Lesson 6 Physical Structure and Composition of Egg
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The document describes the detailed physical structure and composition of eggs. It notes that eggs have 3 main parts - the shell, egg white, and yolk. However, upon closer examination, eggs contain several additional internal structures. These include an air cell, alternating layers of thick and thin egg white, chalaza strands that anchor the yolk, a germinal disc, two membrane layers, and a yolk surrounded by a vitelline membrane. Each of these structures serves an important role and contributes to the egg's nutritional composition and quality.
Lesson 6 Physical Structure and Composition of Egg
- 2. We normally distinguish 3 parts of an egg, the shell, the egg white, and the egg yolk, but a closer scrutiny reveals a much more detailed structure of an egg.
- 5. 1. Shell. The egg‘s outer covering, the shell, accounts for about 9 to 12 % of its total weight depending on egg size. The shell is the egg‘s first line of defense against bacterial contamination.
- 6. 2. Air cell. This is the empty space between the white and shell at the large end of the egg which is barely existent in newly laid egg.
- 7. 3. Albumen/Egg white. Albumen, also called egg white, accounts for most of an egg‘s liquid weight, about 67%.
- 8. This is produced by the oviduct and consists of four alternating layers of thick and thin consistencies.
- 9. From the yolk outward, they are designated as the inner thick or chalaziferous white, the inner thin white, the outer thick white and the outer thin white.
- 10. 4. Chalaza. This is the ropey strands of egg white at both sides of the egg, which anchor the yolk in place in the center of the thick white.
- 11. The twist in the chalaza is meant to keep the germinal disc always on top whichever way the egg may turn. The more prominent the chalazae the fresher is the egg.
- 12. 5. Germinal Disc. This is the entrance of the latebra, the channel leading to the center of the yolk.
- 13. When the egg is fertilized, sperm enter by way of the germinal disc, travel to the center and a chick embryo starts to form. Since table eggs are not fertilized, this is not as easy to recognize as when the egg is fertilized.
- 14. 6. Membranes. There are two kinds of membranes, one just under the shell and the other covering the yolk. These are the shell membrane and the vitelline membrane.
- 15. The air cell formed due to the contraction of egg as it cools, is found between the two layers of this shell membrane. The outer membrane sticks to the shell while the inner membrane sticks to the albumen.
- 16. During storage, the egg losses water by evaporation, causing the air cell to enlarge. The vitelline membrane is the covering that protects the yolk from breaking.
- 17. 7. Yolk. The yolk or the yellow to yellow- orange portion makes up about 33% of the liquid weight of the egg.
- 18. The egg yolk is formed in the ovary. On the surface of the yolk, there is a small white spot about 2 mm in diameter.
- 19. The yolk material serves as a food source for embryonic development. It contains all the fat in the egg and a little less than half of the protein.
- 20. The main protein in the egg yolk is vitelline, a lipoprotein. It also contains phosvitin which is high in phosphorus and has antioxidant properties, and livetin which is high in sulfur.
- 23. Egg is indeed one of nature‘s complete food. It contains high quality protein with all the essential amino acids, all of the vitamins except vitamin C, and many minerals.
- 24. Egg products are particularly good for fortifying food low in protein quality.
- 25. Except for mother‘s milk, eggs provide the best protein naturally available. Egg protein is often used as a reference standard for biological values of their proteins.
- 28. Egg quality Egg quality has two general components: shell quality (exterior quality) and interior egg quality.
- 29. Interior egg quality has direct bearing on the functional properties of eggs while shell quality has direct influence on microbiological quality.
- 30. Egg Grading. Grading is a form of quality control used to classify eggs for exterior and interior quality. In the Philippines, the grade designations are A, B, C, and D.
- 31. Egg Size. Several factors influence the size of the egg: breed, age of hen, weight, feed and environmental factors.
- 32. The environmental factors that lead to smaller eggs are heat, stress and overcrowding.
- 33. The egg sizes are Jumbo, Extra Large, Large, Medium, Small and Peewee. Medium, Large, and Extra Large are the sizes commonly available.