MEAT
SCIENCE
Meat Science 74 (2006) 380–387
www.elsevier.com/locate/meatsci
Influence of rosemary–tocopherol/packaging combination on
meat quality and the survival of pathogens in restructured
irradiated pork loins
K.C. Nam 1, K.Y. Ko, B.R. Min, H. Ismail, E.J. Lee, J. Cordray, D.U. Ahn
*
Animal Science Department, Iowa State University, 1221 Kildee Hall, Ames, IA 50011-3150, United States
Received 28 February 2006; received in revised form 5 April 2006; accepted 6 April 2006
Abstract
Irradiated restructured pork loins treated with rosemary–tocopherol/double-packaging had lower TBARS values than vacuum-packaged control after 10 days of refrigerated storage. The rosemary–tocopherol combination, however, had no effect on the production of
sulfur volatiles responsible for the irradiation off-odor, and color changes in irradiated pork. V7/A3 double-packaging was effective in
reducing the sulfur volatiles significantly. Rosemary–tocopherol combination was highly effective in reducing the volatile hexanal in irradiated restructure pork. Irradiation was effective in reducing Listeria monocytogenes and Salmonella typhimurium inoculated on the surface of restructured pork loin in dose-dependent manner. The irradiation D10 values for L. monocytogenes and S. typhimurium were 0.58
and 0.55 kGy, respectively. During the 20 days of refrigerated storage, L. monocytogenes in both nonirradiated and irradiated samples
grew gradually, but the number of S. typhimurium decreased. The added rosemary–tocopherol, however, showed little bacteriocidal
effects to L. monocytogenes and S. typhimurium.
2006 Elsevier Ltd. All rights reserved.
Keywords: Rosemary–tocopherol; Double-packaging; Meat quality; Survival of pathogen; Irradiation
1. Introduction
The purpose of irradiating meat is to control pathogenic
microorganisms in raw and processed meat products. Irradiation, however, induces chemical changes that can influence quality of meat. Pink color (Millar, Moss,
MacDougall, & Stevenson, 1995; Nam & Ahn, 2002) and
off-odor (Ahn, Nam, Du, & Jo, 2001; Patterson & Stevenson, 1995) are the major color problems produced in poultry meat by irradiation and these problems persists
throughout the storage period under vacuum conditions.
The major volatile compounds responsible for the characteristic off-odor in irradiated meats are sulfur compounds.
*
Corresponding author. Tel.: +1 515 294 6595; fax: +1 515 294 9143.
E-mail address: duahn@iastate.edu (D.U. Ahn).
1
Present address: Exam. Div. of Food and Biological Resources,
Korean Intellectual Property Office, Daejeon 302-701, Republic of Korea.
0309-1740/$ - see front matter 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.meatsci.2006.04.004
Carbon monoxide myoglobin was responsible for the pink
color formation in irradiated light meats (Nam & Ahn,
2003a, 2003b). Irradiation produced sulfur compounds
via the radiolytic degradation of sulfur-containing amino
acids, such as methionine and cysteine (Ahn, 2002; Jo &
Ahn, 2000). Carbon monoxide production from meat components such as asparagine, glyceraldehydes, and phospholipids (Lee & Ahn, 2004) and the increase in reducing
potential by irradiation are the mechanisms involved in
pink pigment formation in irradiated light meats. Because
consumers associate the pink color in cooked meat with
undercooked or contaminated, and off-odor with the formation of undesirable chemical compounds by irradiation,
the prevention of pink color defects and off-odor in poultry
and pork is critical for using irradiation in those meats.
Free radicals generated by irradiation are responsible
for killing microorganisms as well as initiating chemical
changes that cause potential quality problems in meat.
K.C. Nam et al. / Meat Science 74 (2006) 380–387
381
Thus, incorporation of antioxidant compounds that can
chelate free radicals to meat before irradiation would minimize the radiolytic reactions between meat components
and the free radicals. Nam and Ahn (2003b) reported that
gallate or sesamol in combination with a-tocopherol was
the best phenolic antioxidant combinations in preventing
oxidative changes in irradiated raw pork. However, these
phenolic antioxidants as purified forms are not permitted
for use in meat as antioxidants. Oleoresins extracted from
various plants contain large amounts of natural phenolic
antioxidants, however, are listed as GRAS (generally
regarded as safe) flavoring agents, and are commercially
available for use in meat products. Our preliminary study
indicated that rosemary and a-tocopherol combination
had a very strong antioxidant effect in irradiated cooked
pork loins (Nam et al., submitted).
The chemical changes in irradiated meats are highly
dependent upon packaging conditions. Most sulfur volatile
compounds produced by irradiation were highly volatile
and could be eliminated easily by storing the irradiated
meat under aerobic conditions, and the pink color formed
in irradiated turkey breast returned to normal after a few
days of aerobic storage (Nam & Ahn, 2003a). Exposing
meat under aerobic conditions, however, accelerated lipid
oxidation (Ahn et al., 2001). Double-packaging is a concept that combines aerobic and vacuum-packaging conditions to minimize lipid oxidation but maximize the
elimination of off-odor volatiles from irradiated meat during storage (Nam, Min, Lee, Cordray, & Ahn, 2004). However, double-packaging alone was not enough to control
oxidative changes in irradiated meat during storage. Our
preliminary study indicated that rosemary–tocopherol/
V7/A3 double-packaging (vacuum packaged for 7 days
and then aerobically packaged for 3 days) was very effective in preventing oxidative changes in irradiated cooked
ground pork. The objective of this study was to determine
the effects of rosemary oleoresin–tocopherol and doublepackaging combination on lipid oxidation, color, and
volatiles, and the survival of Listeria monocytogenes and
Salmonella typhimurium in irradiated restructured cooked
pork loin.
ately chilled with a cold water shower for 10 min, stored
at 4 C for 4 h, and sliced to 2 mm-thickness. No rosemary–tocopherol combination-added rolls were prepared
as a control.
For the chemical analyses, the restructured pork slices
were double-packaged: pork slices were individually packaged in oxygen permeable bags (polyethylene, 4 · 6, 2 mil.;
Associated Bag Company, Milwaukee, WI) and then a
number of aerobically packaged products were vacuumpackaged in a larger oxygen-impermeable bag (nylon/polyethylene, 9.3 mL O2/m2/24 h at 0 C; Koch, Kansas City,
MO). Vacuum-packaged slices were also prepared as controls. Samples were irradiated at 2.5 kGy using a Linear
Accelerator (Circe IIIR; Thomson CSF Linac, SaintAubin, France) with 10 MeV of energy, 10.2 kW of power
level, and 88.9 kGy/min of average dose rate, stored for 10
days at 4 C. The doubly packaged pork slices were irradiated (2.5 kGy) and stored at 4 C for 7 days and then the
outer vacuum bag was removed after 7 days of storage to
expose the meat to aerobic conditions for the remaining 3
days. The rosemary–tocopherol combination with V7/A3
double-packaging used in this study was selected from
the previous study as the most effective oleoresin antioxidant-packaging systems in preventing oxidative changes
in irradiated cooked pork. Nonirradiated and irradiated
vacuum-packaged samples were prepared as controls.
Lipid oxidation, color, and volatiles of the samples were
determined after 0 and 10 days of storage.
For the microbial studies, the surface of sliced samples
was inoculated with 0.1 mL cocktail stock suspension of
five different strains of L. monocytogenes (Scott A,
H7969, H7596, H7762, and H7962) or S. typhimurium to
give a final cell concentration of 107 CFU/g and vacuum
packaged. The samples were irradiated in duplicate at five
target-dose levels (0.5, 1.0, 1.5, 2.0 or 2.5 kGy) using a Linear Accelerator. Nonirradiated samples were served as controls. Nonirradiated and 2.5 kGy-irradiated samples were
held at 4 C up to 20 days and the survivors were enumerated every 5 days.
2. Materials and methods
Lipid oxidation was determined by a TBARS method
(Ahn et al., 1998). Minced sample (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, NY) for
15 s at high speed. The meat homogenate (1 mL) was transferred to a disposable test tube (13 · 100 mm), and 50 lL
butylated hydroxytoluene (7.2% in ethanol) and 2 mL of
thiobarbituric acid/trichloroacetic acid (20 mM TBA and
15%, w/v, TCA) solution were added. The mixture was
vortex-mixed and then incubated in a 90 C water bath
for 15 min. After cooling, the samples were vortex-mixed
and centrifuged at 3000g for 15 min. The absorbance of
the resulting upper layer was read at 532 nm against a
blank (1 mL DDW + 2 mL TBA/TCA). The amounts of
2.1. Preparation of restructured pork
Pork loin (Longissimus dorsi) muscles from eight different animals were purchased from Meat Lab., Iowa State
University (Ames, IA) and ground through a 5-mm plate.
Ground pork loin muscles were mixed with a commercially
available transglutaminase (TG, 0.3% level, 30 ppm of
enzyme; Ajinomoto, Teaneck, NJ) and rosemary–tocopherol combination (0.05–0.02% of meat weight), stuffed in
10.5-cm fibrous casings, and then stored at 4 C overnight
to allow the action of TG to cross-link proteins, peptides
and primary amines. The rolls were heat-processed in a
smokehouse to an internal temperature of 75 C, immedi-
2.2. 2-Thiobarbituric acid-reactive substances (TBARS)
382
K.C. Nam et al. / Meat Science 74 (2006) 380–387
TBARS were expressed as mg of malondialdehyde (MDA)
per kg of meat.
2.3. Color measurement
CIE color values were measured on the sample surface
using a LabScan colorimeter (Hunter Associate Labs,
Inc., Reston, VA) that had been calibrated against black
and white reference tiles covered with the same packaging
materials as used for samples. The CIE L* (lightness), a*
(redness), and b* (yellowness) values were obtained by an
illuminant A (light source). Two random readings from
both top and bottom locations on a sample surface were
used for statistical analysis.
2.4. Volatile compounds
A dynamic headspace analysis was performed using a
Solartek 72 Multimatrix-Vial Autosampler/Sample Concentrator 3100 (Tekmar-Dohrmann, Cincinnati, OH) connected to a GC/MS (HP 6890/HP 5973, Hewlett-Packard
Co.) according to the method of Ahn et al. (2001). Minced
sample (3 g) was placed in a 40-mL vial, He (40 psi) was
flushed for 3 s, and a Teflon fluorocarbon resin/silicone
septum (I-Chem Co.) was capped airtight. The maximum
waiting time in a loading tray (4 C) was less than 2 h to
minimize oxidative changes before analysis. The meat sample was purged with He (40 mL/min) for 14 min at 40 C.
Volatiles were trapped using a Tenax/charcoal/silica column (Tekmar-Dohrmann) and desorbed for 2 min at
225 C, focused in a cryofocusing module ( 80 C), and
then thermally desorbed into a column for 60 s at 225 C.
An HP-624 column (7.5 m, 0.25 mm i.d., 1.4 lm nominal),
an HP-1 column (52.5 m, 0.25 mm i.d., 0.25 lm nominal),
and an HP-Wax column (7.5 m, 0.250 mm i.d., 0.25 lm
nominal) were connected. Ramped oven temperature was
used to improve volatile separation. The initial oven temperature of 0 C was held for 1.5 min. After that, the oven
temperature was increased to 15 C at 2.5 C per min,
increased to 45 C at 5 C per min, increased to 110 C at
20 C per min, and then increased to 170 C at 10 C per
min and held for 2.25 min at that temperature. Constant
column pressure at 20.5 psi was maintained. The ionization
potential of MS was 70 eV, and the scan range was 19.1–
350 m/z. Identification of volatiles was achieved by the
Wiley library (Hewlett-Packard Co.). The area of each
peak was integrated using ChemStation software (Hewlett-Packard Co.) and the total peak area (total ion
counts · 104) was reported as an indicator of volatiles generated from the samples.
2.5. Microbiological analysis
Sample bags were aseptically open using an alcohol-sterilized scissors, 50 mL of sterile 0.1% peptone was added to
each meat sample, and then pummeled for 1 min at medium speed in a stomacher (400 Laboratory Blender). Sam-
ples were serially diluted with 0.1% peptone water and
surface-plated (0.1 ml) in duplicate on modified oxford
(MOX) agar plates and tryptic soy agar supplemented with
0.6% yeast extract (TSAYE) to enumerate L. monocytogenes. The survivors of S. typhimurium were enumerated by
plating the diluted samples on trypticase soy agar supplemented with 800 ppm nalidixic acid (TSANA). All inoculated agar plates were incubated aerobically at 35 C for
48 h. After incubation, colonies were counted and recorded
as colony forming units per gram (CFU/g) of sample.
Randomly selected bacterial colonies were streaked onto
Modified Oxford (MOX) agar and XLT4 agar for confirming L. monocytogenes and S. typhimurium, respectively. In
situations where L. monocytogenes or S. typhimurium could
not be detected by plating methods, appropriate selective
enrichment broth was used: for L. monocytogenes, pork
samples were enriched in UVM broth (10%, w/v) supplemented with nisin and then plated on MOX agar. Characteristic colonies (colonies surrounded by black halos) were
picked (3 colonies/plate), purified on MOX agar, and
stored on TSA + 0.1% yeast extract.
The number of survivors (log10 CFU/g) in inoculated
samples at each irradiation level, based on colony counts
from MOX or TSANA plates, was plotted against irradiation dose to construct survivor curves for L. monocytogenes
or S. typhimurium. Least-squares analysis of the regression
of the survivor values versus radiation dose was conducted.
The irradiation D10 value, radiation dose required to 90%
reduction of viable L. monocytogenes or S. typhimurium,
was calculated as the reciprocal of the absolute value of
the regression line.
2.6. Statistical analysis
The experiment was a completely randomized design with
four replications. Data were analyzed by the procedure of
generalized linear model of SAS (SAS Institute, 1995): Student-Newman–Keuls’ multiple-range test was used to compare the mean values of treatments. Mean values and
standard error of the means (SEM) were reported (P < 0.05).
3. Results and discussion
3.1. Lipid oxidation
Rosemary + tocopherol with V7/A3 double-packaged
restructured pork loins had lower TBARS values than vacuum-packaged control in both nonirradiated and irradiated samples (Table 1). Although double-packaged
restructured cooked pork was exposed to aerobic conditions for 3 days during the 10-day storage, lipid oxidation
could be controlled by the added rosemary–tocopherol
combination. Other antioxidant combinations such as sesamol + tocopherol and gallate + tocopherol were also proven effective in preventing oxidative changes in aerobically
packaged irradiated turkeys and pork (Nam & Ahn, 2003a,
2003b).
K.C. Nam et al. / Meat Science 74 (2006) 380–387
383
Table 1
TBARS values of irradiated raw pork patties affected by oleoresin + tocopherol and packaging during refrigerated storage
Irradiation (kGy)
AntioxidantA
Packaging
0 day (mg MDA/kg meat)
10 day (mg MDA/kg meat)
SEM
0
2.5
0
2.5
Control
Control
Rosemary + tocopherol
Rosemary + tocopherol
Vacuum
Vacuum
V7/A3B
V7/A3
0.77ay
0.77ay
0.54by
0.50by
1.30ax
1.27ax
0.84cx
1.03bx
0.06
0.06
0.03
0.05
0.03
0.05
SEMC
a–c
x,y
A
B
C
Mean values with different letters within a column are significantly different (P < 0.05).
Mean values with different letters within a row are significantly different (P < 0.05).
Oleoresin–tocopherol combination added at 0.05–0.02% of meat weight.
Aerobically packaged for 3 days at first and then vacuum-packaged for 7 days.
Standard error of the means.
3.2. Color
The color a*-value of restructured pork loins increased
2.5 U by irradiation (Table 2). The prevention of pink
color defects and off-odor in irradiated poultry and pork
is critical. Nam and Ahn (2002) reported that the color
of irradiated turkey breast became pink due to carbon
monoxide–myoglobin complex formation, which was
induced by the production of carbon monoxide and reducing conditions by irradiation. The mechanisms of color
changes in irradiated turkey breast can be similarly applied
to that of irradiated pork loin.
The increased redness in irradiated restructured pork
loins was not stable and the a*-values of irradiated restructured pork loins were lower than that of nonirradiated after
10-day storage (Table 2). However, rosemary–tocopherol
had little effect on the a*-values of restructured pork loin.
L*- and b*-values of restructured pork loins showed
increasing trends during storage regardless of added antioxidant and packaging conditions.
3.3. Volatiles
Irradiation of restructured pork loins increased the
amount of total volatiles by about 25% (Table 3). The
most distinctive changes in volatile profiles by irradiation
were the increase of lipid oxidation products (aldehydes),
and sulfur volatiles (methanethiol, carbon disulfide,
dimethyl disulfide) and 1-alkenes (1-pentene, 1-hexene,
1-hepene, 1-octene), which were newly generated. Ahn,
Olson, Jo, Love, and Jin (1999) reported that the production of 1-heptene and 1-nonene were proportional to irradiation dose and could be used as indicators for
irradiation. Sulfur volatiles were reported to be the most
Table 2
Color values of irradiated raw pork patties affected by oleoresin + tocopherol and packaging during refrigerated storage
AntioxidantA
Packaging
0 day
10 day
SEM
value
0
2.5
0
2.5
SEMC
Control
Control
Rosemary + tocopherol
Rosemary + tocopherol
Vacuum
Vacuum
V7/A3B
V7/A3
66.8y
65.9y
65.5y
66.4y
0.5
70.2x
69.4x
70.0x
69.8x
0.5
0.6
0.5
0.3
0.6
a* value
0
2.5
0
2.5
SEM
Control
Control
Rosemary + tocopherol
Rosemary + tocopherol
Vacuum
Vacuum
V7/A3
V7/A3
7.0cx
9.5ax
7.7bx
9.5ax
0.2
b* value
0
2.5
0
2.5
SEM
Control
Control
Rosemary + tocopherol
Rosemary + tocopherol
Vacuum
Vacuum
V7/A3
V7/A3
13.9by
13.7by
14.9ax
13.7by
0.2
Irradiation (kGy)
L*
a–c
Mean values with different letters within a column are significantly different (P < 0.05).
Mean values with different letters within a row are significantly different (P < 0.05).
A
Oleoresin–tocopherol combination added at 0.05–0.02% of meat weight.
B
Aerobically packaged for 3 days at first and then vacuum-packaged for 7 days.
C
Standard error of the means.
x,y
7.2ax
6.8by
7.4ay
6.5by
0.1
14.5x
14.7x
15.0x
15.1x
0.2
0.2
0.2
0.1
0.2
0.2
0.2
0.2
0.2
384
K.C. Nam et al. / Meat Science 74 (2006) 380–387
Table 3
Volatile profiles of irradiated restructured pork treated by oleoresin–tocopherol combination and packaging at 0 day
Control (total ion counts · 104)
Rosemary + tocopherolA (total
ion counts · 104)
0 kGy
2.5 kGy
0 kGy
2.5 kGy
Acetaldehyde
Methanethiol
1-Pentene
Pentane
Propanal
2-Propanone
Carbon disulfide
2-Methyl propanal
Ethanol
1-Hexene
2-Propanol
Hexane
Butanal
2-Butanone
3-Methyl butanal
2-Methyl butanal
Benzene
1-Heptene
Heptane
Pentanal
Dimethyl disulfide
Toluene
1-Octene
Octane
1-Butanol
Hexanal
Heptanal
1442b
0c
0b
2542a
1152b
4364b
0b
226b
5753ab
0b
436a
264c
150b
515b
485b
218b
0c
0c
346b
2394a
0c
0c
0c
575c
384a
30686a
170b
10110a
1836b
212a
2084a
1439a
6155a
234a
948a
7487a
417a
441a
850b
299a
1182a
1909a
1072a
229a
695b
1295a
2676a
4621a
760b
328b
1433b
235b
30836a
266a
1097b
0c
0b
542b
668c
4409b
0b
185b
5005b
0b
381a
235c
0c
389b
405b
203b
0c
0c
204c
1381b
0c
81c
0c
704c
291ab
15660b
110c
10582a
2278a
156a
1233b
1271b
6932a
211a
989a
7930a
457a
646a
906a
308a
1290a
1930a
1101a
201b
825a
1235a
2627a
2609b
872a
363a
1690a
221b
26878a
281a
363
117
59
249
40
304
30
15
589
15
69
17
12
70
39
25
6
39
45
154
410
29
8
81
37
1543
16
Total
51936b
79792a
31844c
75750a
2240
Compound
a–c
A
B
SEMB
Different letters within a row are significantly different (P 6 0.05); n = 4.
Vacuum-packaged for 7 day then aerobically packaged for 3 day.
Standard error of the means.
important for the off-odor in irradiated pork because their
threshold values are much lower than other volatile compounds (Ahn, Jo, Olson, 2000). The sulfur compounds
were produced through the radiolytic degradation of
sulfur-containing amino acids, such as methionine and
cysteine (Ahn, 2002; Jo & Ahn, 2000). Hashim, Resurreccion, and MacWatters (1995) reported that irradiating
raw chicken breast and thigh produced a characteristic
‘‘bloody and sweet’’ aroma that remained after the thighs
were cooked, but was not detectable after the breasts were
cooked. Ahn, Jo, Du, Olson, and Nam (2000) described
the irradiation odor in raw pork as a ‘‘barbecued cornlike’’ odor while Nam, Prusa, and Ahn (2002) describe
the irradiation odor from pork as sulfury, boiled sweet
corn, or steamed or rotten vegetables. Rosemary–tocopherol combination was effective in reducing lipid oxidationderived volatile compounds but had little effect on the
production of sulfur volatiles in irradiated pork loin at
Day 0.
After 10 days of storage, most sulfur volatiles reduced
regardless of packaging conditions (vacuum or V7/A3
double-packaging). Methanthiol and carbon disulfide
were not found in pork loins and relatively small amounts
of dimethyl disulfide were detected in irradiated restructured pork loins (Table 4). The results are different from
our previous studies and could be attributed to the size
of restructured pork loins, which were sliced into 2 mmthickness and individually packaged. The very thin
restructured pork slices with wide surface should have
provided conditions for the highly volatile sulfur compounds to be evaporated during slicing, packaging, and
sample preparation for the volatiles analysis. Hexanal
was the most predominant volatile compound in irradiated restructured pork loins, which attributed to about
60% of the total volatiles. The addition of rosemary–
tocopherol combination reduced the amount of hexanal
in pork loin to 30% of the irradiated control, indicating
that the treatment was highly effective in controlling lipid
oxidation in irradiated restructured pork loins. Hexanal is
a good indicator of lipid oxidation in meat (Ahn et al.,
1999). Thus, decrease of hexanal production by rosemary + tocopherol treatment means lower oxidative
changes than control as shown in Table 1. Rosemary
extracts contain high levels of phenolic antioxidants and
rosemary–tocopherol combination produced a synergistic
effect in reducing oxidative changes in meat.
K.C. Nam et al. / Meat Science 74 (2006) 380–387
385
Table 4
Volatile profiles of irradiated restructured pork treated by oleoresin–tocopherol combination and packaging at 10 day
Compound
Acetaldehyde
Pentane
Propanal
2-Propanone
2-Methyl propanal
Ethanol
1-Hexene
2-Propanol
Hexane
Butanal
2-Butanone
3-Methyl butanal
2-Methyl butanal
1-Heptene
Heptane
Pentanal
Dimethyl disulfide
Toluene
1-Octene
Octane
1-Butanol
Hexanal
1-Pentanol
Heptanal
Total
a–d
A
B
Control (total ion counts · 104)
Rosemary + tocopherolA (total ion
counts · 104)
0 kGy
0 kGy
2.5 kGy
2.5 kGy
SEMB
2204b
2417a
3215a
15617b
224c
8852a
0c
474b
484a
325b
0c
463c
201c
0c
466b
5663a
0b
0d
0b
985ab
649a
76710a
1716a
642a
7592a
1261ab
3112a
20403a
924a
7280a
194a
462b
596a
331b
1118a
1927a
975a
435a
808a
3628b
235a
301a
171a
1353a
370a
78595a
0b
432ab
1961b
814b
800c
18781ab
278c
9717a
0c
855a
582a
102c
0c
534c
252c
0c
0c
1811c
0b
66c
0b
602b
323a
23503c
410b
159b
8634a
1659ab
2898ab
17282ab
812b
8503a
110b
370b
498a
443a
899b
1596b
792b
303b
701a
3258a
217a
208b
185a
1100a
433a
56065b
0b
411ab
470
319
210
968
25
1480
11
52
49
26
39
57
34
19
54
501
16
12
9
128
89
5327
199
88
118955a
132080a
60988b
106974a
7362
Different letters within a row are significantly different (P 6 0.05); n = 4.
Aerobically packaged for 3 day then vacuum-packaged for 7 day.
Standard error of the means.
3.4. Survival of pathogens
The survival of L. monocytogenes inoculated in restructured pork loins was inversely correlated to the irradiation
dose (Fig. 1a). The numbers of L. monocytogenes in 1.0
and 2.5 kGy treated samples decreased respectively by
1.5 and 4.3 log CFU/g. The dose of irradiation needed
to decrease by 1 log CFU/g of L. monocytogenes number
in restructured pork loins was 0.58 kGy (D10 value). Patterson (1989) showed that c-irradiation D10 values of L.
monocytogenes in poultry meat were from 0.42 to
0.55 kGy depending on strain and plating medium. (Gürsel & Gürakan, 1997) have also observed that the sensitivity special strains of L. monocytogenes to irradiation
varied with different meat substrates. Tarte, Murano,
and Olson (1996) also reported that different strains of
L. monocytogenes had different susceptibilities to e-beam
irradiation and the D10 values ranged from 0.372 to
0.638 kGy in ground pork.
Addition of rosemary–tocopherol combination to irradiated restructured pork loins showed some antimicrobial
effect, but the effect was not statistically significant. Theoretically, addition of antioxidants with free radical scavengers
can reduce the bactericidal effect of irradiation due to their
free radicals-scavenging effects. However, certain antioxidants such as butylated hydroxy anisole (BHA) and propyl
gallate (PG) improved the microcidal effects of irradiation
(Gailani & Fung, 1984; Yousef, Gajewski, & Marth, 1991).
The initial number of L. monocytogenes inoculated on
sliced restructured pork loins was 1.93 · 107 CFU/cm2.
The L. monocytogenes recovered from the inoculated nonirradiated restructured pork loins was 4.72 · 108 CFU/cm2
after 20 days of refrigerated storage. The stationary behavior of L. monocytogenes observed between Day 1 and Day
5 (Fig. 1b) should be due to the high counts of inoculated
population. On the other hand, the starting number of L.
monocytogenes in 2.5 kGy-irradiated restructured pork
loins was 9.26 · 102 CFU/cm2 and gradually increased to
2.88 · 105 CFU/cm2 after 20 days of storage. Therefore,
2.5 kGy of irradiation could keep the restructured pork
loins at less microbial load compared with the nonirradiated control during the 20 days of refrigerated storage.
The added rosemary–tocopherol combination did not
show any significant effects (P > 0.05) on the survival of
L. monocytogenes in nonirradiated and irradiated samples
during the storage. Probably, the amounts of rosemary–
tocopherol combination added were not enough to be effective as antimicrobial or microbial protecting agents because
the antimicrobial actions of irradiation was much more
powerful than that of the additive effects.
Fig. 2a shows the survival and growth of S. typhimurium in irradiated restructure pork loins. The survival of
386
K.C. Nam et al. / Meat Science 74 (2006) 380–387
a. ST in fomed pork by irradiation dose
8
7
7
6
6
5
5
Log CFU
Log CFU
a. LM in formed pork by irradiation dose
8
4
3
4
3
2
2
Co ntro l
Control
Ro semary
1
Rosemary
1
0
0
0 kGy
0.5 kGy
1.0 kGy
1.5 kGy
2.0 kGy
0 kGy
2.5 kGy
0.5 kGy
1.0 kGy
1.5 kGy
2.0 kGy
2.5 kGy
Dose
Dose
b. ST in formed pork during storage
b. LM in formed pork during storage
8
10
7
9
8
6
Log CFU
Log CFU
7
6
5
5
4
Control-0 kGy
Control-2.5 kGy
Rosemary-0 kGy
Rosemary-2.5 kGy
3
4
Co ntro l-0 kGy
3
2
Co ntro l-2.5 kGy
2
Ro semary-0 kGy
1
Ro semary-2.5 kGy
0
0 day
5 days
10 days
15 days
20 days
1
0
0 day
5 days
10 days
15 days
20 days
Storage time
Storage time
Fig. 1. Survival and growth curve for Listeria monocytogenes in restructured pork.
Fig. 2. Survival and growth curve for Salmonella typhimurium in
restructured pork.
S. typhimurium inoculated in restructured pork loins was
also highly dependent upon applied irradiation dose. Irradiation at 1.0 and 2.5 kGy produced about 2.1 and 4.8 log
reductions of S. typhimurium, respectively. The irradiation
D10 value for S. typhimurium in restructured pork loins
was 0.55 kGy.
The survival trend of S. typhimurium in restructured
pork was totally different from that of L. monocytogenes.
The initial number of S. typhimurium CFU/cm2 inoculated
on sliced restructured pork loins at Day 0 was 8.47 · 106
and then, the number decreased to 2.24 · 105 CFU/cm2
after 20 days of refrigerated storage. This had happened
because S. typhimurium is sensitive to low temperature
and the refrigerated temperature inhibited the growth of
the pathogen. The antimicrobial effect of irradiation was
clearly shown in 2.5 kGy-irradiated restructured pork during the storage. The average initial number of S. typhimurium observed in 2.5 kGy-irradiated restructured pork loins
was 1.41 · 102 CFU/cm2, but there were many plates with
no S. typhimurium after 10 days of storage, indicating that
irradiation is a powerful tool for controlling S. typhimurium in restructured pork loins. As shown in L. monocytogenes, added rosemary–tocopherol showed little effect in
killing or protecting S. typhimurium in irradiated restructured pork loin.
4. Conclusions
Athough rosemary–tocopherol combination was effective
in preventing quality changes in irradiated restructured pork
loin the combination had little effect on the survival of
L. monocytogenes and S. typhimurium. Irradiation at
2.5 kGy greatly reduced the number of L. monocytogenes
and S. typhimurium inoculated on the surface of restructured
pork loins, and the irradiation D10 values for L. monocytogenes and S. typhimurium were 0.58 and 0.55 kGy, respectively. L. monocytogenes grew but S. typhimurium
gradually died during the 20-day storage under refrigerated
conditions. Therefore, antimicrobial strategies for cooked
meat products that will be refrigerated should be focused
on controlling L. monocytogenes rather than S. typhimurium.
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