Determination of Critical Oxygen Level in
Packages for Cooked Sliced Ham to Prevent
Color Fading During Illuminated Retail Display
H. LARSEN, F. WESTAD, O. SøRHEIM, AND L.H. NILSEN
T
Introduction
he color of meat products such as cooked ham is of major importance for the consumer acceptance. Hence, color stability
during storage and retail display is important to the meat processing industries and the retailers (Møller and others 2003; Solomon
2004). The increased use of modified atmosphere (MA) packaging
for sliced cooked products has, in general, generated problems with
discoloration when the products are exposed to light in retail display
cabinets. Changing from vacuum to modified atmosphere packaging has introduced headspace volume and residual oxygen in the
package as new factors influencing the color of cooked meat products. Discoloration is, in particular, a problem in the Scandinavian
countries, where the meat processors, due to marketing reasons,
employ transparent packages for sliced meat products.
Denatured nitrosylhemochrome (MbFe(II)NO), which is the pigment giving cooked meat products their pink color, is very sensitive
to light when exposed to oxygen (Andersen and others 1988; Andersen and Skibsted 1992; Cornforth 1994). Light fading of the pigment
is a 2-step process that includes dissociation of nitric oxide (NO)
from heme groups catalyzed by light, followed by oxidation of NO
and heme groups by oxygen (Cornforth 1994). What type of reaction that is degrading MbFe(II)NO upon light exposure is at present
not entirely clear, but free radical processes are suggested to be involved in the degradation process (Møller and others 2002, 2005).
The fading process requires both light and oxygen, and if no oxygen
is present, the NO split from the heme groups by light will not be oxidized, and it can then recombine with the heme (Judge and others
MS 20060036 Submitted 2/10/06, Accepted 4/09/06. Authors are with
MATFORSK, Norwegian Food Research Institute, Osloveien 1, N-1430 Ås,
Norway. Author Nilsen is with Gilde Norge BA, Postbox 397 Økern, 0513
Oslo, Norway. Direct inquiries to author Larsen (E-mail: hanne.larsen@
matforsk.no)
1989; Andersen and Skibsted 1992). Even small amounts of oxygen
in the headspace of the package will cause a color change from fresh
pink to gray within 6 to 8 h when exposed to typical lighting conditions in the refrigerated display cabinets. Other parameters that
influence the extent of photooxidation of the meat pigment is the
gas-to-product-volume ratio (GP ratio), intensity and type of light
exposure, type of meat product (Møller and others 2000, 2003; Jakobsen 2003), quality of the raw meat, and nitrite content (Froehlich and
others 1983; Dineen and others 2000; Møller and others 2003).
Thus, the amount of oxygen available in the package for pigment oxidation is crucial to the color stability of cooked meats.
The amount of oxygen is influenced by the oxygen transmission
rate (OTR) of the package, residual oxygen level after packaging, GP
ratio, and oxygen-consuming microbial activities in the meat products during storage. In order to prevent discoloration, the OTR of
the packaging material (measured on flat film) should be below 30
and 40 mL O 2 /m2 × 24 h for sliced salami and sliced bologna, respectively (Yen and others 1988; Grini and others 1992). Møller and
others (2003) found that the OTR of the packaging material should
be even lower for sliced cooked ham packaged in modified atmosphere, as packages with an OTR value of 10 mL O 2 /m2 × 24 h (flat
material) also clearly showed discoloration in their study. The OTR of
the package should preferably be measured for the finished package
at the storage temperature of intended use. By measuring the OTR
for the finished thermoformed and sealed package using the Ambient Oxygen Ingress Rate (AOIR) method, changes due to stretching
of the material, heat seals, and possible defects created under the
converting process are taken into account (Larsen and others 2000;
Larsen 2004). The OTR of the packages can, by this method, also be
measured at the realistic storage temperature.
Even if several studies are conducted on how the above mentioned factors, separately and in combination, are affecting the
color stability of cooked meats, there is still a need for more precise
S: Sensory & Nutritive Qualities of Food
ABSTRACT: The effect of packages with different oxygen transmission rates (OTR), different gas-to-product-volume
(GP) ratios, and various levels of residual oxygen after packaging on the color stability of cooked ham exposed to
commercial retail light conditions was studied. Sliced cooked ham was packaged in thermoformed packages with
OTR of 0.04 and 0.06 mL O 2 /pkg × 24 h and GP ratios of 2.6 and 4.1. After packaging, the packages were additionally
divided into groups with 4 levels of residual oxygen ranging from 0.09% to 0.46%. The packaged ham was stored
in darkness at 4 ◦ C up to 33 d, and during the storage period samples were withdrawn and exposed to light for
2 d before instrumental and visual color evaluation. In order to maintain an acceptable color of this particular ham
product when exposed to typical retail light conditions, the highest acceptable level of oxygen in the headspace of
the packages was 0.15% oxygen at the time of illumination. This threshold level was independent of the storage time
before light exposure. A residual oxygen level of below 0.15% just after packaging combined with the package with
the lowest OTR (0.04 mL O 2 /pkg × 24 h) and the lowest GP ratio (2.6) was the optimal condition for maintaining the
color of the tested ham product throughout the entire storage period.
Keywords: color stability, cooked ham, photooxidation, residual oxygen concentration
Color fading of cooked ham . . .
knowledge of how these factors interact under commercial light and
temperature retail conditions during the entire shelf-life period. Under commercial conditions, the light exposure period will depend
on several factors, such as the product turnover and the type of
chill cabinet used in the retail stores. However, the product will very
rarely be exposed to light all through the shelf-life period, which is
a commonly used experimental setup seen in the literature.
The objective of the present work was to study the effect of different levels of residual oxygen after packaging, 2 GP ratios, and 2 types
of packaging films with different OTR on the color stability of cooked
sliced ham exposed to commercial retail light conditions. Another
aim in this work was to determine the maximum oxygen content
in the package headspace for this particular product to avoid color
fading during retail display.
Materials and Methods
Product
The cured, cooked meat product used for the packaging experiment was a commercial ham product produced by Gilde Fellesslakteriet (Sarpsborg, Norway). Carcasses of 6-mo-old pigs were used for
the experiment. The raw materials for the ham were pork shoulder
and leg muscles, with the final product containing 4% fat. The recipe
consisted of meat, water, sodium chloride, glucose, sodium diphosphate, carrageenan, sodium ascorbate, sodium nitrite, and spices.
After grinding of the meat through a plate with 25-mm openings,
spices and curing brine were added and the mixture was tumbled
for approximately 6 h before stuffing into polyamide casings with
a diameter of 92 to 94 mm (Naturin GmbH & Co. KG, Germany).
The hams were heated to a core temperature of 74 ◦ C, with consecutive cooling in brine to a core temperature of approximately
−1 ◦ C. After cooling the hams were stored at 2 to 4 ◦ C for 6 to
24 h before slicing and packaging. The residual nitrite content in
the cooked ham was 12.80 mg/kg as analyzed by Norsk Matanalyse (Oslo, Norway) according to European prestandard ENV120143:1998E (spectrophotometrically measured at 540 nm using the diazotization reaction of sulphanilamide and subsequent coupling
with N-(1-naphthyl)ethylenediamine).
Packaging of product
The ham was cut in slices with a thickness of approximately
0.7 mm and packaged in semirigid trays on a commercial thermoforming machine. The packages were evacuated and subsequently
flushed with 100% N 2 before sealing. The size of the packages was
105 mm (width) × 160 mm (length) × 27 mm (depth). The ham
was packaged in 2 different quantities, 120 or 80 g in each package, giving a GP ratio of 2.6 and 4.1, respectively. Both quantities
were packaged in 2 different packages with slightly different OTR.
Although both the packages had relatively low OTR values, they are
denoted high- and low-OTR packages in the following. Combining
the same flexible, transparent top web with 2 different semirigid,
transparent bottom webs made the different packages. The main
distinction of the bottom webs was different thickness of the ethylene vinyl alcohol (EVOH) barrier layer, 3 μm in the high-OTR pack-
age and 8 μm in the low-OTR package. The OTR for the flat webs,
according to information from the film suppliers, were 2 mL O 2 /m2
× 24 h (23 ◦ C, 0% RH) for the top web, <3 mL O 2 /m2 × 24 h (23 ◦ C,
50% RH) for the high-OTR bottom web (high-OTR package) and
1 mL O 2 /m2 × 24 h (23 ◦ C, 50% RH) for the low-OTR bottom web
(low-OTR package). After packaging the ham was transported to
Matforsk for storage and analyses.
Selection and grouping of packages with different
residual oxygen levels
The O 2 concentration in the packages was measured at Matforsk
at the day of packaging. The O 2 concentration was measured by
removing 10-mL gas samples with a 10-mL Plastipak disposable syringe (Becton Dickinson SA, Madrid, Spain) with a TO-120 needle
with side-hole (G Wöhl Consulting AB, Vintrosa, Sweden). The samples were withdrawn through self-sealing Toray rubber seal patches
(Part. nr. T0125; Toray Engineering Co. Ltd., Tokyo, Japan) attached
to the packages. The gas samples were injected into a MOCON/Toray
oxygen analyzer LC-700F (Modern Controls Inc., Minn., U.S.A.) with
a zirconium oxide cell. The system accuracy of the oxygen analyzer
was ±2% in the interval 0.00 to 50.00% O 2 and ±3% in the interval
0.000 to 0.500% O 2 . The oxygen analyzer was calibrated before starting the measurement of the O 2 concentration, toward air (21 mol%)
in the upper part of the scale and a calibration gas containing 0.21
mol% in nitrogen (Yara A/S, Norway) in the lower part of the scale.
In order to obtain the targeted (highest) levels of residual oxygen,
a few mL of air was injected into some of the packages by using
the disposable syringe. After measurement and adjustment of the
residual oxygen in all the packages, the packages with 120 and 80 g
product were divided into 4 groups with different levels of residual
oxygen; see Table 1.
Storage, light exposure, and experimental setup
After dividing the packages in groups with different oxygen levels,
the packages were stored in darkness at 4 ◦ C for up to 33 d. Samples for oxygen analysis (O 2 concentration before light exposure, 3
replicates of each) were removed after 3, 5, 10, 15, 20, and 33 d of
storage, with subsequent light exposure for 48 h before visual color
evaluation.
The light exposure period was selected on the basis of data supplied from another subproject in the study carried out by Østfold Research Foundation (Fredrikstad, Norway). Data from 10 retail stores
showed that the ham product used in the experiment on an average
was exposed to light for approximately 24 h before sale. To simulate
a worst case condition and for practical reasons, an exposure time
of 48 h was chosen in the experimental setup.
The light exposure of the packages with ham was performed at
4 ◦ C with fluorescent light tubes placed at a distance of approximately 20 cm over the shelves. The light tubes were Osram L36W/76
Natura de luxe (Osram AS, Oslo, Norway), which are similar to the
light tubes commonly used in supermarkets and recommended for
lighting of cooked meat products. The light intensity was in the range
5 to 7 watt/m2 (corresponding to 1000 to 1800 lx for these light tubes).
Light intensity was measured by a calibrated spectroradiometer
S: Sensory & Nutritive Qualities of Food
Table 1 --- Target and actual oxygen level in the high and low OTR packages with 120 and 80g ham
Actual range of measured O 2 levels
Target level % oxygen
0.10
0.20
0.30
0.40
High OTR --- 120 g
Low OTR --- 120 g
High OTR --- 80 g
Low OTR --- 80 g
0.12–0.17
0.20–0.22
0.26–0.33
0.38–0.46
0.13–0.17
0.19–0.22
0.28–0.32
0.34–0.40
0.09–0.13
0.18–0.23
0.26–0.32
0.36–0.43
0.09–0.13
0.18–0.23
0.26–0.34
0.35–0.39
Color fading of cooked ham . . .
(Apogee/StellarNet Spectroradiometer with spectraWiz software,
mod. SPEC-UV/PAR, Apogee Instruments Inc., Logan, Utah, U.S.A.).
After 48 h of light exposure, a visual color evaluation was performed by a laboratory panel on the intact packages with ham under
the fluorescent tubes in the chilling chamber. After the visual color
evaluation, the O 2 concentration after light exposure was measured
with subsequent opening of the package and instrumental color
measurements. Microbial analysis was performed at day 0, 12, 22,
and 35. The experimental setup is shown in Table 2.
Analyses of the packages and the product
Measurement of oxygen concentration in the headspace of
the packages. The O 2 concentration in the packages was measured
after various days of storage according to Table 2. Oxygen analyses
were performed before and after illumination by removing 10-mL
gas samples with a syringe through self-sealing patches attached to
the packages as described earlier. New packages were applied for
each measurement during the storage experiment.
Visual color evaluation
Oslo, Norway). Serial 10-fold dilutions were made and 100 μL of the
appropriate dilution were spread on duplicate plates on PCA (Plate
Count Agar; Oxoid AS, Oslo, Norway) for enumerating total counts
of bacteria and MRS Broth (Oxoid AS, Oslo, Norway) with pH adjusted to 5.7 for lactic acid bacteria. The PCA plates were incubated
aerobically at 20 ◦ C for 3 d and the MRS plates aerobically at 30 ◦ C
for 3 d.
Measurement of OTR of the packages
OTR values of 3 replicates of the high- and low-OTR packages
were measured by the AOIR method (Larsen and others 2000). The
packages were flushed with nitrogen, and the O 2 concentration in
the packages was measured after 1 and 5 d of storage at 4 ◦ C and approximately 60% RH outside and 100% RH (5 mL water) inside. The
O 2 concentration was measured by the use of a specially designed
syringe for gas sampling, and the gas sample was injected into the
MOCON/Toray oxygen analyzer LC-700F as described earlier. The
OTR of the packages was calculated according to the equations given
in the paper by Larsen and others (2000).
A 5-member experienced laboratory panel performed the visual
color evaluation. The panelists were habitual with these types of
meat color measurements, and prior to the experiment they were
trained on samples with different discoloration and the application
of the color scale. The color of the ham in the different packages was
evaluated through the transparent top web. A 5-point color scale
was used including: 1 = bright pink, 2 = pink, 3 = slightly pink (still
marketable), 4 = slightly gray (not marketable), and 5 = extremely
gray (not marketable). The area of discoloration was assessed using
a scale of 1 = none, 2 = 0 to 10%, 3 = 11% to 20%, 4 = 21% to 60%,
and 5 = 61% to 100% of the light exposed area (adapted from AMSA
[1991]).
On the basis of two O 2 concentration measurements in the
nitrogen-flushed packages (after 1 and 5 d), the OTR of the packages was determined according to the AOIR method as described
above. Additionally, by using the mathematical framework developed for the AOIR method, the O 2 concentration can be calculated
(predicted) at any time in the nitrogen-flushed packages, as verified
in the paper by Larsen and others (2002). The change in headspace
oxygen concentration was calculated over a time period of 33 d for
empty packages flushed with nitrogen and stored at 4 ◦ C.
Instrumental color measurements
Statistical analysis
∗ ∗ ∗
CIE L a b values (lightness, redness, yellowness) of the ham were
measured with a Minolta Chroma Meter CR-300 (Minolta Camera
Co., Osaka, Japan) with 8-mm viewing port, 20◦ viewer angle, and
illuminant D 65 . Before each measurement, the instrument was calibrated against a white tile (L = 97.16, a = 0.25, and b = 2.09). The measurements were performed through a thin, transparent polyethylene
film, and for each package of ham, measurements on 3 different locations were averaged. The CIE a∗ value (redness) was previously
found to give the best correlation with the visual evaluation of the
color of ham (Andersen and others 1988).Thus, the a∗ value was used
for describing the color in this experiment.
Microbial analysis
Total and lactic acid bacteria counts were performed at day 0, 12,
22, and 35 with 3 replicates of each variant at 2 oxygen levels (lowest
and highest, giving a total of 6 packages from all variants each time
of sampling). Ten grams of ham were mixed with 90 mL of peptone
water solution in a stomacher (Colworth 400, VWR International,
Calculation of oxygen concentration
in nitrogen flushed packages
Statistical analyses were performed with Minitab Release 14
(Minitab Inc., Pa, U.S.A.) and The Unscrambler ver. 9.2 (Camo Process, Oslo, Norway).
Results and Discussion
Changes in headspace oxygen concentration
in the packages during storage
The headspace oxygen concentration in the packages was influenced by the OTR of the package, residual oxygen after packaging,
and microbial activities consuming oxygen during the storage period. The interactions of these 3 phenomena in the ham packages
are summarized in Figure 1.
The OTR of packages without product was measured by the AOIR
method at the test conditions 4 ◦ C with 5 mL water inside. The OTR
values were 0.06 ± 0.00 mL O 2 /pkg × 24 h for the high-OTR package
and 0.04 ± 0.00 mL O 2 /pkg × 24 h for the low-OTR package, showing
that the thicker EVOH layer in the base web of the low-OTR package
Day of measurement
Treatmenta
High OTR --- 120 g
Low OTR --- 120 g
High OTR --- 80 g
Low OTR --- 80 g
O 2 -levels
4
4
4
4
Analyses
O 2 before light exposure
O 2 after 48 h in light
Color measurements
Visual color evaluation
Microbial analysis
0
x
x
3
5
7
x
x
x
x
x
x
10
12
x
15
17
x
x
x
x
x
20
22
x
x
x
33
35
x
x
x
x
x
x
x
x
x
a
High OTR --- 120 g: high OTR package with 120 g ham; Low OTR --- 120 g: low OTR package with 120 g ham; High OTR --- 80 g: high OTR package with 80 g ham;
and Low OTR --- 80 g: low OTR package with 80 g ham. Three replicates were analyzed from each variant.
S: Sensory & Nutritive Qualities of Food
Table 2 --- Experimental setup
Color fading of cooked ham . . .
lowered the OTR value for this package. According to the method
described by Larsen and others (2002), the changes in headspace
oxygen concentration were further calculated over a time period
of 33 d for packages flushed with nitrogen and stored at 4 ◦ C. The
calculated oxygen ingress curves are shown in Figure 1 for the highand low-OTR packages. The oxygen concentration increases more
rapidly in the high-OTR package, reaching approximately 0.60% and
0.75% oxygen in the package, given an initial oxygen concentration
of 0.15% (Figure 1a) and 0.3% (Figure 1b), respectively.
The measured headspace oxygen concentration in the packages
with ham is also shown in Figure 1. The curves are mean values averaged over all the 4 initial oxygen concentrations, showing the average changes in headspace oxygen concentration during the storage
time. Figure 1a shows the average changes with 0.15% oxygen as initial oxygen concentration and Figure 1b with 0.3% as initial oxygen
concentration. Figure 1a and b shows that the oxygen concentration
in all the packages with ham is lower than in the empty packages
and decreases with time due to oxygen consumption by microbial
activity. Figure 2 shows that the total bacteria increased during storage, while headspace oxygen decreased (Figure 1a), indicating that
with bacterial growth, there was a reduction in headspace oxygen
levels. The oxygen concentration decreased in all the packages after
approximately 15 to 20 d of storage, and the microbial load increased
from approximately 104 to 107 colony forming units (CFU) per
gram in the similar time interval. Similar growth curves as recorded
in this work are reported by other researchers with maximum
S: Sensory & Nutritive Qualities of Food
Figure 1 --- Calculated changes in oxygen concentration
in empty packages during 33 d and measured average
changes in oxygen concentration in packages with 120
or 80 g (GP ratio 2.6 and 4.1, respectively) sliced ham during 33 d of dark storage. Figure 1a with 0.15% as initial
oxygen concentration and Figure 1b with 0.3 % as initial
oxygen concentration.
microbial levels of approximately 108 CFU per gram after 20 to 30 d
of storage (Grini and others 1992; Houben and Gerris 1998; Møller
and others 2003). The growth curves for lactic acid bacteria were almost equal (not shown) to the total bacteria curves, suggesting that
most of the bacteria were different types of lactic acid bacteria. Lactic acid bacteria were also found to be dominating at the end of the
storage period in other studies with sliced bologna and ham (Ahvenainen and others 1989; Grini and others 1992; Houben and Gerris
1998).
Figure 1 also shows that the oxygen reduction tends to be highest
in the packages with 120-g product compared to packages with 80 g,
especially for the high-OTR package, probably due to a higher total
microbial activity consuming oxygen in the package. The oxygen
concentration in the high-OTR package was significantly higher (P <
0.005) than in the low-OTR package after 10, 15, and 20 d of storage,
as also observed by the statistical analysis in Table 3. This finding was
in agreement with the higher OTR value of the high-OTR package
compared to the low-OTR package.
Correlation between oxygen concentration
in the packages and color
A visually acceptable product (score 3 = slightly pink) corresponded to a CIE a∗ -value (redness) of 8 (correlation coefficient =
−0.91, P < 0.005) for this particular ham product. Samples with
a∗ -values lower than 8 had various degrees of gray color and were
considered as unfit for sale.
Figure 3 shows the correlation between the oxygen concentration
in the packages before light exposure and the a∗ -value measured after 48 h of light exposure. Figure 3 shows a high negative correlation
(linear correlation coefficient = −0.87, P < 0.005) between the oxygen concentration and the a∗ -value. Both a linear and a nonlinear
correlation line are drawn in Figure 3, and the nonlinear curve seems
to give the best fit. Figure 3 shows that as the oxygen concentration
increased, the a∗ -value decreased. At oxygen concentrations below
approximately 0.14%, most of the samples had higher a∗ -values than
8 and were considered as acceptable (samples within the marked
area in Figure 3). At oxygen concentrations higher than 0.14%, the
samples had different degrees of gray color. Samples with a∗ -values
below approximately 4 were extremely gray.
Figure 4 shows the correlation between the oxygen concentration
in the packages before light exposure and the visual evaluation of
color after 48 h of light exposure. Figure 4 also shows a high correlation (linear correlation coefficient = 0.89, P < 0.005) between the
oxygen concentration and the visual evaluation. Both a linear and a
nonlinear correlation line are drawn for visual color in Figure 4, and
Figure 2 --- Total bacteria counts on the ham in the different packages during 35 d of storage
Color fading of cooked ham . . .
Table 3 --- Effect of film permeability and GP ratio on oxygen concentration and color after various days of storage
Days of dark storage
Factor
3
5
Effect of film and GP ratio on oxygen concentration before light exposure
Package permeability (high or low)
NS
NS
GP ratio 2.6 and 4.1
NS
NS
10
15
20
33
+++
NS
+++
NS
+++
NS
NS
NS
Days of dark storage prior to illumination
Factor
3
5
10
Effect of film and GP ratio on ham color after dark storage and 48 h of light exposure
Package permeability (high or low)
NS
NS
+++
GP ratio 2.6 and 4.1
NS
NS
+
15
20
33
+++
+
+
+
NS
NS
NS: not significant.
+++: P < 0.005.
++: P < 0.01.
+: P < 0.05.
as for the a∗ -value, the nonlinear curve also seemed to give the best fit
for the visual color. At oxygen concentrations below approximately
0.17%, the majority of the samples had acceptable color with visual
color values from 1 (=bright pink) to 3 points (=slightly pink, see
samples within the marked area in Figure 4). At oxygen levels higher
than 0.17%, most of the samples had different degrees of gray color
with visual color values from 3.5 to 5 (=extremely gray).
The instrumental color analysis and the visual color analysis indicated slightly different threshold oxygen levels in the packages before one could record a shift from acceptable to unacceptable color.
The difference was very small, and was considered as insignificant
from a practical point of view. By summing up the results from both
the instrumental analysis and the visual evaluation, a highest acceptable level of 0.15% oxygen in the headspace of the packages
could be set for this particular ham product and lighting conditions to prevent color fading when exposed to light. This threshold
level was independent of the storage time before light exposure. This
finding is in accordance with results obtained by Møller and others
(2000), who found that the limit lay between 0.1% and 0.5% oxygen
in the headspace of the packages for the sliced cooked ham used in
their experiment.
Figure 3 --- Correlation between oxygen concentration in
the packages before light exposure and a∗ -value. The
solid-drawn line shows a linear relationship (r = −0.87),
and the dotted line a nonlinear relationship.
Figure 4 --- Correlation between oxygen concentration in
the packages before light exposure and visual color evaluation. The solid-drawn line shows a linear relationship
(r = 0.89), and the dotted line a nonlinear relationship.
S: Sensory & Nutritive Qualities of Food
quantities) on the oxygen level before light exposure and the ham
color after 48 h of light exposure.
Table 3 shows that the oxygen concentration in the packages was
significantly different at d 10, 15, and 20, where the high-OTR package had the highest O 2 levels (see Figure 1 as well). The different GP
ratios had no significant effect on the oxygen level. The lower oxygen concentration in the low-OTR package also affected the color.
The ham in the low-OTR package had higher a∗ -values than the
high-OTR package after 10, 15, and 20 d of dark storage prior to illumination. Even if the different GP ratios did not show any significant
effect on the oxygen concentration, the GP ratio was found to significantly influence the color of the ham after 10, 15, and 20 d. At
these points of time, the average a∗ -values for the packages with GP
ratios of 4.1 and 2.6 were approximately 4.75 and 6.50, respectively.
The average a∗ -values were accordingly below the acceptance limit
of 8 for both GP ratios, but the ham in the packages with GP ratios
of 4.1 had an overall more gray color due to a more severe oxidation.
Our findings are in accordance with the results obtained by Møller
and others (2003). They stated that a low GP ratio should be used
to maintain a high a∗ -value, but the GP ratio became less important at increased oxygen levels. Above 0.85% oxygen there was sufficient oxygen available for pigment degradation regardless of the GP
Statistical analysis of the effect of films and GP ratio
ratio.
Analysis of variance (ANOVA) was performed analyzing the effect
A higher GP ratio could on the other hand be partially compenof different films and GP ratios (2.6 and 4.1 due to different product sated for by a lower oxygen concentration in the packages. The
Color fading of cooked ham . . .
oxygen analyses performed in this experiment, given in Table 1,
showed that the residual oxygen level in general was slightly lower
in the packages with the highest GP ratio, probably due to better
evacuation of these packages. Both the GP ratio and the oxygen
concentration account for the total amount of oxygen available for
photooxidation (Møller and others 2003).
Oxygen level in the packages before
and after light exposure
Andersen and Skibsted (1992) and Møller and others (2002, 2005)
have shown that there is a linear relationship between oxygen pressure and light-induced degradation expressed as quantum yields. In
aqueous solutions the stoichiometry for photooxidation was even
found to be higher than 1:1, indicating that a different reaction
mechanism is operating for the photooxidation than for the thermal
oxidation. Andersen and Skibsted (1992) describe the mechanism of
photooxidation in 2 steps:
MbNO + hv → MbNO∗
(1)
where an electronically excited state of MbNO is produced by absorption of a photon, which reacts in a bimolecular process with a
ground-state oxygen (triplet state: 3 O 2 ):
MbNO∗ + 3 O2 → MMb + other products
(2)
giving metmyoglobin (MMb) and other products. At present it is
not entirely clear what type of reaction is degrading MbFe(II)NO
upon light exposure, but free radical processes are suggested to be
involved in the degradation process (Møller and others 2002, 2005).
The equations above show that oxygen is consumed in the photooxidative reaction.
Figure 5 shows how the oxygen concentration rapidly decreased
in 1 set of packages with 4 different initial oxygen levels during 48 h
of light exposure.
S: Sensory & Nutritive Qualities of Food
Figure 5 --- Headspace oxygen levels (mL) in packages before and after 48 h of light exposure (high-OTR package,
120 g, 10 d of dark storage before oxygen measurement
and light exposure). The oxygen concentration (%) in the
packages before light exposure and the a∗ -values are also
shown for each level and the dark reference sample.
The highest oxygen consumption was observed in the packages
with the highest oxygen level before light exposure, and the color
of the ham was most severely degraded in these packages as well
as shown in Figure 5. The ham in the packages stored in darkness
did not consume oxygen. Rather, the oxygen concentration slightly
increased due to oxygen ingress through the package.
With oxygen levels lower than approximately 0.15% in the packages (corresponding to 44 and 46 mL oxygen in the packages with
GP ratio of 2.6 and 4.1, respectively), the total amount of oxygen
molecules available for photooxidation was not sufficient to give a
significant visible color degradation for the ham product and package used in this experiment.
In order to avoid color fading of cooked meat products, the meat
processors can choose 2 main strategies; eliminating oxygen or light.
This work has demonstrated that in order to eliminate oxygen to the
essential low level, the packaging material must have a very high oxygen barrier and the running of the packaging machine must be performed by well-trained operators to maintain a low residual oxygen
level after packaging. The oxygen measurements on the day of packaging showed a relatively large variation in the oxygen level in the
packages produced under equal conditions. The variation in residual oxygen level was most probably due to different effectiveness
on the 4 parallel lines of the packaging machine in vacuuming and
flushing the packages. Good maintenance routines for the packaging machines and training of the operators are therefore important
to secure optimal running of the machines.
Andersen and others (1988) found for vacuum-packed sliced
cooked ham that an initial dark storage period of 4 d prior to light
exposure improved the color stability due to efficient depletion of
oxygen in the packages during the 4 d of storage. Oxygen depletion
did not occur until after 15 to 20 d in the packages in this experiment. The product is usually exposed to light in the retail store before
15 d, indicating that an oxygen level above the critical limit would
cause discoloration in many of the packages. A more effective strategy would be to lower the residual oxygen level in the packages at
the time of packaging.
Oxygen can also be eliminated by the use of oxygen absorbers, as
demonstrated by, for example, Grini and others (1992) in packages
for sliced bologna. The largest single category of products currently
employing oxygen absorber technology is sliced meat products such
as smoked and cured meats (Solomon 2004). Different absorber solutions are available, depending on the consumer acceptance in
different countries. The use of oxygen absorbers is not yet common
in Northern Europe due to legal limitations, negative consumer attitudes, and the cost of the absorber.
Eliminating light exposure is another strategy to reduce the problem with discoloration of sliced meat products. The most commonly used packaging solution for sliced meat products in Southern Europe is a printed top web with light barrier combined with
a transparent thermoformed bottom web. The printed top web is
turned toward the fluorescent light tubes and the consumer, but the
consumer can evaluate the product appearance just by turning the
package upside down. A higher headspace oxygen concentration
could probably be accepted by using this solution, and an optimal
running of the packaging machine would be less critical.
I
Conclusion
n order to maintain an acceptable color of this particular ham
product when exposed to commercial retail light conditions, the
highest acceptable level of oxygen in the headspace of the packages was 0.15% oxygen. This threshold level was independent of the
storage time before light exposure. A residual oxygen level of below 0.15% just after packaging combined with the package with the
Color fading of cooked ham . . .
Acknowledgments
The authors thank Gilde Fellesslakteriet and Gilde Vest for the support of product and use of packaging machinery, and Wihuri OY
Wipak (Nastola, Finland) and Bemis Valkeakoski Oy (Valkeakoski,
Finland) for the supply of film materials used in the project. We
would also like to thank Aud Espedal, Karin Solgaard, Anne-Kari Arnesen, and Janina Berg for skilful assistance performing the oxygen
analyses, visual evaluation, and microbial analyses.
The work was performed within a collaboration project between
Matforsk, Østfold Research Foundation, Gilde Norge BA, Prior Hærland AS, and Fjordland AS. The Research Council of Norway provided
financial support.
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S: Sensory & Nutritive Qualities of Food
lowest OTR (0.04 compared with 0.06 mL O 2 /pkg × 24 h) maintained
the color of the tested ham product throughout the entire storage
period. However, if the residual oxygen level just after packaging was
above approximately 0.15%, applying the package with the lowest
OTR did not prevent color degradation when the ham was exposed
to light during the first 10 d of storage. The oxygen level in most of the
packages decreased at the end of the storage time due to microbial
activity, and the color fading of the ham when exposed to light was
hence less pronounced at the end of the storage time. The color of
the ham in packages with the GP ratio of 2.6 was significantly better
than in packages with a ratio of 4.1 after 10, 15, and 20 d of storage.