Egg: Structure, Composition, Spoilage, and Preservation

Definition of Egg

• Egg is a widely consumed and versatile food source that comes from various kinds of animals, mostly chickens.
• Their oval shape, hard shell, and yolk and egg white inner parts are what make them recognizable.
•  High-quality protein, vitamins, and minerals are among the many nutrients found in eggs.
• They are utilized in many different recipes, ranging from easy breakfast options like scrambled or fried eggs to intricate baking and cooking formulas.
•  In many societies, eggs also have cultural and symbolic meaning; they are frequently associated with fertility and new life.

Structure and composition of Egg

• The shell is filled when the egg is freshly laid. The air cell is formed by the contraction of the contents during cooling and moisture loss. A high-quality egg contains only a single small air cell.
• The yolk is well-centered in the albumen and is surrounded by a colorless vitelline membrane. The germinal disc, where fertilization takes place, is attached to the yolk.
• Chalazae are two twisted, whitish cord-like objects found on opposite sides of the yolk. Their function is to support the yolk in the albumen’s center. Although chalazae vary in size and density, they have no effect on cooking performance or nutritional value.
• A large portion of the albumen is thick. The albumen is surrounded by two shell membranes and the shell itself. The shell contains thousands of pores that allow it to “breathe.”

Composition of Egg

Egg yolks contain a lot of protein, fat, and cholesterol. The whites of eggs contain 90% moisture and 10% protein. The yolks contain 50% moisture, 30% fat, and 20% protein.

Nutrition value of egg

The nutritional value of eggs varies according to their size; however, this is not an important factor in determining their quality. Larger eggs, of course, have more nutritional value than smaller ones.

• A single large egg contains 6.5gm of protein, or about 13% of the adult RDA, as well as 80 calories and a good amount of iron, phosphorus, thiamine, and vitamins A, D, E, and K. The egg’s high cholesterol content is a disadvantage as a staple diet food.
• The yolk, which contains approximately 50% water, 34% lipids, fats, and related substances, and 16% protein, as well as traces of glucose and minerals.
• It is composed of roughly 11% shell and 89% interior. The shell’s composition plays a role in terms of food safety, sanitation, and appearance. It is composed of 94% calcium carbonate, 1% magnesium carbonate, 1% calcium phosphate, and 4% organic matter.
•  It is critical to recognize that there is substantial evidence that the diet of the hen may affect the composition of the egg.

Contamination source of egg

Various sources of contamination can contaminate eggs at various stages of production, handling, and storage. Some of the most common sources of contamination are as follows:

Bacteria

• Salmonella: This bacterium can be present inside the egg if the hen that laid it was infected. It can contaminate before the shell forms.
• Campylobacter, E. coli, and Listeria: These bacteria can also be contaminants, because of poor hygiene during production or processing.

Environment and handling

• Dirty or Cracked Shells: Bacteria can enter through cracks in the shell or from the outside if the shell is dirty.
• Cross-Contamination: Happens when eggs meet surfaces or tools that are contaminated with bacteria.
• Hen health: If the hen laying the eggs is infected, the eggs themselves may be contaminated internally before they are laid.
• Storage and temperature: Improper storage conditions, particularly when eggs are stored at temperatures favorable to bacterial growth, can result in contamination.

Spoilage of egg

Non microbial spoilage of egg

• These include moisture loss which results in weight loss during long-term storage.
• Long-term storage causes changes in the physical state of the egg contents.
• They include egg white thinning and yolk membrane rupture.
• As the yolk membrane weakens and breaks, the yolk settles and becomes homogeneously mixed with the egg white.

Microbial spoilage of egg

• Microorganisms must contaminate the shell of an egg, penetrate through the pores in the shell and inner membrane, reach the egg white and yolk, and grow there.
• Some microorganisms can’t grow in egg white but develop in yolk.
• Changes in storage temperature allow organisms to pass through the shell and thus facilitate microbial spoilage.

Bacterial spoilage of egg

• Bacteria are a more common spoilage organism than mold.
• Bacteria cause rot in eggs.
• When bacteria grow within an egg, they decompose the contents and produce byproducts.

Black rot

• It is caused by Proteus and, on occasion, Pseudomonas and Aeromonas.
• Yolks blacken and then break down, giving the whole egg content a muddy brown color.

Green rot

• It’s caused by Pseudomonas fluorescence.
• When exposed to ultraviolet light, green egg white fluoresces.
• In the later stages of spoilage, yolk disintegrates, masking the green color of the egg white.
• The odor is either lacking, fruity, or sweetish.

Red rot

• It is caused by Serratia marcescens.
• These eggs are distinguished by a rod dissociation of the egg white and the surface of the yolk in ammoniacal, or putrefied odor.

Colorless rot

• It may have been caused by Pseudomonas, Acetobacter, Acinetobacter, or coliform.
• In the later stages of spoilage, the yolk disintegrates or has incrustations.

Pink rot

• It is usually caused by Pseudomonas during the late stages of green rot.
• They are like colorless rot, with the exception that the yolk and white are pink. 

Preservation of egg from spoilage

The following methods are used for its preservation:

Removal of microorganisms

• Several methods can be used to remove dirt and feces from eggshells.
• Sand blasting removes dirt and blooms.
• Washing with warm water removes dirt, bloom, and microorganisms, but allows bacteria to penetrate through pores in the shell.
• The use of disinfectant in washing water reduces the number of microorganisms.

Asepsis

• To reduce fecal matter contamination by dust and nest, extreme caution should be applied.
• When breaking eggs for freezing or drying, any spoiled eggs should be discarded, and contamination from equipment can be reduced by sanitizing it.

Drying methods

• Eggs are washed with a chloride solution, then broken to separate the yolk and white.
• The liquid egg is then dried using drum or spray drying methods.
• Glucose should be removed from them before drying because it causes browning.

Chilling methods

• Eggs are commercially stored for six months or more at temperatures ranging from -1.7℃ to -0.55℃ and relative humidity levels of 70-80%.
• If the temperature is higher than -1.67℃, microorganisms penetrate and grow more quickly into them, causing more physical and chemical changes.
• Special treatments, such as impregnation of eggshell with colorless and odorless mineral oil, keep moisture out, slow desiccation, and prevent air penetration while chilling.

Learn more about the

Milk and Milk Products Contamination, Spoilage, Preservation and Packaging Methods (thesciencenotes.com)

Causes and Prevention Strategies for Food Spoilage – The Science Notes

Causes of Meat Spoilage and Effective Preservation Techniques (thesciencenotes.com)