EGGS. Eggs have been known to, and enjoyed by, humans for many centuries. Jungle fowl were domesticated in India by 3200 b.c.e. Records from China and Egypt show that fowl were domesticated and laying eggs for human consumption around 1400 b.c.e., and there is archaeological evidence for egg consumption dating back to the Neolithic age. The Romans found egg-laying hens in England, Gaul, and among the Germans. The first domesticated fowl reached North America with the second voyage of Columbus in 1493.
Eggs are a staple of the diet in most of the world. They are used as main dishes or served as garnish with other parts of the meal. Barer-Stein points out that Austrian and Croatian cuisines are rich in eggs. In Africa eggs are regarded as a symbol of fertility and accompany yams in many festive dishes.
While most discussions of eggs and egg nutrition refer to bird's eggs, it should be borne in mind that many other animal species also lay eggs. Among mammals the echidna and the duck-billed platypus lay eggs. Reptiles, among them lizards, chameleons, and the tuatara of New Zealand, lay eggs, as do toads and frogs. Mollusks and
|Proximate composition of a large raw egg|
|(59 g shell, 33.4 g white, 16.6 g yolk)|
|Nutrient||Whole egg (g)||% in yolk|
crustaceans lay eggs that appear in a variety of forms. The eggs of fish offer a wide range of number and form; for instance, salmon eggs are deposited in a trough prepared by the parent, while eggs of perch are adhesive and stick to water plants.
The number of eggs laid by fish varies widely but increases with age and weight. The salmon may produce 1,000 eggs for every pound of weight. The sturgeon lays about 7,000,000 eggs, whereas the herring and sole produce 50,000 and 134,000 eggs, respectively. The number of eggs laid is proportional to the risk of destruction or loss.
The greatest knowledge of eggs, their physiology and chemistry, comes from observing hens. The hen's reproductive system consists of the ovary, where the yolk develops, and the oviduct where the egg is completed. At birth the female chick has a fully formed ovary containing several thousand tiny ova, or future yolks. The ova begin to develop, one at a time, when the chick reaches sexual maturity. Each yolk is enclosed in its own sac or follicle. The follicle contains a system of blood vessels that supply nourishment to the developing yolk. At ovulation the follicle ruptures, thus releasing the yolk into the oviduct. The yolk passes into the infundibulum (funnel), where fertilization could occur. After about fifteen minutes the yolk passes into the magnum, where, in a period of three hours, albumen is deposited around the yolk. The yolk next passes into the isthmus where two shell membranes are formed in about seventy-five minutes. The egg has now achieved its full size and shape. It now passes into the uterus, where, over a period of nineteen to twenty hours, it acquires its shell, shell color, and outer shell coating. After a few minutes the egg is released via the vagina, cloaca, and vent. During formation the egg moves through the oviduct small end first, but just before laying, it is rotated and laid large end first. It takes the hen twenty-four to twenty-six hours to produce an egg. Within fifteen to thirty minutes after laying, the hen starts the process all over again.
The egg is designed to support life (to bring a chicken into the world) and has been called nature's ideal food. The yolk comprises about one-third of the weight of the egg. The albumen or white of the egg is primarily protein and water. The yolk of a large egg contains fifty-nine calories and the albumen carries about seventeen calories. See Table 1 for the proximate composition of a large raw egg.
Protein is required for synthesis and maintenance of muscles, body organs, nerves, bones, and blood. Protein quality is measured by how efficiently it is used for growth. Only mother's milk has higher quality protein.
Egg protein contains different amino acids, including all the essential amino acids (essentiality of a nutrient means that it cannot be synthesized by humans and must be obtained from the diet).
Fat (lipid) comprises about 10 percent of the total weight of a large egg. The lipid composition of the egg is presented in Table 2.
Saturated, monounsaturated, and polyunsaturated fatty acids comprise 37.5, 46.0, and 16.5 percent of the total, respectively. Oleic acid (18:1) represents 40 percent of egg yolk fatty acids. There is a nutritional ambivalence with regard to eggs. Although the superior quality of egg protein is acknowledged, there is concern regarding the cholesterol content, this despite evidence that moderate intake of egg yolk is generally not harmful. Since polyunsaturated fats reduce plasma cholesterol levels, efforts have been made to increase their presence in the yolk. Feeding laying hens high levels of polyunsaturated fats such as corn or soybean oil will raise the level of their component yolk fatty acids to a slight degree. "Polyunsaturated" eggs are commercially available, but there is little evidence regarding any sustained hypocholesterolemic effect. There are also efforts to increase levels of antioxidant vitamins and fish oil
|Yolk lipids of a large raw egg|
|(59 g shell, 33.4 g white, 16.6 g yolk)|
fatty acids in egg yolk and those products, too, are available to the public.
Hen's eggs are the most common source of egg nutrition. In 1989 the three largest egg-producing countries were China (140,900 × 106 eggs), Russia (84,600 × 106 eggs), and the United States (67,042 × 106 eggs). However, eggs of other avian species are eaten around the world. Hen's eggs contain less fat and less cholesterol than those of the duck, goose, quail, or turkey. The amount of fat in a yolk is partly a function of the size of the embryo and its future requirements. Caviar, for instance, contains almost 18 g of fat per 100 g of edible portion, but its cholesterol content is only 38 percent greater than that of the hen's egg. The major fatty acids of all the yolks are palmitic and oleic.
The egg is an easily available, inexpensive source of high-quality nutrition. It is an especially important source of nutrition for young people, old people, and sick people. Starting about forty years ago, as the relation between blood cholesterol and the risk of coronary heart disease was unfolding, the egg came under fire because of its cholesterol content. The assumption was that eating cholesterol-rich foods led directly to elevations in blood cholesterol and hence in risk of disease. That elevated blood cholesterol presents a risk for heart disease is fairly well established, but a direct link between dietary cholesterol and levels of blood cholesterol is not. Many other aspects of the diet influence blood cholesterol to a greater extent than does dietary cholesterol; principal among them are the amount of saturated fat in the diet and the type and amount of fiber in the diet. There are dietary prescriptions limiting the permissible amount of cholesterol in the diet. These are easy to follow, but are based on relatively little hard data. The emphasis on cholesterol has overshadowed the greater impact of saturated fat. The fat of the egg is relatively unsaturated, or the raw yolk would be solid. A calculated iodine value (measure of unsaturation) of egg yolk is about 72, which is not much below that of olive oil.
The relation of dietary cholesterol to blood cholesterol levels has been studied for many years. In the early 1950s it was demonstrated that while cholesterol levels of subjects with coronary disease were significantly higher than those of control subjects there was no relation to the level of cholesterol in the diet. In 1970 the subjects in the Framingham Study, both men and women, were segregated by serum cholesterol level under 180 mg/dl, over 300 mg/dl, and 181–300 mg/dl. The study found no correlation of serum cholesterol with any dietary component. The Framingham Study also showed that ingestion of one or nine eggs per week had the same influence on serum cholesterol. A study conducted by the NIH over twenty years ago attempted to correlate dietary factors with actual heart attacks in three large studies—Framingham, Puerto Rico, and Hawaii. At no location were there differences in cholesterol intake between those subjects who had suffered a heart attack and those who had not.
Epidemiological studies of dietary cholesterol have to be aware of possible confounding by other dietary factors. Many foods high in cholesterol are also high in saturated fat. High-fat diets are often poor in fiber. Since saturated fat intake has been linked to coronary disease and fiber appears to be protective, both must be considered when evaluating the role of dietary cholesterol.
Data relating dietary cholesterol to heart disease are available. A study of middle–aged men in the Netherlands showed no significant relationship between cholesterol intake and coronary death after ten years. A positive relationship appeared after twenty years of follow-up but was not significant after adjustment was made for standard risk factors, occupation, and energy intake.
More pertinent to the discussion is the relationship, if any, between egg consumption and coronary disease. A study of women in Italy found no association between egg consumption and nonfatal myocardial infarction. A study among Seventh Day Adventists in California found no association between egg consumption and risk of cardiovascular disease, whereas a study of vegetarians in Oxford, England, found a significantly greater risk in those consuming six or more eggs a week than in those eating less than one egg per week.
One study, conducted at Harvard University, was aimed at investigating an explicit relationship between egg consumption and risk of cardiovascular disease. The authors addressed data derived from ongoing studies of more than eighty thousand female nurses and more than forty-three thousand male health professionals. After adjustment for age, body mass index, cigarette smoking, parental history of heart disease, vitamin intake, alcohol use, hypertension, physical activity, energy intake, bacon consumption, and in the women, menopausal status and postmenopausal hormone use, there was no association between egg consumption and coronary heart disease in either group.
The diet-heart hypothesis holds that a diet high in saturated fat and cholesterol and low in polyunsaturated fat leads to hypercholesterolemia and subsequent development of atherosclerosis. Addition of eggs to the usual diet of free living subjects does not affect cholesterolemia. Addition or deletion of eggs in the diet leads to other changes in diet that must be evaluated and corrected for. The Framingham data suggest that addition of eggs to the diet does not affect serum cholesterol levels. The egg is a source of a number of essential nutrients; that, plus its high-grade protein and low price, make the egg a desirable food, especially for the very young, old, and infirm. The presence of a high level of cholesterol in the egg has led to suggestions that it not be included in healthful diets since elevated blood cholesterol is a risk factor for cardiovascular disease. Data are accumulating which show that dietary cholesterol has a minimal effect on blood cholesterol levels. Epidemiological data also show little association between eggs and risk of cardiovascular disease. Eggs can be an important part of the diets of healthy persons.
See also Cholesterol; Combination of Proteins; Lipids; Poultry; Proteins and Amino Acids; Sea Birds and Their Eggs.
American Egg Board, The. The Incredible Edible Egg Eggcyclopedia. Park Ridge, Ill.: American Egg Board, 1999.
Ascherio, Alberto, Eric B. Rimm, Edward L. Giovanucci, Donna Spiegelman, Meir Stampfer, and Walter C. Willett. "Dietary Fat and Risk of Coronary Heart Disease in Men: Cohort Follow-up Study in the United States." British Medical Journal 313 (1996): 84–90.
Barer-Stein, Thelma. You Eat What You Are: People, Culture, and Food Traditions. 2d ed. Willowdale, Ontario: Firefly Books, 1999.
Dawber, Thomas R., Rita J. Nickerson, Frederick N. Brand, and Jeremy Pool. "Eggs, Serum Cholesterol and Coronary Heart Disease." American Journal of Clinical Nutrition 36 (1982): 617–625.
Hu, Frank B., Meir J. Stampfer, JoAnn E. Mason, Eric Rimm, Graham A. Colditz, Bernard A. Rosner, Charles H. Hennekens, and Walter C. Willett. "Dietary Fat Intake and the Risk of Coronary Heart Disease in Women." New England Journal of Medicine 337 (1997): 1491–1499.
Hu, Frank B., Meir J. Stampfer, Eric B. Rimm, JoAnn E. Manson, Alberto Ascherio, Graham A. Colditz, Bernard A. Rosner, Donna Spiegelman, Frank E. Speizer, Frank M. Sacks, Charles H. Hennekens, and Walter C. Willett. "A Prospective Study of Egg Consumption and Risk of Cardiovascular Disease in Men and Women." Journal of the American Medical Association 281 (1999): 1387–1394.
Kannel, William B., and Tavia Gordon. "Section 24: The Framingham Study: Diet and the Regulation of Serum Cholesterol." In The Framingham Study: An EpidemiologicalInvestigation of Cardiovascular Disease. Vol. 24. Bethesda, Md.: U.S. Dept. of Health, Education, and Welfare, 1970.
Kiple, Kenneth F., and Kriemhild Coneè Ornelas. The Cambridge World History of Food. Vol. 2. Cambridge, U.K.: Cambridge University Press, 2000.
Kritchevsky, Stephen B., and David Kritchevsky. "Egg Consumption and Coronary Heart Disease: An Epidemiological Overview." Journal of the American College of Nutrition 19 (2000): 549S–555S.
Kromhout, Daan, and C. deLezenne Coulander. "Diet, Prevalence and 10-year Mortality from Coronary Heart Disease in 871 Middle Aged Men: The Zutphen Study." American Journal of Epidemiology 119 (1984): 733–741.
McNamara, Donald J. "Dietary Cholesterol and Atherosclerosis." Biochimica et Biophysica Acta 1529 (2000): 310–320.
The unfertilized egg is considered an important and inexpensive food source, particularly high in protein, including 0.21 oz (6 g) of complete protein per two-ounce egg. However, it also includes 0.42 oz (12 g) fat, both saturated and unsaturated, which is nearly all located in the yolk. Therefore, if the yolk is separated from the albumen (white) of the egg and the white only used, the egg contains no fat and a fair amount of protein. The egg also contains significant amounts of iron, vitamins A and D, riboflavin, and thiamine. However, that nutritional value does vary somewhat depending on the diet of the laying hen. Annual consumption during the late 1990s averaged 245 eggs for each of the 265 million people in the United States.
While geese, squab, ducks, and turkeys supply edible eggs, the preponderance of eggs eaten come from domesticated chickens bred for laying eggs. The females (mature hens and younger pullets) are raised for meat and egg production and breeds have been developed to fulfill commercial needs. Each of the 235 million laying hens in the United States produces about 300 eggs a year. Farmers are careful to house and feed the chickens to maximize laying and ensure the hen has a relatively long and healthy life. Egg producers also have flocks of hens at different ages, ensuring they have a steady supply of eggs ready for market to provide year-round income.
Eggs are an important ingredient in many recipes. Because the protein in the egg white coagulates as it is heated, eggs are utilized in many recipes as a structural component. Stiffly-beaten egg whites expand as they are heated and are used as leavening in angel-food cakes, souffles, and meringues. In cake batters that utilize the entire egg, the egg acts as leavening as well as providing moisture and firm texture. Soups and sauces use eggs as a thickening agent. Ice creams often include eggs to prevent the formation of ice crystals which can ruin the product. But the plain egg is eaten by millions each day for its own unique flavor and nutritive value—they may be boiled, poached, fried, scrambled, or baked.
Fresh egg production is primary to the egg industry, however, a significant amount of egg production includes eggs purposely broken and used for powdered eggs, frozen eggs, or purchased by food producers for inclusion in food products. (In some fresh egg production plants, accidentally broken eggs are sold to bakeries or other food production plants.)
The egg has been a protein source for centuries. Sometime in the second millennium b.c. Indian wild red jungle fowl, the ancestor of the modern chicken (Gallus gallus) was dispersed throughout Europe, China, and the Middle East. Chickens were brought to the New World on Columbus's second voyage in the late fifteenth century. These imported chickens laid eggs year-round and were considered most valuable for their egg production rather than for meat. Soon family farms raised chickens for the family's consumption of eggs and meat within the household—few families had laying flocks of any size. However, by 1800 chickens began being raised for meat and egg production in increasing numbers in the United States. Until World War II, egg production came from rather small flocks of less than 400 laying hens. After the war, automation and advances in breeding, feeding, and developing efficient henhouses gave rise to modern high-volume chicken farms. Today, a single egg producer may well have a flock of over 100,000 laying hens—and some have flocks of over one million.
The egg itself is the ingredient in egg production. Soaps are used in egg production facilities to clean the shell. Some processors coat the shell in a light film of oil.
However, the hen from which the egg drops can be considered an important part of the raw material. Certain breeds lay the majority of eggs in the United States. A particularly prolific laying hen is the Single Comb White Leghorn. This breed reaches maturity early and can begin laying at 19 weeks of age and continue to lay eggs for about a year. In addition, the Leghorn utilizes feed efficiently, is fairly small, can adapt to a variety of climates and is able to lay a relatively large amount of white eggs, the type most in demand. Plymouth Rock hens, Rhode Island Reds and New Hampshire hens produce brown eggs much favored in New England.
Feed for chickens is generally all-mash, consisting of sorghum grains, corn, and either cottonseed meal or soybean oil depending on availability. Farmers carefully mix the mash so that the chickens get just the right amounts of protein, fat, carbohydrates, vitamins, and minerals. This is essential in that the nutritional quality of the laid egg depends on the feed the chicken was given. All additives to chicken feed must be approved by the federal government (after research on toxicity to animals and humans is determined). Hormones are not given to chickens, but they occasionally require antibiotics. On average, a chicken requires about 4 lb (1.8 kg) of feed to produce a dozen eggs; a Leghorn hen eats about 0.25 lb (0.1134 kg) of chick mash a day.
Some egg farmers have their own egg processing facility on premises. Others contract with egg processing firms in the locale who purchases eggs and processes them. Generally, an egg moves from the egg farm to being ready for public consumption in just a few days.
Laying the eggs
- 1 Hens are kept in cages, usually in groups of three to five. After the egg is laid, the cage is devised so the egg rolls out for easy collection. Eggs are gathered twice a day, generally by automated machinery but are occasionally still gathered by hand. Eggs are gathered as soon as hens lay them because warmer temperatures encourage physical and chemical changes that affect freshness adversely. Thus, many egg farmers refrigerate the eggs immediately after gathering before they are packed for transport to the processor.
Packing the eggs on the farm
- 2 The eggs are then packed on skids that are formed of layers of flats—eggs are packed on flats that include 2.5 dozen eggs, with as many as six flats per layer (layers of six flats are separated by a board). A single skid holds about 30 cases or 900 dozen eggs. These eggs are then sent to the processing plant via truck.
Washing the eggs
- 3 The skids are brought into the production room and the individual flats are put on the conveyor, one at a time. Individual eggs are grasped by small suction cups and placed onto another conveyor belt. Now, the eggs are moved into the grader where they are cleaned with a USDA approved cleanser. They are rotated as brushes and water jets move carefully across the eggs. A fan then dries the eggs.
Candling and grading
- 4 The cleaned eggs are graded in a candling booth which is a dark cubical or room. A penetrating light is shined on the eggs in order to grade them. The egg processor is able to grade the egg during candling. The trained candler can see that an older egg has thinner albumen; thus, the yolk casts a sharp shadow and immediately indicates an older egg. Eggs are graded as "A" (sold for household use or at retail markets), grade "B" (used mostly for bakery operations), or grade "C" (sent to egg breakers who break the shell in order to convert it to other egg products); higher grade eggs have a thick, upstanding albumen, an oval yolk, and a clean, smooth, unbroken shell. Eggs with cracks that are not leaking are removed from the process at this point and are not packaged for household use or retail sale.
Weighing and packing the eggs
- 5 The grading process actually includes the weighing of the eggs as well. At the grading station, a machine weighs each individual egg and remembers what each egg weighs. In the United States, eggs are sized (extra large, large, medium, small) on the basis of a minimum weight rather than by size of egg. Extra large eggs must weigh a minimum of 2.24 oz (64 g), large at least 1.96 oz (56 g), medium at least 1.72 oz (49 g), and small at least 1.47 oz (42 g). The packing machine then assembles cartons based on the weight of individual eggs; thus, the heaviest eggs are "found" and packed into the extra large cartons, the next heaviest tier are packed in to the large-size egg carton, etc. Packaging varies, but is generally either recycled cardboard or a colored polystyrene the egg producers purchase from a packaging manufacturer.
Transportation to trucks
- 6 After packing, the cartons are placed on a conveyor and packed into flats by machine, put into trucks (generally refrigerated) and sent to be sold. One large Pennsylvania egg processing plant processes 45,000 dozen eggs per day.
Quality control happens in all parts of the egg processing. First and foremost, the chicken farmer ensures that his hens are well fed with a diet specifically formulated to provide the best grade of egg. Shell strength, for example is determined by the presence of adequate amounts of vitamin D, calcium, and other minerals in the chick mash. Too little vitamin A can result in blood spots (not harmful to the consumer but render an egg undesirable and unusable to the consumer). Laying hens also require a good supply of clean fresh water. The henhouse is well insulated so that the farmer can control the temperature. Facilities are windowless so that light can be manipulated—egg production is spurred by maintaining a lighted environment 14-17 hours a day. The henhouse should be comfortable for the hens and well-ventilated. Birds are generally kept in cages because they are easier to clean and it is easier to collect eggs from cages; however, some hens are allowed to roam free.
Effective candling is essential to quality control as well. Candling reveals nearly everything that is need to know about the quality of the eggs—age, cracks, clarity (no blood spots). Furthermore, most egg processors can tell much about the quality of the egg just by looking at the shape and color of the shell.
Salmonella is a hazard of the egg industry. However, it is estimated that 90% of eggs are free from salmonella at the time they are laid. Salmonella bacteria occurs after laying. Proper washing and sanitizing of eggs with a government-approved soap eliminates most Salmonella and spoilage organisms that are deposited on the shell from the hen ovaries.
Egg farmers are also careful to refrigerate the eggs as soon as they are gathered just prior to packing. The egg processors, too, move eggs quickly through the processing to packaging to ensure the eggs are clean and fresh for the consumer.
Furthermore, government standards for the grade and size of eggs are strictly adhered to. Flocks are periodically monitored for proper feeding as well as acceptable facility standards. Extension services provide educational materials and new information about raising laying hens to farmers as needed.
Eggs with cracks are removed from the egg processing line. Broken eggs are thrown into a bin and sold for utilization in dog food. Eggs that have cracks but are not leaking are taken away for pasteurization and transformation into liquid egg products (sold in cartons of plastic containers or perhaps even frozen). They may be sold to another processor who transforms them into powdered eggs, or they may be sold to local bakeries for use in goods there. Many egg processors are aware of the ecological problems created by the polystyrene cartons and egg processor encourage recycling of the product packaging.
Where to Learn More
Wexler, Mark. "Eggsquisite!" World Magazine (March 1989): 70.
American Egg Board. http://www.aeb.org/ (June 29,1999).
Manitoba Egg Producers. http://www.mbegg.mb.ca/ (June 29, 1999).
One of the few references to the egg in the Bible, and the only injunction connected with it, is the command to drive away the dam before taking the eggs from the nest (Deut. 22:6). The only other references to birds' eggs are in Isaiah 10:14 and the hatching of the egg of the ostrich through the heat of the sun (Job 39:14). Viper's eggs are mentioned in Isaiah 59:5. In contrast, the egg figures prominently in rabbinical literature, both in halakhah and aggadah.
The egg belongs to two spheres of halakhah: as permitted food and as a standard measure of volume (the tractate of the Talmud called Beẓah ("egg") deals with the laws of the festivals and is so called merely because of the first word of its first Mishnah, which deals with the question of the permissibility of eating an egg laid on the festival).
(1) Although it is nowhere clearly stated in the Bible that eggs are permitted for food (the Talmud sees a reference to it in Deut. 22:6; see Ḥul. 140a), on the principle that "that which emerges from a clean animal is clean and that from an unclean animal unclean" (Bek. 1:2) it is established that the eggs of clean birds are permitted for food, and those of unclean birds, forbidden (Ḥul. 122a). With the formation of the hard shell of the egg, however, even before the egg has been laid, it is regarded as independent and no longer part of its dam, with the result that the prohibition of eating a part of a living animal does not apply to it, nor the law prohibiting the eating of meat with milk (Beẓah 6b). Nevertheless fully formed eggs in a bird which is *terefah or *nevelah are forbidden (Maim., Yad, Maakhalot Asurot, 3:19). The Talmud (Ḥul. 64a) gives the signs of the eggs of permitted and forbidden birds. The former have one end oblate and the other pointed and the white surrounds the yoke, while if both ends are oblate or pointed and the yellow surrounds the white it is the egg of a forbidden bird. The egg is regarded as beginning to form the embryo when a bloodspot appears on the yoke, from which time it is forbidden as food, but the custom has been generally adopted of forbidding eggs if the bloodspot appears even on the albumen (see *Blood).
(2) The bulk of an egg is one of the most common of all the measures of volume in the halakhah. It constitutes the usual quantity of volume to establish liability, e.g., for ritual uncleanness, for the size of the *etrog, for the amount of bread from which *ḥallah must be separated, and many others. It is also the standard whereby all other measurements are calculated, a log being equal to six eggs, a kab to 24, and a se'ah to 144 (see Er. 83a and *Weights and Measures). It is evident however that these relative measurements do not accord with the normal size of an egg. It is accepted that the "egg" is that of the chicken (Yoma 80a) and recourse has had to be made to the theory that the egg thus referred to is a "desert egg" which was much larger than the present day one, and to be on the safe side the standard adopted in the halakhah for the egg of the Talmud is two present-day eggs (Ḥatam Sofer, oḤ, Tesp. no. 127).
The egg is regarded both as having laxative qualities and of bringing about sexual stimulation. The egg, being "round and having no mouth" (opening), is regarded as a symbol of mourning which "is like a wheel which continually revolves in the world, and one must not open one's mouth in complaint" (bb 16b; yd 378:9 of Gen R. 63:14). It is therefore given to mourners at the meal given to them on the return from the burial (se'udat havra'ah) and is eaten at the meal before undertaking the fast of the Ninth of *Av. On the seder night of Passover there has developed the custom of eating an egg dipped in salt water before beginning the festive meal. There is no authority for this custom in the sources; various explanations have been put forward, and Moses Isserles connects it with its mourning aspect. According to him it is in commemoration of the destruction of the Temple with which the paschal sacrifice was discontinued, and it happens that the first day of Passover always falls on the same day of the week as the Ninth of Av of each year (oḤ 476:2). A roasted egg, in memory of the festival offering (ḥagigah), forms part of the Passover plate at the seder.
Krauss, Tal Arch, 1 (1910), 124–6; Eisenstein, Yisrael, 3 (1951), 37–40; et, 3 (1951), 131–45.
[Louis Isaac Rabinowitz]
kill the goose that lays the golden eggs destroy a reliable and valuable source of income; the allusion is to one of Aesop's fables, in which a man killed the goose which laid a single golden egg each day in the belief that he would find a number of eggs inside it, and instead through greed lost his source of wealth. (See also golden goose.)
See also egg, you cannot make an omelette without breaking eggs, there is reason in the roasting of eggs, don't teach your grandmother to suck eggs.