EGGS | Dietary Importance

EGGS | Dietary Importance

EGGS/Dietary Importance See also: Cholesterol: Properties and Determination; Fats: Classification; Lipoproteins; Phospholipids: Properties and Occurre...

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EGGS/Dietary Importance See also: Cholesterol: Properties and Determination; Fats: Classification; Lipoproteins; Phospholipids: Properties and Occurrence; Protein: Chemistry; Triglycerides: Structures and Properties

Further Reading Li-Chan ECY, Powrie WD and Nakai S (1995) The chemistry of eggs and egg products. In: Stadelman WJ and Cotterill OJ (eds) Egg Science and Technology, 4th edn. Binghamton, New York: Haworth Press. Romanoff AL and Romanoff AJ (1949) The Avian Egg. New York: John Wiley. Stadelman WJ and Cotterill OJ (1995) Egg Science and Technology, 4th edn. Binghamton, New York: Haworth Press. Stadelman WJ and Pratt DE (1989) Factors influencing composition of the hen’s egg. World Poultry Science Journal 45: 247–266. USDA (1976) Composition of Foods: Dairy and Egg Products. US Department of Agriculture, ARS, Agriculture Handbook 8.1. Washington, DC: US Government Printing Office.

Dietary Importance W J Stadelman, Purdue University, West Lafayette, IN, USA Copyright 2003, Elsevier Science Ltd. All Rights Reserved.

2009

composed of about 10% shell, 30% yolk, and 60% albumen. The overall composition of the egg is influenced by breeding and age of the hens. Nutrition of the hen has little effect on the proximate analysis but can significantly modify the content of some of the nutrients in eggs.

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Protein and Amino Acids The egg white is a protein system consisting of ovomucin fibers in an aqueous solution of globular proteins. The thick and thin layers of egg white differ only in their ovomucin content. The protein fractions of egg white consist of ovalbumin, conalbumin or ovotransferin, ovomucoid, lysozyme, ovomucin, avidin, ovoglobulins, ovoinhibitor, and flavoprotein. The molecular structure and amino acid mapping of several of these proteins have been documented. (See Protein: Chemistry.) The absence of lipid materials from the albumen makes possible the formulation of foods with the high protein quality of eggs and a complete absence of fats. The amino acid composition of the several component parts of the egg is listed in Table 1. It is apparent that whole egg, egg yolk, and albumen each have a good balance of amino acids. (See Amino Acids: Properties and Occurrence.)

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Introduction 0001

Eggs are a complete food for a developing embryo of a chick. The only vitamin not found in eggs is vitamin C. Most nutrients in the egg are present in variable amounts depending on breeding, feeding, and management of the hens. The amino acids in the proteins of the egg are relatively uniform among eggs so that egg proteins are often used as the standard against which other protein food sources are evaluated in biological value studies.

Nutrient Distribution in the Egg 0002

The proximate analyses of the edible portion of whole eggs, albumen, and yolk are given in Table 1 of the previous article Eggs: Structure and Composition. Values are included for pure yolk and commercially available yolk. The difference is that some albumen is included with the commercial yolk, as evidenced by a change in solids from 51.2% to 44%. The percentages of yolk and albumen vary among eggs from different hens and change with age of the hen, with the percentage of albumen increasing with age of the hen. For rough calculation the egg is

Table 1 Amino acids in the edible portion of eggs (per 100 g) Yolk Amino acid (g)

Wholea

Albumena

Purea

Commercialb

Alanine Arginine Aspartic acid Cystine Glutamic acid Glycine Histidine Isoleucine Leucine Methionine Phenylalanine Proline Serine Threonine Tryptophan Tyrosine Valine

0.696 0.750 1.256 0.290 1.632 0.420 0.296 0.682 1.068 0.390 0.664 0.498 0.930 0.600 0.152 0.510 0.762

0.608 0.572 1.072 0.272 1.400 0.368 0.237 0.596 0.886 0.362 0.614 0.410 0.725 0.479 0.129 0.410 0.671

0.861 1.000 1.639 0.301 2.126 0.518 0.434 0.849 1.470 0.416 0.717 0.699 1.432 0.892 0.199 0.747 0.934

0.818 0.904 1.559 0.301 2.005 0.497 0.402 0.790 1.364 0.416 0.696 0.645 1.307 0.819 0.191 0.686 0.885

a

Based on data from USDA (1989) Composition of Foods: Dairy and Egg Products. USDA Handbook 8.1. Washington, DC: US Department of Agriculture. b Based on data from Cotterill OJ and Glauer JL (1979) Nutrient values for shell, liquid/frozen and dehydrated eggs derived by linear regression analysis and conversion factors. Poultry Science 58: 131–134.

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2010 EGGS/Dietary Importance

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There are four major proteins in egg yolk. These are livetins, phosvitin, lipovitellins, and lipovitellenin. The livetins, (a, b, and w) and phosvitin are classified as high-density lipoproteins; the lipovitellins (a and b) are classed as low-density lipoproteins and lipovitellenin is a very-low-density lipoprotein. The amino acid balance of egg protein is almost perfect to meet human requirements. The amino acid content of eggs can be modified slightly by feeding and management programs. When hens are molted (loss of feathers), the cystine content of the eggs will be slightly reduced because of the high demand for this amino acid in the growth of new feathers.

Lipid Components 0008

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The lipids of the egg are all contained in the yolk in a fresh egg. There may be some migration into the albumen in stale eggs. Nearly all of the yolk lipids are present as lipoproteins. As mentioned above, they are classified as high-density, low-density, and very-lowdensity lipoproteins. The classification is based on percentage of the lipoprotein that is lipid material. The high-density lipoproteins contain very small amounts of lipid, usually less than 10%. Most human nutritionists consider the high-density lipoproteins to be desirable components of a diet. The low-density lipoproteins, a- and b-lipovitellin, contain about 20% lipids; lipovitellenin, the very-lowdensity lipoprotein, has 40% or more lipid content. This latter material is considered to be a harmful ingredient in the human diet as its presence tends to increase the serum cholesterol level. A summary of lipid content is given in Table 2. The saturated fatty acids make up 38% of the total, monounsaturated fatty acids account for 46%, and the polyunsaturated fatty acids comprise the remaining 16%. The amounts of different fatty acids can be dramatically altered depending upon the fatty acids in the diet of the hen. (See Fatty Acids: Properties.) A lipid component that has received much attention is cholesterol. According to the value given in Table 2, one large egg with a 17-g yolk has 213 mg of cholesterol. This is now considered to be the value, but a number of studies indicate a wide range of values, apparently the result of breeding, feeding, or management. Age of the hen will also influence the cholesterol content of an egg. This change is due to the increase in size of the yolk in eggs from older hens. The cholesterol level of an egg, on a per-gram-of-yolk basis, is relatively constant for any hen as long as the feed remains the same. The method of analysis for cholesterol in egg yolk can also influence results, as colorimetric methods generally give higher values than high-pressure liquid chromatography. Extensive

Table 2 Lipid components of the edible portion of eggs (per 100 g)

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Yolk Fatty acids (g)

Wholea

Purea

Commercialb

Saturated (total) 8:0 10:0 12:0 14:0 16.0 18:0 20:0 Monounsaturated (total) 14:1 16:1 18:1 20:1 22:1 Polyunsaturated (total) 18:2 18:3 20:4 20:5 22:6 Cholesterol (mg) Lecithin (g) Cephalin (g)

3.100 0.004 0.004 0.004 0.034 2.226 0.784 0.004 3.810 0.010 0.298 3.472 0.028 0.004 1.364 1.148 0.034 0.142 0.004 0.036 426.0 2.30 0.46

9.554 0.012 0.012 0.012 0.102 6.861 2.416 0.012 11.741 0.030 0.916 10.699 0.084 0.012 4.205 3.356 0.102 0.440 0.012 0.114 1,283.0 6.687 1.319

7.663 0.010 0.010 0.010 0.077 5.524 1.915 0.010 9.444 0.030 0.717 8.623 0.067 0.010 3.374 2.842 0.077 0.352 0.010 0.092 1,038 5.396 1.068

a Based on data from USDA (1989) Composition of Foods: Dairy and Egg Products. USDA Handbook 8.1. Washington, DC: US Department of Agriculture. b Based on data from Cotterill OJ and Glauert JL (1979) Nutrient values for shell, liquid/frozen and dehydrated eggs derived by linear regression analysis and conversion factors. Poultry Science 58: 131–134.

works have been reported indicating that eggs in the diet of humans had slight or no effect on serum cholesterol levels in the blood.

Minerals in Eggs The egg is an excellent source of many of the trace minerals. The mineral content of egg components is listed in Table 3. The exact mineral content of the egg depends on the mineral content of the hen’s diet. The content of all minerals analyzed for could be varied, with the exception of zinc. The minerals of the shell are not considered in the values given in Table 3 as the shell is not a normal item in the human diet. The shell is primarily calcium carbonate and could be a readily available source of calcium. Refer to individual minerals.

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Vitamins in Eggs The vitamins found in the egg vary with the level of vitamins in the layer’s feed. The average levels of vitamins are given in Table 4. The fat-soluble

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EGGS/Dietary Importance tbl0003

Table 3 Mineral components of the edible portion of the egg (per 100 g) Yolk Mineral (mg)

Wholea

Albumena

Purea

Commercial b

Calcium Chlorine Copper Iodine Iron Magnesium Manganese Phosphorus Potassium Sodium Sulfur Zinc

50.0 174.2 0.014 0.048 1.440 10.0 0.024 178.0 120.0 126.0 164.0 1.10

5.998 179.64 0.006 0.003 0.030 11.976 0.003 11.976 143.7 164.7 167.7 0.0

138.552 163.250 0.024 0.133 3.554 6.024 0.072 487.944 96.384 42.168 150.600 3.132

113.276 163.250 0.020 0.108 2.861 5.837 0.059 395.080 106.022 60.008 150.600 2.536

Based on data from USDA (1989) Composition of Foods: Dairy and Egg Products. USDA Handbook 8.1. Washington, DC: US Department of Agriculture. b Based on data from Cotterill OJ and Glauert JL (1979) Nutrient values for shell, liquid/frozen and dehydrated eggs derived by linear regression analysis and conversion factors. Poultry Science 58: 131–134.

Table 4 Vitamin components of the edible portion of eggs (per 100 g) Yolk Vitamin (unit)

specific fatty acids, vitamins, or minerals. An example is the ‘omega egg.’ This egg is obtained by adding a rich source of omega-3 fatty acids (menhaden fish oil, specific algae products, or canola oil). The omega egg producers also increase the vitamin E content of the diet of their hens to get a higher level of both vitamin E and omega-3 fatty acids in the egg. The inclusion of omega-3 fatty acids in the human diet has been reported to reduce the incidence of heart-related problems. Other designer eggs can be produced with increased levels of vitamins, minerals, or other fatty acids. A likely future designer egg is one with increased conjugated linolenic acid.

General Considerations

a

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Wholea

A (IU) 634.0 D (IU) 49.0 E (mg) 1.40 B12 (mg) 1.00 Biotin (mg) 19.96 Choline (mg) 430.12 Folic acid (mg) 46.0 Inositol (mg) 10.78 Nicotinic acid (mg) 0.074 Pantothenic 1.254 acid (mg) Pyridoxine (mg) 0.140 Riboflavin (mg) 0.508 Thiamin (mg) 0.062

Albumena Purea

0.210 7.006 1.257 2.994 4.132 0.093 0.120 0.003 0.452 0.006

Commercial b

1946.0 1580.0 147.6 118.3 4.217 3.390 3.132 2.512 45.662 37.943 1301.00 1050.09 144.58 120.17 23.795 19.891 0.012 0.013 3.807 3.899 0.392 0.639 0.169

0.320 0.595 0.133

a Based on data from USDA (1989) Composition of Foods: Dairy and Egg Products. USDA Handbook 8.1. Washington, DC: US Department of Agriculture. b Based on data from Cotterill OJ and Glauert JL (1979) Nutrient values for shell, liquid/frozen and dehydrated eggs derived by linear regression analysis and conversion factors. Poultry Science 58: 131–134.

vitamins, A, D, and E, are found only in the yolk. Water-soluble vitamins are found in both albumen and yolk components. Refer to individual vitamins.

The exact nutrient composition of eggs is dependent on many factors, including the breed or strain of the hen, age of the hen, diet of the hen, and other management factors. The per capita consumption of eggs in the USA has declined significantly from a high of about 400 eggs per year in 1945 to a level of about 233 in 1990. The annual consumption of eggs then increased slowly to about 245 in 1998. The decrease in egg consumption has been attributed to three factors: (1) increased buying power, which allows consumers to purchase more expensive substitutes for eggs, such as meats and convenience foods; (2) changing eating patterns, including lighter breakfasts; and (3) concerns about the negative health effects of eating eggs – first the cholesterol issue and more recently the inclusion of Salmonella enteritidis, a pathogenic microorganism, in a low percentage of intact shell eggs. In 1990 over 20% of all eggs were converted to egg products prior to sale to the ultimate consumer in the USA. This percentage has increased to over 30% by 1998 as more convenience foods rich in eggs are being offered, as well as the fear of foodborne pathogen. See also: Amino Acids: Properties and Occurrence; Ascorbic Acid: Properties and Determination; Calcium: Properties and Determination; Cholecalciferol: Properties and Determination; Cobalamins: Properties and Determination; Eggs: Structure and Composition; Fatty Acids: Properties; Fats: Classification; Retinol: Properties and Determination; Riboflavin: Properties and Determination; Thiamin: Properties and Determination; Vitamin K: Properties and Determination; Vitamins: Overview; Vitamin B6: Properties and Determination

Designer Eggs 0014

Niche markets for special eggs have resulted in some producers of eggs modifying the nutrient content of the ‘normal’ egg by enriching the diet of the hen with

Further Reading Cotterill OJ and Geiger GS (1977) Egg product yield trends from shell eggs. Poultry Science 56: 1027–1031.

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2012 EGGS/Microbiology Cotterill OJ and Glauert JL (1979) Nutrient values for shell, liquid/frozen and dehydrated eggs derived by linear regression analysis and conversion factors. Poultry Science 58: 131–134. Ginsberg HN, Karnally W, Siddiqyi M et al. (1994) A dose– response study of the effects of dietary cholesterol on fasting and postprandial lipid and lipoprotein metabolism in healthy young men. Artery Thrombosis 14: 576–586. Hu FB, Stampfer MJ, Rimm EB et al. (1999) A prospective study of egg consumption and risk of cardiovascular disease in men and women. Journal of the American Medical Association 281: 1387–1394. Li-Chan ECY, Powrie WD and Nakai S (1995) The chemistry of eggs and egg products. In: Stadelman WJ and Cotterill OJ (eds) Egg Science and Technology, 4th edn, pp. 105– 175. Binghamton, New York: Haworth Publishing. McCharen C (1988) Nutrition and cholesterol. In: Stadelman WJ, Olson VM, Shemwell GA and Pasch S (eds) Eggs and Poultry-Meat Processing, pp. 26–35. Chichester: Ellis Horwood. Stadelman WJ (1977) Egg proteins. In: Graham HD (ed.) Food Colloids, pp. 207–239. Westport, Connecticut: AVI Publishing. Stadelman WJ and Pratt DE (1989) Factors influencing composition of the hen’s egg. World Poultry Science Journal 45: 247–266. USDA (1989) Composition of Foods: Dairy and Egg Products. USDA Handbook 8.1. Washington, DC: US Department of Agriculture. Vorster HH, Bernade AJS, Barnard HC et al. (1992) Egg intake does not change plasma lipoprotein and coagulation profiles. American Journal of Clinical Nutrition 55: 400–410. Watkins BA (1995) The nutritive value of the egg. In: Stadelman WJ and Cotterill OJ (eds) Egg Science and Technology, 4th edn, pp. 177–194. Binghamton, New York: Haworth Publishing. Yoshida A, Naito H, Niiyama Y and Suzuki T (1990) Nutrition: Proteins and Amino Acids. Tokyo: Japan Scientific Societies Press.

the nutrients, elements, and water required for embryo development. Natural selection has put in place a defense system that protects the food stored in the yolk from attack by saprophytic bacteria and microfungi. This article deals with the defense system of the eggs of domestic hens, the only species that has been studied in detail. It deals with the events that culminate in the breakdown of the defense system such that the yolk and white become grossly contaminated and, in most instances, markedly changed in appearance. Eggs may be endowed also with defense against pathogenic microorganisms harbored by the parents or present in the nest environment. This does not appear to have been studied in detail and will not be considered further.

The Defense Physical and chemical systems contribute to the antimicrobial defense of an egg. The former resides mainly in the shell. Both systems occur in the albumen. The yolk appears to contain little, if any, defense against exploitation by saprophytic microorganisms. The physical defense of the egg shell is most easily understood if its component parts are considered as resistances in series (Figure 1). The porous shell of calcite is bounded on its outer surface by a thin layer of glycoprotein, the cuticle (resistance R1). At the moment of lay the cuticle is immature but it rapidly loses its amorphous state and becomes vesicular

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R1 C

S P R2

Microbiology C E Clay, J L Lock, J Dolman, S A Radford and R G Board, University of Bath, Bath, UK This article is reproduced from Encyclopaedia of Food Science, Food Technology and Nutrition, Copyright 1993, Academic Press.

Introduction 0001

The microbiology of eggs must be considered in a biological setting, namely their role in the breeding biology of birds. At the time of lay, an egg contains

SM R3 Figure 1 Schematic drawing of hen’s egg shell in radial section showing the resistances (R1, R2, and R3) to water movement. C, cuticle; P, pore; S, calcitic shell; SM, shell membrane. For further details, see Board RG and Sparks NHC (1991) Shell structure and formation in avian eggs. In: Deeming DC and Ferguson MWJ (eds) Egg Incubation. Cambridge: Cambridge University Press. Reproduced from Eggs: Microbiology, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

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