Lipids in egg yolk

Table 11.12. Fatty acid composition of the lipids in egg yolk - Influence of feed ...

The adaptation of methods for lipid extraction and quantification by colorimetric determination of either ester or cholesterol in egg yolk with some contamination of egg white is described. Results are compared with those obtained by a conventional enzymatic determination. 

Compositional data on commercial egg products and various lipid extracts from egg yolk have been compiled by Gornall and Kuksis (13), Kuksis (11), Schneider (14), and Satirhos et al. (15). Tables 1 and 2 (14, 16) compare the composition of soy and egg lecithins and their fatty acids, and Table 3 (16-21) shows the distribution of phospholipid classes in egg yolk. 

Besides phospholipid composition, the main difference between plant/legume lecithin (e.g., soy) and lecithin in egg yolk is that the former has a higher unsaturated fatty acid content and no cholesterol. Egg lecithin as a commercial ingredient, with the exception of some medical feeding programs, is too expensive for routine use in food (10). In some infant formulas, egg yolk lipids and egg lecithin are used (22). 

Although phospholipid bilayers are better mimics of biomembranes than are micelles, there are few reliable quantitative data on flavonoid antioxidant activities in lipid bilayers. Terao and coworkers compared the antioxidant efficiency of quercetin and catechins (epicatechin and epicatechin gallate) with that of a-Toc in egg yolk PC liposomes using initiation by the water-soluble initiator, ABAP, and analysis of hydroperoxide formation and antioxidant consumption by HPLC. Based on the length of the induction periods and the profile of suppressed hydroperoxide formation, they concluded that quercetin and the catechins were more efficient antioxidants than a-Toc in these bilayers. Apparently the unique behavior of a-Toc in bUayers is responsible for these results (vide supra). In hexane and alcohols solution during suppressed peroxidation of methyl linoleate, the relative antioxidant activities reversed so that the flavonoids were 5-20 times less active... 

The composition of the phospholipid fraction in a lipid extract is rather variable, not only between two species, but also within the same species (Table 5.1). Phosphatidylcholine is the main phospholipid fraction in most phospholipid concentrates. In egg yolk, the phosphatidylcholine content is particularly high in comparison with... 

Phospholipid contents are very similar (about 1 to 2% dry matter) in microbial, plant, and animal tissues. If the content of neutral lipids is low, phospholipids may account for 20 to 40% of lipid extracts (e.g., in marine invertebrates). In egg yolk, 23% of the total lipids are phospholipids and other polar lipids (Kuksis, 1985). On the contfary, in adipose tissue or in oilseeds, the content of phospholipids is between 1 and 3% of total lipids. In oilseeds rich in oil (such as in rapeseed), it is lower than in oilseed with lower oil content (such as soybeans) when the results are expressed in % oil content, but much the same if the content is expressed in terms of total dry matter of the oilseed. Phospholipids are mainly extracted by nonpolar solvents, together with other lipids, and are obtained in the crude oil. However, in the original material, phospholipids are primarily bound to proteins (e.g., in membranes) or may be bound to other tissue components for example, phospholipids interact with chlorophyll pigments, where they may form complexes between the magnesium ion of the chlorophyll molecule and the phospho group of the phospholipids. 

The fat content of egg yolk is approximately 31.8 to 35.5% that is, about a third of the whole egg yolk. Lipids occur in different particles (granules, LDL-micelles) and are almost completely associated with proteins. An overview is provided by Temes (2001). About 70% of the dry mass consists of lipids. Differences in the Upid composition are not only genetically determined, but also occur due to the animals age and diet. The contents of long-chain (C20 and C22) polyunsaturated FA (PUFA) of n-6 and n-3 were 20 to 25% higher in the lipids of egg yolk from younger hens (Nielsen, 1998) (Table 14.1). 

In careful examinations, Rhodes and Lea (1957) obtained the composition of phospholipids. Here, for the first time, lysophospholipids were shown to be natural components of egg lipids and not artifacts produced during sample preparation, as the authors explicitly point out. The PC fraction contains 0.9% plasmalogens the cephalin fraction contains 0.2% phosphatidylserines. Table 14.3 gives an overview of the composition of phospholipids in egg yolk. There is high agreement between the results of all individual authors. Only the results of Holopainen (1972) for sphingomyelins appear to be too high. 

Moschidis et al. (1984) found small amounts of phosphonohpids in the egg yolk. The comparison of the FA composition of minor components shows that in sphingomyelins the amount of palmitic acid is twice that in the egg yolk lipids, whereas there is almost 40% less oleic acid. Linoleic acid at 30% is found at concentrations about three times higher in lysophosphatidylcholines than in egg yolk hpids, and twice as much as in the PC fraction. At 28%, docosanoic acid (C20 o = behenic acid) is present in considerable amounts in gangliosides, as opposed to lower concentrations of oleic and linoleic acids in comparison with the FA spectrum of egg yolk lipids. Momma et al. (1972) also found a high proportion of approximately 67% hydroxy FA in the cerebrosides. Many of the hydroxy FA have a chiral center. In any case. Momma et al. (1972) determined a relatively high concentration (15.7%) of tetracosanoic acid (C24 o = lignoceric acid), while smaller concentrations of the main FA of the egg yolk lipids were found in the cerebrosides. Table 14.5 shows an overview of the FA spectrum of the minor components. 

Dietary fat supplements are freqnently nsed to increase the energy content in the hens ration. Increased dietary fat does not lead to an increase in the fat content in the egg yolk, but the EA and TAG spectra in egg yolk lipids do change in relation to the fat composition of the feed. There is nearly always a connection between diet and the egg yoUc FA spectrnm, bnt the amonnts of FA do not completely correspond. 

Hirata et al (1987) conducted extensive tests comparing the FA composition of egg yolk after feed supplements of vegetable oil and animal fat. The FA sample of the dietary fat supplement is reflected considerably more clearly in TAG than in polar lipids. Table 14.10B reveals that compared to the supplement of animal fats, the proportion of linoleic acid in the TAG of the egg yolk is highest after a 10% soy oil supplementation, and the proportion of oleic acid the lowest this also applies to a coconut oil supplementation. Furthermore, after coconut oil supplementation, the TAG contain substantially more myristic, myristoleic, and patmitoleic acids. Because myristoleic acid (C14,) and pahnitoleic acid (Cjea) are not present in coconut oil, but do occur in egg yolk lipids, the production of a double bond in myristic and palmitic acids by fat metabolism can be assumed. Although coconut oil consists of 50% lauric acid (C,2 o), only small amounts of this acid can be found in the egg yolk. It has been assumed that hens can use only very small amounts of FA with shorter chains for egg yolk lipid production. Shorter FA in the coconut oil do not contribute to egg yolk FA production. The... 

Isomers of conjugated linoleic acids (CLA) have anticancer activity, and immnne-enhancing-weight reductions, and antiatherogenic properties. CLA are fonnd in food prodnced from ruminant animals. The development of CLA-enriched eggs showed a redaction in monoenoic FA and non-CLA PUFA after CLA feeding, and an increase in ERA and DHA. The incorporation rates of different CLA isomers into the whole lipids of egg yolk and TAG, PC, and PE-lipid classes were different. cis-9,trans-ll and cis-l0,trans-l2 CLA were deposited more in TAG, bnt cis-11,trans-l3 CLA was less. There are large differences in the concentrations of cis-8,trans-10 CLA in PC and PE (Table 14.10C). 

The nntritional modihcation of egg yolk lipids primarily involves an increase in the concenhation of mono- and polyenoic fatty acids and a decrease in the cholesterol concentration. Feeding layer diets with increased concentrations of olive oil rich in Ci81 acid resnlted in an increase of this acid in the egg yolk lipids. However, the best resnlts in terms of an increase in the concenhation of PEFA in egg yolk lipids were obtained by snpplemenhng feed with hsh oils or seaweed biomass rich in these acids. 

Nnhitional methods can also be applied to enriching egg yolk lipids with CLA by administering it in adequate doses with feed. However, the effectiveness of nnhitional methods in the reduction of cholesterol in egg yolk lipids is low. The nse of 3-hydroxy-3-methylglutarylo-CoA reductase for this purpose is more effective. In this way, the cholesterol level in egg yolk lipids can be decreased by 30% however, this has consequences in the layers prodnctivity (Pisnlewski et al., 2001 Pszczola et al., 2000). 

Vitellin is present in egg yolk as the lipoprotein called lipovitellin (Chapter 12.4). Both a and P varieties have been identified by chromatographic methods and their molecular weights are believed to be -400,000. Both the lipid and the protein parts of lipovitellin appear to be phosphorylated. The protein parts differ in their P content (a = 0.5%, p = 0.3%). 

Vitellin and vitellenin (Table 12.20) are present in egg yolk as the lipoproteins lipovitellin and lipovitellenin, respectively, the lipid portions of which are mainly the phospholipid lecithin with smaller amounts of cephalin (Chapter 10.3). 

Vhaliin a lipophosphoprotein present in egg yolk tr ether with Phosphovitin (see). It is present in a higher concentration than phosphovitin, but in contrast to this phosphorus-tich protein, V. contains only 1 % phosphate. M yolk, and in neutral salt solutions, V. exists as a dimer (M, 380,000 16-22% lipid). The monomer (JIf, 190,000) consists of two dissimilar chains (L, M, 31,000 Lj, M, 130,000) only L, contains phosphate. 

Except for eicosanoic acid, the proportions of all fatty acids in egg yolk lipids were significantly (P < 0.01) influenced by the dietary CLA (59). The proportions of myristic, palmitic, and stearic acids, and CLA cis-9,trans- CLA and transit),cis-f2 CLA) in egg yolk lipids were increased by dietary CLA, but those of palmitoleic, oleic, linoleic, and linolenic, arachidonic acids, and DHA were decreased. These changes in fatty acid composition of yolk lipids are similar to those reported by Chamruspollert and Sell (60), although the total CLA concentration observed in the current research when a 5% CLA diet was fed (8.5-8.6%) was less than the 11.2% reported by those authors. The decrease in the concentrations of linoleic and linolenic acids in yolk lipids of hens fed CLA likely reflects the relatively low concentration of these fatty acids in the CLA source compared with soybean oil. Decreases in arachidonic acid and DHA in yolk lipids from hens fed CLA also could be related to the low concentration of dietary linoleic and linolenic acids, which serve as precursors to the formation of arachidonic acid and DHA. Another possibility is that CLA may compete with Unoleic and/or linolenic add for A6-desaturase, the rate-limiting step for the conversion of these fatty acids into arachidonic acid and/or DHA in liver microsomes (48). Feeding CLA increased the concentration of stearic acid in yolk lipids. 

Figure 15.2 Separation of hen s egg yolk and Spirulina (see text) following extraction in chloroform-methanol. Development and detection was as described in Figure 15.1. Lanes 1 and 2 show the predominant lipids in the yolk. Lanes 3 and 4 show nnainly the presence of free sterols in the Spirulina. Lanes 5 and 6 show the neutral lipid standards. Abbreviations as in Figure 15.1.

Electrophoretic analyses of the lipid-free samples provide an insight into the proteins and apoproteins (lipoproteins after removal of the hpids, e. g., by extraction with acetone) present in egg yolk and its fractions. A relevant experiment, the results of which are presented in Table 11.9, shows 20 protein zones in the molecular weight... 

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