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 39 to 45F and relative humidities of 75 to 80%.These conditions reduce the evapora
5、tion of water from the egg,which would reduce the eggs weight 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
6、has shown that microbial growth can be curtailed byholding eggs at less than 41F. USDA regulations require eggs to bekept in an ambient temperature below 45F until they reach the con-sumer, to prevent the growth of Salmonella (see October 27, 1992,United States Federal Register). Storage and display
7、 areas must berefrigerated and able to maintain ambient temperatures at 45F.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 weight and is deposited on theexteri
8、or of the 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
9、covers the 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 fi
10、brous shell 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
11、 about 58% of the egg weight.The white consists of a thin, inner chalaziferous layer of firm pro-tein containing 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 i
12、nnerthin, outer thick, and outer thin layers.The yolk constitutes approximately 31% of the egg weight. Itconsists of a yolk (vitelline) membrane and concentric rings of sixyellow layers and narrow white layers (Figure 1). In the intact egg,these layers are not visible. Most of the eggs lipids and ch
13、olesterolare 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 prepara
14、tion of this 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, lb/ft367.5 64.7 64.7Surface tension, psi 6.38 104Freezing point, F 31.2 31
15、.0Specific heat, Btu/lbF 0.772Viscosity, centipoiseThick white 164Thin white 4Electrical conductivity, mho/cm 1048.25 0.07Water activity, % relative humidity 97.8 98.1Source: Burley and Vadehra1989).Fig. 1 Structure of an EggFig. 1 Structure of an Egg34.2 2010 ASHRAE HandbookRefrigerationChemical Co
16、mpositionThe weight of the chicken egg varies from 35 to 80 g or more.The main factors affecting weight and size are the birds age, breed,and strain. Nutritional adequacy of the ration and ambient tempera-ture of the laying house also influence egg size. Size affects theeggs composition, because the
17、 proportion of the parts changes asegg weight increases. For example, small eggs laid by young pulletsjust coming into production will have relatively more yolk and lessalbumen than eggs laid by older hens. Table 2 presents the generalcomposition of a typical egg weighing 60 g.The shell is low in wa
18、ter content and high in inorganic 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 la
19、yercovering the shell surface. 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
20、 gel-like substance consisting ofovomucin fibers and globular-type proteins in an aqueous solution.Ovalbumin is the most abundant protein in egg white. When heatedto about 140F, coagulation occurs and the albumen becomes firm.Several fractions of ovoglobulins have been identified by electropho-retic
21、 and chromatographic analyses. 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
22、 in cakes and candy.Egg white 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; there
23、fore, glucose is enzymatically 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 pr
24、oteins. These very com-plex lipoproteins 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
25、 of eggs differ among the vari-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 sufficien
26、t to meet the bodys demands. 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 significantpo
27、rtion of the recommended daily 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
28、- andpolyunsaturated fatty acids 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
29、modifying the hens diet. Several commer-cial egg products with modified lipids have been marketed.EGG QUALITY AND SAFETYQuality Grades and Weight 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.
30、USDA standards for quality of individual 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
31、 vitelline membrane strength.Classes for shell eggs are shown in Table 4. The average weight 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 pa
32、cked. Eggs are oiled at times to 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 weight that relates to theegg size.Table 2 Composition of Whole EggEg
33、gComponentProtein,%Lipid,%Carbohydrate,%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 PhosphateO
34、rganic MatterShell 11 94.0 1.0 1.0 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 1/8 in. or
35、less in depth3/16 in. or less in depthOver 3/16 in. 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 fai
36、rly well definedOutline plainly visiblePractically 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 Ch
37、eckUnbroken. Adhering dirt or foreign material, 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
38、 than 1/8 in. in diameter).cLeaker has broken or 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 th
39、echaracteristics that affect an eggs acceptability 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 o
40、f yolk shadow under the candling lightand resistance 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
41、.Shell Quality. Strength, texture, porosity, shape, 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
42、 shell damage canbe salvaged only for their liquid 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, nutriti
43、on, length of continuous lay, disease,and environmental 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 shel
44、l texture and thickness dete-riorate as the laying 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 a
45、re factors in microbial pen-etration and loss of 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 additionalprot
46、ection.Eggs have an oval shape with shape indexes (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 s
47、hell.Shells with visible soil or deep stains are 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
48、 reddish-brown pigment, ooporphyrin, which is derived 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 Q
49、uality. Egg white viscosity differs in various areasof 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 c
