
chitosan coated PE film; PECSECVC – chitosan film containing carvacrol coated PE film; CSA – films prepared in the acetic acid solution; CSE-films prepared in hydroalcoholic
acid solution; CSACVC, CSECVC – carvacrol containing films.
Fig. 3. Oxygen and carbon dioxide permeability influenced by relative humidity of (a) polyethylene and chitosan coated polyethylene films and (b) chitosan and carvacrol
activated chitosan self standing film. O2 dry, O2 humid, CO2 dry, CO2 humid. a–hDifferent superscripts indicate significant differences between samples (p < 0.05). PE
– polyethylene; PECSE – chitosan coated PE film; PECSECVC – chitosan film containing carvacrol coated PE film; CSA – films prepared in the aqueous acetic acid solution; CSE
– films prepared in hydroalcoholic acid solution; CSACVC, CSECVC – carvacrol containing films.
network that became more mobile attributed to the presence of
carvacrol microdroplets.
As the RH increased, the PO2 and PCO2 of all samples significantly increased. Despond, Espuche, and Domard (2001) found that
PO2 and PCO2 increased by 12.9 and 172.7 times, respectively,
when RH increased from 0% to 100%. In general, there is a competition between water and gas molecules. The plasticizing and/or
swelling effect of moisture followed by self association of water
molecules (clusters) might have induced rearrangements in the
conformation, changes in crystallinity and mobility of the polymer
chains and thus changed the permeation of gases through the
chitosan film. CSACVC and CSECVC films were less permeable than
CSA and CSE films. There are several explications. First, at higher
RH, TDS for CSECVC films was higher comparing to pure chitosan
films. Thus in humid environment, in the presence of carvacrol
the decrease in PO2 can be associated with the increase of the crystallinity. Indeed, authors reported that in semi-crystalline chitosan,
the mass transfer is primarily function of the amorphous phase, because the crystalline phase is usually assumed to be impermeable
(Ziani, Oses, Coma, & Maté, 2008). Second, at higher RH there is a
competition between water molecules and carvacrol to be bonded
with chitosan chains. Then, as mentioned previously, the plasticization effect of water was stronger in non activated samples and
PO2 and PCO2 increased. CSACVC films (aqueous acid solvent) were
less permeable than CSECVC (hydroalcoholic acid solvent). This can
be attributed to the different molecular orientation of the polymeric chains due to solvent nature (Salame & Steingiser, 1997).
Generally, when comparing gases, higher solubility of CO2 in the
water leads to higher PCO2 in humid conditions. In this study, in
carvacrol containing samples, PCO2 was higher than PO2, while in
the pure chitosan films no significant differences were observed
(Fig. 3b). Similarly, Bae et al. (1998) found that CO2 in wet chitosan
membranes were 15–17 times more permeable than in the dry
chitosan membrane.
3.5. Carvacrol release from chitosan films
In the development of active packaging, the remaining concentration of the active compound during processing and controlledrelease of the same one from packaging materials is of major significance as it will extend the antimicrobial effect of the packaging
film. Since the carvacrol release and diffusion from the chitosan
matrix begins as soon as carvacrol is added to the film-forming
solution, this point has been well reported in our previous study
(Kurek et al., 2012a). Diffusion can occur through the non hydrated
chitosan matrix but will generally be facilitated as the polymer
gradually swells in contact with water vapour. In order to quantify
the effect of RH and temperature on the release of carvacrol from
chitosan based films, release kinetics and apparent diffusivities of
carvacrol were assessed for three relative humidities and three
temperatures. Experimental (amounts of carvacrol) and calculated
(diffusion coefficient) data are given in Fig. 4. The experimental release kinetics at controlled RH (0%, 75% and >96% RH) clearly
showed that the release was greatly accelerated by saturating the
system with the water vapour and increasing the temperature
from 4 to 37 °C. Thus after 60 days, the release was the lowest at
0% RH and the highest at >96% RH. Moreover at 37 °C and RH
>96% already after 2 days more than 98% of carvacrol was lost,
while at 0% RH this was significantly lower (only 2%). At 0% RH
even after 60 days remaining carvacrol content was higher than
85% (for 4 °C), 45% (for 20 °C) and 20% (for 37 °C). As long as the
structure of the films was not significantly changed, a high retention of carvacrol was maintained. Effect of RH on carvacrol diffusivity was attributed to the plasticization of the chitosan matrix by
water. In the high humidity environment, the release of the entrapped compound into the headspace is closely related to the
adsorption of water in the material and hydration of the matrix
(Whorton & Reineccius, 1995). Thermodynamically, different
states of matter may be assigned to the chitosan chain in contact
Fig. 5. Influence of temperature and relative humidity on the diffusivity of
carvacrol in chitosan film.
significantly increased with the temperature and humidity increase. Tunç and Duman (2011) mentioned that temperature is
very effective parameter for controlling the loss of antimicrobial
compounds from biopolymer based films. When sufficient amount
of Ea is available in the system the diffusing molecule jumps from
one position to another. For 0%, 75% and >96% RH, the activation
energies were 71, 167 and 136 kJ/mol respectively. The diffusivity
values were a little bit higher than those obtained for the same
compound in soy protein isolate based-matrix (SPI), where at
30 °C and RH varying between 60% and 100%, D ranged from 0.02
to 1.38 1014 m2/s Chalier, Ben Arfa, Guillard, and Gontard (2009).
4. Conclusion
Fig. 4. Kinetic of carvacrol release from chitosan based film during 2 months
influenced by relative humidity (0%, 75% and 96%) and temperature (a) 4 °C, (b)
20 °C and (c) 37 °C.
with water vapour molecules. With no doubt, hydrophilic content
of the chitosan will affect the intermolecular forces responsible for
diffusion and swelling (Ogawa et al., 2004). Water begins to penetrate the surface of the film, followed by cracks appearing near the
surface, so subsequent release of carvacrol occurs. During the time,
due to its hydrophilicity, chitosan chains are significantly hydrated
meaning that the interaction between water and chitosan increases. This facilitates water diffusion, leads to a greater swelling
and thus to a greater release of carvacrol. The release mechanisms
are quite important from the application point of view. When estimating the shelf life of the active packaging film we want to avoid
active compound loss. On the contrary, as soon as the packaging is
put in the atmosphere of a ‘real fresh food product’, where aw and
thus RH in the packaging is high, accumulated water vapour would
favour the release of carvacrol and thus induce the adsorption on
the food surface. The antimicrobial effect will thus be obtained.
Depending on the RH and temperature, the diffusion coefficients (D) of carvacrol varied from 1.2 1017 (at 0% RH and
4 °C) to 55000 1017 m2/s (at 100% RH and 37 °C) (Fig. 4). The
slower release in the low aw region is most likely due to the lower
mobility of carvacrol molecules in the glassy state of the chitosan
matrix. The activation energy (Ea) for diffusion may be described
as the energy required to create a hole large enough to let by a diffusing molecule. The calculated D was plotted against the reciprocal of absolute temperature (Fig. 5). The diffusion rates
Chitosan films and coatings showed great potential to be used
as active aroma compound support matrices. These matrices can
provide activity to food packaging films, by humidity and temperature induced release mechanisms. Changes in swelling properties,
water vapour permeability and gas permeabilities at high humidity
conditions were mostly influenced by structure reorganisation and
plasticizing effect of water molecules. At low relative humidity gradients, the incorporation of carvacrol induced a plasticization of
chitosan matrix, decreasing its barrier properties. These phenomena were confirmed by the changes in structural properties displayed by thermal analysis. Chitosan coatings significantly
improved the gas permeability properties in dry conditions. For
PECSE films, PO2 was still the lowest at high RH. Contrarily, lesser
improvement was obtained for coatings with carvacrol, especially
in the case of carbon dioxide. The release of the active compound
was strongly enhanced by RH as required for the application. During the film storage, the most important was to avoid the active aroma compound loss. That is the reason why the diffusion
coefficients had to be low at low RH. Contrarily, regardless of the
temperature, as soon as the film is exposed to high humidity (foodstuff), the active compound will be fast released and will provide
an immediate antimicrobial efficiency.
Acknowledgements
The authors wish to thank the colleagues from PAM-PAPC Laboratory. This work was supported by the Ministère de l’Enseignement Supérieur et de la Recherche, the Ministère de l’Economie,
de l’Industrie et de l’Emploi, by the way of Fond Unique Interministériel (project EMAC N° 09 290 6395), the Conseil Régional de
Bourgogne, Direction Generale de la Competitivite de l’Industrie
et des Services DGCIS, Conseil régional Franche-Comté, Conseil
Régional Picardie, Conseil Général Côte d’Or, Conseil Général du
Jura, le Grand Dijon, the competitive clusters Vitagora, IAR et Plastipolis and all private partners involved in this project. The authors
wish to thank Professor JP Gay for English improvement.
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