JFS:
Food Chemistry and Toxicology
Effects of Antioxidants and Packaging on
Lipid and Cholesterol Oxidation and
Color Changes of Irradiated Egg Yolk Powder
ABSTRACT: Electron-beam irradiation significantly increased the oxidation of docosahexaenoic, arachidonic and
linolenic acids, and cholesterol in egg yolk powder. Arachidonic-acid content was reduced from 4.58% to 3.07%,
and total cholesterol oxidation products increased from 11 ␮g/g to 467␮g/g after 5.0 kGy irradiation. Further
oxidation occurred during storage. Vacuum-packaging significantly reduced, but the use of antioxidants had no
effect on the fatty acids and cholesterol oxidation during irradiation and storage. Irradiation caused color change
in egg yolk powder. The Hunter color a- (redness) values decreased from 3.89 to 2.48 and 1.94, respectively, after 2.5
and 5.0 kGy irradiation. Hunter color a- and b-values were also decreased during storage. Vacuum-packaging and
antioxidants significantly reduced color changes.
Key Words: egg yolk powder, irradiation, antioxidants, lipid and cholesterol oxidation, color
Introduction
T
HERE IS INCREASING CONCERN ABOUT
the incidence of salmonellosis caused
by eggs and other poultry products (Serrano and others 1997). Much effort has been
made to control Salmonella in egg products, but irradiation treatment has been
proposed as the most effective way of control (Radomyski and others 1994). With a
dosage above 2.0 kGy, Salmonella and other bacteria in egg yolks can be successfully
controlled (Narvaiz and others 1992).
However, ionizing radiation generates free
radicals that may cause lipid peroxidation
and other chemical changes, which will
deteriorate the quality of the egg products
(Branka and others 1992; Lebovics and
others 1992, Wong and others 1995). Branka and others (1992) reported that irradiation at 3 kGy is the threshold dose for organoleptic changes in dehydrated egg products, and that irradiation dosage and the
presence of oxygen influenced the buildup of lipid hydroperoxides. Lebovics and
others (1992) reported that the concentrations of cholesterol oxidation products
(COPs) produced by 1 kGy ionizing radiation were similar to those formed by the
autoxidation of nonirradiated egg powder
stored under aerobic condition for 1 mo.
Egg yolk lipids include polyunsaturated
fatty acids (PUFAs) and cholesterol, which
both can be oxidized easily following irradiation (Gardner 1989). Lipid oxidation
causes the loss of PUFAs (Kaneda and
Miyazawa 1987) and other nutrients and
also causes carotenoids destruction and
organoleptic
changes
(Katusin-Razem
and others 1992).
© 2000 Institute of Food Technologists
Earlier studies have suggested that lipid oxidation products (LOPs) and COPs
have negative effects on human health.
Dietary COPs and LOPs have been shown
to induce and accelerate the development
of atherosclerosis, 1 of the main causes of
death in the world (Staprans and others
1994, 1998). Staprans and others (1998)
and Vine and others (1997) reported that
LOPs in the diet were absorbed by the
small intestines of humans and rodents
and were incorporated into chylomicrons.
With human studies, it has been demonstrated that the quantity of oxidized lipids
in the diet directly correlated with the levels of oxidized lipids in serum postprandial chylomicrons (Staprans and others
1994). In rodents, dietary LOPs were also
incorporated into serum very-low-density
lipoprotein (VLDL) and low-density lipoprotein (LDL) fractions (Staprans and others 1993), and the levels of oxidized chylomicrons and VLDL + LDL were directly correlated with the quantity of oxidized lipids
in the diet. This indicated that LOPs were
transported by LDL and VLDL (Krut and
others 1997) and provided a mechanism
by which dietary oxidized lipids can affect
the oxidative state of endogenous lipoproteins. Staprans and others (1998) further
showed that oxidized cholesterol in the
diet could also be directly absorbed into
the circulatory system. These results show
that LOPs and COPs in the diet contribute
to serum lipoprotein oxidation. Therefore,
it is important to assess cholesterol and
lipid oxidation of egg yolk powder by irradiation and during storage. Color is an im-
Table 1—The change in color L-value of electron-beam irradiated yolk powder during storage
Vacuum-packaging
Irradiation Storage1
dose(kGy) (d)
Control
0
0
45
90
SEM
90.32 aby
90.76ay
89.98bx
90.38ay
91.09ax
0.17
89.96ay
90.58bx
0.11
89.94ax
90.40bx
0.15
2.5
0
45
90
SEM
90.28by
91.59ax
92.02ax
0.23
91.11ay
91.17ay
92.01ax
0.12
5.0
0
45
90
SEM
90.86ay
91.88ax
90.82ay
0.09
90.84ay
91.36bx
91.01 axy
0.07
VE
BHT
Aerobic-packaging
SEM
Control
VE
90.98ay
BHT
SEM
0.16
0.15
0.16
91.12ay
90.18by
91.40ax
91.99az
0.08
0.14
0.23
0.11
90.73ay 90.98ay 90.16by 0.14
90.95ay 90.47 abz 90.18by 0.16
94.31ax 94.31ax 94.02ax 0.08
0.12
0.13
0.13
90.59by
91.01bx
91.06ax
0.11
0.04
0.14
0.10
90.87az 90.94ay
93.56by 93.75bx
94.58ax 94.01bx
0.13
0.19
93.54ax 93.71ax
91.59ay 91.17ay
0.08
0.13
90.05by
0.18
93.42ax 0.13
az
91.40 0.23
0.20
90.66az 0.12
94.41ax 0.13
93.86by 0.11
0.10
a,bDifferent letters within a row of the same packaging method and irradiation dose are different (P < 0.05). n = 6.
x-zDifferent letters within a column of the same irradiation dosage and storage time are different (P < 0.05). n = 6.
1Samples were stored at room temperature (22 °C) with relative humidity of 73%.
Vol. 65, No. 4, 2000—JOURNAL OF FOOD SCIENCE
625
FoodChemistryandToxicology
M. DU AND D.U. AHN
Antioxidants and Packaging on the Quality of Irradiated Yolk Powder . . .
FoodChemistryandToxicology
portant quality parameter of egg yolk.
Carotenoids, the main pigments in yolk,
can be oxidized by the same mechanism
as lipids and cholesterol. Thus, the influence of irradiation on yolk color needs to
be ascertained (Serrano and others 1997).
BHT (butylated hydroxytoluene), BHA
(butylated hydroxyanisole), and vitamin E
(VE) are widely used in the food industry
to prevent lipid oxidation. They can terminate chain reactions of peroxides by scavenging chain-propagating radicals, and
thus they can prevent lipids from oxidizing (Halliwell 1996; Halliwell and Gutteridge 1989). Li and others (1996) also indicated that the stability of lipids in chicken meat and egg yolk powder decreased
with high contents of PUFAs but improved
with dietary supplementation of tocopherol.
The objective of this research was to
determine the effect of antioxidants (VE
or BHT) and packaging on color and lipids
and cholesterol oxidation of irradiated egg
yolk powder during storage.
Table 2—The change in color a-value of electron-beam irradiated yolk powder during storage
Irradiation Storage1
dose(kGy)2 (d)
C
ness) values of egg yolk powder were
decreased by irradiation. The color of irradiated egg yolk powder looked paler than
nonirradiated sample (Tables 1, 2, and 3).
In the control treatment (no antioxidant
added) with vacuum-packaging, the color
a-value (redness) of egg yolk powder decreased significantly (from 3.89 to 2.48, p <
0.05) after 2.5 kGy irradiation and to 1.94
after 5.0 kGy irradiation (p < 0.05); b-value
(yellowness) decreased significantly (from
26.23 to 21.12 and 20.36, p < 0.05) after 2.5
and 5.0 kGy irradiation, respectively. Although changes in color L values by irradiation were not consistent, significant decrease in color a- and b-values produced
the color of egg yolk powder light. Egg yolk
color is dependent on carotenoids that
contain unsaturated double bonds and
can be oxidized with the same mechanism
as lipid oxidation. The color a- and b-values of egg yolk powder kept decreasing
during storage, indicating possible destruction of pigments by oxidation. Ma
and others (1992) also noted that yolk color was pale after irradiation. Branka and
others (1992) found that the loss of carotenoids was positively correlated with irradiation dosage, which suggests that irradiation-dependent production of free radicals could have contributed to the destruction of carotenoids. Serrano and others (1997) suggested that there would be
an irradiation-dose threshold for causing
color changes in yolk.
Tukey grouping analysis (SAS 1989)
was employed to compare the difference
between vacuum and aerobic packaging.
VE
Aerobic-packaging
BHT
SEM
Control
0.06
0.05
0.03
3.69ax
VE
3.50ax
BHT
SEM
2.30by
2.19by
0.05
2.42ay
2.46ay
0.04
0.07
2.31by 0.03
by
2.29 0.04
0.07
0
45
90
SEM
3.89ax
3.76ay
3.72ax
3.74bx
2.32by
0.05
3.94ax
2.57az
0.04
3.64bx
2.39by
0.03
2.5
0
45
90
SEM
2.48ax
2.19by
1.75bz
0.04
2.35ax
2.46ax
1.78by
0.05
2.45ax
2.29by
1.95az
0.03
0.04
0.04
0.03
2.26bx
1.74ay
1.65by
0.04
2.63ax
1.93ay
1.92ay
0.05
2.60ax 0.06
1.83ay 0.05
1.92ay 0.04
0.04
5.0
0
45
90
SEM
1.94bx
1.53ay
1.38bz
0.04
2.15ax
1.65ay
1.47az
0.03
2.16ax
1.60ay
1.53ay
0.03
0.03
0.04
0.03
2.12 cx
1.33by
1.17bz
0.05
2.44ax
1.60ay
1.30az
0.04
2.29bx 0.04
1.46 aby 0.05
1.19bz 0.02
0.03
0
3.50ax
a-cDifferent letters within a row of the same packaging method and irradiation dose are different (P < 0.05). n = 6.
x-zDifferent letters within a column of the same irradiation dosage and storage time are different (P < 0.05). n = 6.
1Samples were stored at room temperature (22 ºC) with relative humidity of 73%.
Table 3—The change in color b-value of electron-beam irradiated yolk powder during storage
Irradiation Storage1
dose (kGy) (d)
Vacuum-packaging
Aerobic-packaging
Control
VE
BHT
SEM
Control
0
0
45
90
SEM
26.23by
26.56bx
23.32az
0.07
26.65ay
26.85ax
23.51az
0.06
25.26cx
25.15cx
23.44ay
0.10
0.10
0.08
0.07
26.30ax 26.50ax
24.74ay 24.46ay
22.50az 21.63bz
0.12
0.11
24.88bx 0.14
24.47ay 0.09
21.81bz 0.18
0.08
2.5
0
45
90
SEM
21.12bz
22.50ax
21.99ay
0.08
21.69ay
21.63by
21.99ax
0.11
21.99ax
21.81bx
22.00ax
0.18
0.13
0.18
0.06
21.17bz 21.39bz
22.35 aby 22.56ay
23.33bx 24.38ax
0.12
0.08
21.19bz 0.09
21.97by 0.13
23.59bx 0.13
0.17
5.0
0
45
90
SEM
20.36bz 21.39ay
22.13ax 22.32ax
21.13 aby 20.87bz
0.11
0 .11
20.27by
21.47bx
21.53ax
0.13
0.10
0.06
0.17
21.51bx 22.68ax
20.88by 22.29ay
16.04bz 16.86az
0.09
0.10
21.35by 0.06
22.00ax 0.15
16.62az 0.11
0.12
Results and Discussion
OLOR A - ( REDNESS ) AND B - ( YELLOW
Vacuum-packaging
Control
VE
BHT
SEM
a-cDifferent letters within a row of the same packaging method and irradiation dose are different (P < 0.05). n = 6.
x-zDifferent letters within a column of the same irradiation dosage and storage time are different (P < 0.05). n = 6.
1Samples were stored at room temperature (22 °C) with relative humidity of 73%.
Table 4—The concentration of polyunsaturated fatty acids in vacuum-packaged yolk powder
after electron-beam irradiation and storage1
0-d storage
90-d storage
Irradiation
dose (kGy)
Control
VE
BHT
SEM
Control
VE
BHT
Linolenic
0
2.5
5.0
SEM
2.62ax
2.12ay
1.88az
0.04
2.54ax
2.11ay
1.89az
0.06
2.61ax
2.12ay
1.95az
0.05
0.10
0.04
0.04
1.94bx
1.49ay
1.01az
0.02
2.13ax
1.53ay
1.07az
0.03
2.11ax 0.02
1.54ay 0.03
1.13az 0.04
0.04
Arachidonic
0
2.5
5.0
SEM
4.69ax
3.48ay
2.99az
0.10
4.49ax
3.54ay
3.22az
0.07
4.55ay
3.45ax
3.26az
0.08
0.16
0.11
0.07
3.75ax
2.45ay
1.73az
0.05
3.85ax
2.48ay
1.81az
0.05
3.87ax 0.04
2.51ay 0.05
1.79az 0.04
0.05
DHA
0
2.5
5.0
SEM
2.48ax
1.96ay
1.82az
0.04
2.45ax
1.99ay
1.93az
0.05
2.50ax
1.94ay
1.95az
0.07
0.08
0.06
0.07
2.10bx
1.01ay
0.80az
0.04
2.23abx
0.97ay
0.81az
0.04
2.34ax 0.04
0.97ay 0.04
0.85az 0.03
0.05
Fatty acid
SEM
a,bDifferent letters within a row of the same packaging method and irradiation dose are different (P < 0.05). n = 6.
x-zDifferent letters within a column of the same irradiation dosage and storage time are different (P < 0.05). n = 6.
1Samples were stored at room temperature (22 °C) with relative humidity of 73%.
Abbreviation: DHA, docosahexanoic acid
There were significant differences for aand b-value (p < 0.05), indicating that the
color change of egg yolk powder in aerobic-packaging was faster than that in the
vacuum-packaging. Thus, vacuum-pack-
626 JOURNAL OF FOOD SCIENCE—Vol. 65, No. 4, 2000
aging is effective in reducing oxidative
changes during storage. The color a-values in antioxidant (VE or BHT) treatments
were higher than the control treatment,
suggesting that antioxidants reduced the
Table 5—The concentration of polyunsaturated fatty acids in yolk powder of aerobic-packaging after electron-beam irradiation and storage1
Fatty acid
0-d storage
Irradiation
dose (kGy) Control
90-d storage
VE
BHT
0
2.5
5.0
SEM
2.59ax
2.65ax
2.66ax
2.02ay
1.55az
0.08
2.13ay
1.65az
0.06
2.09ay
1.61az
0.06
Arachidonic
0
2.5
5.0
SEM
4.58ax
3.49ay
3.07az
0.12
4.22ax
3.69ay
3.22az
0.11
DHA
0
2.5
5.0
SEM
2.46ax
1.96by
1.77az
0.07
2.39ax
2.06 aby
1.80az
0.07
Linolenic
SEM
Control
0.07
0.04
0.03
1.79bx
VE
2.00ax
BHT
SEM
1.46ay
1.06az
0.02
1.53ay
1.02az
0.03
0.03
1.54ay 0.03
az
0.99 0.04
0.04
4.21ax
3.58ay
3.26az
0.12
0.10
0.09
0.06
3.10bx
2.27ay
1.62az
0.05
3.15 abx
2.28ay
1.67az
0.04
3.25ay 0.04
2.21ax 0.05
1.69az 0.04
0.07
2.45ax
2.13ay
1.83az
0.08
0.07
0.04
0.05
1.53bx
0.98ay
0.62az
0.06
1.82ax
0.97ay
0.77az
0.05
1.92ax 0.05
0.97ay 0.04
0.75az 0.04
0.04
1.99ax
a,bDifferent letters within a row of the same storage time are different (P < 0.05). n = 6.
x- zDifferent letters within a column of the same irradiation dosage are different (P < 0.05). n = 6.
1Samples were stored at room temperature (22 ºC) with relative humidity of 73%.
Abbreviation: DHA, docosahexaenoic acid
Table 6—The content of COPs (␮g/g) in vacuum-packaged yolk powder after electron-beam
irradiation and 90 d of storage1
Dose
(kGy)
0-d storage
COPs
Control
90-d storage
VE
BHT
SEM
Control
VE
BHT
SEM
0
7␣-hydroxycholesterol
7␤-hydroxycholesterol
25-hydroxycholesterol
␣- plus ␤-epoxide
6-ketocholesterol
7-ketocholesterol
2a
5a
0a
2a
1a
1a
2a
4a
0a
3a
0a
0a
1a
4a
0a
2a
0a
0a
1.1
0.6
0.0
0.7
0.2
0.0
442a
97a
0a
49a
0a
28a
142b
73b
0a
22c
0a
14b
117b
64b
0a
32b
0a
0a
10.7
4.8
0.0
3.3
0.0
4.4
2.5
7␣-hydroxycholesterol
7␤-hydroxycholesterol
25-hydroxycholesterol
␣- plus ␤-epoxide
6-ketocholesterol
7-ketocholesterol
85a
57a
13b
93a
21a
41a
49b
36b
15a
78a
11b
84b
54b
49a
16a
66a
10b
42b
2.7
2.7
1.2
8.6
2.1
4.5
522a
121a
2a
134a
0a
168a
430b
94b
0a
48b
0a
142a
475ab
82b
1a
40b
0a
59b
20.1
4.0
0.4
12.3
0.0
12.9
5.0
7␣-hydroxycholesterol
7␤-hydroxycholesterol
25-hydroxycholesterol
␣- plus ␤-epoxide
6-ketocholesterol
7-ketocholesterol
108a
74a
12b
122a
12a
139a
89b
59b
20a
112a
15a
91b
104a
60b
23a
139a
15a
71b
3.7
4.0
2.8
7.4
1.3
7.4
516a
199a
17b
176a
0a
196a
499a
76c
0b
100b
0a
217a
557a
114b
87a
61b
0a
179a
18.0
5.6
14.6
17.7
0.0
21.4
a-cDifferent letters within a row of the same storage time are different (P < 0.05). n = 6.
1Samples were stored at room temperature (22 ºC) with relative humidity of 73%.
Abbreviation: COPs, cholesterol oxidation products
Table 7—The content of COPs (␮g/g) in aerobic-packaged yolk powder after electron-beam
irradiation and 90 d of storage1
Dose
(kGy)
0-d storage
90-d storage
COPs
Control
VE
BHT
7␣-hydroxycholesterol
7␤-hydroxycholesterol
25-hydroxycholesterol
␣- plus ␤-epoxide
6-ketocholesterol
7-ketocholesterol
10a
5b
4b
18a
1a
10a
0b
4a
10a
2a
9a
3b
1b
12a
0a
10a
12a
0b
1.5
1.0
1.0
2.7
1.6
1.0
2.5
7␣-hydroxycholesterol
7␤-hydroxycholesterol
25-hydroxycholesterol
␣- plus ␤-epoxide
6-ketocholesterol
7-ketocholesterol
184a
110a
23a
187a
27a
119a
114b
81b
23a
152b
7b
85b
148ab
73b
20a
162ab
26a
110a
5.0
7␣-hydroxycholesterol
7␤-hydroxycholesterol
25-hydroxycholesterol
␣- plus ␤-epoxide
6-ketocholesterol
7-ketocholesterol
390a
135a
43a
240a
45a
369a
380a
112b
34b
169b
27b
187c
365a
90c
46a
196ab
23b
266b
0
SEM
Control
VE
BHT
SEM
383b
545a
105a
3a
41a
0a
38a
86b
0a
62b
0a
52a
50.6
4.6
2.1
4.9
0.0
9.8
18.4
5.0
2.3
18.8
4.5
5.5
1,503 a 1,249a 1,470 a
73a
67a
60a
0a
1a
0a
74b
58b
57a
0a
0a
0a
303a
180b
194b
116.0
15.6
0.4
14.7
0.0
20.5
27.7
6.9
1.7
15.4
3.1
14.7
1,994 a 1,790b 1,859 b
160a
157a
70b
51a
9b
10b
215a
217a
220a
0a
0a
0a
514a
390b
270c
32.6
5.9
8.0
22.1
0.0
14.8
653a
95ab
3a
41a
0a
36a
a-cDifferent letters within a row of the same storage time are different (P < 0.05). n = 6.
1Samples were stored at room temperature (22 ºC) with relative humidity of 73%.
Abbreviation: COPs, cholesterol oxidation products
Vol. 65, No. 4, 2000—JOURNAL OF FOOD SCIENCE
627
FoodChemistryandToxicology
destruction of pigments both by irradiation and storage.
Fatty-acid composition of egg yolk powder indicated that irradiation significantly
increased the oxidation of PUFAs (Tables 4
and 5). In vacuum-packaging at d 0,
docosahexaenoic-acid (DHA) content of
egg yolk powder decreased from 2.48% of
total egg yolk lipid to 1.96% after 2.5 kGy
and 1.82% after 5.0 kGy irradiation. In aerobic-packaging at d 0, DHA content of egg
yolk powder decreased to 1.96% and
1.77% of total lipid content after 2.5 and
5.0 kGy of irradiation, respectively. The
contents of arachidonic and linolenic acid
also decreased after irradiation, with a
greater decrease after 5 kGy than 2.5 kGy
irradiation (Table 4). Tukey grouping analysis showed that there was significant difference between vacuum- and aerobicpackaging. The decrease of PUFAs was
smaller in vacuum-packaging compared
with aerobic-packaging because the presence of oxygen could stimulate oxidative
changes in irradiated yolk powder. During
storage, the PUFAs were further decreased
with increasing storage time (Tables 4 and
5). After 90 d of storage, the content of
DHA in egg yolk powder irradiated at 5.0
kGy was reduced to 1/3 of its original content to about 1/2 for the 2.5 kGy group and
to about 1/4 for nonirradiated group. The
reason for this accelerated lipid oxidation
could be the free radicals formed by irradiation. Irradiation induced the build-up of
lipid hydroperoxides, which could further
accelerate lipid oxidation (Branka and others 1992) in egg yolk powder. The concentrations of the 3 selected PUFAs in the control, VE- and BHT-treated egg yolk powders showed that antioxidant treatments
prevented the loss of PUFAs in nonirradiated powder but could not prevent the
loss of PUFAs induced by irradiation (Tables 4 and 5).
Several kinds of COPs were formed in
egg yolk powder after irradiation, but significantly more COPs (p < 0.05, data not
shown) were formed after 90 d of storage
(Tables 6 and 7). The formation of COPs
was positively correlated to irradiation
dosage. Data analysis (data not showed)
indicated that there were significant differences between vacuum- and aerobicpackaging in total amount of COPS
formed both immediately after irradiation
and after storage. Vacuum-packaging reduced irradiation-induced oxidation of
cholesterol in egg yolk powder as with color and fatty-acid composition changes.
The composition of COPs in egg yolk powder was also changed during storage; immediately after irradiation, 7␣- and 7␤-hydroxycholesterol, ␣- and ␤- epoxides, and
7-ketocholesterol were the main COPs
formed. This result was the same as previ-
Antioxidants and Packaging on the Quality of Irradiated Yolk Powder . . .
FoodChemistryandToxicology
ous reports (Lebovics and others 1992; Paniangvait and others 1995). After 3 mo of
storage, the amounts of 7␣-hydroxycholesterol and 7-ketocholesterol were increased. Tocopherol or BHT treatment significantly reduced the formation of COPs
in nonirradiated egg yolk powder during
storage but had no effect in irradiated
groups. More COPS were formed in egg
yolk powder after irradiation and storage.
Irradiation also increased the oxidation of
PUFAs and pigments in yolk. Antioxidants
(VE and BHT) were not effective in preventing chemical changes (lipid and cho-
Materials and Methods
Sample preparation
Three different batches of fresh liquid egg yolk (solid content, 49%) were
obtained from a commercial egg processor. Each batch was separated into 3
groups, respectively, and an antioxidant
was added (none, 0.01% BHT, or 0.01%
vitamin E) before spray-drying (APV
Crepaco, Inc., Dryer Division, Attleborofalls, Mass., U.S.A.). This level of antioxidant (0.01%) was selected because
0.02% BHT can prevent oxidative changes in food. The level of antioxidant in egg
yolk powder becomes approximately
0.02% after drying because the solid content in liquid egg is 49%. The inlet temperature of the spray dryer was set at
185 °C, and the exhaust temperature
was maintained at 85 °C by adjusting
the flow rate of liquid yolk to the atomizer. Dried egg yolk powder was either vacuum-packaged in oxygen-impermeable
bags (9.3 mL O2 /m2 /24 h at 0 °C, clear;
Koch) or aerobic-packaged in oxygenpermeable plastic clear bags and stored
at 4 °C before irradiation. Samples packaged in bags were irradiated using the
Linear Accelerator Facility (Circle III Linear Electron Accelerator, Thomson CSF
Linac, Saint-Aubin, France) at 0, 2.5, or 5
kGy of average absorbed dose (true absorbed doses were measured by an alanine dosimetry system). After irradiation, samples were stored at room temperature (22 °C, dark with relative humidity of 73%) for 3 mo. Samples were
prepared 3 times (3 replications) at intervals of 2 to 3 mo. Fatty-acid composition,
cholesterol oxides, and the color of irradiated egg yolk powders were determined
at 0, 45, and 90 d of storage. The color of
egg yolk powder was measured using a
Hunter LabScan Colorimeter (Hunter
Laboratory, Inc., Reston, Va., U.S.A.) and
expressed as color L (lightness), a- (redness), and b- (yellowness) values.
lesterol oxidation and color changes)
caused by irradiation in egg yolk powders
but were effective in nonirradiated
groups. Compared with aerobic-packaging, vacuum-packaging was effective in
preventing adverse changes in egg yolk
powders caused by irradiation.
Conclusions
R
ESULTS FROM THIS STUDY SHOWED
that the lipids of egg yolk powder
were oxidized during storage, and irradiation accelerated the changes greatly. After
Lipid extraction
Lipids were extracted from egg yolk
powder according to the method of Folch
and others (1957). Egg yolk powder (3 g),
50 mL BHT (7.2%), and 30 mL Folch I solution (CHCl3 :CH3 OH = 2:1) were added
to a 50-mL test tube, which was then
capped and mixed well. The homogenate was filtered into a 100-mL graduated cylinder through Whatman no. 1 filter
paper (Whatman Inc., Maidstone, England), and the filter paper was rinsed
twice with 10 mL Folch I solution. After
adding 8 mL of 0.88% NaCl solution, the
cylinder was capped with a glass stopper,
and the content was mixed. The inside
wall of the cylinder was washed twice
with 5 mL of Folch 2 solution (3:47:48/
CHCl 3:CH3 OH:H 2O). After phase separation, the top layer (methanol and water)
of the solution was completely and carefully siphoned off so as not to contaminate the CHCl3 layer. The bottom organic
layer was transferred to a glass scintillation vial and dried in a block heater (1 h
at 50 ºC) under a nitrogen stream. The
dried lipid material was dissolved with
an aliquot of hexane to make 0.2 g fat/
mL hexane, which was used for the next
step.
Separation of cholesterol
oxidation products
A silicic acid (100 mesh), celite-545,
and CaHPO4 .2H 2O (10:9:1, w/w/w) mixture in chloroform was prepared and
packed into a glass column (22 mm X 30
cm with sintered glass frit at the bottom)
to a height of 5 cm. The column was
washed with 10 mL of hexane:ethyl acetate (9:1 vol/vol; solvent I) before loading the sample. Cholesterol oxides were
separated from egg yolk powder by the
method of Ahn and others (1999). The
lipid sample dissolved in hexane (0.2 g)
was loaded onto the silicic-acid column.
Neutral lipids, cholesterol, and phos-
628 JOURNAL OF FOOD SCIENCE—Vol. 65, No. 4, 2000
irradiation, high levels of COPs were
formed, and PUFAs and pigments were
partly destroyed. Thus, pasteurizing egg
yolk powder using irradiation is not highly
recommended. Addition of antioxidants
such as VE and BHT at the 0.01% level significantly reduced the oxidative changes
in nonirradiated egg yolk powders but was
not effective in protecting quality changes
in irradiated egg yolk powders. Vacuumpackaging was more effective in preventing oxidative changes in egg yolk powders
than antioxidant treatments.
pholipids were eluted by passing 40 mL
of solvent I (hexane:ethyl acetate = 9:1,
vol/vol) and 40 mL of solvent II
(hexane:ethyl acetate:ethyl ether = 4:1:2,
vol/vol/vol) through the column. Cholesterol oxides were then eluted with 40
mL of solvent III (acetone:ethyl
acetate:methanol=10:10:1,
1
mL/min
flow rate) and dried under nitrogen gas.
The dried cholesterol oxides were derivatized in a 60 °C water bath for 30 min after adding 50 mL Sylon BFT [99%
bis(trimethylsillyl)trifluoroacetamide+1%
trimethylchlorosilane; Supelco) and 50
mL pyridine. The derivatized samples
were dried under nitrogen, redissolved
in 200 ␮L ethyl acetate, and then used
for gas chromatographic (GC) analysis.
GC analysis of cholesterol oxides
Analysis of cholesterol oxides was performed with a gas chromatograph (GC HP
6890; Hewlett Packard Co., Wilmington,
Del., U.S.A.) equipped with an autosampler and flame ionization detector
(FID). A capillary column (HP-5, 0.25 mm
i.d. ∞ 30 m, 0.25 ␮m nominal; Hewlett
Packard Co.) was used. A splitless inlet
was used to inject samples (0.5 ␮L) into
the column, and a ramped oven temperature was used (260 °C held for 2 min, increased to 290 °C at 3 ºC/min, and then
kept at 290 °C for 18 min). Temperatures
of the inlet and detector were 280 °C. Helium was the carrier gas at a constant flow
of 1.1 mL/min. Detector (FID) air, H2, and
make-up gas (He) flows were 350 mL/
min, 40mL/min, and 43.9 ml/min, respectively. The peaks were identified by standards (Sigma Chemical Co., St. Louis,
Mo., U.S.A.) and also by a Mass Selective
(MS) Detector (Model 5973; Hewlett
Packard Co.) The GC-MS system was
used with the same column conditions as
described above. The ionization potential
of the MS was 70 eV, and the scan range
was 45 to 550. Tentative identification of
Fatty-acid composition analysis
One mL of methylating reagent (boron-trifluoride methanol, Sigma Chemical Co.) was added to 50 ␮L of lipid extract and incubated in a 90 ºC water bath
for 1 h. After cooling to room temperature, 2 mL hexane and 5 mL water were
added, mixed thoroughly, and left at
room temperature overnight for phase
separation. The top hexane layer con-
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Journal paper J-18531 of the Iowa Agriculture and Home
Economics Experiment Station, Ames, IA 50011-3150. Project
No. 3322, and supported by the Iowa Egg Council.
Authors are with the Department of Animal Science, Iowa State University, Ames, Iowa 500113150, U.S.A. Direct correspondence to D.U. Ahn
(E-mail: duahn@iastate.edu).
Vol. 65, No. 4, 2000—JOURNAL OF FOOD SCIENCE
629
FoodChemistryandToxicology
COPs was achieved by comparing mass
spectral data with those of the Wiley library (Hewlett Packard Co.) The area of
each peak was integrated using ChemStation software (Hewlett Packard Co.),
and the amount of COPs was calculated
using an internal standard.