On Food And Cooking Part 97

Maillard Reactions 250F/120C and above Maillard Reactions 250F/120C and above

Sweet (sucrose, other sugars)

Savory (peptides, amino acids) Savory (peptides, amino acids)

Sour (acetic acid)

Floral (oxazoles) Floral (oxazoles)

Bitter (complex molecules)

Onions, meatiness (sulfur compounds) Onions, meatiness (sulfur compounds)

Fruity (esters)

Green vegetables (pyridines, pyrazines) Green vegetables (pyridines, pyrazines)

Sherry-like (acetaldehyde)

Chocolate (pyrazines) Chocolate (pyrazines)

b.u.t.terscotch (diacetyl)

Potato, earthy (pyrazines) Potato, earthy (pyrazines)

Caramel (maltol)

Nutty (furans)

Plus caramelization flavors Plus caramelization flavors

Drawbacks of the Browning Reactions Browning reactions do have some drawbacks. First, many dehydrated fruits are p.r.o.ne to gradual browning over weeks or months at room temperature, because the carbohydrates and amine-containing molecules are especially concentrated (browning caused by enzymes can also be a factor). Small amounts of sulfur dioxide are commonly added to these foods to block these unwanted changes in color and taste. Second, the nutritional value of the foods is slightly reduced because amino acids are altered or destroyed.

Finally, there's evidence that some products of the browning reactions can damage DNA and may cause cancer. In 2002, Swedish researchers found worrisome levels of acrylamide, a known carcinogen in rats, in potato chips, french fries, and other starchy fried foods, apparently the product of reactions between sugars and the amino acid asparagine. The health significance of this and similar findings remains unclear. The ubiquity of browned foods, both today and through thousands of years of history, would suggest that they do not const.i.tute a major threat to public health. And other browning-reaction products have been found to protect against DNA damage! But it's probably prudent to make charred meats and fried snacks occasional pleasures, not everyday ones.

Forms of Heat Transfer Cooking can be defined in a general way as the transformation of raw foods into something different. Most often, we transform foods by heating them - by transferring energy from a heat source into the foods, so that the food molecules move faster and faster, collide harder and harder, and react to form new structures and flavors. Our various cooking methods - boiling, broiling, baking, frying, and so on - achieve their various effects by employing very different materials as the medium through which the heat moves, and by drawing on different forms of heat transfer. There are three ways to transfer heat, and an acquaintance with them will help us understand how particular cooking techniques affect foods the way they do.

Conduction: Direct Contact When thermal energy is exchanged from one particle to a nearby one by means of a collision or a movement that induces movement (for example, through electrical attraction or repulsion), the process is called conduction. conduction. Though it's the most straightforward means of heat transfer in matter, conduction takes different forms in different materials. For example, metals are usually good conductors of heat because, while their atoms are fixed in a lattice-like structure, some of their electrons are very loosely held and tend to form a free-moving "fluid" or "gas" in the solid that can carry energy from one region to another. This same electron mobility makes metals good electrical conductors. But in nonmetallic solids like ceramics, conduction is more mysterious. It seems that heat is propagated not by the movement of energetic electrons - in solids of ionic-or covalent-bonded compounds, the electrons are not free to move - but by the vibration of individual molecules or a portion of the lattice, which is transferred to neighboring areas. This transfer of vibration is a much slower and less efficient process than electron movement, and nonmetals are therefore usually referred to as thermal or electrical Though it's the most straightforward means of heat transfer in matter, conduction takes different forms in different materials. For example, metals are usually good conductors of heat because, while their atoms are fixed in a lattice-like structure, some of their electrons are very loosely held and tend to form a free-moving "fluid" or "gas" in the solid that can carry energy from one region to another. This same electron mobility makes metals good electrical conductors. But in nonmetallic solids like ceramics, conduction is more mysterious. It seems that heat is propagated not by the movement of energetic electrons - in solids of ionic-or covalent-bonded compounds, the electrons are not free to move - but by the vibration of individual molecules or a portion of the lattice, which is transferred to neighboring areas. This transfer of vibration is a much slower and less efficient process than electron movement, and nonmetals are therefore usually referred to as thermal or electrical insulators, insulators, rather than conductors. Liquids and gases, because their molecules are relatively far apart, are very poor conductors. rather than conductors. Liquids and gases, because their molecules are relatively far apart, are very poor conductors.

The conductivity of a material determines its behavior on the stove. The better the conductor, the faster a pan heats up and cools off, and the more evenly heat is distributed across the pan bottom. Uneven heating creates hot spots that can burn foods: during frying, for example, or the boiling down of a puree or sauce.

Conduction Within a Food Heat also travels from the outside to the center of a solid piece of food - a piece of meat or fish or vegetable - by means of conduction. Because the cellular structure of foods impedes the movement of heat energy, foods behave more like insulators than like metals, and heat up relatively slowly. One of the keys to good cooking is knowing how to heat a food to the desired doneness at its center without overheating its outer regions. This is not a simple task, because different kinds of foods heat through at different rates. One of the most important variables is the thickness of the food. Though common sense might suggest that a piece of meat one inch thick would take twice as long to cook through as a half-inch piece, it turns out that it takes somewhere between twice and four times longer, depending on the overall shape: less for a compact chop or chunk, more for a broad steak or fillet. There's no absolutely reliable way to predict how long it will take heat to move from the food surface to its center, so the best rule is to check the doneness frequently. Heat also travels from the outside to the center of a solid piece of food - a piece of meat or fish or vegetable - by means of conduction. Because the cellular structure of foods impedes the movement of heat energy, foods behave more like insulators than like metals, and heat up relatively slowly. One of the keys to good cooking is knowing how to heat a food to the desired doneness at its center without overheating its outer regions. This is not a simple task, because different kinds of foods heat through at different rates. One of the most important variables is the thickness of the food. Though common sense might suggest that a piece of meat one inch thick would take twice as long to cook through as a half-inch piece, it turns out that it takes somewhere between twice and four times longer, depending on the overall shape: less for a compact chop or chunk, more for a broad steak or fillet. There's no absolutely reliable way to predict how long it will take heat to move from the food surface to its center, so the best rule is to check the doneness frequently.

Convection: Movement in Fluids In the form of heat transfer called convection, convection, heat is transferred by the movement of molecules in a fluid from a warm region to a cooler one. The fluid may be a liquid such as water, or it may be the air or other gases. Convection is a process that combines conduction and mixing: energetic molecules move from one point in s.p.a.ce to another, and then collide with slower particles. Convection is an influential phenomenon, contributing as it does to winds, storms, ocean currents, the heating of our homes, and the boiling of water on the stove. It occurs because air and water take up more s.p.a.ce - become less dense - when their molecules absorb energy and move faster, and so they rise when they heat up and sink again as they cool off. heat is transferred by the movement of molecules in a fluid from a warm region to a cooler one. The fluid may be a liquid such as water, or it may be the air or other gases. Convection is a process that combines conduction and mixing: energetic molecules move from one point in s.p.a.ce to another, and then collide with slower particles. Convection is an influential phenomenon, contributing as it does to winds, storms, ocean currents, the heating of our homes, and the boiling of water on the stove. It occurs because air and water take up more s.p.a.ce - become less dense - when their molecules absorb energy and move faster, and so they rise when they heat up and sink again as they cool off.

Radiation: the Pure Energy Of radiant Heat and Microwaves We all know that the earth is warmed by the sun. How does solar energy reach us across millions of miles of nearly empty s.p.a.ce, where there's nothing there to conduct or convect? The answer is thermal radiation, radiation, a process that does not require direct physical contact between heat source and object. All matter emits thermal radiation all the time, though normally we can detect it only when something is very hot. The warmth we feel from sunlight or a stove burner comes from thermal radiation. It's emitted by atoms and molecules which, having absorbed energy, release it again not in the form of faster movement, but as waves of pure energy. a process that does not require direct physical contact between heat source and object. All matter emits thermal radiation all the time, though normally we can detect it only when something is very hot. The warmth we feel from sunlight or a stove burner comes from thermal radiation. It's emitted by atoms and molecules which, having absorbed energy, release it again not in the form of faster movement, but as waves of pure energy.

Radiant Heat Is Invisible "Infrared" Radiation As unlikely as it may seem, radiated heat is close kin to radio waves, microwaves, visible light, and X rays. Each of these phenomena is a part of the As unlikely as it may seem, radiated heat is close kin to radio waves, microwaves, visible light, and X rays. Each of these phenomena is a part of the electromagnetic spectrum, electromagnetic spectrum, waves of varying energies created by the movement of electrically charged particles, often electrons within atoms. Such movement creates electrical and magnetic fields that radiate, or spread out, as waves. And conversely, when such energetic waves. .h.i.t other atoms, they cause increased movement in those atoms. One of the first to recognize that heat radiation is related to light was the English oboist and astronomer William Herschel, who noticed in 1800 that if a thermometer was moved from one end of a prism-produced light spectrum to the other, the highest temperatures would register below the red band, where no light was visible. Because of its position in the spectrum, heat radiation is called waves of varying energies created by the movement of electrically charged particles, often electrons within atoms. Such movement creates electrical and magnetic fields that radiate, or spread out, as waves. And conversely, when such energetic waves. .h.i.t other atoms, they cause increased movement in those atoms. One of the first to recognize that heat radiation is related to light was the English oboist and astronomer William Herschel, who noticed in 1800 that if a thermometer was moved from one end of a prism-produced light spectrum to the other, the highest temperatures would register below the red band, where no light was visible. Because of its position in the spectrum, heat radiation is called infrared (infra infrared (infra is Latin for "below"). is Latin for "below").

Different Kinds of Radiation Carry Different Amounts of Energy Different kinds of radiation carry different energies, and the energy of a given kind of radiation determines the kind of effect that it will have. Different kinds of radiation carry different energies, and the energy of a given kind of radiation determines the kind of effect that it will have.

At the bottom end of the scale, radio waves are so weak that they can only cause increased movement in free electrons. This is why metal antennas and their mobile electrons are necessary to transmit and receive such radiation.

Next come microwaves, which are energetic enough to set polar molecules like water moving faster. (Microwave refers to the fact that their wavelength is shorter than radio wavelengths.) Since most foods are mostly water molecules, microwave radiation is an effective means of cooking. refers to the fact that their wavelength is shorter than radio wavelengths.) Since most foods are mostly water molecules, microwave radiation is an effective means of cooking.

Then there's heat radiation, the cook's standard energy source, which causes the increased movement of nonpolar molecules - including carbohydrates, proteins, and fats - as well as polar water.

Visible and ultraviolet light is capable of altering the orbits of electrons bound in molecules, and so can initiate chemical reactions that cause damage to pigments and fats and the development of stale, rancid flavors. Visible and ultraviolet rays from the sun can ruin the flavor of milk and beer, and ultraviolet rays can burn our skin, damage our DNA, and cause cancer.

X and gamma rays penetrate matter and ionize ionize it, or strip electrons from its molecules. Along with controlled beams of certain subatomic particles, they damage DNA and kill microbes, and are used to "cold-pasteurize" and sterilize some foods. it, or strip electrons from its molecules. Along with controlled beams of certain subatomic particles, they damage DNA and kill microbes, and are used to "cold-pasteurize" and sterilize some foods.

Useful Heat Radiation Is Generated by High Temperatures Because all molecules are vibrating to some extent, everything around us is emitting at least some infrared radiation all the time. The hotter an object gets, the more energy it radiates in higher regions of the spectrum. So it is that glowing metal is hotter than metal that does not radiate visible light, and that yellow-hot metal is hotter than red-hot. It turns out that the rate of infrared radiation is relatively low below about 1,800F/980C, or the point at which objects begin to glow visibly red. Cooking by radiation is thus a slow process except at very high cooking temperatures, those characteristic of grilling and broiling near glowing coals, electrical elements, or gas flames. At typical baking and frying temperatures, conduction and convection tend to be more significant than infrared radiation. But as the oven temperature goes up, the proportion of heat contributed by the radiating oven walls goes up with it. The cook can control this contribution by moving the food close to the walls or ceiling to increase it, or s.h.i.+elding the food with reflective foil, which reduces it. Because all molecules are vibrating to some extent, everything around us is emitting at least some infrared radiation all the time. The hotter an object gets, the more energy it radiates in higher regions of the spectrum. So it is that glowing metal is hotter than metal that does not radiate visible light, and that yellow-hot metal is hotter than red-hot. It turns out that the rate of infrared radiation is relatively low below about 1,800F/980C, or the point at which objects begin to glow visibly red. Cooking by radiation is thus a slow process except at very high cooking temperatures, those characteristic of grilling and broiling near glowing coals, electrical elements, or gas flames. At typical baking and frying temperatures, conduction and convection tend to be more significant than infrared radiation. But as the oven temperature goes up, the proportion of heat contributed by the radiating oven walls goes up with it. The cook can control this contribution by moving the food close to the walls or ceiling to increase it, or s.h.i.+elding the food with reflective foil, which reduces it.

Basic Methods of Heating Foods Pure examples of the three different forms of heat transfer are seldom found in everyday life. All hot utensils radiate heat to some degree, and cooks usually work with combinations of solid containers that conduct and fluids that circulate. As simple an operation as heating a pan of water on the stove involves radiation and conduction from an electrical element (radiation and convection from a gas flame), conduction through the pan, and convection in the water. Still, one kind of heat transfer usually predominates in a given cooking technique and, together with the cooking medium, has a distinctive influence on foods.

The spectrum of electromagnetic radiation. We use both microwave and infrared radiation to cook our foods. (The scale employs a standard scientific abbreviation for large numbers; 105means a 1 followed by 5 zeroes, or 100,000.) Grilling and Broiling: Infrared Radiation Grilling and broiling are the modern, controlled versions of the oldest culinary technique, roasting over an open fire or glowing coals. In grilling, the heat source is below the food; in broiling, above. Though air convection contributes some heat, especially as the distance between heat source and food is increased, broiling is largely a matter of infrared radiation. The heat sources used in these techniques all emit visible light and so are also intense radiators of infrared energy. Glowing coals or the nickel-chrome alloys used in electrical appliances reach about 2,000F/1,100C, and a gas flame is closer to 3,000F/1,600C. The walls of an oven, by contrast, rarely exceed 500F/250C. The total amount of energy radiated by a hot object is proportional to the fourth power of the absolute temperature, so that a coal or metal rod at 2,000F is radiating more than 40 times as much energy as the equivalent area of oven wall at 500F.

This tremendous amount of heat is at once the great advantage and the princ.i.p.al challenge of grilling and broiling. On one hand, it makes possible a rapid and thorough browning of the surface, and so produces intense flavors. On the other, there's a huge disparity between the rate of heat radiation at the surface and the rate of heat conduction within the food. This is why it's so easy to end up with a steak that's charred on the outside and cold at the center.

The key to grilling and broiling is to position the food far enough from the heat source to match the browning rate with the inner conduction rate, or to brown the surface well with intense heat, and then move the food to finish cooking through with a more distant or weaker heat source. This might be a spot on the grill with fewer coals below, or a moderate oven.

Induction CookingAn innovative version of heating with electromagnetic radiation is induction heating. It's an alternative to the stovetop burner or electrical element, and heats the pot that then heats the food. In induction heating, the heating element, under a ceramic cook-top surface, is a wire coil through which a rapidly alternating electrical current flows (between 25,000 and 40,000 cycles per second). The current causes the coil to generate a magnetic field that extends some distance from the coil, and that alternates at the same rate. If a pot made from a magnetic material - cast iron, steel, stainless steel of the proper crystal structure (ferritic) - is placed near the coil, then the alternating magnetic field induces induces an alternating electrical current in the pot. That is, it causes electrons to move in the pot, and that movement rapidly generates heat. an alternating electrical current in the pot. That is, it causes electrons to move in the pot, and that movement rapidly generates heat.Induction heating has two notable advantages over burners and radiant elements. Like microwave heating, it's more efficient, because all the energy goes into the object to be heated, not into the surrounding air. And only the pot and its contents get very hot. The ceramic surface above the induction coil is heated only indirectly by the pot, because its electrons aren't free to be moved by the magnetic field.

Baking: Air Convection and Radiation When we bake a food, we surround it with a hot enclosure, the oven, and rely on a combination of radiation from the walls and hot-air convection to heat the food. Baking easily dehydrates the surface of foods, and so will brown them well provided the oven temperature is high enough. Typical baking temperatures are well above the boiling point, from 300 to 500F/150250C), and yet baking is nowhere near as efficient a means of heat transfer as is boiling. A potato can be boiled in less time than it takes to be baked at a much hotter temperature. This is so because neither radiation nor air convection at 500F transfers heat very rapidly to food. Oven air is less than a thousandth as dense as water, so the collisions between hot molecules and food are much less frequent in the oven than in the pot (this is why we can reach into a hot oven without immediately burning our hand). Convection ovens Convection ovens increase the rate of heat transfer by using fans to force more air movement, and significantly reduce baking times. increase the rate of heat transfer by using fans to force more air movement, and significantly reduce baking times.

Because baking requires a fairly sophisticated container, it was probably a late addition to the culinary repertoire. The earliest ovens seem to have accompanied the refinement of bread making around 3000 BCE BCE in Egypt; they were hollow cones of clay that contained a layer of coals, with the bread stuck onto an inside wall. As a relatively compact metal box easily installed in individual homes, the modern oven dates from the late 19th century. Before then, most meat cooking was done over the fire. in Egypt; they were hollow cones of clay that contained a layer of coals, with the bread stuck onto an inside wall. As a relatively compact metal box easily installed in individual homes, the modern oven dates from the late 19th century. Before then, most meat cooking was done over the fire.

Boiling and Simmering: Water Convection In boiling and its lower-temperature versions, simmering and poaching, food is heated by the convection currents in hot water. The maximum temperature possible is the boiling point, 212F/100C at sea level, which is usually not high enough for these "moist" cooking methods to trigger browning reactions. Despite the relatively low cooking temperature, boiling is a very efficient process. The entire surface of the food is in contact with the cooking medium, and water is dense enough that its molecules constantly collide with the food and rapidly impart their energy to it.

As a cooking technique, boiling probably followed roasting and preceded baking. It requires containers that are both water-and fireproof, and so probably had to await the development of pottery, around 10,000 years ago.

The Boiling Point: A Reliable Landmark It isn't always easy for the cook to recognize and maintain a particular cooking temperature, and reproduce the same temperature reliably. Thermostats, thermometers, and our senses are all fallible. So one of the great advantages of water as a cooking medium is that its boiling point is constant - 212F/100C at sea level - and it's instantly recognizable. The sure sign of boiling water is bubbling. Why? When the water in a pan is heated near boiling, molecules at the bottom, where the pan is hottest, vaporize and become steam, and form regions that are less dense than the surrounding liquid. (The small bubbles that form very early on are pockets of air that had been dissolved in the cold water but became less soluble as the temperature rose.) Because all the pan heat at the boil goes into vaporizing the liquid water, the temperature of the water itself stays the same (p. 816). It's only slightly higher at a full, rolling boil than in a gently bubbling pot, and will not get any higher until the phase change from liquid to gas has been completed. It isn't always easy for the cook to recognize and maintain a particular cooking temperature, and reproduce the same temperature reliably. Thermostats, thermometers, and our senses are all fallible. So one of the great advantages of water as a cooking medium is that its boiling point is constant - 212F/100C at sea level - and it's instantly recognizable. The sure sign of boiling water is bubbling. Why? When the water in a pan is heated near boiling, molecules at the bottom, where the pan is hottest, vaporize and become steam, and form regions that are less dense than the surrounding liquid. (The small bubbles that form very early on are pockets of air that had been dissolved in the cold water but became less soluble as the temperature rose.) Because all the pan heat at the boil goes into vaporizing the liquid water, the temperature of the water itself stays the same (p. 816). It's only slightly higher at a full, rolling boil than in a gently bubbling pot, and will not get any higher until the phase change from liquid to gas has been completed.

The Boiling Point Depends on Elevation The boiling point of water is constant given a constant physical environment, but it varies from place to place and even in the same place. The boiling point of any liquid depends on the atmospheric pressure bearing down on its surface: the higher the pressure, the more energy it takes for liquid molecules to escape the surface and become a gas, and so the higher the temperature at which the liquid boils. Every 1,000 feet/305 meters in elevation above sea level lowers the boiling point about 2F below the standard 212F (or 1C below 100C). And food takes longer to cook at 200 than it does at 212. Even a low-pressure weather front can lower the boiling point, or a high-pressure front raise it, by as much as a degree or two. The boiling point of water is constant given a constant physical environment, but it varies from place to place and even in the same place. The boiling point of any liquid depends on the atmospheric pressure bearing down on its surface: the higher the pressure, the more energy it takes for liquid molecules to escape the surface and become a gas, and so the higher the temperature at which the liquid boils. Every 1,000 feet/305 meters in elevation above sea level lowers the boiling point about 2F below the standard 212F (or 1C below 100C). And food takes longer to cook at 200 than it does at 212. Even a low-pressure weather front can lower the boiling point, or a high-pressure front raise it, by as much as a degree or two.

Pressure Cooking: Raising the Boiling Point The same princ.i.p.al is put to use to The same princ.i.p.al is put to use to speed speed cooking in the pressure cooker. This appliance reduces cooking times by trapping the steam that escapes from boiling water, thereby increasing the pressure on the liquid, and so raising its boiling point - and maximum temperature - to about 250F/120C. This is the equivalent of boiling water in an open pan at the bottom of a pit 19,000 feet/5,800 meters cooking in the pressure cooker. This appliance reduces cooking times by trapping the steam that escapes from boiling water, thereby increasing the pressure on the liquid, and so raising its boiling point - and maximum temperature - to about 250F/120C. This is the equivalent of boiling water in an open pan at the bottom of a pit 19,000 feet/5,800 meters below below sea level. sea level.

The pressure cooker was invented by the French physician Denis Papin in the 17th century.

The Boiling Point Is Increased by Dissolved Sugar and Salt When salt, sugar, or any other water-soluble substance is added to pure water, the boiling point of the resulting solution becomes higher than the boiling point of water, and the freezing point lower than water's freezing point. Both effects are due to the fact that the water molecules are diluted by the dissolved particles, which interfere with the water molecules as they change phase from liquid to gas or liquid to solid. In the case of the boiling point, the solution contains sugar molecules or salt ions that also absorb heat energy, but cannot themselves turn into a gas. So at water's normal boiling point, there is a smaller proportion of molecules with enough energy to escape from the liquid and form a bubble of vapor, and the cook has to add more energy than usual in order to get those bubbles to form. The boiling point and freezing point rise and fall predictably as the concentration of dissolved sugar or salt increases, a fact that is handy for making both sugar candies and ice creams. When salt, sugar, or any other water-soluble substance is added to pure water, the boiling point of the resulting solution becomes higher than the boiling point of water, and the freezing point lower than water's freezing point. Both effects are due to the fact that the water molecules are diluted by the dissolved particles, which interfere with the water molecules as they change phase from liquid to gas or liquid to solid. In the case of the boiling point, the solution contains sugar molecules or salt ions that also absorb heat energy, but cannot themselves turn into a gas. So at water's normal boiling point, there is a smaller proportion of molecules with enough energy to escape from the liquid and form a bubble of vapor, and the cook has to add more energy than usual in order to get those bubbles to form. The boiling point and freezing point rise and fall predictably as the concentration of dissolved sugar or salt increases, a fact that is handy for making both sugar candies and ice creams.

It's true that adding salt to water raises its boiling point, and so speeds cooking. However, it takes one ounce of salt in a quart of water - around the salinity of the ocean - to raise the boiling point a negligible 1F. A Denverite who wanted to boil water at the same temperature as someone in Boston would have to add more than half a pound of salt to that quart of liquid (225 grams to a liter).

Cooking Below the Boil Though the boil is a handy temperature landmark, it's not necessarily the best temperature at which to cook foods in water. Fish and many meats develop an ideal texture at temperatures around 140F/60C. If they're cooked in boiling water, which is 70F hotter, then the outer portions of the food overcook and dry out while the interior heats through. Lower water temperatures reduce this overcooking, though they also prolong cooking times. A water temperature of 180F/80C, verified by thermometer, offers a good compromise between gentle and efficient cooking. Though the boil is a handy temperature landmark, it's not necessarily the best temperature at which to cook foods in water. Fish and many meats develop an ideal texture at temperatures around 140F/60C. If they're cooked in boiling water, which is 70F hotter, then the outer portions of the food overcook and dry out while the interior heats through. Lower water temperatures reduce this overcooking, though they also prolong cooking times. A water temperature of 180F/80C, verified by thermometer, offers a good compromise between gentle and efficient cooking.

Steaming: Heating by Vapor condensation and Convection Though it's less dense than liquid water and so makes less frequent contact with the food, steam compensates for this loss in efficiency with a gain in energy. It takes a large amount of energy to turn liquid water into a gas, and conversely gaseous water releases that same large amount of energy when it condenses onto a cooler object. So molecules of steam don't just impart their energy of motion to the food; they impart their energy of vaporization also. This means that steaming does an especially quick job of bringing the surface of the food up to the boiling point, and an effective job of keeping it there.

Pan-Frying and Sauteing: Conduction Frying and sauteing are methods that heat foods for the most part by conduction from a hot, oiled pan, with temperatures between 350 and 450F/175225C that encourage Maillard browning and flavor development. The fat or oil has several roles to play: it brings the uneven surface of the food into uniform contact with the heat source, it lubricates and prevents sticking, and it supplies some flavor. As is true in broiling, the trick in frying is to prevent the outside from overcooking before the inside is done. The surface is quickly dehydrated by the high temperatures - odd as it sounds, frying in oil is a "dry" technique - while the interior remains largely water and never exceeds 212F/100C. In order to reduce the disparity between outer and inner cooking times, we generally fry only thin cuts of food. It's also common practice to fry meats at a high initial temperature - to sear them - in order to accomplish the browning, and then to reduce the heat while the interior heats through. Yet another way to avoid overcooking the outer portions of the food is to coat it in another material that develops pleasant flavors when fried, and acts as a kind of insulation to protect the inner food from direct contact with high heat. Breadings and batters are such insulators.

How far back frying goes is hard to tell. The rules for sacrifice in Leviticus 2, which dates from about 600 BCE BCE, distinguish between bread baked in an oven and cooked "on the griddle" or "in the pan." Pliny, in the 1st century CE CE, records a prescription for spleen disease that calls for eggs steeped in vinegar and then fried in oil. And by Chaucer's time, the 14th century, frying was common enough to serve as a colorful metaphor. The Wife of Bath says of her fourth husband That in his owene grece I made hym fryeFor angre, and for verray jalousye.By G.o.d! in erthe I was his purgatorie,For which I hope his soule be in glorie.

Deep Frying: Oil Convection Deep frying differs from pan frying by employing enough oil to immerse the food altogether. As a technique, it resembles boiling more than pan frying, with the essential difference that the oil is heated far above the boiling point of water, and so will dehydrate the food surface and brown it.

Microwaving: Microwave Radiation Microwave ovens transfer heat via electromagnetic radiation, but with waves that carry only a ten-thousandth the energy of infrared radiation from glowing coals. This s.h.i.+ft makes for a unique heating effect. Whereas infrared waves are energetic enough to increase the vibratory movement of nearly all molecules, microwaves tend to affect only polar molecules (p. 793), whose electrical imbalance gives the radiation a kind of handle with which to move them. So foods that contain water are heated directly and rapidly by microwaves. But the oven air, composed of nonpolar nitrogen, oxygen, and hydrogen molecules, and nonpolar container materials like gla.s.s, stoneware, and plastic (made of hydrocarbon chains), are unaffected by the microwaves; the food heats them as it heats up.

Here's how a microwave oven works. A transmitter, very much like a radio transmitter, sets up an electromagnetic field in the oven which reverses its polarity some 2 or 5 billion times every second. (It operates at a frequency of either 915 or 2,450 million cycles per second, compared to wall socket currents at 60 cycles, and FM radio signals at some 100 million cycles per second.) Polar water molecules in the food are pulled by the field to orient themselves with it, but because the field is constantly changing, the molecules oscillate back and forth with it. The water transmits this motion to neighboring molecules by knocking into them, and the temperature of the food as a whole quickly rises.

Metal foil and utensils can be put in the microwave oven with moist foods without causing problems, provided they're reasonably large and kept some distance from the walls and each other to prevent arcing. Fine metallic decoration on china will spark and suffer damage. Foil is useful for partly s.h.i.+elding some foods from the radiation, the thin edges of fish fillets, for example.

Microwave ovens are a recent invention. In 1945 Dr. Percy Spencer, a scientist working for Raytheon in Waltham, Ma.s.sachusetts, filed a patent for the use of microwaves in cooking after he had successfully popped corn with them. This kind of radiation had already been used in diathermy, or deep heat treatment for patients with arthritis, as well as in communications and navigation. Microwave ovens became a popular appliance in the 1970s.

Advantages and Disadvantages of Microwaves Microwave radiation has one great advantage over infrared: the fact that it cooks food much faster. Microwaves can penetrate food to a depth of about an inch/2.5 cm, while infrared energy is almost entirely absorbed at the surface. Because heat radiation can travel to the center of foods only by the slow process of conduction, it's easily beaten by microwaves, with their substantially deeper reach. This reach, along with the microwaves' concentration on heating the food and not its surroundings, results in a very efficient use of energy. Microwave radiation has one great advantage over infrared: the fact that it cooks food much faster. Microwaves can penetrate food to a depth of about an inch/2.5 cm, while infrared energy is almost entirely absorbed at the surface. Because heat radiation can travel to the center of foods only by the slow process of conduction, it's easily beaten by microwaves, with their substantially deeper reach. This reach, along with the microwaves' concentration on heating the food and not its surroundings, results in a very efficient use of energy.

Microwave cooking has several disadvantages. One is that, in the case of meats, speedy heating can cause greater fluid loss and so a drier texture, and makes it more difficult to control doneness. This can be partly overcome by pulsing the oven on and off to slow the heating. Another problem is that microwaves cannot brown many foods unless they essentially dehydrate them, since the food surface gets no warmer than the interior. Thin, radiation-concentrating metal sheets in special microwavable food packaging can help heat the food surface to the point that it browns.

Utensil Materials Finally, a brief discussion of the materials from which we make our pots and pans. We generally want two basic properties in a utensil. Its surface should be chemically unreactive so that it won't change the taste or edibility of food. And it should conduct heat evenly and efficiently, so that local hot spots won't develop and burn the contents. No single material provides both properties.

The Different Behaviors of Metals and Ceramics As we've seen, heat conduction in a solid proceeds either by the movement of energetic electrons, or by vibration in crystal structures. A material whose electrons are mobile enough to conduct heat well is also likely to give up those electrons to other atoms at its surface: in other words, good conductors like metals are usually chemically reactive. By the same token, inert compounds are poor conductors. Ceramics are stable, unreactive mixtures of compounds (magnesium and aluminum oxides, silicon dioxide) whose covalent bonds hold electrons tightly. They therefore transmit heat slowly by means of inefficient vibrations. If subjected to the direct and intense heat of the stovetop, ceramics can't distribute the energy evenly. Hot areas expand while cooler areas do not, mechanical stresses build up, and the utensil cracks or shatters. This is why ceramics are generally used only in the oven, where they encounter only moderate and diffuse heat, or are applied in thin coatings on the surface of metals, so that the metals can do the job of distributing the heat evenly.

Spontaneous Ceramic Coatings on Metals It turns out that most of the metals commonly used in kitchen utensils naturally cover themselves with a very thin layer of ceramic material. Metallic electrons are mobile, and oxygen is electron-hungry. When metal is exposed to the air, the surface atoms undergo a spontaneous reaction with atmospheric oxygen to form a very stable metal oxide compound. (The discoloration on silver and copper that we call It turns out that most of the metals commonly used in kitchen utensils naturally cover themselves with a very thin layer of ceramic material. Metallic electrons are mobile, and oxygen is electron-hungry. When metal is exposed to the air, the surface atoms undergo a spontaneous reaction with atmospheric oxygen to form a very stable metal oxide compound. (The discoloration on silver and copper that we call tarnish tarnish is a metal-sulfur compound; the sulfur comes mainly from air pollution.) These oxide films are both unreactive and fairly tough. Aluminum oxide, when it occurs in crystals rather than on pans, makes up the abrasive called corundum, and is also the princ.i.p.al material of rubies and sapphires (the gem colors come from chromium and t.i.tanium impurities). The problem is that these natural coatings are only a few molecules thick, and are easily scratched through or worn away during cooking. is a metal-sulfur compound; the sulfur comes mainly from air pollution.) These oxide films are both unreactive and fairly tough. Aluminum oxide, when it occurs in crystals rather than on pans, makes up the abrasive called corundum, and is also the princ.i.p.al material of rubies and sapphires (the gem colors come from chromium and t.i.tanium impurities). The problem is that these natural coatings are only a few molecules thick, and are easily scratched through or worn away during cooking.

Metallurgists have found two ways to take advantage of metal oxidation at the pan surface. The film over aluminum can be made up to a thousandth of an inch/0.03 mm thick, and so fairly impervious, by a chemical treatment. And iron can be protected by mixing it with other metals that form a tougher oxide surface and so produce stainless steel (p. 791).

Here are brief descriptions of the materials from which most kitchen utensils are made today, and their particular advantages and disadvantages.

Ceramics Earthenware, Stoneware, Gla.s.s Ceramics are varying mixtures of a number of different compounds, notably the oxides of silicon, aluminum, and magnesium. Ceramics are varying mixtures of a number of different compounds, notably the oxides of silicon, aluminum, and magnesium. Gla.s.s Gla.s.s is a particular variety of ceramic whose composition is more regular, and usually includes a preponderance of silica (silicon dioxide). Until fairly recently, these materials were made from naturally occurring mineral aggregates: the word is a particular variety of ceramic whose composition is more regular, and usually includes a preponderance of silica (silicon dioxide). Until fairly recently, these materials were made from naturally occurring mineral aggregates: the word ceramic ceramic comes from the Greek for "potter's clay." The molding and drying of simple clay pottery, or comes from the Greek for "potter's clay." The molding and drying of simple clay pottery, or earthenware, earthenware, dates from about 9,000 years ago, or about the time that plants and animals were first domesticated. Less porous and coa.r.s.e than earthenware, and much stronger, is dates from about 9,000 years ago, or about the time that plants and animals were first domesticated. Less porous and coa.r.s.e than earthenware, and much stronger, is stoneware, stoneware, which contains enough silica and is fired at a high enough temperature that it vitrifies, or becomes partly gla.s.s. The Chinese invented this refinement sometime before 1500 which contains enough silica and is fired at a high enough temperature that it vitrifies, or becomes partly gla.s.s. The Chinese invented this refinement sometime before 1500 BCE BCE. Porcelain Porcelain is a white but translucent stoneware made by mixing kaolin, a very light clay, with a silicate mineral, and firing at high kiln temperatures; it dates from the T'ang Dynasty (618907 is a white but translucent stoneware made by mixing kaolin, a very light clay, with a silicate mineral, and firing at high kiln temperatures; it dates from the T'ang Dynasty (618907 CE CE). This fine ceramic was introduced to Europe with the tea trade in the 17th century, and in England was first called "Chinaware," and then simply "China." The first gla.s.s containers were not molded or blown, but laboriously sculpted from blocks, and date from 4,000 years ago in the Near East.

The Qualities of Ceramic Pots The outstanding characteristic of ceramic materials is chemical stability: they are unreactive, resist corrosion, and don't affect the flavor or other qualities of foods. (One exception to this rule is the fact that clays and glazes sometimes contain lead, which is a nerve poison, and which can be leached out into acidic foods. Imported ceramic containers made with high-lead clays or glazes still occasionally cause cases of lead poisoning.) Ceramic pots tend to be used only in slow, uniform cooking processes, especially oven baking and braising, because direct high heat can shatter them. Heat-resistant forms of gla.s.s incorporate an oxide of boron that has the effect of reducing thermal expansion by a factor of about 3, and for this reason are less affected by thermal shock, though they're still not immune. The outstanding characteristic of ceramic materials is chemical stability: they are unreactive, resist corrosion, and don't affect the flavor or other qualities of foods. (One exception to this rule is the fact that clays and glazes sometimes contain lead, which is a nerve poison, and which can be leached out into acidic foods. Imported ceramic containers made with high-lead clays or glazes still occasionally cause cases of lead poisoning.) Ceramic pots tend to be used only in slow, uniform cooking processes, especially oven baking and braising, because direct high heat can shatter them. Heat-resistant forms of gla.s.s incorporate an oxide of boron that has the effect of reducing thermal expansion by a factor of about 3, and for this reason are less affected by thermal shock, though they're still not immune.

Enamelware In utensils called In utensils called enamelware, enamelware, powdered gla.s.s is fused into a thin layer onto the surface of iron or steel utensils. This was first done to cast iron early in the 19th century, and today enameled metal is widely used in the dairy, chemical, and brewing industries, as well as on bathtubs. In kitchen utensils, the metal diffuses the direct heat evenly, the ceramic layer is thin enough that it can expand and contract uniformly, and it protects the food from direct contact with the metal. Enamelware is reasonably durable, though it still requires some care: the ceramic layer can be chipped or damaged by quenching a hot pan in cold water. powdered gla.s.s is fused into a thin layer onto the surface of iron or steel utensils. This was first done to cast iron early in the 19th century, and today enameled metal is widely used in the dairy, chemical, and brewing industries, as well as on bathtubs. In kitchen utensils, the metal diffuses the direct heat evenly, the ceramic layer is thin enough that it can expand and contract uniformly, and it protects the food from direct contact with the metal. Enamelware is reasonably durable, though it still requires some care: the ceramic layer can be chipped or damaged by quenching a hot pan in cold water.

The Advantages of Poor Conductivity The poor conductivity of ceramic materials is an advantage if the cook needs to keep food hot. Good conductors like copper and aluminum quickly give up heat to their surroundings, while ceramics retain it well. Similarly, ovens with ceramic (brick) walls are unparalleled for the evenness of their heating. The walls slowly absorb and store large quant.i.ties of energy while the oven is heated up, and then release it when the food is placed inside. Modern metal ovens can't store much heat and so must cycle their heating elements on and off. This causes large temperature fluctuations, and can scorch breads and other foods that are baked at high temperatures. The poor conductivity of ceramic materials is an advantage if the cook needs to keep food hot. Good conductors like copper and aluminum quickly give up heat to their surroundings, while ceramics retain it well. Similarly, ovens with ceramic (brick) walls are unparalleled for the evenness of their heating. The walls slowly absorb and store large quant.i.ties of energy while the oven is heated up, and then release it when the food is placed inside. Modern metal ovens can't store much heat and so must cycle their heating elements on and off. This causes large temperature fluctuations, and can scorch breads and other foods that are baked at high temperatures.

Aluminum Aluminum has been used in pots and pans for barely a century, despite the fact that it's the most abundant metal in the earth's crust. It is never found in nature in the pure state, and a good method for separating the metal from its ore wasn't developed until 1890. In cookware, it is usually alloyed with small amounts of manganese and sometimes copper. Aluminum's prime advantages are its relatively low cost, a heat conductivity second only to copper's, and a low density that makes it lightweight and easily handled. Its ubiquitous presence in the form of foil wrappings and beer and soft drink cans testifies to its usefulness. But because unanodized aluminum develops only a thin oxide layer, reactive food molecules - acids, alkalis, the hydrogen sulfide evolved by cooked eggs - will easily penetrate to the metal surface, and a variety of aluminum oxide and hydroxide complexes, some of them gray or black, are formed. These can mar light-colored foods. Today, most aluminum utensils are either given a nonstick coating or are anodized, anodized, a process that involves making the metal the positive pole (anode) in a solution of sulfuric acid, and so forcing the oxidation of its surface to make a thick protective oxide layer. a process that involves making the metal the positive pole (anode) in a solution of sulfuric acid, and so forcing the oxidation of its surface to make a thick protective oxide layer.

Nonstick Coatings and Silicone "Pans"The materials for nonstick coatings were developed around the middle of the 20th century by industrial chemists, and nonstick utensils were introduced in the 1960s. Teflon and its relatives are long chains of carbon atoms with fluorine atoms projecting from the backbone. They produce a plastic-like material with a smooth, slippery surface, and are as inert as ceramics at moderate cooking temperatures. Above about 500F/250C, however, they decompose into a number of noxious and toxic gases. Nonstick utensils therefore need to be used with care to avoid overheating. The coatings have the additional disadvantage of being easily scratched, and food sticks to the scratches.Beginning in the 1980s, flexible nonstick sheets and containers made of silicone have been used by bakers to line metal baking sheets or replace molded metal pans. Silicone is also a long-chain molecule, with a backbone of alternating silicon and oxygen atoms, and small fat-like carbon chains projecting from it. The backbone gives the material its flexibility, and the hydrophobic projections make the surface behave like a permanently well-oiled pan surface. Food-grade silicones decompose at temperatures above about 480F/240C, so like nonstick pans, silicone bakeware must be used with some caution.

Copper Copper is unique among the common metals because it can be found naturally in the metallic state. For this reason it was the first metal to be used in tool making, about 10,000 years ago. In the kitchen, it is prized for its unmatched conductivity, which makes fast and even heating a simple matter. But copper is also relatively expensive, since its conductivity has made it the preferred material for millions of miles of electrical circuitry. It is troublesome to keep polished, because it has a high affinity for oxygen and and sulfur, and forms a greenish coating when exposed to air. Most important, copper cookware can be harmful. Its oxide coating is sometimes porous and powdery, and copper ions are easily leached into food solutions. Copper ions can have useful effects: they stabilize foamed egg whites (p. 102), and the green color of cooked vegetables is improved by their presence. But the human body can excrete copper in only limited amounts, and excessive intake may cause gastrointestinal problems and, in more extreme cases, liver damage. No one will be poisoned by the occasional meringue whipped in a copper bowl, but bare copper isn't a good candidate for everyday cooking. To overcome this major drawback, manufacturers line copper utensils with stainless steel or, more traditionally, with tin. Tin has its own limitations (p. 791). sulfur, and forms a greenish coating when exposed to air. Most important, copper cookware can be harmful. Its oxide coating is sometimes porous and powdery, and copper ions are easily leached into food solutions. Copper ions can have useful effects: they stabilize foamed egg whites (p. 102), and the green color of cooked vegetables is improved by their presence. But the human body can excrete copper in only limited amounts, and excessive intake may cause gastrointestinal problems and, in more extreme cases, liver damage. No one will be poisoned by the occasional meringue whipped in a copper bowl, but bare copper isn't a good candidate for everyday cooking. To overcome this major drawback, manufacturers line copper utensils with stainless steel or, more traditionally, with tin. Tin has its own limitations (p. 791).

Iron and Steel Iron was a relatively late discovery because it exists in the earth's crust primarily in the form of oxides, and had to be encountered in its pure form by accident, perhaps when a fire was built on an outcropping of ore. Iron artifacts have been found that date from 3000 BCE BCE, though the Iron Age, when the metal came into regular use without replacing copper and bronze (a copper-tin alloy) in preeminence, is said to begin around 1200 BCE BCE. Cast iron Cast iron is alloyed with about 3% carbon to harden the metal, and also contains some silicon; is alloyed with about 3% carbon to harden the metal, and also contains some silicon; carbon steel carbon steel contains less carbon, and is heat-treated to obtain a less brittle, tougher alloy that can be formed into thinner pans. The chief attractions of cast iron and carbon steel in kitchen work are their cheapness and safety. Excess iron is readily eliminated from the body, and most people can actually benefit from additional dietary iron. Their greatest disadvantage is a tendency to corrode, though this can be avoided by regular seasoning (below) and gentle cleaning. Like aluminum, iron and carbon steel can discolor foods. And iron turns out to be a poorer conductor of heat than copper or aluminum. But exactly for this reason, and because it's denser than aluminum, a cast iron pan will absorb more heat and hold it longer than a similar aluminum pan. Thick cast iron pans provide steady, even heat. contains less carbon, and is heat-treated to obtain a less brittle, tougher alloy that can be formed into thinner pans. The chief attractions of cast iron and carbon steel in kitchen work are their cheapness and safety. Excess iron is readily eliminated from the body, and most people can actually benefit from additional dietary iron. Their greatest disadvantage is a tendency to corrode, though this can be avoided by regular seasoning (below) and gentle cleaning. Like aluminum, iron and carbon steel can discolor foods. And iron turns out to be a poorer conductor of heat than copper or aluminum. But exactly for this reason, and because it's denser than aluminum, a cast iron pan will absorb more heat and hold it longer than a similar aluminum pan. Thick cast iron pans provide steady, even heat.

"Seasoning" Cast Iron and Carbon Steel Cooks who appreciate cast iron and carbon steel pans improve their easily corroded surface by building up an artificial protective layer. They "season" them by coating them with cooking oil and heating them for several hours. The oil penetrates into the pores and fissures of the metal, sealing it from the attack of air and water. And the combination of heat, metal, and air oxidizes the fatty acid chains and encourages them to bond to each other ("polymerize") to form a dense, hard, dry layer (just as linseed and other "drying oils" doon wood and on paintings). Highly unsaturated oils - soy oil, corn oil - are especially p.r.o.ne to oxidation and polymerizing. To avoid removing the protective oil layer, cooks carefully clean seasoned cast iron pans with mild soaps and a dissolving abrasive like salt, rather then with detergents and scouring pads. Cooks who appreciate cast iron and carbon steel pans improve their easily corroded surface by building up an artificial protective layer. They "season" them by coating them with cooking oil and heating them for several hours. The oil penetrates into the pores and fissures of the metal, sealing it from the attack of air and water. And the combination of heat, metal, and air oxidizes the fatty acid chains and encourages them to bond to each other ("polymerize") to form a dense, hard, dry layer (just as linseed and other "drying oils" doon wood and on paintings). Highly unsaturated oils - soy oil, corn oil - are especially p.r.o.ne to oxidation and polymerizing. To avoid removing the protective oil layer, cooks carefully clean seasoned cast iron pans with mild soaps and a dissolving abrasive like salt, rather then with detergents and scouring pads.

Stainless Steel The important exception to the rule that metals form protective surface coatings is iron, which rusts in the presence of air and moisture. The orange complex of ferric oxide and water (Fe2O3 2 2O) is a loose powder rather than a continuous film, and so does not protect the metal surface from further contact with the air. Unless it's protected by some other means, iron metal will corrode continuously (this is why pure iron is not found in nature). Efforts to make this cheap and abundant element more resistant to rusting resulted in the 19th century in the development of stainless steel, stainless steel, an iron-carbon alloy that - in cookware - is formulated with about 18% chromium and 810% nickel. Chrome is synonymous with bright and permanent s.h.i.+niness because chromium is extremely p.r.o.ne to oxidation and naturally forms a thick protective oxide coat. In the stainless steel mixture, oxygen reacts preferentially with the chromium atoms at the surface, and the iron never gets the opportunity to rust. an iron-carbon alloy that - in cookware - is formulated with about 18% chromium and 810% nickel. Chrome is synonymous with bright and permanent s.h.i.+niness because chromium is extremely p.r.o.ne to oxidation and naturally forms a thick protective oxide coat. In the stainless steel mixture, oxygen reacts preferentially with the chromium atoms at the surface, and the iron never gets the opportunity to rust.

This chemical stability is bought at a price. Stainless steel is more expensive than cast iron and carbon steel, and it's an even poorer heat conductor. The addition of large numbers of foreign atoms apparently interferes with electron movement by causing structural and electrical irregularities in the metal. The transfer of heat in a stainless pan can be evened out by coating the underside of the pan with copper, or by inserting a copper or aluminum plate in the pan bottom, or by making the pan out of two or more layers, with a good conductor just under the surface. Of course these refinements add further to the cost of the utensil. Still, these hybrids are the closest thing we have to the ideal chemically inert but thermally responsive pan.

Tin Tin was probably first used in combination with copper to make the mechanically tougher alloy called bronze. Today tin is generally found only as a nontoxic, unreactive lining in copper utensils. This limited role is the result of two inconvenient properties: a low melting point, 450F/230C, that can be reached in some cooking procedures, and a softness that makes the metal very susceptible to wear. The tin alloy called pewter, pewter, which used to contain some lead and now is made with 7% antimony and 2% copper, is not much used today. which used to contain some lead and now is made with 7% antimony and 2% copper, is not much used today.

Chapter 15.

The Four Basic Food Molecules

Water Water Clings Strongly to ItselfWater Is Good at Dissolving Other SubstancesWater and Heat: From Ice to SteamWater and Acidity: The pH Scale Fats, Oils, and Relatives: Lipids Lipids Don't Mix with WaterThe Structure of FatsSaturated and Unsaturated Fats, Hydrogenation, and Trans Fatty AcidsFats and HeatEmulsifiers: Phospholipids, Lecithin, Monoglycerides Carbohydrates SugarsOligosaccharidesPolysaccharides: Starch, Pectins, Gums Proteins Amino Acids and PeptidesProtein StructureProteins in WaterProtein DenaturationEnzymes This chapter describes the characters of the four chemical protagonists in foods and the cooking process, the molecules referred to constantly in the first fourteen chapters.