FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 18 : Eggs

Deterioration during storage

Fertile eggs get deteriorated more rapidly than infertile eggs. Eggs when stored at room temperature undergo deteriorative changes. The weakening of the yolk membrane occurs after 2 days storage at 37oC, or after 5 days at 25oC, or 20 days at 16oC, 100 days at 2oC.

These changes that occur during deterioration may be grouped as physical and chemical changes.

1. Physical Changes
• Egg white becomes less viscous and spreads rapidly
• The size of the air cell increases, due to moisture loss during storage.
• Water passes from the white to the yolk thus increasing the volume and water content of the yolk resulting in breaking of vitelline membrane.


2. Chemical Changes
• Loss of water
• Loss of carbon dioxide
• Change in pH 7.6 to 9.7 in egg white
• The breakdown of proteins
• Increase in the amount of free ammonia.
• Increase in water-soluble inorganic phosphorus.
• Increase in free fatty acid in yolk fat.
• Deterioration in the flavour of eggs occurs by the invasion of micro-organisms and by the changes that take place in fat and protein.


3. Bacterial Decomposition
• As an egg ages the porosity of the cell increases making possible the infiltration of bacteria and moulds.
• The alkalinity of egg white and the lysozyme serve to reduce this spoilage caused by the micro-organisms.