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What is an egg?

• An egg is a round and oval body laid by the female of many animals. – It consists of an ovum GMK 330 surrounded by layers of membranes and an outer casing, which acts to nourish and protect a developing FOOD COMMODITIES embryo and its nutrient reserve. • Bird eggs are the most 7. EGGS common used. – Chicken, duck, goose, ostrich, quail. – For human consumption, eggs are usually unfertilized (keep away from rooster).

Composition of an egg Egg shell

• An egg has • Complex of protein fibers passing through calcium carbonate five major crystals (1:50). components • Comprised of 98% calcium, 0.9% magnesium and 0.9% : Yolk, phosphorus (occurs as phosphate). Albumen • Coated with protective layer (10 - 30 micron thickness) of (egg white), water soluble protein, known as cuticle or bloom. Shell – Helps to restrict movement of water, gases and microorganisms. membrane, • Contains funnel shaped pores (7000 - 17 000/egg) that pass Air cell, and between the cuticle and the underlying membrane. Shell. • Can have a white or brown coloration. – The pigmentation does not affect the nutritive value of the egg.

Shell membrane Albumen (egg white)

• A protein system consisting of ovomucin fibers in an aqueous • There are 2 membranes inside of shell: solution of globular proteins. – The outer is composed of 6 layers of fibers and • Composed of 4 layers: outer thin, outer thick, inner thin and inner thick (chalaziferous; includes chalazae). sticks to the shell. – Chalazae – • A pair of spiral bands that anchor the yolk in the centre of the thick white The inner is composed of 3 layers of fibers and • Egg white consists of 88.5% moisture, 9.8% protein and 0.0% surrounds the albumen (white). fat – Most of the proteins are glycoproteins, whilst others have lipids bound • The membranes are a second line of defense to them • Other properties of the albumen: against bacteria. – freezing point = - 0.424 0C – coagulating temperature = 61 0C – viscosity at 0 C (Poise)= 25

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Albumen (egg white) Albumen (egg white)

• Ovalbumin – More resistant to thermal denaturation. – In denatured state, it is the structural component of baked foods. – Readily undergoes surface denaturation. • Ovotransferrin (also known as conalbumin) – Glycoprotein. – More readily coagulated by heat. – Heat stability is affected by pH (lowest stability at pH 6). – Less readily coagulated by surface denaturation (compared to ovalbumin). • Ovomucoid – Heat resistant glycoprotein.

Albumen Yolk

• Ovomucin • Composed of particles distributed in a protein solution. – Glycoprotein. – The continuous phase: water solution of several proteins called – Contributes to thickness of white. livetins. – 4x as much ovomucin in thick white compared to thin • Types of yolk particles include: yolk spheres (20 - 40 microns), yolk white. granules (1.0 - 1.3 microns) and low-density lipoprotein, which exist as – Complexes with lysozyme. individual particles distributed evenly throughout yolk. • Globulins • The granules are a complex of lipid-protein particles. – Important for foaming properties. • Vitelline membrane (yolk membrane) • And also: – Surrounds the yolk. – Ovoinhibitor: inhibits trypsin and chymotrypsin – Ovoflavoprotein: binds riboflavin – The fresher the egg, the stronger the membrane. – Avidin: binds biotin • Germinal disc – Lysozyme: can act as an antimicrobial agent – A slight depression on the surface of the yolk.

Yolk Germinal disc and Air cell

• The yolk has 57.0% moisture, 15.5% protein and • Air cell 25.6% fat – The air cell forms at the wide end of the egg after cooling, when – Included in the fat of the yolk is 1.075% cholesterol and the inner membrane pulls away from the outer membranes. 7.31% lecithin – Carbohydrates (0.2 - 1.0%) and minerals (1.1%) make up the minor components – Healthy individuals may consume 1 to 2 eggs per day with only ‘modest’ increases of blood’s cholesterol – The yolk also contains fairly high levels of vitamins A, D, E and K. • Other properties of the yolk: – freezing point = - 0.587 0C – coagulating temperature = 65 0C – viscosity at 0 C (Poise)= 200

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Chemical composition of a fresh egg Chemical composition of a fresh egg

• Egg white : yolk = 2 : 1 • One of the carbohydrates in egg, sialic acid (N-acetylneuraminic acid), may have an inhibitory effect on rotavirus, a major pathogen • Composition: 65% water, 12% protein, 11% fat, and other of infant gastroenteritis minor constituents. • Egg flavor results from the combination of at least 141 components, – Each large egg provides about 70 calories, 4.5 grams of fat (1.5 grams 87 present in whole egg, 75 from egg yolk, and 57 in egg white. saturated fat), and 213 mg cholesterol. Most of these are highly volatile. – Eggs contain one of the highest-quality proteins, due to the amount – Egg is a very complex material. The most important descriptors used for and ratio of essential amino acids. egg and egg flavor are 'sulphury' and 'aldehydic,' followed by ammoniacal, – One egg can supply about 10% of the Daily Value of riboflavin, and metallic and rubbery. also supply modest levels of folate, iron, phosphorous and zinc. • The pH of freshly laid egg white ranges between 7.6 and 7.9, while – Typical egg yolk contains fat-soluble carotenoids that create a golden the yolk has pH of 6.0. yellow color. • The feed given to a chicken can affect its eggs. – Eggs also provide choline, believed to be necessary for brain – Feeds high in carotenoids produce darker yolks. development, to aid liver function and to prevent cancer. – When fed foods such as flaxseed or certain algae with high levels of omega-3 fatty acids, chickens lay eggs that contain an elevated level of these compounds.

Handling of Fresh Eggs Changes of egg quality during storage

• A fresh egg should have the • Yolk gets flatter following characteristics: • Thick white gets thin – Clean, uncracked shell. • Greater amount of thin white – ~5 mm regular shaped air cell. • Albumen height decreases – Clear, firm, upright white • Albumen spreads – A cell centered, free from defects • Odor / flavor deterioration yolk • Air cell gets larger • Eggs begin to deteriorate as soon as they are laid and lose quality very rapidly at room temperature – An egg will age more in one day at room temperature than in one week in the refrigerator

10°C (50°F) 20°C (70°F) 30°C (85°F) 45°C (115°F) Egg quality and changes during storage

• Increase in yolk  due to water movement from white to yolk  Fresh increases yolk volume, stretches vitelline membrane & decreases yolk height. • Albumen pH increases from 7.6 to 9.3  due to loss of CO2 that diffuse through shell pores. – No CO2 in the yolk  no loss of CO2 from the yolk  pH of yolk stays unaffected. – Increased albumen pH  breakdown of thick white (thinning), reduced height,

increased width of the white & increase in quantity of thin white. week 1 – Increased pH  increases water-holding capacity. • Increase in air cell size  due to water vapor evaporating from egg, especially during cooling. • Slight changes in protein and fat of egg  alters the odor and flavor. – In addition, the egg absorbs flavor / odors from the surrounding environment.

• The rate of changes in egg during storage is influenced by the temperature weeks 2 and humidity during storage. – Eggs are best stored slightly above their freezing point i.e. 20 C and at a relative humidity around 90%. – As temperature increases and relative humidity decreases  storage life

decreases. 13 weeks 13

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FunctionsFunctions of Eggs in Foods in Foods Main functions - foaming

• How do foams are formed? White Yolk – Foam is formed by the beating of egg white, which is a colloidal dispersion, where the gas phase is dispersed into the liquid phase  a foam is a stable mass of bubbles. Foaming Emulsification – As you beat egg albumen, the following occurs: 1. Air is incorporated into the albumen forms bubbles  The albumen proteins in the bubble wall start to unfold. Binding Binding 2. The hydrogen bonding side chains of the unfolded protein immerse themselves in the water film & the hydrophobic side chains align themselves towards the air pocket. 3. Once unfolded, the protein molecules bond with each other (coagulate) Interfering Interfering  form a reinforcing network that holds the water in place while shielding it from air. – This phenomenon is termed 'surface denaturation' of Clarification Clarification protein.

Main functions - foaming Stages of Egg Foams

• Both egg white and yolk can trap air to form foams, but the white is much more effective. Foamy Soft Peaks Stiff Peaks – Due to their high-protein & no-fat composition. – The surface-active film-forming proteins in eggs are responsible for this function, especially ovomucin, conalbumin and lysozyme. – Globulins help to reduce surface tension increased viscosity  rapidly incorporate air at the beginning of the whipping process. • They also coagulate during heating  stiff structure. – The ovomucin-lysozyme complex provides foam stability. – Ovalbumin and conalbumin provide heat-setting properties. • Stability of the foam increases with beating time up to a point. – Excessive beating creates an unstable foam due to the breakage of the coagulated egg-protein films. – Overbeaten egg-white foam will curdle  liquid drains from the structure  the air pockets coalesce.

Main functions - foaming Main functions - coagulation

• The rate, time & setting of the beater/mixer influence foaming. • How does coagulation take place? • Temperature affects foaming. – Low temperature increases egg white viscosity increases the time – Heating  increased energy of the protein molecules  breaking of required to incorporate air. bonds  molecules begin to unfold. – Room temperature whipping will increase volume  creates finer air cells. • Acids or acid salts, e.g. cream of tartar, increase foam stability. – The length of the molecule is exposed to other unfolded molecules. • Sugar delays surface coagulation of the proteins  smoother and – As temperature increases, the molecules bump into parts of other more stable foam, although it will delay the onset of foaming and molecules. can reduce the volume. • Salt can also delay foam formation. – These molecules bond to each other, forming a mass of • Yolks and whole eggs contain lipids that must be emulsified to interconnected proteins that has entrapped water molecules within. create a stable foam because free fat can retards foaming. – The liquid has become a solid (coagulation) and takes on an opaque – Lipovitellenin and lipovitellin negatively affect foam formation in egg whites. appearance due to the altered deflection of light by the clustered – The proteins in the yolk do not surface-denature  they do not form a proteins. stiff structure without heat denaturization. • Foams act as part of the leavening system in baked products • The quality of gel depends on the extent of protein bonding. – Heat expands the air pockets  solidifies the protein-based structure.

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Main functions - coagulation Main functions - coagulation

• Differences in coagulation between egg proteins and egg • Usage: components: – In foods e.g. cakes, soufflés and custard, egg-protein coagulation – Ovomucin in thick white and chalazae is more resistant to heat forms a solid or semisolid structure as the proteins are exposed to than other proteins. heat. – Thin white begins to coagulate at about 63 0C. 0 – In other products, such as meat loaves or patties, this characteristic – At 66 C both whites are set. serves as a binder that holds the elements together. – At 66 0C the yolk proteins (livetins) begin to gel  solidifies at ~68 0C. – In breading or when used as an , they help to adhere – At 710 C the entire white and yolk are set. particles such as crumbs or seeds. – Beaten mixture coagulates at ~65 0C. – Frozen batter-coated products use batter with higher egg content in • The exact temperature at which this process occurs order to increase crispiness and retain more moisture due to the film- varies with pH, salt level, interaction of other ingredients forming properties of eggs. and time of exposure. – An egg-protein gel can thicken sauces and gravies. – Adding solutes, like salt or sugar, will raise the temperature required.

Main functions - coagulation Main functions - emulsification

• • The surface-active properties of phospholipids and – Curdling occurs when eggs lipoproteins contained in the yolk  excellent emulsifier. are overcooked. • Egg yolk contains lecithin, which has a fat-soluble • The eggs become over hydrocarbon tail attached to a water soluble head. coagulated  elimination of – The tail immerses itself in the fat droplet and the head immerses itself liquid from the cooked egg  in the continuous phase. tough and rubbery texture. – The head is negatively charged  the droplet becomes surrounded by – Once two proteins join at one a shell of negative charge  the droplets actually repel each other. point, further bonding is likely. • This quality keeps oil suspended in mayonnaise and egg-based • The longer the heating goes dressings and provides emulsification in batters and dough. on the closer and more completely the proteins squeeze together and less room in which to retain water.

Other functions - texturization Other functions - appearance

• Fat in egg yolks  tenderizing effect on baked products, especially cakes. • Eggs can contribute a yellow color to many – Eggs also affect crumb formation. products. – However, when only whites are used in the same type of application, the result is a toughening due to their protein content. • Eggs can provide a desirable brown color to • Eggs can act as humectants in breads and rolls  increase shelf life. baked foods. • In confections, egg whites can control sugar crystallization and promote chocolate smoothness. • Egg washes produce gloss and shine on the – Large protein molecules in egg white inhibit sugar crystallization in surface of baked products. sugar/water mixtures. – Where egg yolks are used, the lipids prevent sugar recrystallization.

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EggEgg Preparation Preparation Preparation of eggs

• Changes in prepared eggs: Dry Heat Moist Heat – The key to eggs: keep the temperature low and/or the cooking time short. – Egg whites and yolks coagulate at “Boiled” different temperatures. • Adding other ingredients to eggs changes their coagulation temperature. – Undesirable color changes may occur during egg preparation. Coddling – Lipid oxidation and the Maillard reaction  contribute to cooked-egg flavors. – Aldehydes and heterocyclic nitrogen- containing compounds (pyrazines, pyradines and thiazoles) also play a major role. • Sulphuric compounds have also been found in cooked egg esp. dimethyl Microwaving disulfide  contributes the oniony or 'bad egg' smell.

References

• Brown, A. 2008. Understanding Food: Principles and Preparation. 3rd ed. Wadsworth Cengage Learning: Belmont, CA. • McWilliams, M. 2009. Food Fundamentals. 9th ed. Pearson Education, Inc.: New Jersey. • Parker, R. 2003. Introduction to Food Science. Delmar: New York.

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