Animal Science: Chicken Eggs Quiz

Eggs - General Information

• Eggs are nearly perfect protein foods, containing high-quality nutrients.
• They are easily digested and provide a significant portion of daily nutrients for growth and body tissue maintenance.
• Eggs are widely used in both the food industry and at home.
• Chicken eggs are the most important type. Other birds' eggs (e.g., geese, ducks, quail, plovers, seagulls) are less significant.

Egg Production

• Global egg production in 1996: approximately 47,338 thousand tons (chicken eggs), along with 4,249 thousand tons (other eggs).

Egg Production per Country

• Key countries and their estimated chicken egg production in 1996 are summarised in the attached data.

Egg Structure, Physical Properties, and Composition

• The egg shell is calcified, porous, and 0.2-0.4mm thick.
• Chicken egg shells are white-yellow to brown. Duck eggs are greenish-white, and most wild bird eggs are noticeably spotted.
• The egg shell's inner surface is lined with two inner and outer membranes.
• Air cell (approximately 5mm in fresh eggs) forms at the large end of the egg and grows larger during storage. The air cell size can be used to determine egg age.

Egg White (Albumen)

• Egg white is an aqueous, faintly straw-tinted gel-like liquid.
• It consists of three fractions with varying viscosities.

Egg Yolk

• The egg yolk is the inner portion of the egg and is surrounded by the albumen.
• The yolk contains an arrangement of alternating layers of dark and light-colored material, which is arranged concentrically.

Chalazae

• The chalazae anchor the yolk, resembling twisted rope-like cords.
• They are twisted clockwise at the large end and counterclockwise at the small end of the egg.

Germinal Disc (Blastoderm)

• The germinal disc (blastoderm) is located on top of the yolk (at the wide end).
• It has a club-shaped latebra (lateral part) form.

Egg Composition

• Data regarding the percentage of total weight, dry matter, and protein content of eggs (shell, white, yolk) is provided in tabular form.
• The shell comprises calcite crystals within an organic framework. It also contains minimal levels of magnesium carbonate and phosphates.

Egg Shell Structure

• The shell structure is divided into the cuticle/bloom, spongy layer, mammillary layer, and pores.
• The cuticle is an extremely thin (10μm) protein layer that's transparent and mucilaginous, coating the outer shell part.
• The spongy layer, which comprises about two-thirds of the shell's thickness, is the main calcareous layer below the cuticle.
• The mammillary layer has small knob-like particles cemented to the spongy layer, with the other side adhering to the shell membrane.
• Tiny canals known as pores run through the shell, apparent as openings (7,000 to 17,000 per egg).

Egg Albumen Proteins

• Egg albumen is comprised of various proteins, at around 10% solution.
• The thick, gel-like albumen has significantly more ovomucin (around four times) when compared to the thin albumen.
• It's a viscous pseudoplastic fluid and its viscosity is dependent on the shearing force.

Egg Albumen Proteins (Detailed)

• Key proteins found in egg white: ovalbumin, ovoglobulin G3, avidin, conalbumin, ovoinhibitor, ovomucoid, flavoprotein ovomucin, ficin inhibitor, ovoglycoprotein, and lysozyme G1, ovomacroglobulin.

• Several albumen proteins exhibit biological activity. Examples include enzymes (lysozymes), enzyme inhibitors (ovomucoid, ovoinhibitor), and complex-forming agents for coenzymes (flavoprotein, avidin).

Ovalbumin

• Ovalbumin is a glycophosphoprotein that contains 3.2% carbohydrates, with 0 to 2 molecules of serine-bound phosphoric acid per protein molecule.
• It has 4 thiols and 1 disulfide group, capable of forming heat-stable S-ovalbumin during egg storage via a thiol-disulfide exchange reaction.
• Relatively susceptible to denaturation (e.g., by shaking or whipping its aqueous solution) through protein unfolding and aggregation during interphase denaturation.

Conalbumin

• Conalbumin (also known as ovotransferrin) resists denaturation at the interphase but undergoes coagulation at lower temperatures.

Ovomucoid

• It consists of a single peptide chain with 3 carbohydrate units bound via asparagine residues.
• It contains 9 disulfide bonds, leading to heat stability.

Egg White Proteins: Lysozyme

• Lysozyme (also named ovoglobulin G1), a widely distributed protein occurring in various animal tissues, secretions, plant latex exudates, and some fungi.
• It's an N-acetylmuramidase enzyme.
• It breaks down cell walls of Gram-positive bacteria by hydrolyzing them.
• It is a polypeptide chain with 129 amino acid residues and 4 disulfide bonds.

Egg White Proteins (Other)

• Ovoglobulins G2 and G3: good foam builders
• Ovomucin: forms fibrils, contributing to a significant increase in the viscosity of egg white at higher concentrations compared to thin albumen.

Carbohydrates, Lipids, and Minerals in Eggs

• Egg yolk contains 32.6% lipids.
• Free carbohydrates are part of the 1% of the dry matter in egg yolk.
• Egg white contains about 0.6% minerals.

Mineral Composition of Eggs

• Data on mineral composition of egg whites and yolks in varying percentages is provided.

Vitamin Content of Eggs

• A tabular summary detailing various vitamin levels (e.g., retinol, thiamine, riboflavin, niacin, pyridoxine, pantothenic acid, biotin, folic acid, tocopherols, and α-tocopherol) in whole eggs, egg white, and egg yolk is given.

Egg Yolk Composition

• Egg yolk is a fat-in-water emulsion.
• It contains about 50% dry water content, consisting of proteins and lipids.
• It contains particles of distinct sizes that are categorized into two groups: yolk droplets, which are similar to fat droplets. Granules, substantially smaller than the yolk droplets, feature consistent size and shape with protein, lipid, and mineral content.

Egg Yolk Separation Methods

• Older egg yolk separation methods involved defatting with various solvents.
• Modern procedures employ ultracentrifugation aided by electrolytes, thereby ensuring the retention of native yolk constituents.

Composition of Egg Yolk Fractions

• Data provides the lipid, protein, and mineral composition of various egg yolk fractions.

Proteins of Granules

• Data details the composition and properties of lipoproteins.
• Lipovitellins can be separated by electrophoretic and chromatographic techniques, based on distinctions in protein-bound phosphorus levels.

Phosvitin

• Phosvitin is a glycophosphoprotein.
• Characterized by a high amount of phosphoric acid bound to serine residues.
• Effectively binds metal ions (e.g., Ca²⁺, Mg²⁺), forming inter-molecular complexes.

Plasma Proteins

• Lipovitellin is a low-density lipoprotein obtained via ultracentrifugation of diluted yolk.
• A variety of components with differing densities in the yolk are separated through fractional centrifugation.
• The lipoid moiety constitutes about 84-90% of the dry matter and contains approximately 74% triglycerides and 26% phospholipids.
• Additional contents like phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, and lysophospholipids are also present.
• Individual components of the plasma proteins may not be entirely characterized.

Livetin

• Livetin is a soluble globular protein.
• Electrophoretic procedures can effectively segment livetin into three fractions (α, β, and χ-livetins), which correspond to serum albumin, a₂-glycoprotein, and γ-globulin in chicken blood.

Other Egg Constituents

• Egg yolk includes trace carbohydrates and free sugars (e.g., glucose), similar to egg white, with a proportionally low content (approx. 1%) bound to proteins.
• Egg Yolks have less minerals than egg whites around 0.2%.

Egg Aroma Substances

• The specific aroma substances in egg white and yolk are still not fully determined, although methylamine, with a sensory threshold of around 25 µg/kg, is linked to a "fishy" odor effect. This compounds forms from the breakdown of choline in fish or soy-based diets.

Egg Storage

• During egg storage, changes occur due to the diffusion of CO₂ through shell pores resulting in an increase in egg white pH.
• The evaporation of water through the shell lowers egg density, enlarging the air cell. This results in a gradual reduction of egg white viscosity and the egg yolk becoming flattened over time.
• These changes affect egg quality and provide indicators for determining egg age (e.g., air cell size), or the presence of off-flavors("stale").
• Cold storage is an important factor in egg preservation.

Egg Products

• Eggs are utilized in many forms (liquid, frozen, dried) for both whole egg, white and yolk as food items, such as semi-finished products (baked goods, noodles), confectionery, pastry, mayonnaise, salad dressing, soup powders, margarine, meat products, ice cream and liqueurs.

Technically Important Properties of Eggs

• Eggs' utility is tied to three key properties:
  1. Coagulation when heated
  2. Foaming ability
  3. Emulsifying properties
• Egg color and aroma are also significant aspects.

Thermal Coagulation of Eggs

• Egg white begins to coagulate at 62 °C, and egg yolk at 65 °C.
• The coagulation temperature is contingent on pH.
• At pH levels above 11.9, egg white gels or sets even at normal temperature but later liquifies.
• All egg proteins except ovomucoid and phosvitin undergo coagulation.
• Conalbumin can be stabilized by complexing it using metal ions despite being sensitive to this process.

Foaming Ability in Eggs

• Egg white whipping results in foam that holds air pockets, useful as a leavening agent in food preparation.
• The increased surface area during whipping leads to protein denaturation and aggregation.
• Particular substances, like ovomucin, create an insoluble barrier between the liquid and air bubbles, stabilizing the foam.
• Egg globulin also plays a role, affecting foam stability through viscosity and surface tension.

Emulsifying Properties of Eggs

• Eggs' emulsifying properties are utilized to produce mayonnaise, creamy salad dressings (using a combination of egg yolk, olive oil, lemon juice, or vinegar, and seasonings).
• The emulsifying capacity is due to low-density lipoproteins and proteins in the egg yolk.

Dried Egg Products

• Egg solids are churned either immediately or after separating egg white and yolk to create dried products.
• A homogenization step is followed by a purification process (using a separator) and a final pasteurization step to create the finished product.
• Carbohydrates are often removed to prevent undesirable color changes and unwanted flavors.

Test your knowledge on chicken eggs with this quiz focusing on their structure, composition, and production statistics. From understanding the function of chalazae to the protein content of egg yolk, this quiz covers various aspects of chicken eggs. Perfect for students of animal science or biology enthusiasts.