Ch.14: Cooking Methods and Utensil Materials

Chapter 14 of the book "On Food and Cooking: The Science and Lore of the Kitchen" by Harold McGee (1984) Browning Reactions and Flavor Forms of Heat Transfer Basic Methods of Heating Foods Utensil Materials

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

1. [Cooking Methods and Utensil Materials] Which chemical transformation do foods undergo under strong enough heat, whatever the cooking method?

   The browning reactions (caramelization and the Maillard reactions). They shape the color and flavor of foods as different as condensed milk, grilled meat, chocolate and beer.

2. [Browning Reactions and Flavor] How do the flavors made by the browning reactions differ from the flavor changes caused by moderate heat?

   Browning creates new flavors; moderate heat only changes the flavors already there. Browning flavors are characteristic of cooking itself. Despite the name, the colors produced range from yellow through red to black.

3. [Browning Reactions and Flavor] What is caramelization?

   The browning of sugar heated on its own, with no other ingredient taking part. Example: table sugar (sucrose) first melts to a thick syrup, then turns light yellow and gradually dark brown.

4. [Browning Reactions and Flavor] How does the taste of table sugar change as it caramelizes?

   Plain sweet, odorless sugar becomes sour, a little bitter and strongly aromatic. Hundreds of new molecules form: sour organic acids, sweet and bitter derivatives, volatile aroma molecules and brown polymers.

5. [Browning Reactions and Flavor] The Maillard reactions start with the reaction of which two kinds of molecule?

   A carbohydrate (free sugar, or sugar in starch) and an amino acid (free, or in a protein). They first form an unstable intermediate, which then changes further into hundreds of products.Named after the French physician Louis Camille Maillard, who described them around 1910.Example: bread crust, roasted coffee beans, chocolate, dark beer and roasted meat owe their color and flavor to Maillard reactions.

6. [Browning Reactions and Flavor] In the Maillard reactions, which sugars are more reactive than table sugar (sucrose)?

   Glucose and fructose.

7. [Browning Reactions and Flavor] Why do Maillard reactions (sugar plus amino acid) give meatier, more complex flavors than caramelization (sugar alone)?

   Amino acids bring nitrogen and sulfur atoms, which sugar alone lacks. Sugars contain only carbon, hydrogen and oxygen; the extra elements make whole new families of aroma molecules possible. Left: caramelization aromas (only C, H, O). Right: Maillard ring molecules containing N or S: pyrroles, pyridines, pyrazines, thiophenes, thiazoles, oxazoles.

8. [Browning Reactions and Flavor] At about what temperature does table sugar start to caramelize noticeably?

   About 330°F/165°C. Starting the first reactions takes a lot of energy, so caramelization is fast only at high temperatures.

9. [Browning Reactions and Flavor] At about what temperature does Maillard browning become noticeable?

   About 250°F/120°C, roughly 100°F/50°C lower than the caramelization of table sugar (330°F/165°C). Like caramelization, it is fast only at high temperatures, because the first reactions need a lot of energy.

10. [Browning Reactions and Flavor] Why do most foods brown only on the surface, and only under dry heat?

    Wet food can't get hotter than 212°F/100°C, which is too cool for browning. Water must evaporate before the food gets hotter than its boiling point.In oil or an oven the surface dries out and reaches the temperature around it, 300 to 500°F (150 to 260°C), while the moist interior stays at most 212°F/100°C.(The exception is water under pressure, as in a pressure cooker.)

11. [Browning Reactions and Flavor] Why are boiled, steamed or braised foods usually paler and milder than the same foods grilled, baked or fried?

    In water or steam the food stays at 212°F/100°C or below, too cool for fast browning. Grilling, baking and frying let the surface dry out and get far hotter.

12. [Browning Reactions and Flavor] Making a stew: how do you get a rich, deep flavor from the browning reactions?

    Fry the meat, vegetables and flour until well browned before adding any liquid. Once liquid is added, the food can no longer get hotter than about 212°F/100°C.Conversely, to keep a food's own flavor in front, avoid high temperatures, since browning flavors are intense but much the same from food to food.

13. [Browning Reactions and Flavor] Browning normally needs temperatures higher than the boiling point. Which three conditions can still produce Maillard color and flavor in moist foods?

    Alkaline conditions Example: persimmon pudding, which contains glucose and alkaline baking soda, turns nearly black after hours of cooking. High concentration of sugars and amino acids Example: a watery meat stock darkens as it is boiled down to a concentrated demiglace. Long cooking times Example: egg whites (alkaline, protein-rich, a trace of glucose, 90% water) turn tan after 12 hours of simmering; balsamic vinegar turns nearly black over years.

14. [Browning Reactions and Flavor] Why does beer wort (the water extract of barley malt) darken and gain flavor when boiled for hours?

    It is rich in sugars and amino acids, so Maillard browning goes on slowly even at the boil. Both come from the germinated barley grain.

15. [Browning Reactions and Flavor] Which molecule gives caramelized sugar its butterscotch note?

    Diacetyl. Other caramel aromas: maltol (caramel), esters (fruity), acetaldehyde (sherry-like), acetic acid (sour), furans (nutty).

16. [Browning Reactions and Flavor] The Maillard products that give chocolate, roasted and earthy potato notes (and some green-vegetable notes) are the pyrazines.

    Other Maillard flavors: savory (peptides, amino acids), floral (oxazoles), onion and meaty (sulfur compounds), green vegetables (pyridines and pyrazines). Maillard browning also produces the caramelization flavors.

17. [Browning Reactions and Flavor] Why do dried fruits slowly turn brown at room temperature?

    Drying concentrates their sugars, so slow browning reactions run even at room temperature. The sugars react with amine-containing molecules over weeks or months. Browning caused by the fruit's enzymes can add to it.

18. [Browning Reactions and Flavor] What is added to dried fruits to keep them from slowly browning?

    Small amounts of sulfur dioxide ( SO2\mathrm{SO_2}SO2​ ). It blocks the slow changes in color and taste that the concentrated sugars and amine-containing molecules would otherwise cause.

19. [Browning Reactions and Flavor] How do browning reactions lower the nutritional value of foods?

    They alter or destroy amino acids, though only slightly.

20. [Browning Reactions and Flavor] Which possible carcinogen forms when starchy foods such as potato chips and french fries are fried?

    Acrylamide. It forms when sugars react with the amino acid asparagine. Swedish researchers found it in 2002; it causes cancer in rats.McGee (2004): its significance for human health was unclear (browned foods have been eaten for millennia, and some browning products protect DNA), but charred meats and fried snacks are best kept occasional.

21. [Forms of Heat Transfer] On the molecular level, what does heating a food do?

    Its molecules move faster and collide harder, until they react and form new structures and flavors.

22. [Forms of Heat Transfer] What are the three ways heat is transferred?

    Conduction, convection and radiation. Cooking methods differ in which of these dominates and in the medium (metal, water, oil, air) the heat passes through.

23. [Forms of Heat Transfer] What is conduction?

    Transfer of heat from a particle to the particle next to it. By collision, or by a motion that sets the neighbor moving (e.g. through electrical attraction or repulsion).

24. [Forms of Heat Transfer] Why are metals good conductors of heat?

    Some of their electrons move freely through the metal and carry energy with them. These electrons are loosely held and move between the fixed atoms. The same mobile electrons make metals good electrical conductors.

25. [Forms of Heat Transfer] How does heat move through a nonmetallic solid such as a ceramic?

    As vibrations of molecules, passed on to neighboring molecules. The electrons in a ceramic are held in ionic or covalent bonds and cannot move freely, so they cannot carry heat.

26. [Forms of Heat Transfer] Why are nonmetals such as ceramics thermal insulators rather than conductors?

    Passing on vibrations carries heat far more slowly than a metal's free electrons do. The same lack of free electrons makes nonmetals electrical insulators too.

27. [Forms of Heat Transfer] Why are liquids and gases poor conductors of heat?

    Their molecules are relatively far apart.

28. [Forms of Heat Transfer] On the stove, what does a pan made of a better heat conductor do differently?

    It heats up and cools down faster, and spreads heat more evenly across its bottom. A poor conductor develops hot spots, which burn food, e.g. while frying or boiling down a sauce or puree.

29. [Forms of Heat Transfer] Why do solid pieces of food heat through slowly?

    Their cell structure slows the flow of heat: foods behave more like insulators than metals. Heat reaches the center of a piece of meat, fish or vegetable only by conduction.

30. [Forms of Heat Transfer] A piece of meat 1 inch thick, compared with one 1/2 inch thick: how much longer does it take to cook through?

    Between 2 and 4 times as long, not just twice. Closer to 2 times for a compact chop or chunk, closer to 4 times for a broad, flat steak or fillet. Since this can't be predicted reliably, check doneness often.

31. [Forms of Heat Transfer] What is convection?

    Heat carried by a moving liquid or gas from a warmer region to a cooler one. The warm molecules move along and pass their energy to slower molecules by colliding with them. It combines conduction with mixing.

32. [Forms of Heat Transfer] Why do convection currents form in heated air or water?

    Warmed fluid expands and becomes less dense, so it rises; as it cools it becomes denser and sinks.

33. [Forms of Heat Transfer] What is thermal radiation?

    Heat sent out as electromagnetic waves, which need no contact or medium. Atoms and molecules emit it after absorbing energy. All matter emits it all the time, but we feel it only from very hot things, like the sun or a stove burner.

34. [Forms of Heat Transfer] How did William Herschel discover in 1800 that heat radiation is related to light?

    A thermometer just beyond the red end of a prism's rainbow got hottest of all. No light was visible there. Hence the name infrared: Latin infra means "below".

35. [Forms of Heat Transfer] What do radio waves, microwaves, infrared, visible light and X rays have in common?

    All are electromagnetic radiation, differing in energy (frequency). They arise from moving electric charges (often electrons in atoms); when they reach other atoms, they make those atoms move more. The spectrum by frequency in cycles per second; cooking uses microwaves and infrared.

36. [Forms of Heat Transfer] What does each kind of electromagnetic radiation do to matter, from lowest to highest energy?

    Radio waves Move only free electrons. Example: transmitting and receiving them needs metal antennas. Microwaves Make polar molecules, like water, move faster. Example: watery foods heat in a microwave oven. Infrared (heat radiation) Makes nonpolar molecules (carbohydrates, proteins, fats) and polar water move faster. Example: the heat from glowing coals or a broiler element. Visible and ultraviolet light Change the orbits of electrons bound in molecules, which starts chemical reactions. Example…

37. [Forms of Heat Transfer] Why does sunlight ruin the flavor of milk and beer, while infrared does not?

    Visible and UV light shift electrons in molecules, which starts chemical reactions. These damage pigments and fats and create stale, rancid flavors.Infrared only makes molecules move faster.

38. [Forms of Heat Transfer] Below about what temperature is cooking by infrared radiation slow?

    Below about 1,800°F/980°C, the temperature at which objects start to glow red. So radiation cooks quickly only near glowing coals, electric elements or gas flames (grilling, broiling); at baking and frying temperatures, conduction and convection matter more.

39. [Forms of Heat Transfer] Which is hotter: red-hot or yellow-hot metal?

    Yellow-hot. The hotter an object, the more energy it radiates, and the higher in the spectrum that energy reaches.

40. [Forms of Heat Transfer] In an oven, how can the cook increase or decrease the heat a food gets by radiation from the walls?

    Increase: move the food closer to the walls or ceiling. Decrease: shield it with reflective foil. The share of heat from radiating walls grows as the oven temperature rises.

41. [Basic Methods of Heating Foods] Heating a pan of water on an electric stove: which forms of heat transfer take part?

    Radiation and conduction from the element, conduction through the pan, convection in the water. With a gas flame: radiation and convection from the flame instead. Most cooking combines forms of heat transfer, but one usually dominates.

42. [Basic Methods of Heating Foods] What is the difference between grilling and broiling?

    Grilling heats the food from below; broiling heats it from above.

43. [Basic Methods of Heating Foods] How do grilling and broiling mainly heat the food?

    By infrared radiation from glowing coals, a gas flame or an electric element. Hot air (convection) adds some heat, more so the farther the food is from the heat source.

44. [Basic Methods of Heating Foods] About how hot are glowing coals, compared with the walls of an oven?

    Coals: about 2,000°F/1,100°C. Oven walls: rarely above 500°F/250°C. Electric broiler elements are about as hot as coals; a gas flame reaches about 3,000°F/1,600°C.

45. [Basic Methods of Heating Foods] The total energy radiated by a hot object is proportional to which power of its absolute temperature TTT ?

    T4\boldsymbol{T^4}T4 (the fourth power) Example: a coal at 2,000°F (about 1,366 K) radiates more than 40 times as much as the same area of oven wall at 500°F (about 533 K): (1366/533)4≈2.564≈43(1366 / 533)^4 \approx 2.56^4 \approx 43(1366/533)4≈2.564≈43

46. [Basic Methods of Heating Foods] Why do grilled steaks easily end up charred outside and cold in the center?

    Radiation heats the surface much faster than conduction can carry heat into the interior.

47. [Basic Methods of Heating Foods] Two ways to grill or broil a thick piece of food evenly?

    Distance Place the food far enough from the heat source that surface browning keeps pace with conduction inside. Sear, then move Brown the surface over intense heat, then finish cooking with weaker heat. Example: a moderate oven, or a cooler zone of the grill where the coals are sparser.

48. [Basic Methods of Heating Foods] How does an induction cooktop heat a pot?

    A magnetic field from a coil induces an electric current in the pot, and the current heats the pot. The coil sits under the ceramic top; its current, and so the field, alternates 25,000 to 40,000 times per second.

49. [Basic Methods of Heating Foods] Which pots work on an induction cooktop?

    Pots of magnetic material: cast iron, steel, and some stainless steels. The stainless steel must have the right (ferritic) crystal structure to be magnetic.

50. [Basic Methods of Heating Foods] Two advantages of induction heating over burners and radiant elements?

    Efficiency All the energy goes into the pot, none into the surrounding air. The cooktop itself stays relatively cool Only the pot and the food in it reach high temperatures. The ceramic top warms just by touching the pot, because its electrons cannot move freely in the magnetic field.

51. [Basic Methods of Heating Foods] How does an oven heat food in baking?

    By radiation from the oven walls together with convection of hot air. Typical oven temperatures are 300 to 500°F (150 to 250°C), high enough to dry and brown the surface.

52. [Basic Methods of Heating Foods] Why does a potato cook faster in boiling water (212°F/100°C) than in a 500°F/250°C oven?

    Water is over 1,000 times denser than air, so far more hot molecules strike the potato. Radiation at oven temperatures also transfers heat slowly. This is why a hand can briefly reach into a hot oven without being burned.

53. [Basic Methods of Heating Foods] How does a convection oven shorten baking times?

    Fans move the hot air faster past the food, which increases the rate of heat transfer.

54. [Basic Methods of Heating Foods] What were the earliest known ovens like?

    Hollow clay cones with a layer of coals inside, and the bread stuck to the inner wall. They appeared in Egypt around 3000 BCE, along with more refined bread making.The compact metal home oven dates only from the late 19th century; before then most meat was cooked over the fire.

55. [Basic Methods of Heating Foods] Why is boiling an efficient way to heat food, despite its low temperature?

    Dense water touches the whole surface, and its molecules strike the food constantly. Each collision passes energy to the food; convection currents carry the heat through the water. Simmering and poaching are lower-temperature versions.

56. [Basic Methods of Heating Foods] Why did boiling probably come after roasting but before baking?

    It needs a pot that is waterproof and fireproof, so it had to wait for pottery. Pottery appeared about 10,000 years ago. Baking needs an even more elaborate container, the oven.

57. [Basic Methods of Heating Foods] Why does the temperature of boiling water stay constant, even at a vigorous rolling boil?

    All the extra heat goes into turning water into steam, not into raising the temperature. The temperature can rise only once all the liquid has evaporated. A rolling boil is only slightly hotter than a gentle one.

58. [Basic Methods of Heating Foods] What are the small bubbles that appear in a pan of water long before it boils?

    Air that was dissolved in the cold water. Warm water holds less dissolved air than cold water, so the air comes out as it heats.

59. [Basic Methods of Heating Foods] What are the bubbles in boiling water made of?

    Water vapor (steam), formed at the bottom of the pan, where it is hottest.

60. [Basic Methods of Heating Foods] Why does the boiling point of a liquid depend on the air pressure?

    Higher pressure makes it harder for molecules to escape as gas, so boiling needs more heat. Even weather fronts shift the boiling point of water by a degree or two.

61. [Basic Methods of Heating Foods] How much does each 1,000 ft (305 m) of elevation lower the boiling point of water?

    About 2°F (1°C). Example: Denver, at about 5,300 ft (1,600 m): about 212−5.3⋅2≈201212 - 5.3 \cdot 2 \approx 201212−5.3⋅2≈201 °F, i.e. about 100−5=95100 - 5 = 95100−5=95 °C. Food cooks more slowly there.

62. [Basic Methods of Heating Foods] How does a pressure cooker cook faster?

    Trapped steam raises the pressure, so the water boils at about 250°F/120°C. That is the boiling point in an open pan 19,000 ft (5,800 m) below sea level. The device dates from the 1600s; its inventor was Denis Papin, a French physician.

63. [Basic Methods of Heating Foods] What does dissolving salt or sugar in water do to its boiling and freezing points?

    Raises the boiling point and lowers the freezing point, more so the higher the concentration. Useful in candy making (boiling point indicates sugar concentration) and in making ice cream.

64. [Basic Methods of Heating Foods] Why does dissolved sugar or salt raise the boiling point of water?

    The dissolved particles absorb heat energy but cannot evaporate. So at 212°F a smaller share of molecules has enough energy to escape into a vapor bubble, and more heat is needed.

65. [Basic Methods of Heating Foods] How much does salting pasta water at about seawater strength (1 ounce per quart, about 30 g per liter) raise its boiling point?

    About 1°F, which is negligible. To make water in Denver boil at Boston's temperature, a liter would need more than 225 g of salt.

66. [Basic Methods of Heating Foods] Why is boiling water often too hot for cooking fish and meat?

    The outside overcooks and dries out before the center is done. Fish and many meats have their best texture around 140°F/60°C, 70°F below the boil.

67. [Basic Methods of Heating Foods] What water temperature cooks fish and meat gently but still fairly quickly?

    About 180°F/80°C, checked with a thermometer. Cooler water overcooks the outside less but takes longer.

68. [Basic Methods of Heating Foods] Steam is much less dense than liquid water. Why does steaming still heat food surfaces quickly?

    Steam condensing on the food releases a lot of heat (its heat of vaporization). This comes on top of the steam's energy of motion, and brings the surface quickly to the boiling point.

69. [Basic Methods of Heating Foods] How is food mainly heated in pan-frying and sautéing?

    By conduction from a hot, oiled pan. The pan is at about 350 to 450°F (175 to 225°C), hot enough for Maillard browning.

70. [Basic Methods of Heating Foods] What three roles does the fat play in pan-frying?

    Contact It fills the gaps between the uneven food surface and the pan, so the whole surface is heated evenly. Lubrication It keeps the food from sticking. Flavor It adds some flavor of its own.

71. [Basic Methods of Heating Foods] Why is frying in oil a "dry" cooking method, even though the food sits in liquid?

    The hot oil quickly dries out the food's surface. Meanwhile the interior stays watery and no hotter than 212°F/100°C.

72. [Basic Methods of Heating Foods] In frying, how do cooks keep the outside from overcooking before the inside is done?

    Thin cuts Less distance for heat to travel inward. Searing Brown at high heat first, then lower the heat while the interior cooks. Coatings Breadings and batters insulate the food from the hot pan and develop flavor themselves.

73. [Basic Methods of Heating Foods] Deep frying, like boiling, immerses the whole food in a hot liquid. What does deep frying do that boiling cannot?

    It browns the food's surface. The oil is far hotter than 212°F/100°C, so it dries the surface out.

74. [Basic Methods of Heating Foods] Why do microwaves heat watery food, but not the oven air or glass and plastic containers?

    Microwaves move only polar molecules, like water. Air ( N2\mathrm{N_2}N2​ , O2\mathrm{O_2}O2​ ), glass, stoneware and plastic (hydrocarbon chains) are nonpolar. The containers warm only by contact with the hot food.Microwaves carry only about a ten-thousandth of the energy of infrared from glowing coals.

75. [Basic Methods of Heating Foods] How does a microwave oven heat food, molecule by molecule?

    Water molecules keep flipping to follow a reversing electric field, and bump their neighbors. Water molecules are polar, so they turn to align with the field, which reverses some 2 or 5 billion times per second.Frequencies: 915 or 2,450 million cycles per second (compare 60 for wall current, about 100 million for FM radio).

76. [Basic Methods of Heating Foods] Can metal go into a microwave oven?

    Yes, if the pieces are fairly large and sit with moist food. Keep them away from the walls and from each other to prevent sparks. Thin metal trim on china sparks and gets damaged. Foil can shield thin parts, like the edges of fish fillets.

77. [Basic Methods of Heating Foods] How was cooking with microwaves discovered?

    Percy Spencer of Raytheon popped corn with microwaves, and patented the idea in 1945. Microwave ovens became popular in the 1970s.

78. [Basic Methods of Heating Foods] Why does microwaving cook food faster than infrared radiation?

    Microwaves reach about 1 inch (2.5 cm) into the food; infrared stops at the surface. From the surface, heat moves inward only by slow conduction.Microwaves also heat only the food, not its surroundings, so they use energy efficiently.

79. [Basic Methods of Heating Foods] Two drawbacks of microwave cooking?

    Dry meat, hard-to-control doneness Fast heating makes meat lose more fluid. Pulsing the oven on and off slows the heating. Little browning The surface gets no hotter than the interior, so it browns only if it dries out. Packaging with thin metal sheets that concentrate the radiation can make the surface hot enough to brown.

80. [Utensil Materials] Which two properties do we want in a pot or pan?

    A chemically unreactive surface, and good, even heat conduction. No single material has both.

81. [Utensil Materials] Why are good heat conductors such as metals usually chemically reactive?

    The mobile electrons that conduct heat are also easily given up to other atoms at the surface. Conversely, inert compounds hold their electrons tightly and conduct heat poorly.

82. [Utensil Materials] What are ceramics chemically?

    Mixtures of oxides: mainly silicon dioxide, aluminum oxide and magnesium oxide.

83. [Utensil Materials] Why do ceramics conduct heat poorly?

    Their bonds hold the electrons tightly, so heat moves only by slow vibrations.

84. [Utensil Materials] Why can a ceramic pot crack on a stovetop burner?

    Hot spots expand while cooler parts don't, and the strain cracks the pot. Ceramic can't spread intense heat evenly. So ceramics are used in the oven (moderate, even heat) or as thin coatings on metal, which spreads the heat.

85. [Utensil Materials] What thin coating forms on its own on most kitchen metals exposed to air?

    A film of metal oxide, formed as surface atoms react with oxygen from the air. The film is a ceramic: stable, unreactive and fairly tough.

86. [Utensil Materials] Why doesn't the natural oxide film on a metal pan protect the pan well?

    It is only a few molecules thick, so cooking easily scratches or wears it away.

87. [Utensil Materials] What is the tarnish on silver and copper?

    A metal-sulfur compound, not an oxide; the sulfur comes mainly from air pollution.

88. [Utensil Materials] Aluminum oxide forms the protective film on aluminum pans. Which materials consist of aluminum oxide crystals?

    Corundum (an abrasive), and rubies and sapphires. Traces of chromium and titanium in the crystal give the gems their colors.

89. [Utensil Materials] Two ways metallurgists turn surface oxidation into a durable protective layer?

    Anodized aluminum A chemical treatment thickens the oxide film to up to 0.001 inch (0.03 mm). Stainless steel Iron is alloyed with metals that form a tougher oxide surface.

90. [Utensil Materials] How does glass differ from other ceramics?

    Its composition is more regular, usually mostly silica (silicon dioxide). The word "ceramic" comes from the Greek for "potter's clay".

91. [Utensil Materials] How do earthenware, stoneware and porcelain differ?

    Earthenware Simple molded and dried clay pottery; porous and coarse. Dates from about 9,000 years ago, when plants and animals were first domesticated. Stoneware Contains enough silica and is fired hot enough to vitrify (become partly glass); less porous, much stronger. Invented in China before 1500 BCE. Porcelain White, translucent stoneware of kaolin (a very light clay) mixed with a silicate mineral, fired at high temperature. T'ang Dynasty (618 to 907 CE); reached Europe with the 17th-centur…

92. [Utensil Materials] What is the main health risk from some ceramic cookware?

    Lead in clays or glazes, which acidic foods can leach out. Lead is a nerve poison; imported high-lead ceramics still occasionally cause poisoning. Otherwise ceramics are chemically very stable.

93. [Utensil Materials] What makes heat-resistant glass less prone to cracking from thermal shock?

    An added boron oxide, which reduces thermal expansion about threefold. It still isn't immune.

94. [Utensil Materials] What is enamelware?

    Iron or steel cookware with a thin layer of powdered glass fused onto its surface. The glass layer keeps the food from touching the metal. It was first applied to cast iron early in the 19th century.

95. [Utensil Materials] Plain ceramic pots can crack on a stove burner. Why doesn't the glass layer of enamelware crack there?

    The metal underneath spreads the heat evenly, and the thin glass expands evenly with it. It can still chip if a hot pan is plunged into cold water.

96. [Utensil Materials] When is the poor conductivity of ceramics an advantage?

    For keeping food hot, and in brick ovens, which store a lot of heat and release it evenly. Metal ovens store little heat and must cycle their elements on and off; the temperature swings can scorch breads baked at high heat.

97. [Utensil Materials] Why has aluminum been used for pans only for about a century, though no other metal is as plentiful in the earth's crust?

    It never occurs as pure metal, and a good method to extract it from ore came only around 1890. Cookware aluminum is alloyed with small amounts of manganese and sometimes copper.

98. [Utensil Materials] Advantages of aluminum cookware?

    Low cost, heat conductivity second only to copper, and low density (light weight).

99. [Utensil Materials] Why can unanodized aluminum discolor light-colored foods?

    Its thin oxide layer lets food react with the metal, forming gray or black compounds. Acids, alkalis and hydrogen sulfide (e.g. from cooked eggs) reach the metal; the dark compounds are aluminum oxide and hydroxide complexes.

100. [Utensil Materials] What is anodizing aluminum?

     Using electric current in an acid bath to grow a thick oxide layer on aluminum. The aluminum is made the positive pole (anode) in a sulfuric acid solution, which forces its surface to oxidize.