Egg Technology and Nutritional Value

An egg is an organic vessel in which an embryo first begins to develop. In most birds, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside until the embryo can survive on its own. The term egg is restricted to the animal kingdom. Similar reproductive structures in other kingdoms are called spores.

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Structure and Composition of Eggs

An egg consists of three major parts: the shell, the white or albumen, and the yolk. Most poultry eggs are oval, with one end rounded (the aerus) and the other more pointed (the taglion). This shape results from the egg being forced through the oviduct.

Muscle contractions in the oviduct behind the egg push it forward. The egg wall is still shapeable, and the pointed end develops at the back. Structurally, eggs are divided into distinct parts: the shell, the white (albumen), and the vitellus (egg yolk, contained within various thin membranes).

The shell forms the outer protective layer, composed mainly of calcium carbonate. Its porous nature allows a developing embryo to obtain oxygen. The shell color may vary from white to brown, with no indication of the quality of the egg’s contents.

Inside the shell is a viscous, colorless liquid called egg white, which in a fresh egg is divided into regions of thick and thin white, accounting for about 60% of the total egg weight. In the center of the egg is the yolk, a thick yellow or orange oil-in-water emulsion stabilized by lecithin.

The yolk is suspended in the white, held in position by the chalazae. The larger end of the egg contains the air space that forms when the contents cool and contract after being laid. The egg is not a complete food, as it lacks carbohydrates and vitamin C.

1. Egg Shell

The shell is the outermost covering of an egg, consisting of 94 to 97% calcium carbonate. The remaining 3-6% comprises organic matter and shell pigments (blue or brown pigments), seen as the shell color.

i. Factors Causing Quality Changes in Egg Shell During Storage

Changes in the quality of the eggshell during storage are caused by:

1. Chemical reactions
2. Microbiological reactions
3. Physical phenomena, such as absorption of unwanted odors under adverse conditions

Changes that occur during storage of shell eggs include:

1. Loss of weight due chiefly to loss of moisture from the albumen, but also partly due to loss of CO2, NH4, N2, and H2S gases
2. Increase in air cell size, as moisture loss increases the volume of the air cell
3. Decrease in specific gravity due to the increase in air cell size
4. Mottling on the shell surface due to uneven moisture distribution
5. Decrease in the amount of thick white due to the breakdown of the fibrous glycoprotein ovomucin
6. Increase in yolk size because of water movement from the albumen to the yolk due to osmotic pressure differences
7. Change in flavors
8. Increase in pH, particularly in the albumen, which rises from approximately 7 to 10 or 11 due to CO2 loss

As the egg ages, the firmness of the white decreases, and when broken, the yolk flattens, and the content decreases. The quality of eggs in their intact state is assessed by candling, which involves holding the egg in the path of a strong light to examine its interior.

Candling detects cracks in the shells, the size and mobility of the yolk, the size of the air cell, blood spots, meat spots, microbiological defects, and germination. However, only obvious defects are detectable by candling.

ii. Grades of Shell Eggs

Quality is defined as the sum of the characteristics of a given food item that influence its acceptability.

2.Egg White (Albumen)

The white or albumen is the watery, gelatinous material around the yolk, made up of four layers: outer thin, outer thick, inner thin, and inner thick. The gross composition of the edible portion of an egg consists of 60 parts by weight of white to 40 parts of yolk. Whole egg is the mixture of white and yolk in natural proportion.

The egg white is primarily proteins with unique properties, including ovomucin, globulin, lysozyme, ovalbumin, ovomucoid, and conalbumin. Conalbumin is purely albumin, lysozyme is purely globulin, and ovomucin and ovomucoid are globulins conjugated with carbohydrates, differing only in molecular weight. The white also contains small amounts of avidin.

The main proteins of the yolk are phosphor-proteins, lipovitellin, and lipovitellenin, comprising about 30% of the total egg yolk solids. The phosphorus content is in the form of phosphoric acid esterified with the hydroxyl groups of hydroxy amino acids. Fats constitute two-thirds of the solids in the yolk, comprising triglycerides, phospholipids, and cholesterol.

The main phospholipid is lecithin (phosphatidylcholine), with small amounts of phosphatidylethanolamine and phosphatidylserine. Most lipids are in the low-density lipoprotein fraction, which, in unaltered egg yolk liquid, is finely dispersed.

It is postulated that triglycerides form the inner core of highly emulsified low-density lipoproteins, surrounded by a phospholipid shell, with protein molecules wrapped around it.

3. Egg Yolk

The egg yolk is a complex mixture of lipids and proteins, further complicated by the inclusion of whites. Most lipids are in the low-density lipoprotein fraction, which, in unaltered egg yolk liquid, is finely dispersed.

It is postulated that triglycerides form the inner core of highly emulsified lipoproteins, surrounded by a phospholipid shell, with protein molecules wrapped around it. All lipids in unaltered egg yolk are associated with lipoproteins.

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Nutritional Importance of Eggs

1. Value of Eggs as Food

Eggs possess two important characteristics that make them valuable as food:

1. Eggs are among the most nutritious foods consumed and can be prepared in many ways, making them essential in baked goods, candies, and other foods.
2. Eggs function in various ways to provide desirable characteristics to food products, including leavening, binding, thickening, emulsifying, tenderizing, moisture retention, adding flavor and color, and improving nutrition.

A whole liquid egg contains about 73.7% water, 12.9% protein, 11.5% fat, and 1.4% carbohydrate, making it high in water, protein, and fat. Eggs are rich in B-complex vitamins, provide a fair amount of minerals, and contain traces of carbohydrates.

The protein in eggs is the highest quality found in any food, with a high biological value, containing all essential amino acids required by the body. Eggs supply valuable amounts of iron, phosphorus, fat, vitamin A, calcium, vitamin D, riboflavin, thiamine, and biotin.

However, eggs are not a complete food, as they lack sugar and vitamin C. Avidin is nutritionally significant, as it binds with biotin, rendering it unavailable to the body. A large egg yolk contains more than two-thirds of the recommended daily intake of 300 mg cholesterol. Nutrients in eggs can play a role in weight management and muscle strength.

Nutrient Composition of Eggs

2. Functional Properties of Eggs

i. Leavening Power: Also known as aerating, foaming, or whipping properties, this refers to the ability to incorporate air by itself or in a mixture with other ingredients and hold the aerated structure long enough to be set by heat, drying, or other means. Foam formation results from surface denaturation of protein, involving unfolding and spreading of protein as a monomolecular layer over the surfaces.

ii. Binding and Thickening: Egg proteins coagulate during heating, enabling eggs to bind pieces of food together or thicken foods like custard. Coagulation, sometimes referred to as denaturation or gelation, is one of the most important properties of eggs. Eggs go through stages of heat coagulation: denaturation, flocculation, and insolubility.

iii. Emulsifying Power: Egg yolk, whole egg, and egg white are good emulsifiers, containing natural emulsifiers like lecithin and phospholipids, making them important in preparing permanent food emulsions like mayonnaise, salad creams, spreads, and dips.

iv. Antimicrobial Action: Freshly laid eggs are coated with mucin, which prevents bacterial invasion. When this seal is broken by washing, the egg becomes vulnerable to microbes and moisture. Natural antimicrobials include ovalbumin and lysozyme, which dissolve the cell membranes of some bacteria, acting as effective germicidal agents. The lower the pH, the greater their effectiveness, providing an additional reason to prevent CO2 loss from eggs. Avidin binds biotin, a nutrient needed by some microorganisms, inhibiting their growth. Ovalbumin binds iron, making it unavailable for microbial growth.

v. Tenderizing: Eggs contribute to smoothness, moistness, and desirable texture in baked products by retarding sugar crystallization. This property, known as shortening, adds softness and crispness due to the high fat content in the yolk.

vi. Moisture Retention: In baked goods, eggs help retain moisture during baking and storage by binding ingredients together, creating a barrier that limits moisture escape.

vii.Flavor: Eggs have a distinct flavor that affects baked products and enhances mouthfeel.

viii. Color: Eggs add color (browning) to products in which they are used.

Egg-Derived Products

Several types of dried egg products are available, summarized under four major headings:

1. Dried egg white
2. Dried plain whole egg and yolk
3. Dried blends of whole egg and yolk with carbohydrates
4. Special types of dried egg products

1. Manufacture of Egg White Powder

The production of dried egg products is a sophisticated process. Spray drying is the most common method for producing egg powder, though some egg white is pan-dried, and some whole egg is freeze-dried. Eggs are preheated to 60°C and sprayed into a drying chamber through which air between 121 and 149°C passes.

The powder separates from the air and is continuously removed from the drying chamber. Prior to drying, egg white is often subjected to a controlled bacterial fermentation process.

Dried egg products are typically used in mixes and quantity food production, as water removal lowers water activity (aw), retarding chemical reactions and inhibiting microorganism growth.

Spray-dried egg white is commonly used for its foaming and gelling properties. Dehydration successfully preserves egg white by stopping microbial growth and slowing chemical reactions.

Commercial dried egg white products include spray-dried and pan-dried types, with spray-dried egg white available in instant-dissolving forms. Agglomerated instant egg white offers good dispersing and rapid dissolving properties when added to water.

Most dried egg white products are available in whipping or non-whipping types, depending on required functional properties. Whipping egg white powders, used in biscuits, cakes, and meringues, often contain whipping aids like sodium lauryl sulfate, triethyl citrate, xanthan gum, or sodium oleate at about 0.1% based on egg white solids, depending on regulatory requirements.

Pan-dried egg white, producing flake-type material with about 14% moisture, is mainly used for aerated confectioneries. Due to the heat sensitivity of egg white proteins, temperature control during drying is critical to preserve native characteristics, including gelling and stable foam production.

Studies indicate that spray drying strongly affects foaming properties, with heat transfer and air-product interfaces being more responsible than shear rates. Moderate drying conditions (air inlet temperature <120°C and short residence time) yield egg products with good foaming, gelling, and emulsifying properties.

2. Advantages of Dried Egg Products

Dried egg products offer several advantages:

1. Easy handling and storage at low cost
2. Ready to use immediately without thawing
3. Hygienic handling
4. Easy removal from containers without scraping
5. No bacterial growth in powder at room temperature, provided it is kept dry
6. Good uniformity
7. Precise control over water used in formulation
8. No loss during use, as dried egg is added directly to the batch
9. No need for special transfer or storage equipment
10. Reduced weight and volume (moisture content reduced from ~74% to 2-4%), concentrating food value
11. Nonperishable, unlike fresh or liquid eggs
12. Storage life of 5-10 years, extendable with airtight containers under anaerobic conditions or cool storage (6-12 months without cold storage)
13. No contamination risk from shell breakage
14. Less storage space required compared to fresh eggs, with simpler handling and stacking

3. Disadvantages of Dried Egg Products

A possible disadvantage of powder compared to liquid egg is the loss of fresh flavor and certain functional properties, such as aerating power, unless treated with a non-reducing sugar prior to heating, which limits use due to the sweetened nature of the powder.

4. Storage of Dried Egg Products

Dried egg products are easy to store and relatively stable at room temperature. Dried egg whites can be held under almost any storage condition for an indefinite period.

Products containing whole egg and yolk should be refrigerated for long-term storage, though some are stable at room temperature, especially those with glucose removed.

5. Factors Affecting Quality of Dried Egg Products

The quality of dried egg products is affected by:

1. Quality of eggs used
2. Handling methods
3. Sanitation practices
4. Processing conditions
5. Pasteurization procedures
6. Drying methods
7. Storage conditions

Glucose (0.3-0.5%) is removed from liquid egg white prior to drying by fermentation (using yeast or bacterial culture) or oxidation to gluconic acid (using a glucose oxidase-catalase enzyme system).

Glucose removal ensures stability in dried egg whites. Without removal, glucose reacts with amino acids, causing browning, insoluble proteins, off-odors, and loss of functional properties. Plain whole egg and yolk can be pasteurized and dried without glucose removal, making them less stable.

Important physical properties include bulk density, dispersibility, solubility, and reconstituted viscosity.

A. Reconstitution of Dried Egg Products

i. Whole Egg Powder: 13 g of whole egg powder plus 39 g of water is equivalent to one shell egg. Pre-blend with small amounts of other powders (sugar, starch, flour) to avoid lumpiness, or pre-blend all powder ingredients before adding water.

ii. Egg White Powder: 4 g of egg white powder plus 29 g of water is equivalent to one shell egg white. Pre-blend with small amounts of other powders (sugar, starch) to avoid lumpiness, or pre-blend all powder ingredients before adding water.

iii. Egg Yolk Powder: 9 g of egg yolk powder plus 11 g of water is equivalent to one shell egg yolk. Pre-blend with small amounts of other powders (sugar, starch, flour) to avoid lumpiness, or pre-blend all powder ingredients before adding water.

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