Egg yolk comprises mostly 25-33 percent of fat and 15-17 percent protein and the remaining water. Solid content of yolk is about 50 percent. Yolk is a dispersion containing a variety of particles distributed uniformly in a protein solution. On high speed centrifugation yolk separates itself into granules and a clear supernatant called plasma. Granules are composed of 70 percent α and β lipovitellins, 16 percent phosvitin and 12 percent low-density lipoprotein. Phosvitin, lipovitellin complex is a basic unit of granules. Plasma is composed of the globular protein livetin and a low density lipoprotein fraction.
The major proteins in egg yolk are lipoproteins which include lipovitellins and lipovitellinin. The lipoproteins are responsible for the excellent emulsifying properties of egg yolk, when it is used in such products as mayonnaise.
Lipovitellins:These are high-density lipoproteins comprising 16 to 18 percent of egg yolk solids. They can be separated into two fractions α and β lipovitellins. Each fraction contains 40 percent neutral lipids and 60 percent phospholipids.
Phosvitin: This comprises 5-6 percent of yolk solids. This is rich in phosphorus and accounts for 80 percent of the protein phosphorus of yolk. It is rich in serine and phosphate is esterified to this amino acid. Phosvitin binds tightly ferric ions and forms a soluble complex and is thus the iron carrier of the yolk.
Natural bactericide protecting the egg from bacterial invasion.
It is heat sensitive, has foam forming property.
Avidin pH 10.0
Combines with biotin, render it unavailable.
Becomes inactive by cooking.
Livetin: This comprises 4-10 percent of yolk solids. This consists of three components α, β and γ livetins. This is a water-soluble protein and is rich in sulphur. Components of this protein differ in their molecular weight.
Low-density lipoprotein: Low-density lipoprotein of egg yolk has a density of 0.98. It consists of 74 percent neutral lipids and 26 percent phospholipids. It can be separated into components LDL1 and LDL2. A number of enzymes are also present in the protein fraction.
Fat in the egg yolk It contains triglycerides, phospholipids and lipoproteins. Lipoproteins are complexed with phospholipids and cholesterol. The main phospholipids are lecithin. The main fatty acids in the triglycerides of the egg yolk are oleic, palmitic and linoleic and stearic in order. Table -----shows fatty acid composition of egg yolk. In view of the presence oh highly unsaturated fatty acid in egg yolk fat, it is prone to undergo oxidation easily and develop off flavour. Egg yolk viewed under the microscope is seen to consist of a relatively a small number of larger spherical bodies called granules and numerous smaller less regular shaped bodies called mycelles. Both granules and mycelles are suspended in the liquid or plasma portion of the yolk. Egg yolk does not contain free fat as such inspite of its high content of triglycerides. Instead most of the triglycerides and a high proportion of the phospholipids are found in the mycelles. Mycelles contain an encased layer of phospholipid surrounding a layer of protein. Thus these mycelles are in the nature of micro emulsion. Almost 90 percent of the fat of the egg is found in these mycelles.