Module 7. Egg and egg processing
Lesson 26
EGG: STRUCTURE,
COMPOSITION AND QUALITY
26.1 Introduction
Egg is a complete food consumed throughout the world. Eggs of various birds may be eaten however eggs of hen and duck are most commonly consumed. Egg protein contains all the essential amino acid and has got highest biological value. Thus egg proteins are considered by the WHO to be the reference protein, to which all other proteins are compared. An average weight of hen egg is about 2 ounce i.e. 57g
26.2 Structure and Composition of the Egg
Whole Egg can be divided into three major components
1. Shell
2. Egg white
3. Egg yolk
26.3 Composition of Egg
Table 26.1 Composition of egg
|
Component |
% |
Water % |
Protein % |
Fat % |
Ash % |
|
Whole egg |
100 |
65.5 |
11.8 |
11 |
11.7 |
|
Egg white |
58-60% |
88 |
11 |
0.2 |
0.8 |
|
Egg yolk |
31-33% |
48 |
17.5 |
32.5 |
2 |
|
Shell |
9 – 12% |
|
|
|
|
26.3.1 Shell
It is the Outer covering of Egg, contributes to 9 – 11% of the whole egg weight. 94% of the egg shell is composed of calcium carbonate. It Acts as a barrier against harmful bacteria and other contaminants. Shell has got numerous pores on its surface, permitting moisture and carbon dioxide to move out and oxygen to move in as egg ages. Strength of the shell indicates the quality of the egg and strength is influenced by mineral and vitamin content of the hen’s diet, mainly calcium, phosphorous and Vitamin D. Inside surface of the Shell is lined by a mucous layer also known as protective layer called cuticle or bloom. Cuticle preserves the freshness of the egg by blocking the pores on the shell.
26.3.2 Egg white
It is also known as Egg albumin, it contributes to about 65% of the egg’s liquid weight. It contains more than half of the egg’s total protein. Egg white becomes thin as an egg ages because protein changes in character. Hence fresh eggs sit up tall and firm in the pan while older ones tend to spread out. As the egg ages, carbon dioxide escapes out, so the albumin becomes more transparent than the fresh ones
26.3.3 Egg yolk
The yolk or yellow portion makes upto about 33% of the liquid weight of the egg. It contains all of the fat in the egg and about 45% of total egg proteins.
26.4 Structure of the Egg
Source: USDA
Fig. 26.1 Structure of the egg
26.4.1 Factors affecting quality of egg
1. Age of the egg
2. Storage atmosphere
3. Temperature of storage
4. Relative humidity
5. Pre-treatments given before storage
26.4.2 Testing the quality of egg
1. Water test: this test is based on the principle of density. Eggs are simply dropped into water. Good quality eggs sink to bottom and poor quality ones float. Loss in weight is due to dehydration and thus increased air cell size.
2. Sensory test: Cracked, smelly, rough surface indicates poor quality
3. Candling test: This is the most commonly used test to determine the spoilage of egg. The egg shell is porous and allows the light to pass through. The eggs to be tested are placed in front of sharp, bright light and assessed for following factors
a. Cracks on the shell
b. Air cell size and position
c. Albumin and yolk position and firmness
d. Presence of any blood clot or foreign elements in the egg
26.4.3 Nutritional qualities of egg
Eggs are nutritionally rich and one of the nature’s most complete food. They provide 173Kcals/100g.
26.4.4 Proteins
Egg contains high quality complete protein with all the essential amino acid in well balanced proportion. Biological value of egg protein is 100, indicating all of the protein consumed is retained by the body. Egg white, also known as Albumin, contributes to the 60% of total proteins present in Egg and the rest is from Egg yolk. Nearly 50% of the protein in egg white is Ovalbumin, followed by Conalbumin (13%), Ovomucoid (10%), Lysozymes (3.5%), Ovomucin (2%), Avidin, Ovoglobulin and Ovoinhibitor.
Vitellin is the protein present in Egg yolk, which is present in a lipoprotein complex as lipovitellin and lipovitellinin. Phosvitin phosphorus-containing and Livetin sulfur-containing protein are also present in egg yolk. Avidin is protein present in the egg while which binds the vitamin biotin thus makes it unavailable to the body.
26.4.5 Fat
Fat is concentrated in the Egg yolk. Yolk fat can be divided into 3 major parts i.e. Triglycerides, phospholipids and lipoproteins. Lipoproteins are present in conjugation with phospholipids. The primary phospholipid present in egg is lecithin and major sterol is cholesterol.
The major Triglycerides present in egg yolk are
Oleic acid 38.45%
Palmetic acid 23.50%
Linoleic acid 16.4%
Stearic acid 14.0% of total fatty acids
26.4.6 Carbohydrate
Glucose, Mannose and galactose are present in small amounts. This will take part in maillard reaction producing an undesirable brown discoloration in both dried and cooked egg whites.
26.4.7 Micronutrients
The major micronutrients present in egg are vitamins and minerals. Egg is the rich source of all known vitamins except vitamin C. All the fat soluble vitamins A, D, E are concentrated in yolk. Minerals such as iron, phosphorus, zinc, iodine, potassium, sodium, chlorine and sulphur are present in good amounts. Iron present in egg is bound to conalbumin and therefore its absorption is poor while Zinc is present in the most abundant biologically active form.
References
Avantina Sharma, 2006. Food science and technology, IBDC publications, Lucknow
Vijaya Khader, 2001, Food science and technology, ICAR, New delhi