Structure, Composition and Nutritive value of eggs | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Structure, Composition and Nutritive Value of Eggs

Structure of an Egg

• The egg comprises various intricate parts that are essential to understand for handling and working with eggs effectively.
• Germ spot: This is a critical part of the egg.
• Shell: The outer covering of the egg.
• Vitelline membrane: A membrane surrounding the yolk.
• Concentric layers of white yolk and yellow yolk: Layers of different types of yolk within the egg.
• Chalaza: The twisted, rope-like structure that anchors the yolk in the center of the egg.
• Germinal disc (blastodisc): Essential for egg fertilization and development.
• Yellow yolk and white yolk: Components of the egg yolk.
• Air space: Found at the wider end of the egg.
• Inner shell membrane: The membrane located just beneath the shell.
• Outer shell membrane: Another membrane present outside the inner shell membrane.
• Outer loose albumen and inner loose albumen: Different parts of the egg white.
• Dense albumen: The thick part of the egg white.

Composition of an Egg

• An egg consists of various components, each serving a specific purpose in the overall structure and function of the egg.
• Proteins: Eggs are rich in high-quality proteins, essential for growth and repair in the body.
• Fats: Eggs contain fats that provide energy and aid in the absorption of fat-soluble vitamins.
• Vitamins and minerals: Eggs are a good source of essential vitamins like vitamin A, D, E, and B vitamins, as well as minerals like iron and zinc.
• Water: Eggs contain a significant amount of water, contributing to their overall composition.

Nutritive Value of Eggs

• Eggs are considered a nutrient-dense food, meaning they provide a high amount of nutrients relative to their calorie content.
• Proteins in eggs are of high biological value, containing all essential amino acids needed by the body.
• Eggs are a good source of choline, a nutrient important for brain health and development.
• The fats in eggs include healthy unsaturated fats, which are beneficial for heart health when consumed in moderation.

Egg Structure Overview

• Shell: The shell of an egg, constituting around 11% of the total egg weight, primarily comprises calcium carbonate. It serves as a protective barrier for the egg's contents and facilitates gaseous exchange through numerous tiny pores distributed across the surface. Additionally, the shell is coated with a powdery substance known as cuticle, which plays a role in preventing excessive moisture loss from the egg and acts as a physical barrier against microorganisms. The cuticle also possesses bacteriostatic properties.
• Shell Membranes: There are two layers of membranes - inner and outer - with a combined thickness ranging from 0.01 to 0.02 mm. The outer membrane, slightly thicker than the inner one, typically remains attached to the shell except in the region where the air cell develops. These membranes act as crucial defenses against microbial intrusion into the egg.
• Albumen: The egg white is not uniform in composition and consists of at least four distinct layers. It constitutes approximately 58% of the egg's weight. The layers include a thin albumen layer beneath the inner shell membrane, a thick layer surrounding the yolk, a layer of inner thin albumen, and a chalaziferous layer of thick albumen encircling the yolk. These layers serve various functions such as providing firmness and holding the yolk in place.
• Yolk: The yolk is the yellowish spherical structure enclosed within the egg white. Its color, ranging from light pale to reddish yellow, depends largely on the diet of the hens. The yolk comprises a colorless vitelline membrane that encases the yolk mass. It also contains concentric rings of light and dark yolk materials, influenced by the feeding habits of the hens. A neck-like structure called the latebra secures the germinal disc in position.

Biology of Avian Eggs

External Characteristics of Avian Eggs

• Avian eggs can be identified by external features such as size, shape, color, and texture.
• These characteristics can vary due to factors like heredity, physiology, and the environment.

Composition of Avian Eggs

• Table 6 presents the approximate composition (per 100g) of the edible portion of eggs from various species:
• 
  
  • Chicken: Moisture - 73.7%, Protein - 12.9%, Lipids - 13.1%, Carbohydrates - 0.9%, Energy - 163 kcal
  • Duck: Moisture - 70.4%, Protein - 13.3%, Lipids - 13.1%, Carbohydrates - 0.7%, Energy - 191 kcal
  • Goose: Moisture - 70.4%, Protein - 13.9%, Lipids - 11.5%, Carbohydrates - 1.5%, Energy - 185 kcal
  • Guinea fowl: Moisture - 72.8%, Protein - 13.5%, Lipids - 14.5%, Carbohydrates - 0.8%, Energy - 170 kcal
  • Pigeon: Moisture - 72.8%, Protein - 13.8%, Lipids - 13.3%, Carbohydrates - 0.8%, Energy - 170 kcal
  • Quails: Moisture - 73.7%, Protein - 13.1%, Lipids - 12.0%, Carbohydrates - 1.1%, Energy - 161 kcal
  • Turkey: Moisture - 72.6%, Protein - 12.0%, Lipids - 12.0%, Carbohydrates - 1.7%, Energy - 170 kcal

Weight of Egg Content

• Table 7 provides the average weight of egg content for different bird species:
• 
  
  • Chicken: Albumen - 58g, Shell - 31g
  • Duck: Albumen - 80g, Shell - 11g
  • Turkey: Albumen - 85g, Shell - 10g
  • Quail: Albumen - 40g, Shell - 17g
  • Guinea fowl: Albumen - 200g, Shell - 17g
  • Goose: Albumen - 10g, Shell - 40g
  • Pigeon: Albumen - 58g, Shell - 80g

Nutritional Components of Avian Eggs

• The biological value of egg protein is high at 95, compared to 85 for milk and around 70 for most meat proteins.
• Table 8 details the composition of 100g of a whole egg, highlighting the distribution of nutrients and vitamins.

Composition of Hen's Egg

• Overview: Hen's eggs are rich in various nutrients essential for human health.
• Macronutrients:
  • Protein: Eggs are a great source of high-quality protein, with about 11 grams per egg.
  • Fat: The fat content in eggs is relatively low, with approximately 11 grams per 100 grams.
  • Carbohydrates: Eggs contain a minimal amount of carbohydrates, around 0.9 grams per 100 grams.
  • Water: Eggs have a water content of about 65.5%.
• Essential Vitamins:
  • Pantothenic Acid: Present in eggs at a concentration of 0.76 grams per 100 grams.
  • Choline: Eggs contain choline at a concentration of 0.31 grams per 100 grams.
  • Niacin: Found in eggs at a concentration of 0.80 grams per 100 grams.
  • Pyridoxine: Eggs contain pyridoxine at a concentration of 0.95 grams per 100 grams.
  • Biotin: Present in eggs at a concentration of 0.78 grams per 100 grams.
• Minerals:
  • Calcium: Eggs provide calcium at a concentration of 0.22 grams per 100 grams.
  • Phosphorus: Present in eggs at a concentration of 0.93 grams per 100 grams.
  • Sodium: Eggs contain sodium at a concentration of 11.5 grams per 100 grams.
  • Potassium: Found in eggs at a concentration of 74.5 milligrams per 100 grams.
• Essential Minerals:
  • Iron: Eggs contain iron at a concentration of 2.4 milligrams per 100 grams.
  • Zinc: Present in eggs at a concentration of 3.8 micrograms per 100 grams.
  • Iodine: Found in eggs at a concentration of 4.18 micrograms per 100 grams.
• Nutritional Value: An average chicken egg weighs about 57 grams and is a valuable source of quality protein and essential vitamins and minerals.

Egg Nutrition Overview

• Eggs are rich in essential nutrients crucial for human health.
• One egg (without the shell) contains:
• 
  
  • Protein - 6.1 g
  • Fats and lipids - 5.5 g (including unsaturated fatty acids)
  • Calories - 77 kcal
  • Various vitamins and minerals such as calcium, phosphorus, iron, manganese, Vitamin A, Vitamin D, Vitamin E, choline, and more.

Nutritional Value of Eggs

• Eggs are considered one of nature's most complete protein foods.
• They contain a well-balanced proportion of all essential amino acids.
• Egg whites contain various proteins like ovomucin, ovoalbumen, conalbumen, ovoglobulin, ovomucoid.
• Egg yolks are rich in glycerides, lecithin, cholesterol, vitamins, and minerals.
• Egg yolks provide a high ratio of nutrient value to calories.

Microbial Spoilage of Eggs

• Eggs start deteriorating immediately after being laid.
• Changes during storage include loss of weight, formation of air cells, and conversion of thick albumen to thin albumen.
• Proper storage conditions impact the quality of eggs over time.

Preservation and Maintenance

• Preservation of eggs can be categorized into maintaining the quality of shell eggs and processed eggs.
• The quality of eggs is essential for consumer acceptability.
• Egg quality encompasses various characteristics influencing consumer perception.

By understanding these key points, we can appreciate the nutritional benefits of eggs, their susceptibility to spoilage, and the importance of proper preservation techniques to maintain their quality and safety.

Summary and Explanation of Egg Cleaning and Preservation Methods

Egg Cleaning: Best Practices and Techniques

• Dirty eggs are covered with bacteria, which can spoil the eggs if they penetrate them.
• Slightly dirty eggs can be dry cleaned using abrasives like emery or rough cloth.
• Wet washing with water warmer than the eggs is effective but requires caution to prevent bacterial contamination.
• Precautions for wet cleaning include using compatible detergent, thorough rinsing, and ensuring eggs are completely dry before storage.

Methods for Egg Preservation

• Several preservation methods exist, some applicable at the farm level and others commercially.
• Water-glass, limewater, and oil smearing are common techniques for preserving eggs.
• Limewater involves mixing lime and salt in water to create a solution for submerging eggs.
• Water-glass is a mixture of sodium silicate and water, used to preserve eggs in covered containers.
• Oil smearing can also be used to maintain egg quality during storage.

Importance of Quality in Egg Preservation

• Preservation methods cannot improve poor egg quality, so it is crucial to start with good-quality eggs.
• Preservation should begin soon after collecting eggs, as egg quality deteriorates rapidly after laying.

Commercial Methods of Preserving Eggs

• Cold Storage:

  Storing eggs in cold storage helps in preserving their freshness. The temperature should be maintained close to -2°C, as going below this can harm the quality of the eggs. Lower temperatures slow down the rate of quality decline.

• Thermostabilization with Oil:

  This method involves treating eggs with oil to enhance their shelf life. It is one of the commonly used techniques for preserving shell eggs.

• Storing Eggs under CO₂ Atmosphere:

  Preserving eggs in an atmosphere with controlled carbon dioxide levels can help in maintaining their freshness for a longer duration.

• Irradiation of Eggs:

  Irradiation is another method used for preserving eggs, which helps in killing bacteria and increasing their shelf life.

General Requirements for Storing Shell Eggs

• Atmosphere of the storage room should be free from any odors. Activated charcoal can be used to absorb any unwanted smells.
• Cold storage temperature should be maintained close to -2°C to slow down the quality decline. Lower temperatures lead to a slower degradation of egg quality.
• Relative humidity in the egg storage room should be between 80 to 85 percent to prevent mold growth and maintain freshness.

Table 9: Effect of age and store temperature on the Haugh Unit of Eggs

• Age in Days

  • Non-refrigerated Mean Temperature 15.5°C
  • Refrigerated Mean Temperature 10°C

• 1

  4

  7

  10

  13

  16

  19

  • IAS(M) 80
  • 70
  • 65
  • 60
  • 57
  • 49
  • 53
• 
  
  • 1.51
  • 80
  • 74
  • 73
  • 71
• 
  
  • K 71
  • 69
  • 69
  • 0.56
• 
  
  • Av. decline

Oiling and Pasteurization of Eggs

• Oiling of Eggs:
  • Coating eggs with oil helps in better storage in cold conditions by preventing moisture and CO2 loss, minimizing odour uptake, microorganism penetration, and inhibiting mold growth.
  • Quality oil, odorless and colorless, is used for sealing, typically light mineral oils.
  • Egg surfaces must be clean before oil application, excess oil should be drained, and recirculated oil filtered and heated to ensure germ-free reuse.
  • Improvements include thermostabilization in heated oil by dipping eggs in hot oil at 55-98°C for 16 minutes, draining excess oil, and drying before packing.
• Pasteurization of Eggs:
  • Immersing eggs in hot water at specific time and temperature destroys spoilage microorganisms.
  • Optimal heat treatment includes 62.5°C for three minutes or 64°C for two minutes, enhancing the quality of thick albumen while high temperatures may cause cloudiness in egg white due to coagulation.
  • Pasteurization with hot water allows washing, pasteurization, and storage, differing from thermostabilization in hot oil.

Marketing Systems for Poultry Products

• Marketing Systems Overview:
  • Three main methods: Direct marketing, Indirect marketing, and Marketing cooperatives.
• Elements of Marketing:
  • Includes procurement, transportation, grading, packing, storage, seasonal surplus holding, wholesale and retail movement, consumer delivery, and public education on egg consumption through media.

Indian Standards for Table Eggs

Grade A Eggs

• Extra Large: Eggs weighing 60 grams and above
• Large: Eggs weighing 53-59 grams
• Medium: Eggs weighing 45-52 grams
• Small: Eggs weighing 38-44 grams

Grade B Eggs

• Extra Large: Eggs weighing 60 grams and above
• Large: Eggs weighing 53-59 grams
• Medium: Eggs weighing 45-52 grams
• Small: Eggs weighing 38-44 grams

Shell Characteristics

• Clean, unbroken, and practically regular in shape
• Up to 4 mm in depth, sound, and regular

Air Cell

• Clean to moderately attained, sound, and slightly abnormal
• Clear may be slightly weak, 8 mm in depth

White

• Clear, reasonably firm, 8 mm in depth, slightly weak

Yolk

• Fairly well-centered, distinct outline, free from defects
• May be slightly off-centered, slightly visible outline

Grading of Eggs

Grading of eggs involves classifying table eggs into categories based on size, quality, freshness, and market desirability.

Advantages of Grading

• Reduces confusion and uncertainty about quality
• Satisfies both producers and consumers
• Provides a uniform measure of quality differences
• Makes market price reports easily understandable
• Motivates production of quality eggs due to price differences
• Essential for proper labeling and marketing
• Provides a basis for settling quality disputes

Grading is crucial for ensuring that eggs meet specific quality standards and are appropriately labeled for the market.

Grading of Eggs

• Disadvantages of Egg Grading

  • There is no universally accepted grading system for eggs globally, leading to variation in grading standards from place to place.
  • Eggs are perishable, so their quality may decline from the time of grading to consumption.
  • Grading process demands skilled personnel and specialized equipment, increasing production costs.

• Market Egg Grading Criteria

  Eggs are graded based on size, quality, and other factors:

  • Size categories include extra-large, large, medium, and small, with variations in weight ranges.
  • Quality factors considered in grading include shape, cleanliness, shell quality, freshness, and candling qualities.
  • Some countries also consider shell color in their grading standards.

• Standard Grades in Different Regions

  Various regions have their own grading systems:

  • Indian Standards:
    • Based on egg weight, with grades for quality and weight, totaling eight grades.
  • U.S.D.A. Standards:
    • Categories include jumbo, extra-large, large, medium, small, and peewee.
    • Weights are measured in ounces, with conversions to grams.
  • Agmark Standards:
    • Precise standards set by the Agricultural Produce Grading and Marketing Act of 1937 for chicken and duck eggs.
    • Grades labeled as AS (M), A, B, and C, with specific weight criteria.

• Quality Factors in Agmark Grading

  Common quality factors for all Agmark grades:

  • Shell must be undamaged, stain-free, and normal in shape.
  • Egg contents should be free of blemishes like blood spots, meat spots, and germ development.
  • Yolk must be centered, with faint outlining and not freely movable.
  • Egg white should be translucent, clear, and not watery.
  • Air cell depth must not exceed 3/8 inch.