1、34.1CHAPTER 34EGGS AND EGG PRODUCTSSHELL EGGS 34.1Egg Structure and Composition. 34.1Egg Quality and Safety 34.2Shell Egg Processing . 34.5Effect of Refrigeration on Egg Quality and Safety 34.5Packaging . 34.8Transportation 34.8EGG PRODUCTS 34.8Egg Breaking 34.8Refrigerated Liquid Egg Products . 34.
2、10Frozen Egg Products 34.11Dehydrated Egg Products 34.12Egg Product Quality. 34.12Sanitary Standards and Plant Sanitation. 34.12BOUT 69% of the table eggs produced in the United StatesAare sold as shell eggs. The remainder are further processed intoliquid, frozen, or dehydrated egg products that are
3、 used in food ser-vice or as an ingredient in food products. Small amounts of furtherprocessed eggs are converted to retail egg products, mainly mayon-naise, salad dressings, and egg substitutes. Shell egg processingincludes cleaning, washing, drying, candling for interior and exte-rior defects, siz
4、ing, and packaging. Further processed eggs requireshell removal, filtering, blending, pasteurization, and possiblyfreezing or dehydration.After processing, shell eggs intended for use within severalweeks are stored at 4 to 7C and relative humidities of 75 to 80%.These conditions reduce the evaporati
5、on of water from the egg,which would reduce the eggs mass and hasten breakdown of thealbumen (an indicator of quality and grade). Shell eggs are alsorefrigerated during transportation, during short- and long-term stor-age, in retail outlets, and at the institutional and consumer levels.Research has
6、shown that microbial growth can be curtailed byholding eggs at less than 5C. USDA regulations require eggs to bekept in an ambient temperature below 7.2C until they reach theconsumer, to prevent the growth of Salmonella (see October 27,1992, United States Federal Register). Storage and display areas
7、must be refrigerated and able to maintain ambient temperatures at7.2C.SHELL EGGSEGG STRUCTURE AND COMPOSITIONPhysical StructureThe parts of an egg are shown in Figure 1, and physical proper-ties of eggs are given in Table 1.The shell is about 11% of the egg mass and is deposited on theexterior of th
8、e outer shell membrane. It consists of a mammillarylayer and a spongy layer. The shell contains large numbers of pores(approximately 17 000) that allow water, gases, and small particles(e.g., microorganisms) to move through the shell. A thin, clear film(cuticle) on the exterior of the shell covers t
9、he pores. This materialis thought to retard the passage of microbes through the shell andserves to prevent moisture loss from the eggs interior. The shapeand structure of the shell provide enormous resistance to pressurestress, but very little resistance to breakage caused by impact.Tough fibrous sh
10、ell membranes surround the albumen. As theegg ages, cools, and loses moisture, an air cell develops on the largeend of the egg between these two membranes. The size of the air cellis an indirect measure of the eggs age and is used to evaluate inte-rior quality.The white (albumen) constitutes about 5
11、8% of the egg mass. Thewhite consists of a thin, inner chalaziferous layer of firm proteincontaining fibers that twist into chalazae on the polar ends of theyolk. These structures (Figure 1) anchor the yolk in the center of theegg, also known as the inner thick. The albumen consists of innerthin, ou
12、ter thick, and outer thin layers.The yolk constitutes approximately 31% of the egg mass. It con-sists of a yolk (vitelline) membrane and concentric rings of six yel-low layers and narrow white layers (Figure 1). In the intact egg,these layers are not visible. Most of the eggs lipids and cholesterola
13、re bounded into a lipoprotein complex that is found more in thewhite layers. The yolk contains the germinal disc, which consists ofabout 20 000 cells if the egg is fertile. However, eggs produced forhuman consumption are not fertile because the hens are raised with-out roosters.The preparation of th
14、is chapter is assigned to TC 10.9, Refrigeration Appli-cation for Foods and Beverages.Table 1 Physical Properties of Chicken EggsProperty Whole Egg Albumen YolkSolids, % 26.4 11.5 52.5pH (fresh eggs) 7.6 6.0Density, kg/m31080 1035 1035Surface tension, Pa 4.4Freezing point, C 0.44 0.55Specific heat,
15、kJ/(kgK) 3.23Viscosity, mPasThick white 164Thin white 4Electrical conductivity, mS/m 82.5 0.7Water activity, % relative humidity 97.8 98.1Source: Burley and Vadehra1989).Fig. 1 Structure of an EggFig. 1 Structure of an Egg34.2 2010 ASHRAE HandbookRefrigeration (SI)Chemical CompositionThe mass of the
16、 chicken egg varies from 35 to 80 g or more. Themain factors affecting mass and size are the birds age, breed, andstrain. Nutritional adequacy of the ration and ambient temperatureof the laying house also influence egg size. Size affects the eggscomposition, because the proportion of the parts chang
17、es as eggmass increases. For example, small eggs laid by young pullets justcoming into production will have relatively more yolk and less albu-men than eggs laid by older hens. Table 2 presents the general com-position of a typical egg weighing 60 g.The shell is low in water content and high in inor
18、ganic solids,mainly calcium carbonate as calcite crystals plus small amounts ofphosphorus and magnesium and some trace minerals. Most of theshells organic matter is protein. It is found in the matrix fibersclosely associated with the calcite crystals and in the cuticle layercovering the shell surfac
19、e. Protein fibers are also present in the porecanals extending through the shell structures to the cuticle, and inthe two shell membranes. The membranes contain keratin, a proteinthat makes the membranes tough even though they are very thin.Egg albumen, or egg white, is a gel-like substance consisti
20、ng ofovomucin fibers and globular-type proteins in an aqueous solution.Ovalbumin is the most abundant protein in egg white. When heatedto about 60C, coagulation occurs and the albumen becomes firm.Several fractions of ovoglobulins have been identified by electropho-retic and chromatographic analyses
21、. These proteins impart excellentfoaming and beating qualities to egg white when making cakes,meringues, candies, etc. Ovomucin is partly responsible for the vis-cous characteristic of raw albumen and also has a stabilizing effecton egg-white foams, an important property in cakes and candy.Egg white
22、 contains a small amount of carbohydrates. About halfis present as free glucose and half as glycoproteins containing man-nose and galactose units. In dried egg products, glucose interacts withother egg components to produce off-colors and off-flavors duringstorage; therefore, glucose is enzymaticall
23、y digested before drying.The yolk comprises one third of the edible portion of the egg. Itsmajor components are water (48 to 52%), lipids (33%), and proteins(17%). The yolk contains all of the fatty material of the egg. The lip-ids are very closely associated with the proteins. These very com-plex l
24、ipoproteins give yolk special functional properties, such asemulsifying power in mayonnaise and foaming and coagulatingpowers in sponge cakes and doughnuts.Nutritive ValueEggs are a year-round staple in the diet of nearly every culture.The composition and nutritive value of eggs differ among the var
25、i-ous avian species. However, only the chicken egg is consideredhere, as it is the most widely used for human foods.Eggs contain high-quality protein, which supplies essentialamino acids that cannot be produced by the body or that cannot besynthesized at a rate sufficient to meet the bodys demands.
26、Eggsare also an important source of minerals and vitamins in the humandiet. Although the white and yolk are low in calcium, they containsubstantial quantities of phosphorus, iron, and trace minerals. Ex-cept for vitamin C, one or two eggs daily can supply a significantportion of the recommended dail
27、y allowance for most vitamins,particularly vitamins A and B12. Eggs are second only to fish liveroils as a natural food source of vitamin D.Fatty acids in the yolk are divided into saturated and unsaturatedin a ratio of 1:1.8, with the latter further subdivided into mono- andpolyunsaturated fatty ac
28、ids in a ratio of 1:0.3. Eggs are a source ofoleic acid, a monounsaturated fatty acid; they also contain polyun-saturated linoleic acid, an essential fatty acid. The fatty acid com-position of eggs and the balance of saturated to unsaturated fattyacids can be changed by modifying the hens diet. Seve
29、ral commer-cial egg products with modified lipids have been marketed.EGG QUALITY AND SAFETYQuality Grades and Mass ClassesIn the United States, the Egg Products Inspection Act of 1970 re-quires that all eggs moving in interstate commerce be graded for sizeand quality. USDA standards for quality of i
30、ndividual shell eggs areshown in Table 3. The quality of shell eggs begins to decline imme-diately after the egg is laid. Aging of the egg thins the albumen and in-creases the size of the air cell. Carbon dioxide migration from the eggincreases albumen pH and decreases vitelline membrane strength.Cl
31、asses for shell eggs are shown in Table 4. The average mass ofshell eggs from commercial flocks varies with age, strain, diet, andenvironment. Practically all eggs produced on commercial poultryfarms are processed mechanically. They are washed, candled, sized,then packed. Eggs are oiled at times to
32、extend internal quality whenthey are to be transported long distances over a number of days.Although eggs are sold by units of 6, 12, 18, or 30 per package, thepackaged eggs must maintain a minimum mass that relates to the eggsize.Table 2 Composition of Whole EggEggComponentProtein,%Lipid,%Carbohydr
33、ate,%Ash,%Water,%Albumen 9.7-10.6 0.03 0.4-0.9 0.5-0.6 88.0Yolk 15.7-16.6 31.8-35.5 0.2-1.0 1.1 51.1Whole egg 12.8-13.4 10.5-11.8 0.3-1.0 0.8-1.0 75.5Note: Shell is not included in above percentages.Percentof EggCalcium CarbonateMagnesiumCarbonateCalcium PhosphateOrganic MatterShell 11 94.0 1.0 1.0
34、4.0Source: Stadelman and Cotterill (1990).Table 3 U.S. Standards for Quality of Shell EggsQuality FactorAAQualityAQualityBQualityShell Clean Clean Clean to slightly stainedaUnbroken Unbroken UnbrokenPractically normal Practically normal AbnormalAir cell 3 mm or less in depth5 mm or less in depthOver
35、 5 mm in depthUnlimited movement and free or bubblyUnlimited movement and free or bubblyUnlimited movement and free or bubblyWhite Clear Clear Weak and wateryFirm Reasonably firm Small blood and meat spots presentbYolk Outline slightly definedOutline fairly well definedOutline plainly visiblePractic
36、ally free from defectsPractically free from defectsEnlarged and flattenedClearly visible germ development but no bloodOther serious defectsFor eggs with dirty or broken shells, the standards of quality provide two additional qualities. These are:Dirty CheckUnbroken. Adhering dirt or foreign material
37、, prominent stains, moderate stained areas in excess of B quality.Broken or cracked shell but membranes intact, not leaking.caModerately stained areas permitted (1/32 of surface if localized, or 1/16 if scattered).bIf they are small (aggregating not more than 3 mm in diameter).cLeaker has broken or
38、cracked shell and membranes, and contents are leaking or free toleak.Source: Federal Register, 7CFR56, May 1, 1991. USDA Agriculture Handbook 75, p. 18.Eggs and Egg Products 34.3Quality FactorsBesides legal requirements, egg quality encompasses all thecharacteristics that affect an eggs acceptabilit
39、y to a particular user.The specific meaning of quality may vary. To a producer, it mightmean the number of cracked or loss eggs that cannot be sold, or thepercentage of undergrades on the grade-out slip. Processors associ-ate quality with prominence of yolk shadow under the candling lightand resista
40、nce of the shell to damage on the automated grading andpacking lines. The consumer looks critically at shell texture andcleanliness and the appearance of the broken-out egg and considersthese factors in their relationship to a microbially safe product.Shell Quality. Strength, texture, porosity, shap
41、e, cleanliness,soundness, and color are factors determining shell quality. Ofthese, shell soundness is the most important. It is estimated thatabout 10% of all eggs produced are cracked or broken betweenoviposition and retail sale. Eggs that have only shell damage canbe salvaged only for their liqui
42、d content, but eggs that have bothshell and shell membrane ruptured are regarded as a loss and can-not be used for human consumption. Shell strength is highlydependent on shell thickness and crystalline structure, which isaffected by genetics, nutrition, length of continuous lay, disease,and environ
43、mental factors.Eggs with smooth shells are preferred over those with a sandytexture or prominent nodules that detract from the eggs appearance.Eggs with rough or thin shells or other defects are often weaker thanthose with smooth shells. Although shell texture and thickness dete-riorate as the layin
44、g cycle progresses, the exact causes of thesechanges are not fully understood. Some research suggests thatdebris in the oviduct collects on the shell membrane surface, result-ing in rough texture formation (nodules).The number and structure of pores are factors in microbial pen-etration and loss of
45、carbon dioxide and water. Eggs without a cuticleor with a damaged cuticle are not as resistant to water loss, waterpenetration, and microbial growth as those with this outer protein-aceous covering. External oiling of the shell provides additionalprotection.Eggs have an oval shape with shape indexes
46、 (breadth/length 100) ranging from 70 to 74. Eggs that deviate excessively fromthis norm are considered less attractive and break more readily inpackaging and in transit. Egg shape is changing to a more roundedshape, which is resulting in a stronger shell.Shells with visible soil or deep stains are
47、not allowed in a high-quality pack of eggs. Furthermore, soil usually contains a heavyload of microorganisms that may penetrate the shell, get into thecontents, and cause spoilage.Shell color is a breed characteristic. Brown shells owe their colorto a reddish-brown pigment, ooporphyrin, which is der
48、ived fromhemoglobin. The highest content of the pigment is near the surfaceof the shell. White shells contain a small amount of ooporphyrin,too, but it degrades soon after laying by exposure to light. Brown-shelled eggs tend to vary in color.Albumen Quality. Egg white viscosity differs in various ar
49、easof the egg. A dense layer of albumen is centered in the middle andis most visible when the egg is broken out onto a flat surface. Rawalbumen has a yellowish-green cast. In high-quality eggs, the whiteshould stand up high around the yolk with minimum spreading ofthe outer thin layer of the albumen. Albumen thickness in thefreshly laid egg is affected by genetics, duration of continuous pro-duction, and environmental factors. Albumen quality generallydeclines with age, especially in the last part of the laying cycle.Breakdown of thick white is a continuing process
