Eggs PDF

EGGS EGGS Eggs are one of nature’s nearly perfect protein foods and have other high quality nutrients. are readily digested and can provide signif ic ant portion of the nutrients required daily for growth and maintenance of body tissues are utilized in many ways both in food industry and the...

EGGS EGGS Eggs are one of nature’s nearly perfect protein foods and have other high quality nutrients. are readily digested and can provide signif ic ant portion of the nutrients required daily for growth and maintenance of body tissues are utilized in many ways both in food industry and the home. Chicken egg is the most important. Other birds w/c are less significant are: geese ducks quail plovers seagulls The term “eggs” generally relates to chicken eggs World Production of eggs, 1996 (1,000 t) Continent Chicken eggs other eggs World 43,159 4,179 Africa 1,697 6 America, North-, Central- 6,502 1 America, South- 2,575 25 Asia 22,911 4,075 Europe 9,269 70 Oceania 203 2 Country Chicken Country Chicken eggs eggs China 13,995 UK 614 USA 4,501 Netherlands 593 Japan 2,562 Turkey 560 Russian Fed. 1,747 Iran 520 India 1,540 Italy 680 Brazil 1,400 Spain 696 Mexico 1,266 France 1,018 Germany 836 Country other eggs China 3,453 Russian Fed. 17 Brazil 24 Thailand 287 Vietnam 80 Philippines 60 Bangladesh 28 Romania 23 Malaysia 14 Structure, Physical Properties and Composition The egg is surrounded by a 0.2-0.4 mm thick calcareous and porous shell. Shells of chicken eggs are white-yellow to brown, duck’s are greenish to white, and those of most wild birds are characteristically spotted. The inside of the shell is lined with two closely adhering membranes (inner and outer).  The two membranes separate at the large end of the egg to form an air space, the so called air cell. Air cell is approx. 5 mm in diameter in fresh eggs and increases in size during storage, hence can be used to determine the age of the eggs. The egg white (albumen) is an aqueous, faintly straw -tinted, gel-like liquid, consisting of three fractions that differ in viscosity T he i nne r p or ti on of the e g g , the e g g yol k, i s surrounded by albumen.  A thin but very f irm layer of albumen (chalaziferous layer) closely surrounds the yolk and it branches on opposite sides of the yolk into two chalazae that extent into the thick albumen. The chalazae resemble two twisted ropelike cords, twisted clockwise at the large end of the egg and counterclockwise at the small end. They serve as anchors to keep the yolk in the center. In an opened egg the chalazae remain with the yolk.  The germinal disc (blastoderm) is located at the top of a clubshaped latebra on one side of the yolk. The yolk consist of alternate layers of dark- and light- colored material arranged concentrically. The average weight of a chicken egg is 58 g. Its main components are water (~ 74%), protein (~ 12%), and lipids (~ 11%). Fraction % total weight Dry matter Protein Shell 10.3 98.4 3.3 Egg white 56.9 12.1 10.6 Egg yolk 32.8 51.3 16.6 Fraction Fat Carbohydrates Minerals Shell 95.1 Egg white 0.03 0.9 0.6 Egg yolk 32.6 1 1.1 Shell consists of calcite crystals embedded in an organic matrix or framework of interwoven protein f ibers and spherical masses (protein-mucopolysaccharide complex) in a proportion of 50:1 the re are al so sm al l am o unts o f m ag ne si um carbonate and phosphates.  Shell structure is divided into four parts: a. the cuticle or bloom b. the spongy layer c. the mammillary layer d. the pores Cutic le- the outerm ost shell c oating w/ c is an extremely thin (10um), transparent, mucilaginous protein layer Spongy Layer - a calcareous layer w/c is a matrix comprising two-thirds of the shell thickness - below the thin cuticle Mam m illary layer- c onsists of a sm all layer of compressed, knob-like particles, with one side f irmly cemented to the spongy layer and other side adhering closely to the outer surface of the shell membrane - the shell membrane is made of two layers (48 and 22 um, respectively) - each layer of the shell membrane is an interwoven network of protein polysaccharide f ib ers; the outer layer adheres closely to the mammillary layer Pores- tiny pore canals w/c extend through the shell are seen as minute pores or round openings (7,000 -17,000 per egg) the cuticle protein partially seals the pores, but they re m a i n p e rm e a b l e to g a se s w hi l e re st ri c t i ng penetration of microorganism Albumen a 10% solution of various proteins. The thick, gel-like albumen differs from thin albumen only in its approx. four-fold content of ovomucin a pseudoplastic fluid its viscosity depends on shearing force  the surface tension (12.5% solution, pH 7.8, of freshly laid egg is 7.6-7.9 and rises to 9.7 during storage due to diffusion of solubilized CO2 through the cell the rise is time and temperature dependent example: a pH of 9.4 was recorded after 21 days of storage at 3 -350C Proteins Proteins of egg white: ovalbumin ovoglobulin G3 avidin conalbumin o v o g l o b u l i n G 2 ovoinhibitor ovomucoid flavoprotein ovomucin ficin inhibitor ovoglycoprotein lysozyme G1 ovomacroglobulin  several albumen proteins have biological activity: a. enzymes- lysozymes b. enzyme inhibitor- ovomucoid, ovoinhibitor c. complex-forming agents for some coenzymes- flavoprotein, avidin biological roles may be related to protection of the egg from microbial spoilage  Ovalbumin - main albumen protein, crystallized by Hofmeister in 1890 - a glycophosphoprotein with 3.2% carbohydrates and 0-2 moles of serine-bound phosphoric acid per mole of protein. - contains 4 thiol and 1 disulfide group; During storage of eggs, heat-stable S-ovalbumin is formed from native protein, probably by a thiol-disulf ide exchange reaction - relatively readily denatured, for example, by shaking or whipping its aqueous solution. - this is an interphase denaturation w/c occurs thro ug h unf o l d i ng and ag g re g ati o n o f p ro te i n molecules. Conalbumin (Ovotransferrin) - this protein, unlike ovalbumin, is not denatured at the interphase but coagulates at lower temperatures. - It is consists of one peptide chain and contains one oligosaccharides unit made of 4 mannose and 8 N- acetylglucosamine residues. - the occasional red discoloration of egg products during processing originates from a conalbumin-iron complex; the complex is fully dissociated at a pH less than 4 - it has the ability to retard growth of microorganism  Ovomucoid - ion exchange chromatography or electrophoresis reveals 2 or 3 forms of protein, which apparently differ in their sialic acid contents. - T h e c a r b o h y d r a t e m o i e t y, c o n s i s t s o f 3 oligosaccharide units bound to protein through asparagine residues. The protein has 9 disulfate b o nd s a nd t he re f o re , st a b i l i t y a g a i nst he a t coagulation - it can be isolated from the supernatants of heat- coagulated albumen solutions, and then precipitated by ethanol or acetone - it inhibits bovine but not human trypsin activities - the proportion of regular structural elements is high (26% of  -helix, 46% of ß-structure, and 10% of ß-turn)  Lysozyme (Ovoglobulin G1) - is widely distributed and is found not only in egg white but in many animal tissues and secretions, in latex exudates of some plants and in some fungi. - This protein with three known components, is an N- acetylmuramidase enzyme that hydrolyzes the cell walls of Gram-positive bacteria - consists of a peptide chain with 129 amino acid residues and 4 disulfide bonds.  Ovoglobulins G2 and G3 - proteins w/c are good foam builders Ovomucin - this protein, of which three components are known, c an apparently form f ib rillar struc tures and so c ontrib ute to a rise in v isc osity of alb um en, particularly of the thick, gel-like egg white, where it occurs in a fold in a four-fold higher concentration than in fractions of thin albumen. - has been separated into a low-carbohydrate(15%), - fraction and a high-carbohydrate(50%) -fraction - is heat stable. It forms a water insoluble complex with lysozyme. - dissociation of the complex is pH dependent. - it is important in connection with the thinning of egg white during storage of eggs.  Flavoprotein - this protein binds f irmly with ribof lavin and probably functions to facilitate transfer of this coenzyme from blood serum to egg. Ovoinhibitor - this protein is, like ovomucoid, a proteinase inhibitor. It inhibits the activities of trypsin, chymotrypsin and some proteinase of microbial origin  Avidin - is a basic glycoprotein. - in its amino acid sequence, 15 positions are identical with lysozyme - it is a tetramer consisting of four identical subunits, each of which binds one mole of biotin. - in its form in egg white, is practically free of biotin, and presumably fulfills an anti bacterial role.  Ficin Inhibitor (Cystatin C) - chicken egg Cystatin C consists of one peptide chain with a ca. 120 amino acid residues. - the two i som ers known d i f fer i n thei r isoelec tric point and im m unologic al properties  Other Constituents  Lipids - its content of albumen is negligible.  Carbohydrates - are partly bond to protein and partly free. - free carbohydrates include glucose and mannose, g a l a c t o s e , a ra b i n o s e , x y l o s e , r i b o s e a n d deoxyribose. There are no free oligosaccharides or polysaccharides. - bound carbohydrates were covered previously with proteins. - Mannose , g al ac tose and g l uc osam i ne are predominant, and sialic acid and galactosamine are also present.  Minerals - the mineral content of egg white is 0.6%.  Mineral composition of eggs Egg white(%) Egg yolk(%) Sulfur 0.195 0.016 Phosphorous 0.018 0.543-0.980 Sodium 0.161-0.169 0.070-0.093 Potassium 0.145-0.167 0.112-0.360 Magnesium 0.009 0.032 -0.128 Calcium 0.008-0.02 0.121-0.262 Iron 0.0009 0.0053-0.011  Vitamin content of eggs Vitamin Whole egg Egg white Egg yolk Retinol (A) 0.22 0 1.12 Thiamine 0.11 trace 0.29 Riboflavin 0.3 0.27 0.44 Niacin 0.1 0.1 0.1 Pyridoxine (B6) 0.12 trace 0.3 Pantothenic acid 1.59 0.14 3.72 Biotin 0.025 0.007 Folic acid 0.051 0.016 0.15 Tocopherols 1.0 0 3.0 Tocopherol 0.46 EGG YOLK - is a fat-in-water emulsion with about 50% dry water content, and consisting of proteins and lipids. Water transfer from egg white drops the solid content weeks. - is a pseudoplastic non- Newtonian f luid with a viscosity which depends on the shear forces applied. - it contains particles of differing size that can be classified into two groups: > yolk droplets of hi g h v ari ab l e si ze, wi th a diameter range of 40-40 um. They are resemble fat droplets, consists mostly of lipids and some have p ro t e i n m e m b ra n e s. T h e y a re a m i x t u re o f lipoproteins with a low density. > Granules that have a diameter of 1.0-1.3 um,i.e., they are substantially smaller than yolk droplets, and are more uniform in size but less uniform in shape. They have structure and consist of proteins but also contain lipids and minerals.  older methods of yolk separation, which included at least partial defatting with various solvents, led to lipoprotein destruction and through it to artifacts of varying composition. Yolk studies are now based on ultracentrifugation, when necessary in the presence of electrolytes, which provides native yolk fractions. The granules are separated from the plasma by ultra centrifugation of diluted yolk solution.  After NaCl addition, the granules are separated further into a low density lipoprotein fraction and a lipovitellin-phosvitin complex.  The latter can be separated into its constituents by chromatographic techniques.  In the presence of NaCl the plasma can be further separated by centrifugation into a floating, low density lipoprotein fraction and a water soluble livetin fraction.  Composition of egg yolk granules and plasma fractions Fraction Lipid Protein Minerals Egg yolk 63.5 32.4 2.1 Granules 6.9 16.1 1.4 Lipovitellins 3.5 12.3 Phosvitin 4.6 LDL 2.5 0.3 Plasma 59.3 11.9 1.5 Livetins 10.6 LDL 59.4 6.6  Proteins of Granules Lipovitellins -fractions represents high density lipoproteins. - Its lipid moeity is 22% of dry matter and consists of 35% triglycerides, approx. 60% phospolipids and close to 5% cholesterol and cholesterol esters - i t c a n b e s e p a ra t e d b y e l e c t ro p h o re t i c a n d c hro m ato g rap hi c m e tho d s i nto the i r a- and B- components, which differ in their protein bound phosphorous content. - The protein components probably also contain carbohydrates. - At a pH < 7 lipovitellins occur as dimers with a molecular weight of 400 kdal. - Lipovitellins occur in the yolk as a complex with phosvetin.  Phosvetin -is a glycophosphoprotein with an exceptionally high amount of phosporic acid bound to serine residues. - ef fic iently binds metal ions. Inter molecular complexes are formed through cross linkageges in the the presence of Ca2+ and Mg2+.  Plasma Proteins  Lipovitellenin - is obtained as a f loating, low density lipoprotein by ultracentrifugaration of diluted yolk. - Several components with varying densities can be separated by fractional centrifugaration. - The lipid moeity represents 84-90% of the dry matter and consists of 74% triglycerides and 26% phospolipids. - The latter contain predominantly phosphatidyl choline phospatidyl choline, phospatidyl ethanolamine as well as sphingomyelin and lysophospholipids. - The molecular weight of lipovitellin is several million dal. -The individual components of this plasma protein are not well characterized.  Livetin - the water-soluble globular protein fraction can be separated electrophoretically into  -,  - and  - livetins. -These have been proven to correspond to chicken b l ood se rum p rote i ns, I.e. se rum al b um i n, a 2 - glycoprotein and y-globulin. Lipids - egg yolk contains 32.6% of lipids -These lipids occur as the lipoproteins, are closely associated with the proteins occurring in yolk. Lipid fraction a b Triacylglycerols 66 Phospholipids 28 Phosphatidyl choline 73 Phosphatidyl ethanolamine 15.5 Lysophosphatidyl choline 5.8 Sphingomyelin 2.5 Lysophosphatidyl ethanolamine 2.1 Plasmalogen 0.9 Phosphatidyl inositol 0.6 Cholesterol, Cholesterol esters and other compounds 6 a As percent of total lipids b.As percent of phospholipid fraction Other constituents Carbohydrates - egg yolk carbohydrates are about 1% of the dry matter, with 0.2% bound to proteins. The free carbohydrates present in the addition to glucose are the same monosaccharides identified in egg white. Minerals (Slide 34) Vitamins (Slide 35) Aroma Substances - the typical aroma substances of the egg white and the egg yolk are still unknown. - The “f ishy” aroma defects that can occur in eggs is caused by methylamine, which has an odor threshold of 25uf/kg (pH 7.8). - Trim ethylam ine is form ed by the m icrobial degradation of choline,eg., on feeding f ish meals or soy meal. Storage Of Eggs A series of changes occurs in eggs during storage. The diffusion of CO2 through the pores of the shell, which starts soon after the egg is laid, causes a sharp raise in pH, especially in egg white. The gradual evaporation of water through the shell causes a decrease in density and the air cell enlarges. The viscosity of the egg with white drops. The yolk is compact and upright in a fresh egg, but it f la ttens during the storage.  After the egg is cracked and the contents are release onto a level surface, this f lattening is expressed as yolk index, the ratio of yolk height to diameter. Furtherm ore, the v itellin m em brane of the yolk becomes rigid and tears readily once the egg is opened. Of importance for egg processing is the fact that several properties change, such as egg white whipping behavior and foam stability. In addition, a “stale” flavor develops.  These changes are used for determination of the age of an egg, f lo ating test, f lash candling,egg white viscosity test, measurement of egg air cell size, refractive index,and sensory assay of the “stale” flavor. The lower the storage temperature and the lower the losses of CO 2 and water, the lower the quality loss during storage of eggs.therefore, cold storage is an important part of egg preservation. Egg Products Egg products in liquid, frozen or dried forms are made from whole eggs, white or yolk. They are uti l i ze d f ur the r as se m i - e nd p ro d uc ts i n the manufacturing of baked goods, noodles, confectionery, pastry products, mayonnaise and other salad dressings, soup powders, margarine, meat products, ice creams and liqueurs. Technically important properties The many uses of egg products are basically the results of three properties of eggs: 1. Coagulation when heated 2. Foaming ability 3. Emulsifying properties. The coloring ability and aroma of egg should also be mentioned. Thermal Coagulation  Egg white begins to coagulate at 62 OC and egg yolk at 65 OC  the coagulation temp. is influenced by pH  at pH at or above 11.9 egg white gels or sets even at room temperature, though after a while the gel liquiefies.  all egg proteins coagulate, except ovomucoid and phosvitin  conalbumin is particularly sensitive, but can be stabilized by complexing it with metal ions  due to their ability to coagulate, egg products are important food-binding agents Foaming ability  Whipping of egg white builds a foam which entraps air and hence is used as a leavening agent in many food products.  Due to a large surface area increase in the liquid/air interphase, proteins denature and aggregate during whipping.  I n p ar ti c ul ar, ov o m uc i n f o rm s a f il m o f insoluble materials between the liquid lamella and air bubble, there by stabilizing the foam.  Egg globulin also contributes to this effect by increasing the f luid viscosity and by decreasing the surface tension, both effects of importance in the initial stage of the whipping process.  In angel cake, egg white without ovomucin and globulins leads to long whipping times and cakes with reduces volume  an excessive ovomucin content decreases the thermal stability (expansion of air bubbles)of the foam Emulsifying Effect  the e m ul si f yi ng of the e g g i s uti l i ze d , f or example, in production of mayonnaise and of c ream y salad dressing s ( m ade by b eating a mixture of egg yolk, olive oil, lemon juice or vinegar , and seasonings  LD-Lipoproteins and proteins are responsible for the emulsifying action of the eggs Dried Products  the liquid contents of eggs is mixed or churned either immediately or only after egg white and yolk separation.  Homogenization is followed by a purification step using centrifuges(separator) and then by a pasteurization step  the sugars are removed prior to egg drying to prevent reaction between amino components and reducing sugars, thereby avoiding undesired brown discoloration and Faulty aroma.  sugar are removed from the egg white after pasteurization, usually by microbiological sugar fermentation.