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2
*
Animal Sciences Department and
Department of Food Microbiology and Toxicology, University of Wisconsin-Madison, Madison, Wisconsin 53706
2To whom correspondence should be addressed at 260 Animal Sciences, 1675 Observatory Drive, University of Wisconsin-Madison, Madison, WI 53706-1284. E-mail: mcook{at}facstaff.wisc.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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1- and 1.5-fold
greater, respectively, compared with those fed CO. OO prevented
CLA-induced increases in 16:0 and 18:0 and the decrease in
18:1(n-9) in yolk. Fertile eggs were stored at 4°C for 2 or 10 wk and
analyzed for pH or mineral levels. Dietary CLA caused abnormal pH
changes of albumen and yolk when eggs were stored at 4°C. The pH of
yolk and albumen from CO-fed hens after 10 wk of storage was 6.12
± 0.12 and 9.06 ± 0.03, respectively, versus 7.89 ± 0.25 and 8.32 ± 0.16, respectively, in eggs from CLA-fed
hens. OO prevented CLA-induced abnormal changes in the pH of
albumen and yolks. Eggs from CLA-fed hens had greater iron, calcium
and zinc concentrations and lower magnesium, sodium and chloride
concentrations in albumen relative to those from hens fed CO. OO
prevented CLA-induced mineral exchange between yolk and albumen,
presumably by reducing the yolk saturated fatty acids, which are
believed to disrupt the vitelline membrane during cold storage. This
study suggests that the adverse effects of CLA may be due to the
increased level of saturated fatty acids. However, because the addition
of olive oil also lowered egg CLA content, the direct role of egg CLA
on egg hatchability and quality cannot be ruled out.
KEY WORDS: • chickens • conjugated linoleic acid • conjugated linoleic acid • egg quality • embryonic mortality
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, Ip et al. 1991 and 1994
) and antiatherogenic (Lee et al. 1994
, Nicolosi et al. 1997
) effects in animal
models. CLA also was found to have potent immune modulation activity
characterized by increased lymphocyte blastogenesis (Chew et al. 1997
, Miller et al. 1994
) and
macrophage-killing ability (Chew et al. 1997
,
Cook et al. 1993
). Researchers reported that CLA
decreased the catabolic effects of immune stimulation in mice, rats and
chickens without any adverse effects on the immune response
(Cook et al. 1993
, Miller et al. 1994
).
Recently, it was shown that CLA reduced body fat content and increased
lean body mass in pigs and rodents (Dugan et al. 1997
, Park et al. 1997
).
Major dietary sources of CLA in the human diet are dairy products and
meat from ruminant animals (Chin et al. 1992
). CLA can
be produced by the rumen bacteria as an intermediate via
biohydrogenation of polyunsaturated fatty acids
(PUFA)3 (Kepler et al. 1966
). CLA
has been shown to be enriched in dairy products by processing or
microbial fermentation (Aneja and Murthi 1990
,
Shantha et al. 1995
).
Chin et al. (1994
) investigated the effects of CLA
during gestation and lactation of rats. They showed that CLA was
incorporated into milk fat as well as fetal and neonatal tissues and
that feeding CLA to rats during gestation and lactation improved the
postnatal weight gain of rat pups (Chin et al. 1994
).
Recently, Chamruspollert and Sell (1999
) showed that
chicken eggs could be enriched in CLA to as high as 11% by feeding
hens 5% CLA in the diet. However, it is not possible to simply use CLA
in laying hen diet and obtain an egg enriched with CLA because of the
adverse effects of dietary CLA on the reproduction of laying hens
(Lee 1996
) and on the quality of eggs stored in cold
room temperatures (<15°C) (Ahn et al. 1999
,
Lee 1996
). The ingestion of CLA by laying hens not only
resulted in the incorporation of CLA isomers into egg yolk but also
increased the level of saturated fatty acids (SFA) and decreased the
level of monounsaturated fatty acids (MUFA) (Ahn et al. 1999
, Chamruspollert and Sell 1999
, Lee 1996
). Dietary CLA also was shown to cause complete mortality
in chick embryos from laying hens fed 0.5% CLA (Aydin et al. 1999b
, Lee 1996
).
Yolk fat as a source of energy and essential nutrients has a crucial
role in the avian embryonic development (Freeman and Vince 1974
, Romanoff 1960
). Significant alteration of
yolk fatty acid composition can have drastic effects on embryonic
survival. Donaldson and Fites (1970
) reported that
cyclopropene fatty acids in quail diet drastically increased 18:0,
decreased 18:1(n-9) and induced embryonic mortality. Also, a high level
of myristic acid (15%) in laying hen diet was reported to
significantly depress egg production and hatchability of fertile eggs
(Machlin and Dudley 1962
).
Because CLA decreases the oleate [18:1(n-9)] content of egg yolks,
one of the objectives of this study was to determine whether
CLA-induced mortality could be prevented by feeding olive oil (OO)
[high in 18:1(n-9)]. Dietary CLA has also been shown to cause
hardening of egg yolks and discoloration of yolks and albumen in eggs
stored at 4°C (Ahn et al. 1999
, Lee 1996
). Hence, another objective of the study was to determine
whether OO would prevent CLA-related changes in egg quality by
restoring the 18:1(n-9) content in the egg yolk.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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represents the dietary treatments and fatty acid profile of CLA. The
level of CLA used in this study was selected because previous results
demonstrated that 0.5 g CLA/100 g diet induces 100% embryonic
mortality and causes significant changes in yolk fatty acids. It has
been shown that conjugated linoleic acid (0.5 g CLA/100 g diet) in
laying hen diet caused 50 and 41% decreases in the levels of egg
yolk 16:1(n-7) and 18:1(n-9), respectively (Lee 1996
).
The use of 10 g OO/100 g diet was used because this level provided
sufficient 16:1(n-7) and 18:1(n-9) to compensate for CLA-induced
decrease in 16:1(n-7) and 18:1(n-9). To maintain isocaloric diets in
the presence of OO (10 g OO/100 g diet), wheat middlings were used as a
diluent. Previous work has demonstrated that feeding hens up to 89%
wheat middlings has no adverse effects on egg quality as does feeding
CLA (Patterson et al. 1988
). The low fat and high OO
basal diets contained the following calculated levels of the minerals
of interest, respectively: 3.5 and 3.5 g Ca/100 g diet, 0.15 and
0.14 g Mg/100 g diet, 0.21 and 0.25 g Na/100 g diet, 0.34 and
0.33 g Cl/100 g diet, 201 and 108 mg Fe/kg diet and 46 and 75 mg
Zn/kg diet. The experimental diets were mixed every week. Laying hens
were exposed to a 16:8-h light/dark daily lighting schedule and given
free access to water and feed for the duration of the experiment. After
the 74-d feeding period, all laying hens consumed the control diet (CO)
for 10 d. Laying hens were artificially inseminated weekly with
0.05 mL of pooled semen collected from New Hampshire roosters
immediately before insemination. Eggs were collected daily, held at
15°C for 24 h and then incubated at 37°C and 85% relative
humidity in an incubator. Eggs were candled weekly to detect fertility
and embryonic death. Hatchability and embryonic mortality were computed
as a percentage of total fertile eggs on each day.
|
). Fatty acid methyl esters (FAME) were prepared by
reaction with 4 mL of HCl in 100 mL of methanol for 20 min at 60°C,
and the composition of FAME was determined by gas chromatography
(Chin et al. 1992
). FAME were identified by comparison
of retention times with standards and expressed as percentage of total
FAME. For yolk and albumen pH measurements, 10 whole eggs were collected from each treatment and were stored at 4°C for 2 or 10 wk. After the period of storage, eggs were broken and separated into yolks and albumen. The albumen and yolk samples were stirred with a glass rod during pH measurements (Accumet pH meter 910; Fisher Scientific, Pittsburgh, PA).
Five whole eggs from each treatment were stored at 4°C for 10 wk, and yolks and albumen of the eggs were separated and analyzed for mineral content with an inductively coupled plasma emission spectrophotometer (ICP) at the Department of Soil Science, University of Wisconsin-Madison. The yolk and albumen of the eggs were freeze-dried, and minerals were analyzed on a dry weight basis. Briefly, dried yolk and albumen samples (0.5 g) were put into a 50-mL Folin digestion tubes. Five ml of a 6:1 mixture of concentrated HNO3:HClO4 acids was added into the Folin tubes; tubes were heated to 100–120°C for 3 h and then the temperature was raised to 215°C and the tubes were heated until dense white fumes appeared in the space above the solution. After that, the tubes were removed from the digestion block, placed in a cooling rack and allowed to cool. After the tubes were cooled, the solutions were diluted to the 50-mL calibration mark and analyzed by ICP.
Statistical analysis.
Statistical analysis of fatty acids and pH of yolk and albumen was
performed by two-way ANOVA (SAS Institute 1994
). We
tested CLA treatment, OO and OO x CLA treatment effects for each
variable, and pairwise comparisons were made to compare treatment
differences.
In statistical analysis of hatchability (%), the hatchability for
85 d was reported for each dietary treatment. To compare the
hatchability of dietary treatments across the time, the time in the
study was divided into three periods (d 1–7, 8–74 and 75–85), For
each period, we fitted regression lines for each dietary treatment.
Therefore, the difference between any treatments was examined by
t test on two parameters (slope and intercept). A
different slope or intercept indicated that two regression lines were
different (Fig. 1
,
2
). For statistical analysis of minerals of egg yolk and albumen, a
multivariate one-way ANOVA (MANOVA) model was fitted to each
mineral dataset. Differences were considered significant at
P < 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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).
However, CLA caused 100% embryonic mortality in the fertile eggs after
the 6-d feeding period. The additional of 10% OO completely prevented
CLA-induced embryonic mortality. When CLA-fed hens were fed the
control diet (CO), hatchability improved to a level similar to the
other dietary groups within 7 d (Fig. 2)
.
Feeding CLA in a low fat diet significantly increased SFA and decreased
MUFA in the egg yolk (Table 2
). The 14:0 content of yolk of eggs from CLA group was significantly
greater than that in eggs from the CO group. The ratios of 16:0/16:1
and 18:0/18:1(n-9) in egg yolk from the CLA group were 1- and
1.5-fold greater, respectively, compared with eggs from those fed
CO. The addition of OO (CLA + OO) completely prevented the
CLA-induced increase in 14:0, 16:0 and 18:0 and the CLA-induced
decrease in 18:1(n-9) in egg yolk but not that of 16:1(n-7). Olive oil
significantly decreased 16:1(n-7) compared with CO. Although dietary
CLA had no effect on the concentration of yolk 18:2 (n-6), the level of
20:4(n-6) in the yolk was significantly lower in eggs from CLA-fed
hens. Hens fed CLA had yolks with a 16-fold more in total CLA (as a
percentage of FAME) compared with yolks from hens fed CO (0.11 and
1.91%, respectively). Egg yolks from hens fed CLA + OO had 64% less
yolk CLA compared with those fed CLA.
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10 d of storage at
4°C (Aydin 2000
).
Another prominent biological effect of dietary CLA was the effect on
yolk and albumen pH of eggs stored at 4°C for 2 or 10 wk (Table 3
). Yolk pH was higher and albumen pH was lower in the eggs from
CLA-fed hens relative to the other groups. Yolk and albumen pH was
similar in eggs from hens fed CO, CO + OO and CLA + OO (6.08 ± 0.05 and 9.04 ± 0.02, respectively). Furthermore, when eggs from
CLA-fed hens were stored at room temperature (21°C), the pH of
the albumen and yolk was not different from that of any other group
(not shown).
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. Eggs from CLA-fed laying hens had significantly greater
concentrations of Mg2+, Na+
and Cl- and lower concentrations of
Ca2+, Zn2+ and
Fe3+ in the yolk than other groups. In contrast,
eggs from laying hens fed the CLA diet had greater concentrations of
Fe3+, Ca2+ and
Zn2+ and lower concentrations of
Mg2+, Na+ and
Cl- in albumen relative to eggs from hens
fed CO, CO + OO and CLA + OO. Total Fe3+ levels
in the albumen of the eggs from CLA-fed laying hens was 22-fold
greater than those from hens fed CO. Feeding OO along with CLA (CLA +
OO) completely prevented changes in the mineral content of egg yolk and
albumen.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, Lee 1996
). The mechanism
for chick embryonic mortality resulting from maternal ingestion of CLA
has yet to be determined. When laying hens were fed a diet containing
0.5% CLA, yolks hardened and fertile eggs failed to hatch, possibly
due to increased 16:0 and 18:0 and decreased 18:1(n-9) in the egg yolk.
The relative percentage of 16:1(n-7) and 18:1(n-9) of eggs from the CLA
group decreased 40 and 31%, respectively, compared with the CO
group (Table 2)
. It is not known whether the changes in fatty acid
profiles of yolk are the result of a mixture of CLA isomers or specific
isomers. Lee et al. (1998
) suggested that the
trans-10, cis-12 CLA isomer reduced the ratio of
MUFA to SFA by inhibiting liver stearoyl-coenzyme A desaturase
enzyme activity, an enzyme that catalyzes the insertion of a double
bond between the C9 and C10 atoms of either 16:0 or 18:0 in the
formation of 16:1(n-7) and 18:1(n-9), respectively (Cook 1991
).
Yolk fat, as a source of energy and essential nutrients, has a crucial
role in avian embryonic development (Freeman and Vince 1974
, Romanoff 1960
). Donaldson and Fites (1970
) reported that a significant alteration in yolk fatty
acid composition could have drastic effects on embryonic survival.
Similar to the results reported here, cyclopropene fatty acids in a
quail diet significantly increased 18:0, decreased 18:1(n-9) and
induced chick embryonic mortality (Donaldson and Fites 1970
). Oleic acid [18:1(n-9)] is one of the major fatty acids
in egg yolk and accounts for 40% of the total fatty acids in the
egg (Noble 1987
). Noble et al. (1984
)
reported that the accumulation of cholesterol esters, mainly
cholesterol oleate, that occurred within the liver during embryonic
development arose from synthesis in the yolk sac membrane. There is a
specific requirement of 18:1(n-9) for yolk lipid uptake and metabolism
by the embryo through its esterification with cholesterol (Noble et al. 1987
, Shand et al. 1993
). Researchers
have suggested that 18:1(n-9) plays an important role in the survival
of avian embryos (Noble and Cocchi 1990
). Hatchability
was adversely affected when 18:0 represented >12% and 18:1(n-9)
represented <40% of the total fatty acids in the yolk and when the
ratio of 18:0 to 18:1(n-9) exceeded 0.25 (Tullet 1990
).
In the present study, the ratios of 16:0/16:1(n-7) and 18:0/18:1(n-9)
were 1- and 1.5-fold greater, respectively, in the eggs from
CLA-fed hens relative to control. If the high ratio of SFA to UFA
significantly interfered with the absorption of yolk fatty acids by the
developing embryo, embryonic mortality would ensue, because 90% of the
embryonic energy requirement is derived from yolk fat oxidation
(Boell 1955
).
In the present study, dietary CLA increased the level of SFA and
induced chick embryonic mortality in the fertile eggs stored at 15°C
for 24 h before incubation. We also observed that even when eggs
were incubated immediately after oviposition (without storing eggs at
15°C), they failed to hatch (unpublished observation). Feeding OO
(CLA + OO) completely prevented CLA-induced embryonic mortality in
fertile eggs. This was probably due to restoration of the levels of
16:0, 18:0 and 18:1(n-9) in the egg yolk. Hepatic lipogenesis was
reduced in the laying hens fed a diet supplemented with 30% safflower
oil (Naber and Biggert 1989
). Feeding fats high in
18:1(n-9) to laying hens reduced the conversion of 18:0 to 18:1(n-9)
(Donaldson 1966
). These studies suggest that when hens
are provided with dietary fat, they use dietary fat instead of
synthesizing it in the liver for yolk development. When OO [high in
18:1(n-9)] was supplemented to diet (OO + CLA), the 18:1(n-9) levels
of egg yolk were restored but not that of 16:1(n-7). Also, by using OO
along with CLA in the diet (CLA + OO), the amount of CLA in the eggs
was increased 5-fold without causing undesired reproductive
consequences. However, eggs from hens fed CLA + OO had 64% less yolk
CLA relative to those fed CLA. When hens fed 0.5% CLA were fed the CO
diet (with no CLA), the ratio of 16:0/16:1(n-7) and 18:0/18:1(n-9) in
yolk was restored after 6 d of feeding, but CLA levels in the yolk
(as g/100 g FAME) were 80% lower than levels during CLA-feeding
period (Aydin et al. 1999b
). In addition, the adverse
effects of dietary CLA on the hatchability of eggs were reversible
within 6 d when the diet containing CLA was substituted with a
control diet (CO) not supplemented with CLA. These data suggest that
there is a relationship between the ratio of SFA to MUFA in the yolk
and embryo viability. The yolk fatty acid composition data suggested
that embryonic mortality observed in the eggs from CLA-fed laying
hens was the result of an increase in the ratio of SFA to MUFA.
Recently, we showed that a combination of CLA and canola oil (2 g CLA
plus 4 g canola oil/100 g diet) resulted in 56% greater CLA in
the egg yolk compared with eggs from CLA-fed chickens (Aydin 2000
). Unlike the CLA diet (0.5 g CLA/100 g diet), the
combination of canola oil and CLA did not cause embryonic mortality
(Aydin 2000
).
Another notable effect of dietary CLA was the development of
discoloration of albumen and yolks when the shell eggs were stored at
4°C. The texture of egg yolks exhibiting the color defect was
described as rubbery, pasty or viscous (Lee 1996
). The
mechanism for color defects of yolks and albumen has yet to be
determined. Lee (1996
) suggested that the discoloration
of eggs from hens fed a diet containing 0.5% CLA could result from an
increased permeability of the vitelline membrane of eggs due to
CLA-induced changes in fatty acid composition. Abou-ashour and Edwards (1970
) suggested that a higher 18:0 content of the
egg yolk fatty acids would probably increase the permeability of
vitelline membrane. The pink discoloration of albumen can be attributed
to a combination of ovotransferin, egg albumen protein and yolk iron
that diffuses into the egg albumen (Schaible and Bandemer 1946
). In addition to iron, we found that dietary CLA also
caused calcium and zinc to move from yolk into albumen and magnesium
and sodium to move from albumen into yolk (Table 4)
. When yolks and
albumen of the fresh eggs from CLA-fed laying hens were separated
and stored at 4°C for 1 mo, no color changes were observed in either
yolks or albumen of the eggs (unpublished observation). This
observation suggested that the discoloration of yolk and albumen of
eggs stored at cold temperature might be associated with the increased
permeability of vitelline membrane due to the altered fatty acid
composition such that minerals move down their concentration gradient.
Surprisingly, chloride moved up its concentration gradient in the eggs
from CLA-fed hens. Dietary OO (CLA + OO) completely prevented the
mineral exchange between yolk and albumen as well as the discoloration
of egg albumen and yolks.
The pH values are usually maintained near the divergent ranges of
6.2–6.5 for the yolk and 8.6–9.2 for the albumen after storage
(Heath 1977
), but in eggs of hens fed CLA, albumen and
yolk pH tended to equilibrate (7.89 and 8.32, respectively) when stored
at 4°C for 10 wk (Table 3)
. The pH change occurred only after storage
at 4°C. Abnormal pH changes and discoloration of albumen and yolk did
not develop in CLA-fed laying hens when shell eggs were stored at
21°C for 10 wk (data not shown). We have also shown that the adverse
effects of CLA on pH of egg yolks and albumen and hardening of the egg
yolks at low temperatures were prevented by the addition of OO to the
CLA diet (CLA + OO). Egg yolk from hens fed CLA + OO remained
liquid when cooled below room temperature unlike yolk in eggs from hens
fed CLA with no added OO. Physical changes observed in egg yolk from
hens fed a diet supplemented with CLA (0.5 g/100 g) during cold
temperature are probably associated with a higher concentration of SFA
in the yolk lipids. We also observed that color defects of egg albumen
and yolks were associated with the length of CLA feeding period in
laying hens and the length of the storage at 4°C. After 7 d of
CLA feeding, color defects developed in eggs stored at 4°C for 
1
wk. Discoloration of albumen and yolk may be associated with the
increased ratio of SFA to MUFA in egg yolk (after 1 wk of the CLA
feeding period). We showed that when hens from the CLA group were fed
the control diet (no supplemental CLA) for 7 d, the ratio of SFA
to MUFA returned to levels seen in control eggs (Aydin et al. 1999b
) and no discoloration in either the yolks or albumen of
eggs (when they were stored at cold storage temperatures) was observed.
The substantial change in ingredients that made up the high oil basal
diet, CO + OO and CLA + OO, may have been responsible for preventing
CLA-induced changes in egg hatchability and egg quality apart from
OO. Previous work (Chamruspollert and Sell 1999
,
Lee 1996
) has shown that CLA induces abnormal changes in
the egg even when added to diets similar to the high oil basal diet
used in this study. We have also observed that other UFA prevent
CLA-induced changes in egg hatchability and quality when added to
the diet containing 5 g CLA/kg of diet (Aydin et al. 1999b
). In another study where only minor changes were made to
accommodate the inclusion of 20 g canola oil/kg diet (accomplished
by the addition of 21 g of wheat middlings/kg diet), we found that
the canola oil reduced CLA-induced embryonic mortality
(Aydin 2000
). The only minerals of interest that
differed between the low fat and high OO basal diets were iron (201 and
108 mg/kg diet, respectively) and zinc (46 and 75 mg/kg diet,
respectively). Because iron and zinc did not differ in albumen and
egg yolk when hens were fed these basal diets (a comparison of CO
versus CO + OO, Table 4
), it would appear unlikely that the difference
in the basal content of iron and zinc influenced the results obtained.
Feeding hens 59–218 mg zinc/kg diet has no influence on egg zinc and
iron levels (Stahl et al. 1988
). In addition, hens fed
dietary zinc of 28–2028 mg/kg diet had no impact on hatchability
(Stahl et al. 1990
). Hence, the prevention of
CLA-induced changes in egg hatchability and quality was most likely
attributed to the added dietary OO and not due to the basal diet.
In conclusion, dietary CLA in a low fat diet caused 100% embryonic mortality by causing higher SFA and lower UFA in the egg yolk. Dietary CLA also adversely effects the quality of eggs stored at cold temperature. The present study also suggested that the addition of OO [rich in 18:1(n-9)] prevented CLA-related changes in the pH of yolk and albumen and mineral exchange between yolk and albumen by maintaining the ratio of SFA to UFA.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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3 Abbreviations used: CLA, conjugated linoleic
acid; CO, corn oil; FAME, fatty acid methyl esters; MUFA,
monounsaturated fatty acids; OO, olive oil; PUFA, polyunsaturated fatty
acids; SFA, saturated fatty acids.
Manuscript received April 10, 2000. Initial review completed May 18, 2000. Revision accepted November 28, 2000.
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