Ch.2: Eggs

Chapter 2 of the book "On Food and Cooking: The Science and Lore of the Kitchen" by Harold McGee (1984) The Chicken and the Egg Egg Biology and Chemistry Egg Quality, Handling, and Safety The Chemistry of Egg Cooking Basic Egg Dishes Egg-Liquid Mixtures: Custards and Creams Egg Foams Pickled and Preserved Eggs

245 flashcards · Other · by lakesare

Part of Public › Books › On Food and Cooking (Harold McGee) · 16 courses

1. [Eggs] Where does the yellow color of egg yolk come from?

   From yellow plant pigments (xanthophylls) in the hen's feed, mainly alfalfa and corn. It is not beta-carotene, the orange pigment of carrots. Producers deepen the color by adding marigold petals to the feed. In the plant, xanthophylls shield the photosynthesis machinery from too much sunlight.McGee: "An egg is the sun's light refracted into life."

2. [Eggs] Why can a cook turn eggs into so many different textures, from a meringue to a dense custard?

   Raw egg is mostly water with loose proteins, which the cook can bond into many kinds of networks. Example: beaten whites become a foam; yolks hold oil and water together in mayonnaise; eggs diluted in milk set into a custard.

3. [Eggs] Besides being cooked on their own, which jobs do eggs do in other dishes?

   Emulsify They hold oil and water together in smooth sauces. Example: mayonnaise, hollandaise. Build structure Their proteins set into foams and gels. Example: meringue, custard. Refine texture Example: smoother candies and ice creams. Glaze Example: a shine on pastries. Clarify Example: clearing meat stocks and wines.

4. [The Chicken and the Egg] Biologically, what makes a reproductive cell an egg rather than a sperm?

   The egg is the larger, less mobile cell, and it carries food for the embryo. It also receives the sperm. So eggs, like milk and plant seeds, exist to feed a new organism, which is why they are so nutritious.

5. [The Chicken and the Egg] Which step in evolution made separate egg and sperm cells necessary?

   The rise of many-celled organisms, about a billion years ago. The earliest sexual organisms (probably single-celled algae) simply paired up and swapped DNA; a many-celled body cannot do that.McGee: "Ultimately, we owe our soufflés and sunny-sides-up to the invention of sex."

6. [The Chicken and the Egg] What did the reptile egg (about 300 million years ago) have that eggs laid in the sea lacked?

   A leathery skin against drying out, and enough food for the embryo to develop fully on land.

7. [The Chicken and the Egg] What did the bird egg add to the older reptile egg?

   A hard mineral shell and chemical defenses against microbes. So bird embryos can develop in very dry places, and for the cook bird eggs keep for weeks with little care.

8. [The Chicken and the Egg] Which wild birds are the ancestors of the chicken?

   Jungle fowl of tropical and subtropical Southeast Asia and India. The chicken species, Gallus gallus, is only 3 to 4 million years old; eggs existed nearly a billion years before the first birds. Chickens were domesticated in Southeast Asia before 7500 BCE.

9. [The Chicken and the Egg] What is an indeterminate layer?

   A bird that keeps laying until a certain number of eggs is in its nest, so it replaces every egg that is taken. The chicken is one. Wild jungle fowl lay clutches of about 12 eggs a few times a year; an industrial hen, whose eggs are removed daily, lays about one egg a day for a year or more.

10. [The Chicken and the Egg] The word "yolk" comes from the Old English word for yellow.

    It goes back to an Indo-European root meaning "to gleam", which also gave glow and gold. "Egg" comes from a root meaning "bird".

11. [The Chicken and the Egg] What was the 19th-century "hen fever"?

    A chicken-breeding craze in the West (about 1850 to 1900), set off by showy Chinese breeds; hundreds of new breeds were developed. The chicken changed more in those 50 years than in its whole earlier history as a species. Most of that diversity has since disappeared.

12. [The Chicken and the Egg] How many eggs does a typical industrial laying hen produce?

    250 to 290 eggs, in about a year of laying. She is hatched in an incubator and lives on wire under artificial light.

13. [The Chicken and the Egg] How much feed does it take to produce a pound of eggs in industrial farming?

    Less than three pounds of feed. A pound of broiler chicken takes less than two pounds. Both are therefore cheap among animal foods.

14. [The Chicken and the Egg] What are the drawbacks of industrial egg production?

    Possibly less flavor, more spread of salmonella, and poor living conditions for the hens. Salmonella spreads partly because "spent" hens are processed into feed for the next generation of layers. The flavor loss from soy and fish-meal feeds has been hard to show in taste tests.

15. [The Chicken and the Egg] What does an egg label saying "free-range" guarantee?

    Not much: it can mean only a slightly larger cage or brief access to the outdoors. Switzerland is an exception: its law requires that all hens have free access to the outdoors.

16. [Egg Biology and Chemistry] How much of her body weight does a laying hen turn into eggs in a year?

    About 8 times her body weight (each egg is about 3% of it). That is about 100 times a human's reproductive effort per day. A quarter of the hen's daily energy goes into egg-making (a duck's: half).

17. [Egg Biology and Chemistry] How long does a yolk take to mature in the hen's ovary?

    About ten weeks. The yellow yolk, mostly fats and proteins made in the hen's liver, is added only in the last week. A hen that feeds once or twice a day makes yolks with visible dark and light layers.

18. [Egg Biology and Chemistry] Once the ovary releases a yolk, how long does the hen take to build the white, membranes and shell around it?

    About 25 hours, most of it (about 14 hours) for the shell. 2 to 3 hours: about half the white is added in four alternately thick and thin layers. 1 hour: the two shell membranes. 19 to 20 hours in the uterus (shell gland): 5 hours pumping in water and salts, then about 14 hours depositing calcium carbonate.

19. [Egg Biology and Chemistry] What are the chalazae of an egg?

    Two twisted cords of white that anchor the yolk to both ends of the shell. They hold the yolk in the middle while letting it rotate, so as much cushioning white as possible lies between the embryo and the shell. Shell, two membranes, air cell at the blunt end, thick and thin white, chalazae, yolk and germ cell.

20. [Egg Biology and Chemistry] How does air reach the chick embryo through the eggshell?

    Through about 10,000 pores, mostly at the blunt end, together equal to a hole about 2 mm across. At first they are plugged by the cuticle, a thin protein coating that slows water loss and keeps out bacteria; the cuticle gradually cracks so the chick gets enough oxygen.

21. [Egg Biology and Chemistry] What determines the color of an eggshell?

    The hen's breed (her genes). The pigments are chemical relatives of hemoglobin. Example: Leghorns lay white eggs, Rhode Island Reds brown, the Chilean Araucana blue.

22. [Egg Biology and Chemistry] Do brown eggs differ from white eggs in taste or nutrition?

    No; shell color has nothing to do with taste or nutrition. Color depends only on the hen's breed.

23. [Egg Biology and Chemistry] How does the air cell at the blunt end of an egg form?

    The egg cools after laying, its contents shrink, and the inner shell membrane pulls away from the outer one. The hen's body is at 41°C (106°F). The two membranes are attached everywhere except at the blunt end.

24. [Egg Biology and Chemistry] In a cracked-open raw egg, why does the small white spot of the germ cell (the hen's reproductive cell) usually lie on top of the yolk?

    The yolk under the germ cell is lighter, so that side floats up. Under the germ cell runs a channel of "white yolk", which is less dense than yellow yolk. It is the small soft spot that stays in the middle of a hard-cooked yolk, and it is rich in iron.

25. [Egg Biology and Chemistry] What share of an egg's weight and of its calories does the yolk hold?

    Just over a third of the weight, but three-quarters of the calories. It also holds all of the fat and cholesterol, and most of the egg's iron, thiamin and vitamin A.

26. [Egg Biology and Chemistry] Why is a duck's yolk more orange than a hen's?

    It contains beta-carotene and the reddish pigment canthaxanthin, while a hen's yolk is colored by yellow xanthophylls. Wild ducks get canthaxanthin from small water insects and crustaceans; farmed ducks get it from feed supplements.

27. [Egg Biology and Chemistry] Which enzyme in egg yolk can make a set, starch-thickened pie filling turn runny?

    Amylase, an enzyme that digests starch.

28. [Egg Biology and Chemistry] Why does a yolk cooked intact turn crumbly?

    The yolk is packed with tiny spheres, and heat sets each sphere into a separate grain. The spheres are about 0.1 mm across, each in its own membrane. A yolk broken before cooking lets the spheres move apart, so it cooks less granular.

29. [Egg Biology and Chemistry] Why does a raw yolk become clearer and thicker when you add a pinch of salt?

    Salt breaks the yolk's light-scattering granules into pieces too small to scatter light. The granules are too small to see or to break by beating. Example: salt added to yolks when starting mayonnaise. A yolk granule falling apart in a salt solution (electron microscope).

30. [Egg Biology and Chemistry] What gives egg yolk its power to emulsify (hold oil and water together)?

    Tiny particles with a fat core coated in protein, cholesterol and lecithin (lipoproteins). Most are low-density lipoproteins (LDLs), essentially the same particles measured in human blood cholesterol tests.

31. [Egg Biology and Chemistry] What is the egg white made of?

    Nearly 90% water; almost all the rest is protein. Traces of minerals, fats, vitamins and glucose. Riboflavin gives raw white a slight yellow-green tint. The white is nearly two-thirds of the shelled egg's weight.

32. [Egg Biology and Chemistry] Egg white contains only about a quarter gram of glucose. When does that glucose matter?

    In very long or alkaline cooking it browns the white by reacting with its proteins. Example: Middle Eastern hamindas (eggs cooked 6 to 18 hours) and Chinese "thousand-year" eggs have tan to brown whites. The glucose is too little to taste sweet.

33. [Egg Biology and Chemistry] Besides feeding the embryo, what is the main natural role of the egg-white proteins?

    Chemical defense against microbes and predators. At least four proteins block digestive enzymes, at least three bind vitamins, one binds iron, one inhibits viruses, one digests bacterial cell walls.

34. [Egg Biology and Chemistry] What do the egg-white proteins ovotransferrin, lysozyme and avidin do in the egg?

    Ovotransferrin Binds iron, so bacteria cannot use it (12% of white protein). Lysozyme An enzyme that digests bacterial cell walls (3.5%). Avidin Binds the vitamin biotin (0.06%).

35. [Egg Biology and Chemistry] What does the protein ovomucin do in the egg white?

    It makes the thick white thick, though it is under 2% of the white's protein. The thick white is about 40 times more viscous than the thin white. Ovomucin makes poached and fried eggs compact; it breaks down as the raw egg ages, so the white thins.

36. [Egg Biology and Chemistry] What is chemically special about ovalbumin, the main egg-white protein (54% of it)?

    It is the only egg protein with reactive sulfur groups. These sulfur groups shape the flavor, texture and color of cooked eggs. Ovalbumin was the first protein ever crystallized in a laboratory (1890).

37. [Egg Biology and Chemistry] Why do very fresh eggs need slightly less cooking than eggs a few days old to reach the same consistency?

    Ovalbumin, the main white protein, becomes more heat-resistant in the first days after laying.

38. [Egg Biology and Chemistry] Why does a whole egg set at a higher temperature than egg white alone?

    Ovotransferrin, the first white protein to set, becomes more heat-stable when it binds the yolk's iron.

39. [Egg Biology and Chemistry] Why do egg whites beaten in a copper bowl turn golden?

    The white protein ovotransferrin binds copper, and the protein-copper pair is colored. Example: a pinch of ground iron supplement in the whites gives a pink meringue.

40. [Egg Biology and Chemistry] Nutritionally, why are cooked eggs better than raw ones?

    Cooking disables the white's anti-nutritional proteins (enzyme blockers, vitamin and iron binders). Laboratory animals fed raw egg lose weight. Cooked eggs have a uniquely balanced set of amino acids, the essential fatty acid linoleic acid, and the antioxidant pigments lutein and zeaxanthin.

41. [Egg Biology and Chemistry] How much cholesterol does a large egg contain?

    About 215 mg, compared with about 50 mg in an equal portion of meat. A feed rich in sitosterol (a plant relative of cholesterol) lowers egg cholesterol by a third, still far above most foods.

42. [Egg Biology and Chemistry] Why does an egg contain so much cholesterol?

    Cholesterol is part of every animal cell membrane, and the embryo must build many millions of cells.

43. [Egg Biology and Chemistry] Why does eating eggs in moderation have little effect on blood cholesterol?

    Saturated fat raises blood cholesterol far more than eaten cholesterol, and yolk fat is mostly unsaturated. Yolk phospholipids also reduce the absorption of yolk cholesterol.Still, over 60% of a whole egg's calories come from fat.

44. [Egg Biology and Chemistry] Is a fertilized egg more nutritious than an unfertilized one?

    No, no detectable difference. By the time the egg is laid, the germ cell has become tens of thousands of cells, yet its diameter has only grown from 3.5 to 4.5 mm; refrigeration stops further development.

45. [Egg Biology and Chemistry] Which egg protein usually causes egg allergy?

    Ovalbumin, the main protein of the white. Yolks are far less allergenic.

46. [Egg Biology and Chemistry] What do pediatricians commonly advise about eggs for babies?

    No egg whites before age one; yolks are fine for nearly all infants. Allergy to egg white often develops early in life.

47. [Egg Biology and Chemistry] What are commercial cholesterol-free egg substitutes made of?

    Real egg whites mixed with an imitation yolk. The imitation yolk: vegetable oil, milk solids, gums for thickness, coloring, flavoring, vitamins and minerals.

48. [Egg Quality, Handling, and Safety] What makes an egg high-quality for the cook?

    A strong shell, a firm yolk with a strong membrane, and plenty of thick white. A hen near the end of her laying year produces weaker shells and whites.

49. [Egg Quality, Handling, and Safety] Checking an egg's interior by holding it in front of a bright light is called candling.

    It shows cracks, blood spots, "meat spots" in the white and large air cells. Twirling the egg: if the yolk's shadow stays indistinct, the yolk membrane and thick white are strong.

50. [Egg Quality, Handling, and Safety] What does a USDA egg grade (AA, A, B) tell you?

    The egg's quality when it was packed at the farm; not its freshness or size. Quality falls with age, so a fresh grade A egg can beat an old grade AA. Check the sell-by date (usually four weeks after packing). AA, A and B eggs in a pan: lower grades have less thick white and spread more.

51. [Egg Quality, Handling, and Safety] When is a grade AA egg worth paying more for than a grade A egg?

    When shape or foaming matters: poached and fried eggs, meringues, soufflés. AA has more thick white and a stronger yolk membrane, which leaks less yolk into the white (yolk reduces foaming). For scrambled eggs or custard used soon, A is fine.

52. [Egg Quality, Handling, and Safety] Why do both white and yolk become more alkaline as an egg ages?

    Carbon dioxide (dissolved as carbonic acid) escapes as gas through the shell's pores. White: pH 7.7 to about 9.2 or higher. Yolk: pH 6.0 to 6.6.

53. [Egg Quality, Handling, and Safety] Why is the white of a fresh egg cloudy, but that of an old egg clear?

    Fresh white has protein clusters that scatter light; in old, more alkaline white the proteins stay apart.

54. [Egg Quality, Handling, and Safety] How does the ratio of thick to thin white change as an egg ages?

    From about 60 : 40 to below 50 : 50. Older eggs therefore spread more in the pan.

55. [Egg Quality, Handling, and Safety] Why does an old egg's yolk become flat and fragile?

    Water moves from the white into the yolk by osmosis, stretching the yolk's membrane. The yolk has more dissolved molecules than the white, which draws the water in: about 5 mg a day at refrigerator temperature. The added water also thins the yolk.

56. [Egg Quality, Handling, and Safety] How can you test an egg's freshness in a bowl of water?

    A fresh egg sinks; the older the egg, the higher its blunt end rises; an egg that floats is very old. The egg loses water through the shell (about 4 mg a day even when oiled), so the air cell grows and the egg becomes less dense. A new egg's air cell is under 3 mm deep.

57. [Egg Quality, Handling, and Safety] What is the only culinary advantage of an older egg?

    It is easier to peel when hard-cooked. The aging changes probably help the chick: a more alkaline white resists microbes, and a thinner white lets the yolk and embryo rise toward the shell's oxygen and calcium.

58. [Egg Quality, Handling, and Safety] Why are U.S. eggs washed before sale?

    The shell picks up thousands of bacteria on its way out of the hen; washing removes them. The egg passes through the cloaca, the hen's shared opening for eggs and droppings. Eggs are washed in warm water and detergent.

59. [Egg Quality, Handling, and Safety] How much faster does an egg deteriorate at room temperature than in the refrigerator?

    One day at room temperature equals about four days refrigerated. Salmonella also multiplies much faster when warm, so buy eggs from the cooler and keep them cold.

60. [Egg Quality, Handling, and Safety] Where in the refrigerator should eggs be kept?

    On an inner shelf, not in the door. Shaking from the door opening and closing thins the white.

61. [Egg Quality, Handling, and Safety] What kind of container is best for eggs in the refrigerator?

    An airtight container. It slows moisture loss and keeps out the odors of other foods better than a loose carton (but it concentrates the eggs' own stale flavor).

62. [Egg Quality, Handling, and Safety] Which storage position gives better-centered yolks in hard-cooked eggs?

    Lying on their sides. Probably because both chalazae (yolk cords) then pull equally against gravity. Position does not affect the quality of the white.

63. [Egg Quality, Handling, and Safety] Why do egg yolks frozen on their own thaw into a thick paste?

    Freezing makes the yolk proteins clump together. The paste no longer mixes easily with other ingredients. Whites freeze fairly well, losing only a little foaming power.

64. [Egg Quality, Handling, and Safety] What can you mix into egg yolks before freezing so they stay fluid after thawing?

    Salt, sugar or acid. Per half liter of yolks: 5 g salt, or 15 g sugar, or 60 ml lemon juice; whole eggs need half these amounts.

65. [Egg Quality, Handling, and Safety] Why is the bacterium Salmonella enteritidis (a food-poisoning cause recognized around 1985) so alarming for egg eaters?

    It can be inside intact, clean, high-grade eggs. In the early 1990s about 1 egg in 10,000 was estimated to carry it in the U.S.; the share is now much lower, but not zero.

66. [Egg Quality, Handling, and Safety] What heating makes an egg dish safe from salmonella?

    At least 60°C (140°F) for 5 minutes, or 70°C (160°F) for 1 minute. Yolks stay runny at 60°C but set at 70°C.

67. [Egg Quality, Handling, and Safety] How are eggs pasteurized?

    By careful heating to 55 to 60°C (130 to 140°F), just below where egg proteins start to set. Shell eggs, liquid eggs and dried whites can all be bought pasteurized.

68. [Egg Quality, Handling, and Safety] What do eggs lose when they are pasteurized?

    Some foaming and emulsifying power, some heat stability, and some fresh flavor. For most uses they still replace fresh eggs adequately.

69. [The Chemistry of Egg Cooking] Why is a raw egg a liquid?

    Its proteins stay folded and apart, floating in plenty of water. There are about 1,000 water molecules per protein molecule. In the white most proteins carry a negative charge and repel each other; in the yolk some repel and some are bound up in fat-protein particles.

70. [The Chemistry of Egg Cooking] What happens to egg proteins when heat turns the liquid egg into a moist solid?

    The proteins unfold and bond into a network that traps the water in small pockets. Folded proteins (left), unfolded by heat (center), bonded into a network (right).

71. [The Chemistry of Egg Cooking] Why does egg white turn from transparent to opaque when cooked?

    The bonded proteins cluster densely enough to deflect light.

72. [The Chemistry of Egg Cooking] Besides heat, which treatments make egg proteins bond and set?

    Acid, salt, and beating into a foam. All of them reduce the proteins' tendency to stay apart. Combining treatments (e.g. acid and heat) gives textures from tough to delicate, opaque to clear.

73. [The Chemistry of Egg Cooking] Why do overcooked eggs turn rubbery, and egg-milk mixtures curdle?

    The proteins bond too tightly and squeeze water out of their network. Plain eggs lose that water as steam and get rubbery; mixtures separate into hard protein lumps and liquid.

74. [The Chemistry of Egg Cooking] At what temperatures does egg white thicken and set?

    Thickens at 63°C (145°F), sets tender at 65°C (150°F), firms at about 80°C (180°F). The first setting is due to ovotransferrin, the most heat-sensitive white protein (only 12% of the total); the firming at 80°C is due to ovalbumin, the main white protein.

75. [The Chemistry of Egg Cooking] At what temperatures does egg yolk thicken and set?

    It thickens at 65°C (150°F) and sets at 70°C (158°F).

76. [The Chemistry of Egg Cooking] At about what temperature does whole beaten egg (yolk and white mixed) set?

    Around 73°C (165°F). Like the white and the yolk on their own, well below boiling, which is why gentle heat gives tender eggs.

77. [The Chemistry of Egg Cooking] Why do the yolk cords (chalazae) stay liquid in scrambled eggs after everything else has set?

    They are rich in ovomucin, the most heat-resistant egg-white protein.

78. [The Chemistry of Egg Cooking] Why do milk and sugar raise the temperature at which an egg mixture thickens?

    They dilute the proteins, so the proteins must be hotter before they meet and bond. Hotter proteins move faster and meet more often. Example: 1 egg + 1 cup milk + 1 tablespoon sugar begins to thicken at about 78 to 80°C (175 to 180°F) instead of about 70°C.

79. [The Chemistry of Egg Cooking] Why is a custard much more delicate than plain cooked egg?

    The egg proteins must hold about six times more liquid, so their network is sparse and fragile. In a custard, the proteins of one egg surround 18 to 20 tablespoons of liquid instead of 3. Overheating therefore easily breaks the network. Egg proteins alone (left), diluted by milk (center), set into a sparse network (right).In eggnog or advocaat the proteins are so dilute that they cannot set at all and only add body.

80. [The Chemistry of Egg Cooking] Do acid and salt toughen eggs?

    No: they make egg proteins set at a lower temperature, but more tenderly. Example: Moroccan cooks beat eggs with lemon juice before long cooking so they don't become leathery.

81. [The Chemistry of Egg Cooking] Why do acid and salt make egg proteins set earlier but more tenderly?

    They let the proteins bond early, while still mostly folded, so they cannot tangle tightly. Normally the proteins' negative charges keep them apart. Acid lowers pH (less charge); salt ions cluster around charged parts of the proteins. Acid also suppresses the sulfur bonding that yolk and some white proteins use to set.

82. [The Chemistry of Egg Cooking] Which affects egg flavor more: age and storage, or the hen's diet and freedom to range?

    Age and storage: aroma is weakest right after laying and grows stronger with storage. In fresh eggs the white gives the sulfury note, the yolk a sweet, buttery one.

83. [The Chemistry of Egg Cooking] Why can brown eggs taste fishy?

    Brown-egg hens turn choline from some feeds into fishy trimethylamine, which passes into the egg. These breeds cannot break down the choline in rapeseed and soy feed; their gut bacteria turn it into trimethylamine.

84. [The Chemistry of Egg Cooking] Which compound gives cooked eggs their characteristic eggy smell?

    Hydrogen sulfide ( H2S\mathrm{H_2S}H2​S ). It forms mainly in the white above 60°C (140°F), when unfolding proteins release sulfur. It is one of 100 to 200 aroma compounds identified in cooked eggs; small amounts of ammonia also form.

85. [The Chemistry of Egg Cooking] What makes a cooked egg smell more sulfury?

    Longer time above 60°C (140°F), an older egg, and a higher pH. All three increase the hydrogen sulfide released from the white's proteins.

86. [The Chemistry of Egg Cooking] What makes a cooked egg smell less sulfury?

    Lemon juice or vinegar, and storing the egg after cooking. Acid reduces hydrogen sulfide production; hydrogen sulfide is volatile, so it escapes from cooked eggs over time.

87. [Basic Egg Dishes] Why is boiling a poor way to cook eggs in the shell?

    Bubbling cracks the shells, and the outer white overcooks before the yolk is done. Boiling water (100°C) is far above the temperatures at which egg proteins set.

88. [Basic Egg Dishes] At what water temperature should hard-cooked eggs be cooked?

    A simmer without bubbles, 80 to 85°C (180 to 190°F). Soft-cooked eggs, cooked briefly, can use barely bubbling water just short of the boil.

89. [Basic Egg Dishes] How can you tell whether an intact egg is raw or cooked without breaking it?

    Spin it on its side: a cooked egg spins fast and smoothly, a raw one wobbles. The liquid inside a raw egg moves separately from the shell and resists the spin.

90. [Basic Egg Dishes] How long do eggs cook in the shell for each degree of doneness, from runny to firm?

    Oeuf à la coque 2 to 3 minutes; semi-liquid throughout. Coddled ("soft-boiled") 3 to 5 minutes; barely firm outer white, milky inner white, warm yolk; eaten with a spoon from the shell. Mollet 5 to 6 minutes; semi-liquid yolk, but white firm enough to peel and serve whole. Hard-cooked 10 to 15 minutes; firm throughout. At 10 minutes the yolk is dark yellow, moist and pasty; at 15, light yellow, dry and granular.

91. [Basic Egg Dishes] Why are very fresh eggs hard to peel after hard-cooking?

    A fresh egg's white is less alkaline, and at that pH it sticks to the shell membrane. At pH about 9.2 (several days in the refrigerator) eggs peel easily. For fresh eggs: half a teaspoon of baking soda per quart of cooking water (more sulfury flavor). Poking a pinhole in the shell makes little difference.

92. [Basic Egg Dishes] What is the green-gray layer sometimes found on the surface of a hard-cooked yolk?

    Ferrous sulfide, formed where sulfur from the white meets iron from the yolk; harmless. Heat unfolds the white's proteins and, in alkaline conditions, frees their sulfur.

93. [Basic Egg Dishes] How do you keep hard-cooked yolks from turning green?

    Use fresh eggs, cook them as briefly as possible, and cool them quickly. Older eggs have a more alkaline white, which frees sulfur faster; high temperatures and long cooking make more ferrous sulfide.

94. [Basic Egg Dishes] Why do older eggs, hard-cooked, tend to have off-center yolks?

    As the egg ages, the white loses water and becomes denser, so the yolk rises. Fresh, high-grade eggs (small air cell, much thick white) give the best-centered yolks; storing eggs on their sides helps somewhat.

95. [Basic Egg Dishes] Eggs en cocotte sit in small dishes in simmering water, shielded from direct heat. Why do they still cook as fast as baked eggs?

    Water transfers heat faster than oven air.

96. [Basic Egg Dishes] How do you get a neat, compact poached egg?

    Use a very fresh egg, water just below the boil, and drain off the thin white first. Thin white spreads before it sets; boiling turbulence spreads it more. A perforated spoon drains the thin white. Salt and vinegar in the water speed setting but cause shreds and a ragged film.

97. [Basic Egg Dishes] In a restaurant trick, poached eggs sink in boiling water with vinegar and salt, and float up when done. Why?

    Vinegar makes CO2\mathrm{CO_2}CO2​ bubbles that stick to the egg; salt makes the water dense enough to float it. The vinegar reacts with bicarbonate in the thin white.About half a tablespoon of vinegar and 1 tablespoon of salt per quart (8 g and 15 g per liter), at the boil; the egg rises after about three minutes.

98. [Basic Egg Dishes] How do you keep the runny yolk of a poached egg while killing any salmonella?

    Hold the finished egg in water at 65°C (150°F) for 15 minutes, not letting it fall below 63°C (145°F).

99. [Basic Egg Dishes] Why does a fried egg spread more than a poached egg?

    It is heated only from below, so its white sets more slowly. Fresh, high-grade eggs and draining off the thin white help.

100. [Basic Egg Dishes] What pan temperature gives a pale, tender fried egg?

     About 120°C (250°F): butter has stopped sizzling but has not browned. A hotter pan gives a browned, crisp, more flavorful but less tender egg.