JFS:
Food Chemistry and Toxicology
Mechanisms of Pink Color Formation in
Irradiated Precooked Turkey Breast Meat
K.C. NAM AND D.U. AHN
Food Chemistry and Toxicology
ABSTRACT: Precooked turkey breast meat was aerobically packaged or vacuum-packaged and irradiated at 0, 2.5,
or 5.0 kGy. CIE color, reflectance, oxidation-reduction potential (ORP), gas production, and lipid oxidation were
determined at 0, 7, and 14 d. Irradiation increased redness of vacuum-packaged meat, and the redness was distinct
and stable under vacuum. Irradiation decreased ORP and produced carbon monoxide (CO). This indicated that the
pink color was caused by the heme pigment-CO complex formation. The reflectance of meat and the absorption
spectra of myoglobin solution supported the assumption that denatured CO-myoglobin is the pigment in irradiated
precooked turkey breast.
Keywords: irradiation, color, orp, co, precooked turkey breast
Introduction
C
OLOR IS THE MAJOR SENSORY ATTRIBUTE
determining consumer acceptance of
meat. The normally expected color for
cooked poultry breast meat is grayish
brown. Whenever cooked poultry breast
meat shows pink or red color, consumers
suspect that the meat is undercooked or
contaminated. The pink defect in cooked
meat could be produced by incompletely
denatured myoglobin or oxymyoglobin,
reduced globin hemochromes of wellcooked meat, contamination with nitrite or
nitrate, or the absorption of combustion
gases such as nitric oxide (NO) or carbon
monoxide (CO) (Cornforth and others
1986). This pink defect should be focused
on poultry breast meat because it is more
susceptible to pink color formation than
highly pigmented beef in the presence of
sodium nitrate (Heaton and others 2000).
Irradiation is an excellent method to
improve microbial safety of meat, but it
can have a negative effect on meat color,
especially that of cooked meat. Irradiated
raw chicken and turkey breast muscles
had increased redness and the increased
pink color was stable during refrigerated
storage (Millar and others 1995; Nanke
and others 1998). The brown color of
cooked meat can also be partially converted to red by ionizing radiation. Hanson
and others (1963) observed an objectionable red color in radiation-sterilized
cooked chicken meat in the absence of oxygen. Jo and others (2000) found a significant increase in redness in cooked irradiated pork sausages.
Identifying the pigment responsible
for the pink color is a prerequisite to pre600
venting the color problem in irradiated
cooked meat. Nanke and others (1998)
speculated that the pink color produced in
irradiated raw pork and turkey was induced by the formation of an oxymyoglobin-like pigment. Millar and others (1995)
found that irradiated chicken breasts underwent a definite change during irradiation from the usual brown/purple color to
a more vivid red/pink color, and postulated that the red/pink color might be a ferrous myoglobin derivative such as carboxy-myoglobin or nitric oxide-myoglobin
rather than oxymyoglobin. These studies,
however, were conducted with fresh meat,
and the identification and characterization of the pigments produced by irradiation were not done.
The denatured heme pigments in
cooked poultry breast muscles are chemically determined by the status of heme
iron and the sixth ligand molecule attached to heme-iron. Ahn and Maurer
(1990a,b) reported that many ligands can
bind with denatured heme pigments and
that the binding of heme pigments with
ligands could increase the intensity of
pink color in oven-roasted turkey breast.
For denatured heme pigments to impart a
pink color in fully cooked meat, irradiation
should provide reducing conditions plus
ligand molecule(s) with strong binding affinity to the heme iron. Nam and Ahn
(2002) found that irradiation of raw turkey
breast decreased oxidation-reduction potential (ORP) and produced gas compounds that can act as a sixth ligand of
myoglobin. Therefore, we can hypothesize
that production of certain gas compounds
and increased reducing conditions in-
JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 2, 2002
duced by irradiation can be responsible
for the pink color forming in precooked
turkey breast.
Color changes in irradiated raw meats
have been reported in many irradiation
studies, but little information is available
on the color changes of cooked meat. The
increase of redness by irradiation in raw
meats varies depending on species, muscle type, irradiation dose, and packaging
environment (Ahn and others 1998a). No
attempt, however, was made to elucidate
the mechanisms of color changes and to
characterize the color compounds in irradiated precooked meat. To identify the
pigment in irradiated precooked meat
would be important to establish methods
to control or modify the color in irradiated
cooked meat.
The objectives of this study were to determine the effects of packaging and storage on pink color formation in irradiated
precooked turkey breast and to elucidate
the mechanism involved in the generation
of color compounds responsible for the
pink defect.
Materials and Methods
Sample preparation and
irradiation
Pectoralis major muscles from 50 turkeys were randomly grouped for 8 replications. The muscles were ground twice
through a 3-mm plate and meat rolls (diameter 12cm, length 20cm) were prepared. The rolls were then cooked in a
smoke house to an internal temperature
of 75 8C. After chilling in running cold water for 1 hr, the rolls were sliced to 2.5-cm© 2002 Institute of Food Technologists
thick pieces, then aerobically repackaged
in polyethylene oxygen-permeable (4” 3
6”, 2-mil, Associated Bag Company, Milwaukee, Wis.,U.S.A.) or vacuum-packaged
in oxygen-impermeable bags (nylon/polyethylene, 9.3 mL O2/m2/24 hr at 0 8C;
Koch, Kansas City, Mo.,U.S.A.). After packaging, the meat portions were irradiated
using a Linear Accelerator (Circe IIIR; Thomson CSF Linac, Saint-Aubin, France).
The target doses of irradiation were 0, 2.5,
and 5.0 kGy. The energy and power level
used were 10 MeV and 10 kW, respectively,
and the average dose rate was 95.5 kGy/
min. The max/min ratio was approximately 1.28 for 2.5 kGy and 1.18 for 5.0 kGy. To
confirm the target dose, 2 alanine dosimeters per cart were attached to the top and
bottom surface of a sample. The alanine
dosimeter was read using a 104 Electron
Paramagnetic Resonance Instrument
(Bruker Instruments Inc., Billerica,
Mass.,U.S.A.). The irradiated samples
were stored at 4 8C for up to 14 d. During
the storage, samples were exposed to a
light source (Philips, fluorescent 40W
Cook White). Color value, reflectance
scanning, gas production, ORP, and lipid
oxidation of the samples were determined
at 0, 7, and 14 d of storage.
Color measurement and
reflectance scanning
The surface and internal CIE color L(lightness), a-(redness), and b-(yellowness) values of samples were obtained
(AMSA 1991) with a LabScan spectrophotometer (Hunter Associated Labs., Inc. Reston, Va., U.S.A.) that had been calibrated
against a black and a white reference tile
covered with the same packaging bags
used for samples. For the internal color
measurements, the center of meat samples
was cut immediately before reading. An average value from 2 random locations on
each sample surface was used for statistical
analysis. Reflectance spectra were obtained from the scanning mode of the LabScan spectrophotometer over the range of
400 to 700 nm wavelength with an interval
of 10 nm. Reflectance spectra at 7 days
were reported here. Illuminant source and
other conditions were the same as in measuring CIE color values. Data from 8 reflectance spectra at each wavelength were averaged by treatment and converted into a
reflectance curve using an Excel program
(Microsoft Corp., Seattle, Wash.,U.S.A.).
ORP and lipid oxidation
The method of Moiseeve and Cornforth (1999) was modified to determine
the change of ORP in meat samples using
a pH/ion meter (Accumet 25; Fisher Scientific, Fair Lawn, N.J., U.S.A.). A platinum
electrode filled with a 4M KCl solution saturated with AgCl was tightly inserted in
the center of a meat sample. A small pore
was made by a cutter before the insertion
of an electrode to minimize effect of oxygen in air. A temperature-reading sensor
was also inserted to compensate for the
temperature effect. ORP readings (mV )
were recorded in exactly 3 min after the insertion of an electrode to stabilize and
equilibrate the reaction between electrode and sample.
Lipid oxidation was determined by the
method of thiobarbituric acid-reactive
substances ( TBARS) measurement (Ahn
and others 1998b). Minced meat (5 g) was
placed in a 50-mL test tube and homogenized with 15 mL deionized distilled water
(DDW ) using a Brinkman polytron ( Type
PT 10/35; Brinkman Instrument Inc.,
Westbury, N.Y., U.S.A.) for 15 s at high
speed. The meat homogenate (1 mL) was
transferred to a disposable test tube (13 3
100 mm) and butylated hydroxytoluene
(7.2%, 50 mL) and thiobarbituric acid/
trichloroacetic acid (15 mM TBA/15% TCA)
solution (2 mL) were added. The mixture
was vortexed and then incubated in a
90 8C water bath for 15 min to develop color. After cooling for 10 min in cold water,
the sample was vortexed and centrifuged
at 3,000 3 g for 15 min at 5 8C. The resulting upper layer was determined at 531 nm
against a blank containing 1 mL DDW and
2 mL TBA/TCA solution. The amounts of
TBARS were expressed as mg malondialdehyde per kg meat.
mainly analyzed during the storage.
The method of Furuta and others (1992)
was modified to detect carbon-related gases. Minced meat portions (10 g, 1 to 2 mm
thick) was placed in a 24-mL wide-mouth
screw-cap
glass
vial
with
a
Teflon*fluorocarbon resin/silicone septum
(I-Chem Co., New Castle, De., U.S.A.). The
vial was microwaved for 10 s at full power to
release gas compounds from meat sample.
After 5 min cooling in room temperature,
the headspace-gas (200 mL) was withdrawn
using an air-tight syringe and injected into
a split inlet (split ratio 9:1) of a GC. A Carboxen-1006 Plot column (30 m 3 0.32 mm
i.d.; Supelco (Bellefonte, Penn., U.S.A.) was
used, and a ramped oven temperature was
programmed (50 8C, increased to 180 8C at
25 8C/min, increased to 200 8C at 508 C/
min). Helium was the carrier gas used at a
constant flow of 2.4 mL/min. FID equipped
with a nickel catalyst (Hewlett-Packard Co.,
Wilmington, Del., U.S.A.) was used and the
temperatures of inlet, detector, and nickel
catalyst were set at 250, 280, and 375 8C, respectively. Detector (FID) air, H2, and
make-up He gas flows were 400, 40, and 50
mL/min, respectively. The identification of
gaseous compounds was achieved using
standard gases and GC-MS (Model 5873;
Hewlett-Packard Co.), and the area of each
peak was integrated by using Chemstation
software (Hewlett-Packard Co.). To quantify the amount of a gas released, a peak
area (pA*sec) was converted to a concentration (ppm) of gas in the headspace (14
mL) from 10 g meat compared to CO2 concentration (330 ppm) in air.
Analysis of gas compounds
Absorption spectra of myoglobin
derivatives
Carbon monoxide (CO), nitric oxide
(NO), and hydrogen sulfide (H2S) gas standards were purchased from Aldrich (Milwaukee, Wis., U.S.A.), and hydrogen (H 2),
methane (CH 4), and carbon dioxide (CO 2)
from Praxair (Danbury, Conn., U.S.A.) to
identify gas compounds generated by irradiation in meat samples. The standard gases were analyzed using a gas chromatograph (GC, Model 6890; Hewlett-Packard
Co., Wilmington, Del., U.S.A., USA)
equipped with either flame ionization detector or thermal conductivity detector with
or without a Nickel catalyst. A Supel-Q or
Carboxen-1006 Plot column (30 m 3 0.32
mm i.d.; Supelco, Bellefonte, Pa., U.S.A.)
was used to determine sulfur or carbon gas
compounds, respectively. Among the gas
compounds detected in meat samples, the
production of carbon monoxide, methane,
and carbon dioxide were irradiation dosedependent. Thus, these carbon gases were
To compare the reflectance minima of
meat samples with absorption maxima of
natural and denatured myoglobin derivatives were prepared. Equine myoglobin
(Sigma Chemical Co., St. Louis, Mo.,
U.S.A.) was dissolved in 0.1 M citratephosphate buffer (pH 6.0) to make 2 mg/
mL myoglobin solution. Myoglobin solution (3 mL) was placed in a wide-mouth
screw-cap glass vial with a Teflon*fluorocarbon resin/silicone septum. Half of the
samples were microwaved for 15 s to denature the protein. The solution was converted to a reduced form by adding 200 mL sodium hydrosulfite (10% Na 2S2O 4).
Immediately after injection of the ligand
gas (3 mL; oxygen, carbon monoxide, or nitric oxide) into the pigment solution (using
an air-tight syringe), the solution was
scanned in the range of 400 to 700 nm using a spectrophotometer (Beckman DU
640; Beckman Instruments, Inc., Fuller-
Vol. 67, Nr. 2, 2002—JOURNAL OF FOOD SCIENCE
601
Food Chemistry and Toxicology
Color of irradiated precooked turkey meat . . .
Color of irradiated precooked turkey meat . . .
Table 1—CIE color values of precooked turkey breast meat with different packaging, irradiation dose, and storage
Aerobic packaging
Storage
0
Food Chemistry and Toxicology
Surface color
L-value
0 Week 81.25
1 Week 80.73
2 Week 81.21
SEM
0.50
a-value
0 Week
6.85x
1 Week
6.25y
2 Week
6.06y
SEM
0.28
b-value
0 Week 18.82 ay
1 Week 18.57 ay
2 Week 19.59x
SEM
0.19
Internal color
L-value
0 Week 83.98xy
1 Week 84.91x
2 Week 81.79y
SEM
0.77
a-value
0 Week
8.09c
1 Week
8.56b
2 Week
8.48
SEM
0.36
b-value
0 Week 14.96x
1 Week 14.58x
2 Week 13.53y
SEM
0.25
Vacuum packaging
2.5 kGy
5.0 kGy
SEM1
0
80.23
80.87
81.06
0.50
81.37
80.92
81.29
0.41
0.44
0.55
0.42
7.39x
6.28y
6.04y
0.29
7.42x
6.86y
6.43y
0.32
18.54 ay
17.72 bz
19.43x
0.20
83.82
82.66
82.16
0.93
9.16bx
9.85ax
7.84y
0.37
15.88x
15.49x
14.12y
0.39
2.5 kGy
5.0 kGy
SEM
81.28
81.33
79.58b
0.50
80.60
80.97
80.61ab
0.66
80.81
80.60
81.52a
0.74
0.57
0.79
0.53
0.21
0.33
0.34
7.24by
7.36cy
8.16bx
0.20
9.67ax
8.49by
9.71ax
0.26
9.87a
9.76a
10.08a
0.23
0.29
0.20
0.19
17.96by
17.58by
19.75x
0.19
0.18
0.13
0.24
19.07 ax
17.85 ay
19.10 ax
0.25
17.52b
16.69b
17.77b
0.36
16.99 bx 0.20
14.81cz 0.28
15.98cxy 0.39
0.28
82.99
82.84
81.85
0.80
0.78
0.66
1.03
84.36a
83.50
82.79
0.69
84.78a
84.01
82.41
0.77
81.49b
81.39
82.98
0.89
10.81ax
10.50ax
8.10y
0.40
0.24
0.38
0.46
7.84cy
9.42bx
7.04cy
0.30
9.47b
9.79b
8.82b
0.27
14.97x
15.39x
13.69y
0.33
0.29
0.33
0.36
15.65y
16.65x
13.82z
0.26
15.82x
15.86x
13.28y
0.46
0.80
0.94
0.59
12.40 ax 0.45
11.55 ax 0.21
9.40ay 0.17
0.62
15.94x
16.11x
12.75y
0.53
0.46
0.48
0.34
1 Standard error of the means.
Table 2—Oxidation-reduction potential (ORP) and TBARS values in precooked
turkey breast meat with different packaging, irradiation dose, and storage
Aerobic packaging
Storage
0
2.5 kGy
5.0 kGy
ORP (mV)
0 Week
-19ay
-49by
-62by
1 Week
102ax
68bx
75bx
2 Week
113ax
84bx
82bx
SEM
7
6
4
TBARS (mg malondialdehyde/kg meat)
0 Week
2.01y
4.09y
3.71z
1 Week
7.11x
9.43x
6.25y
2 Week
6.91 bx
9.76ax
9.83ax
SEM
0.51
0.90
0.60
Vacuum packaging
SEM1
0
6
7
4
-48ay
-49ay
-18ax
4
-71ay
-53bxy
-41abx
5
-104by
-50abx
-59bx
11
8
7
8
0.39
0.80
0.77
1.49x
1.88ax
1.01y
0.12
1.22
1.27b
0.93
0.07
1.05
1.30b
0.98
0.09
0.14
0.08
0.08
2.5 kGy 5.0 kGy
SEM
1 Standard error of the means.
a,bDifferent letters within a row with same packaging are different (p , 0.05).
x-zDifferent letters within a column with same irradiation dose are different (p , 0.05).
ton, Ca., U.S.A.). The scanning interval
was 1 nm and the experiments were replicated 4 times. Absorbance data at each
wavelength were averaged and converted
into a graph using a spreadsheet program
(Excel, Microsoft Corp., Seattle, Wash.,
U.S.A.).
Statistical analysis
The experimental design was to deter602
mine the effects of irradiation, packaging,
and storage time on color change, gas production, ORP, and lipid oxidation in samples during the 14 d of storage. Data were
analyzed using SAS software (SAS 1985) by
the generalized linear model procedure;
the Student-Newman-Keuls’ multiple
range test was used to compare differences among means. Mean values and standard error of the means (SEM) were re-
JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 2, 2002
ported. Significance was defined at p
0.05. Pearson’s correlation coefficients between color values, irradiation dose, storage time, ORP, CO, and TBARS within the
same packaging environment were calculated.
Results and Discussion
Color values
Irradiation increased the redness (avalue) of precooked turkey breast except
for the surface color with aerobic packaging ( Table 1). The increased redness was
more distinct in vacuum than under aerobic condition, and it was also greater inside than on the surface. With aerobic
packaging, irradiation did not influence
the surface color of cooked turkey breast.
The surface color was grayish brown regardless of irradiation, and the a-values
decreased due to color oxidation during
storage. The pink color intensity of the inside of cooked meat was stronger in irradiated meat than the nonirradiated, and the
a-value was irradiation dose-dependent.
The pink color inside of aerobically packaged meat, however, mostly discolored to
brown or yellow regardless of irradiation at
2 weeks because of pigment oxidation.
Under vacuum packaging, the increased redness was irradiation dose-dependent both at surface and inside, and it
was stable during the storage. The increased pink color was not occurred in any
localized area, but was uniformly found
throughout the meat. The result is consistent with that of Luchsinger and others
(1991) who reported that increased red
color in irradiated pork was more intense
and stable with vacuum packaging than
aerobic conditions during refrigerated
storage. Satterlee and others (1971) reported that the presence of air slightly inhibited the formation of red color in irradiated bovine metmyoglobin solutions.
Therefore, the red color formed by irradiation produced in mainly anoxic conditions
and the pigment generated by irradiation
cannot be regarded as only an oxygen-related pigment.
The lightness (L-value) of precooked
turkey breast was not much different regardless of packaging, irradiation, and
storage time. Irradiation decreased the
surface yellowness (b-value) of precooked
turkey breast with both vacuum and aerobic packaging. Regardless of irradiation, bvalue of aerobically packaged meat surface increased, but that of the inside
decreased with storage. The internal color
a-values were higher than those of surface
whereas the internal color b-values were
Color of irradiated precooked turkey meat . . .
ORP and lipid oxidation
ORP and lipid oxidation (TBARS) were
determined to elucidate oxidative changes in heme pigments of precooked turkey
breast ( Table 2). Irradiation decreased
ORP of precooked turkey breast in both
aerobic and vacuum packaging, but vacuum-packaged turkey breast had significantly lower ORP value than the aerobically packaged at 0 wk. Cornforth and others
(1986) reported that hemochrome formation was promoted by reducing conditions
and prevented by oxidizing conditions.
Shahidi and others (1991) showed that irradiation increased the reducing potential
of sodium ascorbate. We presume that irradiation and anaerobic conditions can
provide heme pigments in meat with
strongly reducing environments. We also
believe that both undenatured and denatured heme pigments in cooked turkey
may have been involved in heme-complex
formations (with ligands available under
the conditions), which will be important
for the pink color formation. Swallow
(1984) reported that hydrated electrons, a
radiolyzed radical produced by irradiation, could act as a very powerful reducing
agent, and react with ferricytochrome to
produce ferrocytochrome. The decreased
ORP by irradiation in aerobically packaged
meat, however, was not low enough to produce the distinct pink color.
The ORP increased faster under aerobic
than vacuum conditions during storage.
Within each packaging condition, however,
irradiated samples had lower ORP than the
nonirradiated during the storage. Vacuum
packaging maintained the decreased ORP
conditions produced by irradiation during
the 14 d of storage. The color of irradiated
meat was still pinker than nonirradiated
ones even after 14 d of storage under vacuum. The surface pink color generated by irradiation was stable during the storage
with vacuum packaging. This indicated
that some compounds that can make the
sixth ligand of heme pigments were generated by irradiation.
TBARS values were not directly related
to the pink color generated by irradiation.
Irradiation and storage effects on lipid oxidation were detected only in aerobically
packaged turkey breast. With aerobic
packaging, precooked irradiated turkey
breast had higher TBARS values than the
nonirradiated. The TBARS values of aerobically packaged meat increased with storage and the increase was greater by irradi-
ation. Irradiation and storage time did not
affect the lipid oxidation in vacuum-packaged meat. Although lipid oxidation was
not directly related to the pink color of irradiated precooked turkey breast, the high
TBARS values in aerobically packaged
meat partially explained the low a-values
compared with the vacuum-packaged.
Lipid oxidation proceeded along with pigment oxidation during aerobically pack-
(a)
Food Chemistry and Toxicology
lower than those of surface. This results
show that the pigment inside the meat
was not fully denatured.
(b)
Figure 1—Reflectance spectra of aerobically packaged precooked turkey breast
meat as affected by irradiation dose at 7 days of storage (A, surface; B, inside).
Vol. 67, Nr. 2, 2002—JOURNAL OF FOOD SCIENCE
603
Color of irradiated precooked turkey meat . . .
Table 3—Gas production in precooked turkey breast meat with different packaging, irradiation dose, and storage
Aerobic packaging
Storage
0
2.5 kGy
Vacuum packaging
5.0 kGy
SEM1
0
2.5 kGy
5.0 kGy
SEM
456ax
261ay
227y
23
16
22
30
227cx
154cy
130cy
14
370bx
336bxy
289by
14
575ax
558ax
450ay
16
12
14
18
26ax
9y
7y
1
1
3
3
34b
29b
26b
1
67ax
54ay
52ay
2
1
1
1
(ppm 2 )
Food Chemistry and Toxicology
Carbon monoxide
0 Week
220cx
319bx
1 Week
230bx
210by
2 Week
134y
181y
SEM
21
25
Methane (ppm)
0 Week
5cy
13bx
1 Week
9x
8y
2 Week
8x
8y
SEM
1
1
Carbon dioxide (ppm)
0 Week 3483by
4029b
1 Week 3922y
3661
2 Week 8496x
3683
SEM
816
159
5041a
3983
5961
1155
192
258
1377
4c
4c
3c
1
5970bx
6280bx
4874by
294
7300abx
6632bxy
5841ay
285
8732ax
7967axy
6522ay
477
417
384
180
1 Standard error of the means.
2 Gas concentration in headspace (14 mL) from 10 g meat.
a-cDifferent letters within a row with same packaging are different (p , 0.05).
x-zDifferent letters within a column with same irradiation dose are different (p , 0.05).
Table 4—Pearson correlation coefficientsa between color values and other factors in precooked turkey breast meat
Aerobic packaging
L–value
Surface color
Irradiation dose
Storage time
ORP b
TBARS value
Carbon monoxide
Internal color
Irradiation dose
Storage time
ORP
TBARS value
Carbon monoxide
Vacuum packaging
a–value
b–value
L–value
a–value
b–value
–0.12
–0.03
0.12
0.08
0.06
0.46
–0.83**
–0.88**
–0.70*
0.85**
–0.38
0.51
0.38
0.31
–0.60
–0.32
–0.32
–0.41
0.52
0.21
0.88**
0.05
–0.55
–0.77*
0.76*
–0.87**
0.12
0.45
0.25
–0.73*
–0.19
–0.64*
–0.55
–0.53
0.07
0.40
–0.34
–0.33
–0.36
0.69*
0.12
–0.76*
–0.55
–0.50
0.52
0.05
0.25
0.26
0.33
–0.55
0.80*
–0.40
–0.79*
0.12
0.88**
0.05
–0.80*
–0.39
0.71*
0.11
a n = 72.
b Oxidation reduction potential.
*Value with significant correlation (p , 0.05).
**Value with significant correlation (p , 0.01).
aged storage.
Therefore, ORP of meat is better indicator than TBARS values to express the reducing conditions produced by irradiation. Free radicals produced by irradiation
promoted the lipid oxidation of precooked
turkey breast meat, while generated a reducing condition for heme pigments in irradiated meat.
Production of gas compounds
Irradiation, as well as cooking, produced carbon-containing gases such as
CO, CH 4, and CO 2 (Table 3). The gases
were also detected in nonirradiated meat
samples, but they increased proportionally with irradiation dose. CO is a strong field
ligand to heme pigments, whereas CH4 or
CO2 is a very stable gas. Therefore, CO can
form complexes with heme pigments re604
sponsible for the pink color in irradiated
precooked turkey breast. Watts and others
(1978) reported that fresh raw meat exposed to low levels of CO gas turned red
color with the formation of CO-myoglobin,
which is similar to the deep red color of
blood of persons poisoned by CO inhalation. Irradiation generated CO in both aerobically and vacuum-packaged meat, but
vacuum-packaged turkey breast showed
more CO value than aerobically packaged.
The CO generated by cooking did not
influence the redness of nonirradiated
cooked turkey breast, but the CO produced by irradiation significantly increased the redness via the formation of
CO-heme pigment complex. The ORP of
nonirradiated cooked turkey breast was
too high for the heme pigments to form
complexes with ligands. Progressive loss of
JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 2, 2002
redness in meat with increasing cooking
temperature results from progressive denaturation of pigments and the brown color in cooked meat is occurred due to oxidation and denaturation of heme pigments
(Judge and others 1989). Thus, for cooked
to maintain stable pink color, strongly reducing conditions are required. During
the storage, the amount of CO decreased
in aerobically packaged irradiated turkey
breast. Most CO gas produced by irradiation escape and/or dilute under aerobic
conditions. Under vacuum-packaged conditions, on the other hand, almost all the
CO formed by irradiation and the pink irradiation color remains in the meat, even
after 14 d of storage.
Although small, the amounts of CH4 increased with irradiation and it was more irradiation dose-dependent than CO at 0
wk. Therefore, CH4 can be used as an indicator for irradiation dose. Furuta and others (1992) also reported that radiolytic CO
gas was detected in irradiated beef, pork,
and poultry meat. CO 2 was also detected
in meat in proportion to irradiation dose.
However, it cannot be used as an irradiation indicator because it is commonly used
as a gas mixture for modified atmosphere
packaging of meat. Seideman and others
(1984) reported that CO2 was beneficial in
suppressing bacterial growth, but it induced a grayish tinge in fresh meat,
whereas CO2 gas had no significant effect
on the discoloration of the red color in irradiated precooked turkey breast.
Reflectance spectra
The reflectance spectra of aerobically
packaged precooked turkey breast were
different on the surface than on the inside. On the surface of aerobically packaged precooked turkey breast, irradiation
did not affect the reflectance spectra (Figure 1A). The spectra did not show any
characteristic reflectance minima between
500- and 600-nm regions. Thus, the pigments should consist mostly of denatured
metmyoglobin or hemichrome.
The inside of aerobically packaged
nonirradiated meat, however, had a reflectance minimum, while 5-kGy-irradiated
meat had 2 distinct reflectance minima
between 500- and 600-nm regions (Figure
1B). This indicated that the main pigment
in nonirradiated meat samples should be
denatured
reduced-heme
pigment,
whereas the pigments in irradiated ones
would be denatured heme pigments with
the sixth ligand occupied. The reflectance
minima of irradiated precooked meat became shaper with increasing irradiation
dose. Irradiated samples had higher re-
flectance values than the nonirradiated at
630- and 650-nm regions at which red color represents (AMSA 1991).
Under vacuum, the surface and inside
reflectance spectra of irradiated meat
were similar (Figure 2A and 2B). But nonirradiated meat samples had a reflectance
minimum at 550 nm on the surface and 2
a)
b)
reflectance minima at 550 and 570 nm inside. Irradiated precooked turkey breast
had 2 distinct reflectance minima at 540
and 565 nm. The wavelengths of reflectance minima in 5-kGy-irradiated meat
samples were shorter than those of 2.5kGy-irradiated meats. Thus, irradiation
shifted the reflectance minima into shorter wavelengths. The reflectance minima
were sharper with increasing irradiation
dose, and irradiated samples had higher
reflectance values than the nonirradiated
ones at the red color region, which provide
a strong color contrast. Tarladgis and
Ehtashan-Ud-Din (1965) reported that
the absorption maxima of both a- and bbands shifted to shorter wavelengths with
radiation, while that of Soret (400 nm)
band moved toward longer wavelength.
The shifted reflectance minima in irradiated precooked turkey breast were compared with the absorption maxima of natural and denatured myoglobin derivatives
(Figure 3A and 3B). When reduced-myoglobin solution was flushed with CO, O2, or
NO, the absorption minima of CO-myoglobin (541, 577 nm) were positioned at
shorter wavelengths than those of oxymyoglobin (543, 580 nm) or NO-myoglobin (547, 578 nm), respectively. The absorption spectra of denatured myoglobin
solution showed that O2 and NO gas could
little bind to denatured myoglobin whereas CO had strong binding capability to the
denatured heme pigment (Figure 3B).
Therefore, CO detected in irradiated meat
samples can be considered as a sixth
ligand of denatured heme pigment, and
denatured CO-myoglobin can be suggested as a heme pigment responsible for the
pink color in irradiated vacuum-packaged
precooked turkey breast.
Correlation
Figure 2—Reflectance spectra of vacuum-packaged precooked turkey breast
meat as affected by irradiation dose at 7 days of storage (A, surface; B, inside)
Table 4 shows Pearson correlation coefficients between CIE color values and other
factors in irradiated precooked turkey
breast. In vacuum-packaged precooked
turkey breast, the a-values of both surface
and inside were positively correlated with
the irradiation dose and the amount of CO
gas produced. Although significant correlation between a-value and ORP was found
in only inside meat color, the increased avalues by irradiation were highly correlated
with ORP of meat surface at 0 wk (r = -0.73).
Therefore, the increased a-values of irradiated precooked meat with vacuum packaging could be attributed to the decreased
ORP and the formation of heme pigmentCO complex. The result also showed that
the pink pigment formed by irradiation
was considerably stable against the oxida-
Vol. 67, Nr. 2, 2002—JOURNAL OF FOOD SCIENCE
605
Food Chemistry and Toxicology
Color of irradiated precooked turkey meat . . .
Color of irradiated precooked turkey meat . . .
tion during the storage. In aerobically
packaged precooked turkey breast, the avalue of meat surface was not affected by
irradiation because of oxidation.
Conclusions
T
HE MECHANISM OF COLOR CONVERSION OF
precooked turkey breast meat by ionizing radiation can be explained as follows:
a)
Food Chemistry and Toxicology
irradiation provides reducing conditions
and produces CO to which heme pigments
can bind and increase the intensity of
pink color. Therefore, CO-heme pigment
can be a major color component responsible for the pink color in irradiated precooked turkey breast, and the pigment
formed was stable with vacuum packaging. Nevertheless, CO-Mb alone cannot
explain all the irradiated meat color. More
precise and broader analytical techniques
are needed to identify and characterize
other specific heme compounds produced
by irradiation.
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b)
Figure 3—Absorption spectra of various myoglobin (Mb) forms in solution (A,
natural; B, after heating)
606
JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 2, 2002
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Recieved 9/12/01
Journal Paper No. J-19216 of the Iowa Agriculture and Home
Economics Experiment Station, Ames, IA. Project No. 3706,
supported by the Hatch Act and S-292.
Authors are with the Animal Science Department,
Iowa State University Ames, IA 50011-3150. Direct
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