Article
pubs.acs.org/JAFC
Migration of Components from Cork Stoppers to Food: Challenges in
Determining Inorganic Elements in Food Simulants
T. Corona, M. Iglesias, and E. Anticó*
Department of Chemistry, University of Girona, 17071 Girona, Spain
S Supporting Information
*
ABSTRACT: The inorganic elements potentially migrating from cork to a food simulant [a hydroalcoholic solution containing
12 and 20% (v/v) ethanol] have been determined by means of inductively coupled plasma (ICP) with atomic emission and mass
spectrometric detection. The experimental instrumental conditions were evaluated in depth, taking into account spectroscopic
and nonspectroscopic interference caused by the presence of ethanol and other components in the sample. We report
concentrations ranging from 4 μg kg−1 for Cd to 28000 μg kg−1 for Al in the food simulant (concentrations given in kilograms of
cork). The values found for Ba, Mn, Fe, Cu, and Zn have been compared with the guideline values stated in EU Regulation 10/
2011. In all cases, cork met the general safety criteria applicable to food contact material. Finally, we have proposed water as an
alternative to the hydroalcoholic solution to simplify quantification of the tested elements using ICP techniques.
KEYWORDS: elemental composition, cork, food simulant, migration, matrix interferences, spectroscopic interferences
■
INTRODUCTION
Cork is a natural product obtained from the bark of Quercus
suber, a common species in the Mediterranean region. Because
of its unique physical properties, such as elasticity and low
permeability, cork has long been used in the production of cork
stoppers, frequently used in the wine industry to seal wine
bottles.1 The cork stopper fabrication process involves various
steps: the stripping of the cork plank from the tree stem, a first
rest or maturation in the field or factory, followed by boiling
and resting in open air, a further boiling step and resting in the
store room with a high relative humidity, and finally elimination
of the outer corkback and the cork material cut and shaped
according to use (stoppers for still wine and disks for sparkling
wine). Surface modification is also performed using paraffins
and other additives.2
The chemical characterization of cork has been investigated
mainly with respect to organic compounds.3,4 However, little
attention has been paid to determining its elemental
composition. The sources of the inorganic elements present
in cork bark and cork stoppers may differ. On the one hand,
plants and trees can accumulate trace elements, especially heavy
metals, and act as passive receptors; the uptake of nutrients and
trace elements through the roots has been extensively studied.5
On the other hand, contamination from atmospheric particles,
pesticides, and the cork stopper fabrication process itself may
also contribute to the distribution of metals in cork material.
Some studies have addressed the mineral composition of cork
material and its relationship with mineral nutrition, the climate,
or tree characteristics.6 In these cases, it is mainly nutrients that
are analyzed. In addition, some authors have used tree barks as
bioindicators of heavy metal pollution in the atmosphere
because of their ability to accumulate metals. The role of bark
as a cation exchanger has been highlighted.7
Another very important issue to consider is the interaction of
cork with wine when cork stoppers are used to seal wine
bottles. European Regulation No. 1935/2004 (repealing
© 2014 American Chemical Society
Directives 80/590/EEC and 89/109/EEC) requires that food
contact materials are safe and do not transfer their components
into food in quantities that could endanger human health,
change food composition in an unacceptable way, or deteriorate
the taste and odor of the food.8 Annex I of the regulation
mentioned above lists the groups of materials that may be
covered by specific measures, including cork. Specific
regulations for cork are listed in Resolution ResAP(2004)2,
adopted by the Committee of Ministers, in its composition
restricted to Representatives of the States members of the
Partial Agreement in the Social and Public Health Field.9
Among other recommendations, the document states that
Directives 82/711/EEC, 85/572/EEC, 93/8/EEC, 97/48/
EEC, and 2002/72/EEC and their future amendments should
be applied, and that there should be verification of compliance
with the quantitative restriction according to the conditions laid
out in “Technical document No.2-test conditions and methods
of analysis for cork stoppers and other cork materials and
articles intended to come in contact with foodstuffs”. In this
respect, a migration test should be performed under conditions
simulating long-term storage (10 days at 40 °C) using a food
simulant consisting of a 12% ethanol solution. The potential
migrants from agglomerated cork stoppers associated with
synthetic products (additives, surface treatments, and lubricants) have been previously studied in line with this
approach.10
In general, elemental concentration in food simulant
solutions obtained from corks is expected to be very low, and
for this reason, extremely sensitive analytical techniques are
needed. A common technique for determining elements in
aqueous matrices is by means of inductively coupled plasma
Received:
Revised:
Accepted:
Published:
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January 15, 2014
May 22, 2014
May 26, 2014
May 26, 2014
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Prior to analysis, a precise weight of cork soak was taken, and the
desired amount of the internal standard (IS) solution was added. A
small volume of HNO3 was also added to acidify the sample.
To determine the total content of elements in cork samples, the
following procedure was followed.16 Nine milliliters of concentrated
HNO3 and 1 mL of hydrogen peroxide were added to a 0.5 g cork
sample placed in a Teflon reactor. Once the vessel was sealed, it was
placed in the microwave oven and the following program was run: 5
min to reach 180 °C, 10 min at 180 °C, and finally a cooling period.
After cooling to ambient temperature, the reactor was opened, and the
resultant solution was quantitatively transferred to a vial, where the
total weight was carefully measured.
To prepare the samples for analysis, the necessary amount of the IS
solution was added to 5 g of the digested solution. The samples were
measured using the prepared calibration set, as explained below.
To avoid the presence of ethanol in the samples and standards, we
tried other migration treatments. In particular, we evaluated the
possibility of performing the migration tests by using water instead of a
hydroalcoholic solution at different temperatures.
Determination of Elements in the Food Simulant. For Minor
and Trace Element Determination (Ba, Mn, and Al). Standard
solutions were prepared in water or a hydroalcoholic solution. The
calibration standards were prepared from individual standard solutions
in 1% HNO3, containing Y (1 mg L−1) or Rh (3 mg L−1) as an internal
standard, as indicated. IS correction was conducted by taking into
account the intensity of the line for the measured element and for the
internal standard. Additionally, because of the different behavior
traditionally observed for ionic and atomic lines, each ionic line was
corrected using an ionic line from the internal standard and each
atomic line was corrected using an atomic line15 (see Table 1).
(ICP), which allows reliable results and high sample
throughput. Depending on concentration level, different
detection systems are available, such as ICP-AES (atomic
emission spectroscopy) for elements present at concentrations
higher than ∼0.1 mg L−1 and ICP-MS (inductively coupled
plasma with mass spectrometry detection) for those present at
lower concentrations. The latter technique shows good
analytical performance but suffers from interferences of
different types, such as matrix and spectroscopic interferences.11 In the particular case of a hydroalcoholic solution, the
presence of a carbon source such as ethanol is of special
concern. For example, some polyatomic interferences have
been described for Cr (12C40Ar and 13C40Ar).12 Additionally,
the determination of elements with low ionization potential
such as Zn, Se, and As is also challenging because of the
increase in their degree of ionization in the plasma when an
additional carbon source is simultaneously present.13−15 To
overcome these problems, several strategies were developed,
including complex sample treatment procedures, the use of
interference equations, collision cell devices, and highresolution instruments.15
In this study, we have assessed the elemental composition of
a food simulant originating from cork soaks with two
objectives: (1) providing a reliable method for their
determination, avoiding, if possible, the use of ethanol in
calibration standards, and (2) ascertaining whether cork may be
considered a safe material for use as a stopper for bottled wine
according to the aforementioned EU directive.
■
Table 1. Selected Atomic and Ionic Lines for ICP-AES
Measurements
MATERIALS AND METHODS
Reagents. The reagents used were analytical grade suprapur
quality: nitric acid (Suprapur, Merck, Darmstadt, Germany) and
hydrogen peroxide (Trace Select, Fluka, Gilligham, Dorset, U.K.).
Water obtained from a Milli-Q purifier system (Millipore Corp.,
Bedford, MA) was used throughout the study. For matrix matching
standards, ethanol absolute (UV-IR-HPLC) PAI was used, from
Panreac.
Monoelemental ICP standard solutions (1000 or 5000 mg L−1 for
Ba) for each element studied were purchased from Pure Chemistry,
ROMIL, UKAS calibration.
Apparatus. An ETHOS PLUS Milestone microwave with an HPR1000/10S high-pressure rotor (Sorisole, Bergamo, Italy) was employed
for acid digestion of samples.
A sequential inductively coupled plasma atomic emission
spectrometer (ICP-AES, Liberty RL, Varian) in radial configuration
was used for major and minor element determination. For trace
element determination, a quadrupole-based ICP-MS system (Agilent
7500c, Agilent Technologies, Tokyo, Japan) was used, equipped with
an octapole collision reaction cell.
Sample Collection. Samples consisting of cork granules (samples
C1 and C2) and cork slabs (C3) were obtained from local producers
in Girona, Spain. In all cases, the cork material was obtained from the
cork residues discarded in the final stages of the production process,
i.e., shaping of the cork slab to obtain the cork stoppers or cork disks.
Theses residues are ground in the manufacturing to obtain cork
granules intended to be used in the fabrication of different synthetic
cork and other cork types (samples C1 and C2). In the case of cork
sample 3, we obtained from the producer cork slices that were ground
in our laboratory using a conventional grinder (Moulinette D56,
Moulinex España, Barcelona, Spain).
Sample Treatment. The migration tests were performed
according to the European Directive using a hydroalcoholic (HA)
solution [12% (v/v) ethanol] as a food simulant. Four grams of cork
granules or previously ground cork slabs was immersed in 100 mL of a
food simulant solution at 40 °C for a period of 10 days.10 The solid
parts were then separated by filtering the sample through glass wool.
element
line type
wavelength (nm)
IS
line type
wavelength (nm)
Ba
Zn
Cu
Fe
Mn
Al
II
II
I
II
II
I
455.403
202.551
324.754
259.940
257.610
396.152
Y
Y
Rh
Y
Y
Rh
II
II
I
II
II
I
371.030
371.030
343.489
371.030
371.030
343.489
For Ultratrace Element Determination (Cr, Fe, Ni, Cu, Zn, Pb, Cd,
As, and Se). From the commercial standard solutions, an intermediate
solution in 1% (w/w) HNO3 containing the measured elements over
the appropriate range of concentrations was prepared and used to
obtain the calibration set, either in water or in a hydroalcoholic
solution.
The isotope 103Rh was used for internal standard correction after
checking that it was not present in the samples at the concentration
range studied (results from a semiquantitative analysis). The
concentration of IS in the standard solutions prepared for calibration
and in the samples was around 3.7 ng g−1.
The experimental conditions for determining this set of elements
were optimized taking into account different criteria: abundance of the
selected isotope, correction of isobaric and polyatomic interferences
under the vented or pressurized collision cell, type and flow rate of the
gas used in the pressurized collision cell, correction of matrix
interferences by the IS used, and finally the presence of other matrix
interferences that cannot be corrected with the IS.
Octopole and quadrupole potentials were varied according to cell
conditions, being fixed at −12 and −11.1 V, respectively, when a
pressurized cell was used and −6.4 and −4.5 V, respectively, under
vented conditions.
Some other parameters, such as ion lens settings, were adjusted
daily to obtain the maximal sensitivity and precision. The oxide level
(CeO+) and doubly charged ion level (Ce2+) were kept under 1 and
3%, respectively.
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Figure 1. Calibration curves for Al and Mn with and without IS.
Statistical Analysis. The statistical package provided by Excel was
used to perform analysis of variance to evaluate whether differences
between treatments were significant (p < 0.05).
Table 2. Main Isotopes and Possible Interferences
isotope
■
56
Fe
57
Fe
60
Ni
62
Ni
63
Cu
65
Cu
RESULTS AND DISCUSSION
Evaluation of Experimental Conditions for Determining Elements. The elements of interest were selected
111
Cd
Pb
52
Cr
53
Cr
75
As
76
Se
78
Se
80
Se
64
Zn
66
Zn
68
Zn
208
interferences
40
Ar16O, 40Ca16O, 40Ar15N1H, 38Ar18O, 37Cl18O1H
40 16 1
Ar O H, 40Ca16O1H, 40Ar17O, 38Ar18O1H, 38Ar19F
44
Ca16O, 23Na37Cl, 43Ca16O1H
46 16
Ti O, 23Na39K, 46Ca16O
31 16
P O2, 40Ar23Na, 36Ar12C14N1H
49 16
Ti O, 32S16O21H, 40Ar25Mg, 40Ca16O1H, 36Ar14N21H, 32S33S,
32 16 17
S O O, 33S16O2, 12C16O37Cl, 12C18O35Cl, 31P16O18O
−
−
40 12
Ar C, 38Ar14N, 35Cl17O, 35Cl16O1H
40 13
Ar C, 37Cl16O
40 35
Ar Cl
40 36
Ar Ar
40 38
Ar Ar
40 40
Ar Ar
40 24
Ar Mg, 64Ni36Ar12C16O, 38Ar14N2
40 26
Ar Mg, 38Ar28Si, 38Ar14N2
40 14
Ar N2, 36Ar32S, 36Ar16O2, 38Ar14N16O
derived from the sample itself. In a previous work dealing with
wine samples,17 we evaluated the influence of the presence of
ethanol and demonstrated that use of an adequate IS allows the
calibration set to be prepared in water instead of a HA solution.
For this reason, we devoted much effort here to studying
whether choosing an adequate IS makes it possible to
appropriately correct the different types of interferences. The
elements determined by ICP-AES will be discussed separately
from those measured by ICP-MS in the following sections.
Metals Determined by ICP-AES (Ba, Mn, and Al). Table 1
shows the selected atomic and ionic lines taken for measure-
Figure 2. Results obtained for Mn determination in a cork soak under
different calibration conditions.
according to the results of preliminary semiquantitative studies
conducted in our laboratory. To quantify these elements using
ICP-AES and ICP-MS techniques, matrix interferences and
spectral interferences are of great concern because of the
presence of ethanol and other carbon sources in the matrix
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standards in a hydroalcoholic solution with IS. The results
obtained (Figure 2) demonstrate that the aqueous standards,
even with IS, do not allow the correct determination of this
element. On the other hand, when ethanol standards are used,
the same result is obtained with and without IS, so the presence
of ethanol is the main matrix effect in manganese
determination.
Metals Determined by ICP-MS (Cr, Fe, Ni, Cu, Zn, Pb, Cd,
As, and Se). For those elements present at lower concentrations, ICP-MS was used. In this case, spectroscopic and
nonspectroscopic interferences are of particular concern and
different situations can be envisaged: (1) when matrix
interference due to the presence of ethanol in the samples is
corrected by use of the appropriate IS, (2) when polyatomic
interferences exist that are corrected by the use of a pressurized
cell and the conditions in the cell need to be optimized, and (3)
elements with a high ionization potential that also need special
instrumental conditions.
We performed separate experiments depending on the
characteristics of each element.
Cadmium, Lead, Iron, Nickel, and Copper. For lead and
cadmium, no important polyatomic interferences have been
described. Therefore, the most abundant isotope was selected
for their measurement. Calibration curves were measured in
water and hydroalcoholic solutions both with IS, and results
showed that the use of the internal standard allows appropriate
correction for the presence of ethanol in the solution. In both
cases, without using the pressurized collision cell, calibration
could be performed with standards prepared in water.
Additionally, the values obtained for Cd and Pb, measured in
a sample obtained from soaked cork, showed no significant
differences when both calibration sets were used and with IS
Table 3. Isotope and Cell Conditions Selected for ICP-MS
Measurements
isotope
111
Cd
208
Pb
56
Fe
60
Ni
63
Cu
53
Cr
66
Zn
75
As
78
Se
conditions
vented cell
vented cell
He at 2 mL min−1
He at 2 mL min−1
He at 2 mL min−1
He at 2 mL min−1
He at 0.5 mL min−1 and H2 at 3 mL min−1
He at 0.5 mL min−1 and H2 at 3 mL min−1
He at 0.5 mL min−1 and H2 at 3 mL min−1
ment. The main problem associated with determining the
elements is sample composition due to the presence of the
alcohol, which may cause differences in nebulization efficiency
and aerosol transport compared to those of water samples.
Figure 1 shows calibration curves for Al and Mn (see Table S1
of the Supporting Information for calibration parameters). As
we can see, the addition of Rh and Y as internal standards
allows for an appropriate correction of matrix interference due
to the presence of ethanol for Al. Similar results were obtained
for Ba. For Mn, calibration curves are statistically different [p <
0.05 (see Table S2 of the Supporting Information)] for both
matrices even in the presence of the internal standard, which
means that matrix effects other than nebulization efficiency and
transport contribute to the response at the selected wavelength.
To verify this fact, a sample was measured in triplicate and the
manganese concentration was determined using the calibration
plots shown in Figure 1, i.e., aqueous standards, aqueous
standards with IS, standards in a hydroalcoholic solution, and
Figure 3. Calibration curves for 56Fe under different conditions (pressurized cell with He at 2 mL min−1 and IS correction).
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observed in all six cases (two isotopes for each element): the
use of the pressurized cell with He at 2 mL min−1, as previously
described,18 allows for a good correction of polyatomic
interferences, while matrix interferences deriving from the
presence of ethanol can be mostly overcome by employing IS
correction. The most abundant isotope providing better
sensitivity was then chosen (Table 3). In Figure 3 and Table
S3 of the Supporting Information, the case of 56Fe is depicted
by way of example.
The experimental conditions were verified by analyzing the
three metal cations in a cork soak (Figure 4). When the results
obtained from calibration with water standards were compared
with those obtained with the hydroalcoholic solution, no
significant differences were obtained for Ni and Cu if the IS is
used for quantification. For Fe, the matrix interferences are not
adequately corrected, and it is therefore compulsory to perform
calibration using hydroalcoholic solutions. It should be taken
into account that the cork soak solution has a complicated
composition, so other matrix effects, different from those
related to ethanol content, may be present.
Chromium. As explained in the Introduction, determination
of chromium by means of ICP-MS is hampered by the presence
of spectroscopic interferences due to the presence of carbon or
chlorine in the sample matrix (see Table 2). Several strategies
for overcoming these interferences have been proposed in the
literature.12 When the instrumentation available was taken into
account, the use of pressurized cell conditions with He was
chosen as the best alternative. As preliminary work, different He
flow rates in the collision reaction cell were tested to obtain a
high signal background ratio (SBR). A rate of 2 mL min−1 was
finally chosen as a compromise between a low level of
interferences and sufficient analyte signal. Calibration curves,
with standards in water or a hydroalcoholic solution, were
obtained under different conditions (vented and pressurized
cell conditions and with and without IS) for the most abundant
chromium isotopes, 52Cr and 53Cr. Additionally, the effect of
the presence of chloride was also evaluated, because Cl has
been determined to be an abundant element in cork.19
The addition of helium in chromium determination
decreases spectroscopic interferences, which can be observed
in Figure 5. However, the presence of a fairly important
spectroscopic interference is still observed when 52Cr is
measured. In contrast, the use of He at a rate of 2 mL min−1
in the collision cell virtually eliminates the interference on 53Cr.
The use of IS does not result in much difference in this case.
Because chromium was present at a concentration level
below the limit of detection in our particular samples, it could
not be determined in the cork soak.
Arsenic, Zinc, and Selenium. These elements are also
difficult to determine by means of ICP-MS because of their
high ionization potential, which produces a low degree of
ionization in the plasma. Additionally, they also suffer from
spectroscopic interferences (see Table 2). It is already known
that some of these elements show an important increase in
sensitivity in ICP-MS when a quite large amount of C is present
in the plasma source.13 This effect has been explained as a
charge transfer process from C ions to these elements and has
been extensively studied for As and Se.20 Some studies also
include other elements with high ionization potentials, like
Zn.21
The presence of important polyatomic interferences in As,
Se, and Zn measurements makes it necessary to use pressurized
cell conditions. Some studies found in the literature conclude
Figure 4. Results obtained for determination of 56Fe, 60Ni, and 63Cu in
a cork soak under different conditions.
correction. Some differences were encountered when the values
obtained for calibration with ethanol were compared with the
results obtained for calibration with ethanol and IS, probably
due to matrix interferences other than ethanol.
For Ni, Fe, and Cu, common polyatomic interferences are
known (see Table 2). Calibration data were compared for the
most abundant isotopes under different conditions (internal
standard and vented or pressurized cell). Similar behavior is
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Figure 5. Intensity for 52Cr and 53Cr under different conditions in the collision cell. IS is used in all cases.
recoveries are listed in Table 4. Recovery values were obtained
by using calibration curves with hydroalcoholic standards, with
and without the IS correction. If we compare the data in the
table for those elements measured with ICP-AES, the need to
use the internal standard becomes clear because recoveries in
the range of 130−176% were obtained without IS.
On the other hand, no statistical differences were obtained
between the results for the other measured elements
(recoveries obtained using calibration with IS compared with
those without IS), except for As and Se. Taking into account
the fact that the use of an IS is recommended in ICP-MS to
account for variations in the plasma signal during one single
run, we adopted this procedure to prepare the calibration set in
the hydroalcoholic matrix.
Migration Tests. To calculate the percentage of an element
that has migrated to the simulant, we previously determined the
total concentration of each element in the three different cork
samples (C1−C3). To do this, we used microwave-assisted acid
digestion, establishing the condition described in the
experimental part. Table 5 shows the concentrations found.
We have also included some values obtained from the literature
in the table. As shown in the table, few data are available for
microelements. The results reported from PIXE analyses
obtained from the bulk layer of a cork stopper19 do not
significantly differ from those obtained in our work in the case
of Cu, while for Fe and Mn, PIXE concentrations are lower
than ours. Additionally, when Pb is determined in a cork
stopper used to seal a bottle of a French wine,22 a relatively
high concentration at the end in contact with the wine was
found, compared with the concentration at the center of the
cork. In this context, it has to be considered that lead content in
wines is regulated,23 and for that reason, its concentration in
that the use of H2 together with a small amount of He in the
reaction cell (H2 at 3 mL min−1 and He at 0.5 mL min−1) gives
good results for the determination of As and Se.15 We therefore
decided to use these conditions in our measurements. In the
case of Zn, we compared the results obtained using He at 2 mL
min−1 (same conditions used for Cr, Ni, Fe, and Cu) and H2 at
3 mL min−1 and He at 0.5 mL min−1 (same conditions used for
As and Se). Both gave similar results, so we decided to use the
latter because of the greater sensitivity obtained in this case.
Upon comparison of the calibration curves obtained with and
without ethanol and using IS, the internal standard does not
correct properly for any of these elements, as expected. As
already mentioned, the signal enhancement produced, due
probably to a higher degree of ionization in the plasma, which
in turn is related to the presence of C atoms from the matrix, is
observed only for elements with high ionization potentials; this
is not the case with Rh (our IS). The use of ethanol matrix
standards to obtain the calibration curves is therefore once
again compulsory. When a cork soak was measured (see Figure
6) and the results were compared, we again observed a
difference between calibration in an aqueous matrix and
calibration in a hydroalcoholic solution, both with IS. When
standards in water are used, the concentration of these three
elements is overestimated.
In Table 3, final collision cell conditions are summarized for
the determination of arsenic, zinc, and selenium.
Method Validation. Because of the lack of certified cork
soaked material, we validated the final conditions by performing
recovery experiments. Three cork soaks were obtained, one for
each sample type (C1−C3), and element concentrations were
measured. The desired amount of stock solution was then
added and a new measurement performed. Spiked levels and
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Table 4. Recovery Results (R) Obtained for Spiked Samples
Al
Ba
Mn
Cr
Fe
Ni
Cu
Zn
As
Se
Pb
Cd
added (μg/kg of food
simulant)
R% without IS
(SD)a
R% with IS
(SD)a
624
55
168
5.7
31−33.7
31−34.4
31−34.3
31−34.1
6−6.6
5.6−6.1
6−6.5
6−6.5
146
131 (5)
176 (24)
85 (6)
119 (7)
108 (6)
107 (7)
110 (6)
118 (4)
116 (4)
92 (8)
96 (3)
119
86 (4)
103 (9)
102 (6)
109 (2)
99 (2)
98 (4)
98 (4)
106 (2)
104 (1)
83 (6)
106 (8)
a
Three cork soaks (n = 3) were used, corresponding to each sample
type (C1−C3), except for Ba and Mn (n = 2; C1 and C2) and Al (n =
1; C2).
Figure 6. Results obtained for
calibration conditions.
66
Zn,
75
As, and
78
majority of producers obtain the cork material from forests in
Andalusia, Extremadura, or Portugal. In this respect, it is hard
to determine the precise origin of the material. It is also worth
mentioning the differences between cork granules (corks 1 and
2) and cork slabs (cork 3), which may be attributed to the
fabrication process applied to obtain cork granules, i.e.,
contamination due to the grinding process, when the cork
material remains in close contact with the metallic parts of the
machine for long periods of time.
The amount of each element transferred from the cork
sample to the food simulant solution is presented in Table 6.
The results are given as micrograms of the element measured in
the solution divided by the total amount of cork used for the
experiment. The percentage of the element migrated from the
cork to the solution is also given. As we can see, the
percentages vary from 0.6% for Ba to 67% for Se. Although this
last migration percentage seems quite high, some studies in the
literature reported Se migration values of up to 37% from yeast
in hot water (50 °C) for 24 h.24 It should be mentioned that
selenoamino acids and inorganic Se (mainly selenite and
selenate) are water-soluble.25
Prior to 2011, no reference values were established by the
EU to determine whether these results might be considered
safe. Recently, the EU has approved a new directive (EU
directive 10/2011)26 establishing maximal migration values for
Ba, Cu, Fe, Mn, and Zn. Additionally, because cork stoppers are
considered to be a material that comes into contact with wine,
the new directive recommends the use of a food simulant
consisting of 20% (v/v) ethanol in water. For this reason, we
have measured the amount of the element that migrates not
only in a 12% ethanol/water mixture but also in a hydroalcoholic solution with 20% ethanol. The values obtained for
sample C2 are listed in Table 7. As we can see, the cork samples
analyzed give migration results well below the maximum
established, Ba and Mn being the most critical elements with
respect to the maximal concentrations allowed. It is also worth
mentioning that, in general, concentrations in a hydroalcoholic
solution containing 20% ethanol are slightly higher than those
in 12% ethanol, which may indicate that the solubility of
metallic species increases at higher ethanol concentrations.
Migration Tests Using Water. Finally, we tested other
conditions to perform the migration test using only water as a
food simulant. In this way, we were able to avoid the presence
Se under different
cork material and the migration to wine deserve special
attention. With respect to the values we found for samples C1−
C3, high variability was obtained between samples, presumably
depending on the provenance of the cork. Catalonia is one of
the most important regions producing cork stoppers, but the
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Article
element
concentration in the food simulant
(μg/kg of cork) (n = 3)
% migration
Al
Ba
Mn
Cr
Fe
Ni
Cu
Zn
As
Se
Cd
Pb
28000 (3000)
860 (70)
3300 (200)
not determined
1600 (100)
70 (30)
1400 (50)
700 (20)
41.1 (0.6)
5.4 (0.3)
4 (2)
14 (3)
3.5
0.6
10.6
not determined
1.1
10.4
28.0
0.8
6.6
67.2
10.3
3.7
37.0 (0.8)
42.3 (2)
50 (10)
−
−
−
8 (0.3)
8.86 (0.04)
6.9 (0.8)
−
−
−
380 (20)
350 (40)
246 (8)
−
60
this work
this work
this work
6
22
19
Table 6. Results from the Migration Test with Sample C1
[mean (SD)]
Table 7. Migration Results Compared with Data from EU
Directive 10/2011 (C2; n = 3)a
670 (60)
520 (40)
630 (40)
−
−
−
620 (10)
23 (4)
30 (4)
−
−
−
Cdb
Nib
Asb
Seb
Pbb
ref
Journal of Agricultural and Food Chemistry
element
HA 12% ethanol
HA 20% ethanol
EU Directive 10/2011
Ba
Cu
Fe
Mn
Zn
0.038 (0.009)
0.052 (0.003)
0.101 (0.005)
0.30 (0.02)
0.080 (0.003)
0.37 (0.01)
0.18 (0.01)
0.43 (0.02)
0.116 (0.004)
1.0 (0.2)
1
5
48
0.6
25
660 (50)
330 (60)
710 (20)
−
−
−
Concentration in the food simulant given in milligrams per kilogram
of food simulant [mean (SD)].
Table 8. Results from the Migration Test and Different
Treatments (C1; n = 3) [mean (SD)]
Milligrams per kilogram of cork. bMicrograms per kilogram of cork
83 (6)
92 (6)
1.2 (0.4)
18−9.9
−
<3.6
element
W at 40 °C
W at 55 °C
W at 75 °C
HA at 40 °C
Ala
Baa
Mna
Feb
Nib
Cub
Cdb
Pbb
Znb
Asb
Seb
16.2 (0.9)
0.2 (0.03)
1.6 (0.2)
930 (10)
37 (4)
720 (30)
2.8 (0.2)
4.83 (0.08)
280 (30)
43 (2)
2.9 (0.3)
49 (6)
0.9 (0.2)
6.8 (0.5)
2340 (60)
100 (20)
1960 (60)
6.2 (0.3)
9.3 (0.6)
837.0 (0.1)
33 (2)
3.1 (0.3)
120 (20)
3.5 (0.2)
18.8 (0.5)
12800 (300)
195 (8)
3140 (50)
23 (0.6)
28 (1)
2720 (30)
43 (0.9)
4.0 (0.3)
28 (3)
0.86 (0.07)
3.3 (0.2)
1600 (100)
70 (30)
1400 (50)
4 (2)
14 (3)
700 (20)
41.1 (0.6)
5.4 (0.3)
a
Milligrams per kilogram of cork. bMicrograms per kilogram of cork.
of ethanol in the samples, resulting in fewer spectroscopic and
nonspectroscopic interferences. Obviously, water is expected to
have less extraction power than hydroalcoholic solutions. To
address this difference, we increased the temperature (40−75
°C) while retaining the total contact time (10 days). Results are
listed in Table 8, where those values that are similar to or
slightly higher than those obtained for the food simulant
[hydroalcoholic solution with 12% (v/v) ethanol] are underlined. As a general trend, we observe an increase in the amount
of the element that migrated from the cork when the
temperature is increased. With the exceptions of As and Se,
the migration tests with water at 55 °C resulted in
concentrations higher than those obtained with a HA solution.
Taking into account the fact that most of the interferences in
ICP-MS analysis of a food simulant are related to the presence
of ethanol, we can propose the use of water at 55 °C as an
alternative for making the analytical procedure easier.
a
140 (10)
26 (2)
16.2 (0.3)
142−185
−
3.7 (1.9)
5 (2)
6 (1)
9 (4)
12−10
−
9.3 (1.5)
C1 (n = 4)
C2 (n = 4)
C3 (n = 4)
800 (100)
68 (6)
27 (8)
−
−
−
136 (4)
142 (6)
6.2 (0.2)
−
−
−
31 (4)
35 (2)
11.7 (0.4)
22−64
−
8.7 (2.3)
Fea
Cua
Mna
Baa
Ala
Table 5. Contents of Elements in Different Cork Samples [mean (SD)]
Zna
Crb
a
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Journal of Agricultural and Food Chemistry
■
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ASSOCIATED CONTENT
S Supporting Information
*
Tables S1−S3. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Funding
The Spanish National Research Program supported this work
through Project CGL2010-22168-C03-03.
Notes
The authors declare no competing financial interest.
■
■
ACKNOWLEDGMENTS
We are grateful to Dr. E. Besalú for helping with the statistical
analysis of the data.
ABBREVIATIONS USED
ICP-AES, inductively coupled plasma atomic emission spectroscopy; ICP-MS, inductively coupled plasma mass spectrometry; HA, hydroalcoholic; OIV, Organisation Internationale de
la Vigne et du Vin; IS, internal standard; SD, standard
deviation; PIXE, particle-induced X-ray emission
■
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