On Food And Cooking: The Science And Lore Of The Kitchenby Harold McGee
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Harold McGee's On Food and Cooking is a kitchen classic. Hailed by Time magazine as "a minor masterpiece" when it first appeared in 1984, On Food and Cooking is the bible to which food lovers and professional chefs worldwide turn for an understanding of where our foods come from, what exactly they're made of, and how cooking transforms them into something new and delicious.
Now, for its twentieth anniversary, Harold McGee has prepared a new, fully revised and updated edition of On Food and Cooking. He has rewritten the text almost completely, expanded it by two-thirds, and commissioned more than 100 new illustrations. As compulsively readable and engaging as ever, the new On Food and Cookingprovides countless eye-opening insights into food, its preparation, and its enjoyment. On Food and Cooking pioneered the translation of technical food science into cook-friendly kitchen science and helped give birth to the inventive culinary movement known as "molecular gastronomy." Though other books have now been written about kitchen science, On Food and Cooking remains unmatched in the accuracy, clarity, and thoroughness of its explanations, and the intriguing way in which it blends science with the historical evolution of foods and cooking techniques.
Among the major themes addressed throughout this new edition are:
- Traditional and modern methods of food production and their influences on food quality
- The great diversity of methods by which people in different places and times have prepared the same ingredients
- Tips for selecting the best ingredients and preparing them successfully
- The particular substances that give foods their flavors and that give us pleasure
- Our evolving knowledge of the health benefits and risks of foods
On Food and Cooking is an invaluable and monumental compendium of basic information about ingredients, cooking methods, and the pleasures of eating. It will delight and fascinate anyone who has ever cooked, savored, or wondered about food.
- Da Jia Chu Ban She
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On Food and Cooking
By Harold J. McGee
Scribner Book CompanyCopyright © 1984 Harold J. McGee
All right reserved.
Milk and Dairy Products
THE NATURE OF MILK
The History of Dairying
What better subject for the first chapter than the food with which we all begin our lives? By our very biological nature as mammals (from the Latin mamma, meaning "breast"), we humans take mother's milk as our first food, and have done so from the beginning of our species, a million or more years ago. Less easy to ascertain is when we first drank the milk of other mammals with any regularity. This probably came well after the domestication of animals, which were first of all a source of meat and skins. The archaeological evidence suggests that sheep and goats were domesticated in Eurasia around 11,000 and 9,500 years ago respectively, at least a millennium before the larger, much less manageable cattle, which came under human control about 8,500 years ago. The smaller animals would have been much easier to milk, and were probably the first dairy animals. Rock drawings from the Sahara show that dairying was known by 4000 B.C., and what appear to be the remains of cheese have been found in Egyptian tombs dating back to 2800 B.C. In any case, dairy products are a relatively recent addition to the human diet, roughly contemporaneous with such other innovations as bread, beer, and wine.
By the time that the Old Testament began to be set down, roughly 3,000 years ago, milk-and its products had become familiar enough to serve as metaphors or analogies for less immediate, more abstract conditions. The Promised Land is described over and over again as a land "flowing with milk and honey": a durable image of plenty. And in a sentence that suggests both creation and violence -- the bringing forth of form and substance -- Job asks God rhetorically, "Hast thou not poured me out as milk, and curdled me like cheese?" The process of curdling seems to have been especially intriguing to those who pondered the transition from chaos to order. Aristotle used it in On the Generation of Animals to explain human conception.
The male provides the "form" and the "principle of the movement," the female provides the body, in other words the material. Compare the coagulation of milk. Here, milk is the body, and the fig-juice or rennet contains the principle which causes it to set.
A much more recent and wonderfully strange version of the cheese analogy of creation has been uncovered by the Italian scholar Carlo Ginzburg in the records of the Inquisition. According to his study, The Cheese and the Worms, a miller named Menocchio, from the town of Friuli, was put to death in 1599 for various heretical views, including this account of the making of earth and heaven:
I have said that, in my opinion, all was chaos, that is, earth, air, water, and fire were mixed together; and out of that bulk a mass formed -- just as cheese is made out of milk -- and worms appeared in it, and these were angels. The most holy majesty declared that these should be God and the angels....
Still other analogies live on unnoticed in contemporary English. From the Greek for milk, gala, came galaxis, "milky way," the origin of our "galaxy." And from the Latin lac came lactuca, ancestor of our "lettuce" (which exudes a milky sap when cut from its roots).
Dairy products were important foods all over early Europe, though preferences varied from region to region. Neither fresh milk nor butter was very popular in Greece or Rome, while cheese was. The reverse was true of northern Europe and Asia. Milk and butter would have spoiled quickly in the Mediterranean climate, which offered the olive as an alternative source of oil. The Greeks and Romans commonly referred to the barbarians as "milk-drinkers" (Greek: galaktopotes), so remarkable did this habit seem to them. In the 5th century B.C., the Greek historian Herodotus described the Massagetai, inhabitants of the Caucasus, in this way: "They sow no crops but live on livestock and fish, which they get in abundance from the river Araxes; moreover, they are drinkers of milk." And 500 years later, the Roman Pliny said that butter was considered to be "the most delicate of foods among barbarous nations, and one which distinguishes the wealthy from the multitude at large." He wondered at the fact that "the barbarous nations, who live on milk, do not know or disdain the value of cheese," but also noted that the cheeses most favored at Rome came from the provinces that are now parts of France and Switzerland. By Pliny's account, fresh milk was as much a cosmetic as a food, at least among the ruling classes.
Milk is valued for giving a part of its whiteness to the skin of women. Poppea, wife of Domitius Nero, took 500 nursing asses everywhere in her travelling party, and soaked herself completely in a bath of this milk, in the belief that it would make her skin more supple.
Over a thousand years later, another writer from the rim of the Mediterranean reported oh the strange uses to which northerners put milk. The Venetian Marco Polo traveled to, from, and in China between 1271 and 1295, and observed the nomadic Tartars as they prepared milk from their mares (horses being more mobile and versatile than cattle). The Tartar armies, wrote Polo,
can march for ten days together without preparing meat, during which time they subsist upon the blood drawn from their horses, each man opening a vein and drinking from his own. They make provisions also of milk, thickened or dried to the state of a hard paste, which they prepare in the following manner. They boil the milk, and skimming off the rich or creamy part as it rises to the top, put it into a separate vessel as butter; for so long as that remains in the milk, it will not become hard. The milk is then exposed to the sun until it dries. [When it is to be used,] some is put into a bottle with as much water as is thought necessary. By their motion in riding, the contents are violently shaken, and a thin porridge is produced, upon which they make their dinner.
Besides this precursor of powdered milk, the Tartars also enjoyed a dairy product "with the qualities and flavor of white wine." Not living the settled life that makes it possible to brew beer from grain or wine from grapes, they coupled the alcoholic fermentation of yeasts with milk to produce koumiss. A similar beverage has been made in the Balkans for many centuries; there it is called kefir.
Changes in the handling of milk came very slowly from the Middle Ages through the eighteenth century. Milking, churning, and cheese making, all hard work, were done by hand, and, at least in England, done mostly by women. The word dairy was originally dey-ery, with dey meaning a woman servant in Middle English (in Old English, it meant "kneader," "maker of bread"; "lady" shares this root). In other European languages, the words equivalent to "dairy" have, appropriately, something to do with milk.
The making of cheese, yogurt, and other fermented products was largely uncontrolled, with microbes from the air or left over from the previous batch, whether desirable or not, colonizing the milk. Apparently none of these foods was known in North America until the arrival of the Europeans, and the first dairy herd was not established here until about 1625. While farmers may have enjoyed wholesome milk, city-dwellers generally saw only watered-down, adulterated, disease-carrying milk hauled in open containers through the streets (see Tobias Smollett's description on page 501). With the Industrial Revolution, of course, much changed. Railroads, steam power, and refrigeration made fresher milk available to a larger population. Milking machines and automatic churners appeared in the 1830s, specialized cheese factories in the 1850s, margarine in the 1870s. By the turn of the century, purified bacterial cultures were being used to control the quality of cheese more closely. Today, dairying has split up into several very big businesses, with nothing of the dey left about them.
Milk in the Diet
Milk is a white, opaque liquid secreted by female mammals to nourish their newly born offspring. In a sense, it takes on the duties performed by the mother's blood during gestation, and early students of human development and medicine often spoke of these two fluids as related to each other. Milk is an especially valuable food, then, because, like the contents of the chicken egg, it is meant to be the sole sustaining food of the animal during part of its early life. Though nearly all milks contain the same battery of substances -- water, proteins, fat, milk sugar, or lactose, various vitamins and minerals -- the relative proportions of these substances vary greatly from species to species. Generally, animals that grow rapidly are fed with milk high in protein and minerals, and those that need to develop a thick layer of insulating fat (in particular, seals, sea lions, and walruses) receive high-fat, low-sugar or no-sugar milks. A calf doubles its weight at birth in 50 days, a human infant in 100 -- and cow's milk contains about three times the protein and minerals of mother's milk.
Two other differences between human and bovine milks are worth mentioning. Human milk is more easily digested both because there is less protein in it, and because less of that protein curdles in stomach acid (molecules at the center of the curd are not as easily reached by digestive enzymes as are those that remain in solution). Homogenization, pasteurization, and cooking all cause milk proteins to form weaker, looser curds than they normally do, and so improve the digestibility of cow's milk. Second, human milk alone contains the so-called "bifidus factor," an as-yet-unidentified substance that promotes the growth of Lactobacillus bifidus. This harmless bacterium populates the infant's digestive tract and excretes as a waste product lactic acid, which helps to inhibit the growth of other, harmful microbes. This growth factor, together with antibodies from the mother against such ingestible pathogens as polio, salmonella, and coliform bacteria, help the baby over the transition from a shielded development in the womb to life in an environment filled with germs.
In the animal world, humans are exceptional for drinking milk of any kind after they have started eating solid food. In fact, those people who drink milk after infancy are the exception within the human species. This is not simply a matter of tradition or choice. The milk sugar lactose, which accounts for about half of the calories in milk, is a disaccharide; each of its molecules consists of one glucose and one galactose unit joined together. It is found in no other food: in fact, the only sources of lactose currently known aside from the mammal's breast are forsythia flowers and a few tropical shrubs, which produce it only in very small quantities. Now, all multiunit sugars, including table sugar (sucrose), must be broken down into their components by digestive enzymes in the small intestine before they can be absorbed and used by the body. The lactose-breaking enzyme, lactase, reaches its maximum levels in the human intestine shortly after birth, and thereafter declines slowly, with a steady minimum coming between 1 1/2 and 3 1/2 years of age. The logic of this trend is obvious: it is a waste of resources for the body to produce an enzyme when it is no longer needed. Once most mammals are weaned, they never encounter lactose again, and this was also true of humans until the beginnings of animal husbandry. But if substantial amounts of lactose are consumed in the absence of lactase, then the sugar passes through the small intestine without being absorbed and reaches the colon intact. Some colonic bacteria ferment lactose, producing carbon dioxide gas in the process, and thereby cause real distress. The presence of sugar in the colon can also cause water retention, which may be manifested in a bloated feeling or diarrhea.
It was not until the late 1960s that Western medical science realized that adult lactase deficiency -- the inability to digest milk sugar -- was the rule rather than the exception, and was not really a deficiency at all but the natural state of affairs. It took this long because most Westerners, in particular those of northern European background, are capable of digesting lactose in adulthood. Their lactase levels, and those of a couple of nomadic African tribes, do not drop off as drastically as those of the rest of the world's population. The "ethnic chauvinism," as it has been called, of assuming that everyone could digest milk led to such policies as shipping surplus powdered milk to famine areas where it could do as much harm as good. This attitude was overcome only when researchers noticed that about 70% of American blacks are intolerant of lactose, while the figure for whites is closer to 10%. A few years of study showed that lactose-tolerant adults are a distinct minority on this planet.
This is not to say that only a minority can eat dairy products. Most lactose-intolerant adults can consume about a pint of milk per day, which provides valuable amounts of several nutrients, without severe symptoms. (This is not true of those people who are allergic to milk proteins. Lactose intolerance is not an allergy.) And cheese, yogurt, and other cultured foods are practically free of lactose because the fermenting bacteria use it as fuel. But the practice of dairying would have been unlikely to develop in populations with a low tolerance for milk itself. It has been suggested that the genetic trait of continuing lactase production arose in the people of northern Europe because it conferred the advantages of increased calcium intake and improved absorption of that mineral (one of the effects of lactose in the small intestine) on a group whose dark, cold environment developed little vitamin D in the skin. Of course, the ability to drink milk also simply widens the range of foods on which people can survive. In any case, lactose tolerance is probably a very recent adaptation. Mother's milk has been drunk on our branch of the phylogenetic bush for millions of years, but sheep, goats, and cows have been milked only for a few thousand.
The Milking Cycle
A cow must give birth before it will begin to give milk. The mammary glands are activated by hormones produced at the end of pregnancy, and are stimulated to continue secreting milk by regular milkings. It is possible to initiate the secretion of milk by administering hormones to the cow artificially, but this technique is not used; dairy herds do need to be perpetuated. The interval between insemination and birth is 282 days, and even with continuous milking a cow will dry up about 10 months after calving. The optimum sequence for milk production is to milk a cow for 10 months after it calves, let it go dry for 2 months, have it calf again, and then repeat the cycle. In order to meet this schedule, the cow is bred 90 days after calving.
The first fluid secreted by the mammary glands is not milk, but colostrum, a clear solution of concentrated protein, vitamins, and antibodies. As soon as the colostrum flow has ceased and the milk is salable -- a matter of a few days -- the calf is put on a diet of reconstituted and soy milks, and the Cow is milked by the farmer two or three times a day to keep the secretory cells working at full capacity. Many factors can influence the particular balance of nutrients and the taste of the milk, including the breed and age of the animal; its feed, the stage in its lactation period, the season of the year, and even odors in the barn.
The Composition of Milk
Both physically and chemically, milk is a complex material. Globules of milk fat, complexes of protein and salts, and dissolved sugar, vitamins, and other salts and proteins all swim in the water that accounts for the bulk of the fluid. The fat and proteins are by far the most important components, and we will examine them in some detail. A few words only about the remainder. A wide range of salts is found in milk, with sodium, potassium, calcium, magnesium, chloride, phosphate, sulfate, citrate, and bicarbonate ions among the more populous. Milk is also slightly acidic, with a pH between 6.5 and 6.7. Both the salt concentrations and the acidity affect the behavior of the proteins, as we shall see. All of the recognized vitamins are present in milk, though some, such as vitamin C, come only in tiny quantities. Vitamin A and its chemical precursor carotene are carried in the fat globules and give milk and butter a yellowish cast; riboflavin, which has a greenish color, can sometimes be seen in skim milk or in the whey that separates during cheese making. Finally, milk contains lactose, a sugar that is only one tenth as soluble in water as table sugar. As a result of this low solubility, lactose crystals form readily in such products as ice cream and condensed milk, and do not dissolve very quickly on the tongue: they can give a persistently gritty, "sandy" texture to these foods.
Milk fat is important nutritionally, aesthetically, and economically. It carries the fat-soluble vitamins, essential fatty acids, and about half the calories; it contributes to milk's characteristic taste and texture; and the higher the milk fat content, the more butter or cream can be made from the milk, and so the higher the price it will bring. Most cows secrete more fat in winter, more as a particular lactation period gets on, and more at the end of a particular milking than at the beginning. Specialized cells in the mammary gland release it in the form of globules that are 1 to 5 microns, or about 0.0001 inch, in diameter. This is close to the size of the cells themselves, and in fact each globule brings with it a remnant of its origin: a thin membrane or outer sheath that was part of the cell's membrane as it squeezed the globule out (see page 50 for a view of the globule membrane through the scanning electron microscope). The membrane is composed largely of proteins and phospholipids (phosphate-fatty acid complexes) which stabilize the globules by preventing them from pooling together into one large mass of fat; it also carries most of the vitamin A and cholesterol.
While the globule membranes may prevent the sheathed fat from coalescing into blobs, they do appear to cause the discrete globules to cluster together. When fresh milk is allowed to stand for a while, the fat globules tend to rise and form a fat-rich layer at the top of the container. This phenomenon is called creaming, and it was the means by which cream was obtained from milk until the last century, when centrifuges were developed to do the job much more rapidly and thoroughly. The globules rise, of course, because fat is lighter than water. Given the viscosity of the water phase and the size of the globules, it should be possible to calculate how fast creaming will occur in milk; but the actual rate greatly exceeds the theoretical prediction. It is thought that the membranes are responsible for this, by causing the initial aggregation of globules into larger masses that can rise more rapidly. Few of us see creaming any more because most milk is now homogenized precisely to prevent it from separating into layers.
Two Protein Classes: Curds and Whey
There are a great many different kinds of protein -- perhaps dozens -- floating around in milk, from those meant to nourish the calf, to the globulins that fight disease organisms, to various enzymes that break down and build up other molecules. One of these is the enzyme that synthesizes lactose from its constituent units. The presence of such biochemical machinery has led to the description of milk as an "unstructured tissue," since even after it leaves the animal's body it retains some of the characteristics of a living system. When it comes to the culinary behavior of milk, we can fortunately reduce the protein population to two basic groups: little Miss Muffet's curds and whey. The two are distinguished by their reactions to acid and rennin, an enzyme from the stomach of a calf that is used to make cheese. The curd protein, casein, coagulates and forms solid clumps, while the whey proteins, principal among them lactoglobulin, remain suspended in the liquid.
Take casein first. It is not a single kind of molecule, but consists of several different components that are bound up, together with some calcium and phosphate ions, into bundles about 1/10 micron (a few millionths of an inch) across. These bundles are called micelles, and along with the larger fat globules, they give milk its milky, opalescent appearance by deflecting light rays as they pass through the liquid. The major component of casein itself has a subunit that somehow stabilizes that component and keeps it dispersed in separate micelles. When this protein subunit is removed from the casein micelles -- and this is just what the calf enzyme rennin does -- then the micelles react with free calcium ions, which act as bridges between them. The micelles clot together and form the curd that is treated further to make cheese.
Casein proteins will also coagulate to form curds under other conditions, and the extraction of a chemical subunit need not be involved. If enough acid is added to milk -- whether in the form of fruit or vegetable tissue, or the lactic acid produced by certain bacteria -- curdling will occur. For all proteins there is a particular range of surrounding acidity that will cause them to aggregate, while outside this range the molecules have a net electrical charge that repels them from each other. The pH of milk is normally about 6.5, and if it is lowered to about 5.3, the casein micelles lose their negative charge and clump together. Added salt will have the same effect since it releases positive and negative ions into the solution and changes the electrical environment, but the threshold level for curdling is unbearably salty to the palate. Milk is normally resistant to heat curdling, but small amounts of acid or salt can sensitize the casein fraction and cause it to clump when cooked.
Apart from its function as a food, casein has been exploited for its adhesive properties in the manufacture of paper, plastics, and glue.
In contrast to casein, the whey proteins, and especially the antibodies, are quite resistant to denaturation and curdling, perhaps because it is important that they remain intact in the gut to do their protective work. It takes sustained temperatures near boiling and a pH of about 4.6 to coagulate the whey proteins, conditions that are generally met only when whey cheeses are being made. Less drastic heating does, however, involve the whey in important changes that affect the character of the milk. For one thing, the lactoglobulin contains most of the sulfur atoms in milk along its side chains. When it is heated to about 165°F (74°C), these atoms are sufficiently exposed to the surrounding liquid that they react with hydrogen ions and form hydrogen sulfide (H2S), a gas with a powerful aroma that in small quantities contributes to the cooked aroma of many foods. And in the same temperature range, the lactoglobulin apparently unfolds into a form that interferes with the aggregation of casein micelles. As a result, the process of clotting is slowed down, and the final curd is softer than usual. This turns out to be one of the problems with using pasteurized milk in cheese making, but is an advantage in making yogurt, as we shall see.
The Flavor of Milk
The flavor of milk is normally rather bland -- or subtle -- and alterations in it are easily noticed and quickly off-putting. The major contributors to the flavor of fresh milk are the slightly sweet lactose, the salts, and the light odors of short-chain fatty acids and sulfur compounds. Undesirable off-flavors can be caused by several different conditions. They may be native to the milk because the cow breathed foul air or are tainted feed. The growth of bacteria that excrete lactic acid will sour milk, while the by-products of other microbes can give it stale, bitter, or putrid flavors.
UNFERMENTED DAIRY PRODUCTS
Milk is sold in a variety of forms today, but very few of us ever see raw milk any more. Ordinary drinking milk is routinely subjected to pasteurization, homogenization, and vitamin fortification, and condensed products to even more drastic treatments.
Most milk sold for direct human consumption has been pasteurized, or heated hot and long enough to destroy all disease-causing organisms and most others as well. Since it is by design a very nutritious substance, milk is quite hospitable to microbes, and is easily contaminated by contact with bovine or human skin and with milking equipment. Tainted milk has been known to transmit such serious diseases as tuberculosis and undulant fever to people. In the 1820s, long before the germ theory of disease had been proposed, some books on domestic economy advocated boiling all milk before using it. The great French scientist Louis Pasteur studied the spoilage of wine and beer in the 1860s and developed a heat treatment that preserved these fluids without greatly injuring their flavor. Pasteurization was not applied to milk in the United States until around the turn of the century, but by the 1940s individual states had begun to require it. Nowadays, with a system of distribution that includes handling large quantities at once, shipping fairly long distances, and stocking in supermarkets, pasteurization is a practical necessity. It extends the shelf life of milk not only by killing microbes, but also by inactivating enzymes native to milk, especially the fat-splitters, whose slow but steady activity can make it unpalatable.
There are many different combinations of temperature and time that pasteurize milk, but a few are by far the most commonly employed. One standard method is to heat the milk to 144°F (62°C) and hold it there for 80 minutes; another keeps it at 160°F (71°C) for 15 seconds. The first has the advantage of staying well below the temperature at which a cooked flavor develops, and while the second flirts with this limit, it is much faster. Pasteurization does not normally liberate the strongly aromatic gas hydrogen sulfide from the protein lactoglobulin, but it does change the flavor slightly by evaporating away some volatile molecules and creating new ones. Ultra-pasteurization, in which a temperature of 280°F (188°C) is held for one second, is a more severe treatment that does leave behind a cooked flavor. It is normally applied only to cream, which we tend to use more slowly and keep longer than we do milk.
This treatment, whose name comes from the Greek for "of the same kind," involves forcing the milk at high pressure through a very small nozzle onto a hard surface; it breaks the fat globules up into more uniform particles about a quarter of their original size. Homogenization was developed in France around 1900 to prevent creaming, because once the fat globules have aggregated and risen, it is difficult to redisperse them evenly in the milk. When broken down to about a micron in diameter, the individual globules are too small to rise alone, and because their surface area has multiplied beyond the covering capacity of their membranes, some of the other dissolved milk proteins fill in, and apparently interfere with globule aggregation. As a result, the fat remains evenly dispersed in the milk. Homogenized milk is whiter, blander, less stable to heat, and more sensitive to spoilage by light than unhomogenized milk, and the process can also have undesirable effects on the whipping of cream and on cheese making, as we shall see.
Fresh milk is never homogenized as is, because it will go rancid in a matter of minutes. When stripped of some of its protective membrane, the fat is exposed to the activity of fat-splitting enzymes in the milk, and these quickly produce unappetizing quantities of odiferous free fatty acids. The enzymes are inactivated by high temperatures, and accordingly all milk is pasteurized before or simultaneously with homogenization.
In addition to being pasteurized and homogenized, most milk is fortified with the fat-soluble vitamins A and D; it is a good source for these vitamins, riboflavin, calcium and phosphorus, and high-quality protein. The diversity that we see in the dairy case comes with a number of further alterations in milk composition. Low-fat milks are made by centrifuging some of the globules off before homogenization. Whole milk is about 4% fat, low-calorie milks 1 or 2% ("99% fat-free" doesn't mean that 99% of the fat has been removed, but only that the percentage has been lowered from 4 to 1), and skim milk between 0.1% and 0.2%. Additional milk solids can be used to fortify low-fat milks with more protein and restore some of the body that is lost when the fat is removed. "Acidophilus" milk is designed for people with lactose intolerance; it has been cultured with Lactobacillus acidophilus, a bacterium that consumes the lactose and produces lactic acid in the process, and sometimes has a sour taste.
Storage and Cooking
Milk is a highly perishable food. Even Grade A pasteurized milk contains tens of millions of bacteria in every half-gallon, and will sour quickly unless refrigerated. A temperature just above freezing would be ideal, though few home refrigerators are set that low.
It is less well known that the flavor of milk can be altered by the electrical energy carried in ordinary daylight. In the chemical reaction called "autoxidation," light energizes an oxygen atom, which then invades the long, regular chains of carbon and hydrogen atoms in the fats and disrupts their structure (see page 608). Fragments of the hydrocarbon chain split off and react with each other to produce various small, volatile, and odorous molecules; oily, fishy, metallic, and other unpleasant overtones accumulate. Indirect daylight is intense enough to fuel this deterioration. Direct sunlight, with its far greater intensity, causes not only autoxidation, but "sunlight flavor" as well. This burnt or cabbagelike flavor is the result of a reaction between an amino acid, methionine, and the vitamin riboflavin, which is destroyed in the process. So for both nutritional and gustatory reasons, clear glass or plastic containers of milk should be kept in the dark as much as possible.
Whether fluid milk is used to make a soup or a sauce, scalloped potatoes or hot chocolate, the tendency of its proteins to coagulate can cause problems. The skin that forms on the surface of boiled milk or cream soups is a complex of casein and calcium, and results from evaporation of water at the surface and the subsequent concentration of protein there. If you skim off the skin, you remove significant amounts of valuable nutrients. Skin formation can be minimized by covering the pan or whipping up a little foam; both actions slow evaporation. Milk scorches easily because relatively dense complexes of casein micelles and whey proteins fall to the pan bottom, stick, and burn. A moderate flame or a double boiler is the best safeguard against scorching. Large foreign molecules -- starch, vegetable or fruit cellulose and hemicelluloses, sugars, fats -- can also cause some general coagulation, and so a curdled appearance, by providing sites at which the milk proteins can gather and begin to clump, And acid in the juices of all fruits and vegetables, and phenolic compounds in such things as potatoes and tea, make the milk proteins more sensitive to coagulation and curdling at cooking temperatures. Fresh milk and careful control of the burner are the best weapons against curdling.
Condensed milk products are useful because they supply milk fat and solids in concentrated form, and are treated to keep for at least several months. Ordinary condensed or evaporated milk is made by rapidly evaporating off about half its water, not by heating it, but by putting it under a vacuum. The resulting liquid is then sterilized so that it will keep indefinitely, and then homogenized. The high temperature reached in sterilization, together with the greater concentration of lactose, causes that sugar to undergo some browning, and this gives evaporated milk its characteristic note of caramel. In sweetened condensed milk, the caramel flavor is replaced by more intense sweetness. Sugar is added in the amount of about 2 pounds per 10 pounds of condensed milk, a level that makes the liquid uninhabitable for microbes by raising its osmotic pressure (see page 170). Accordingly, sweetened condensed milk need not be sterilized. Powdered or dried milk, of course, is the result of taking evaporation to the extreme. With all but a tiny fraction of the original water removed, it is impossible for microbes to grow on it. Most powdered milk is made from low-fat milk because the fat quickly goes rancid in contact with solid salts and plenty of atmospheric oxygen, and because it tends to coat the particles of protein and so makes subsequent mixing with water difficult. Powdered milk will keep for several months in dry, cool surroundings. It is primarily used in and best suited to batters and doughs for baked goods, where the lack of water is an advantage and the lack of flavor no great disadvantage. Without any fat or volatile aroma molecules left, dried milk is not very appealing when mixed with water and sampled straight.
The word cream comes from the Greek chriein, which means "to anoint," and which is also the root of Christ ("the anointed one"). The link between ancient ritual and rich food is oil, the substance used to anoint the chosen, and the defining element of cream. Cream is a form of milk in which the fat globules have become more concentrated than usual, whether by rising to the top in a bottle or spinning off from the heavier water phase in a centrifuge. There are three grades of cream marketed today: light cream is between 18 and 80% butterfat, light whipping cream between 80 and 36%, and heavy whipping cream between 36 and 40%. Whole milk, by contrast, is closer to 4% fat. "Half-and-half" is, as its na batters and doughs for baked goods, where the lack of water is an advantage and the lack of flavor no great disadvantage. Without any fat or volatile aroma molecules left, dried milk is not very appealing when mixed with water and sampled straight.
The word cream comes from the Greek chriein, which means "to anoint," and which is also the root of Christ ("the anointed one"). The link between ancient ritual and rich food is oil, the substance used to anoint the chosen, and the defining element of cream. Cream is a form of milk in which the fat globules have become more concentrated than usual, whether by rising to the top in a bottle or spinning off from the heavier water phase in a centrifuge. There are three grades of cream marketed today: light cream is between 18 and 80% butterfat, light whipping cream between 80 and 36%, and heavy whipping cream between 36 and 40%. Whole milk, by contrast, is closer to 4% fat. "Half-and-half" is, as its name suggests, intermediate in composition between milk and cream; it must be at least 10.5% fat. Cream is chiefly valued for its thick, smooth texture and rich taste, and in some ways it is a handier cooking ingredient than milk. Because its proteins have been greatly diluted by fat globules, it is less likely to form a skin when heated or even boiled down for a sauce, and it is fairly immune to curdling in the presence of acidic or salty foods. Perhaps most important, cream can be whipped into a stable foam.
On the other hand, cream is infamous for its seeming fickleness when whipped. Sometimes it doubles its original volume, rising in soft, light, long-lived peaks, and other times it stays stubbornly liquid, even to the point of turning into a mixture of buttermilk and butter. It has its reasons, and these have to do with the nature of foam reinforcement in cream. Like whipped egg whites, whipped cream is a foam of air and water that is stabilized by the proteins contained in the liquid. When the beating action introduces air bubbles into the liquid, some proteins are caught in the walls of the bubbles, and because of the imbalance of forces there, the molecules are distorted from their normal shape; they react with each other, and form a thin film of coagulated molecules (for more detail, see the discussion of beating egg whites in chapter 2). This film gives the liquid foam a solid if very delicate reinforcement, and prevents it from collapsing under the force of gravity -- for a few seconds, or indefinitely, depending on the materials. Milk foams are very unstable and collapse almost immediately after they form, because the milk proteins simply don't unfold and coagulate very much, and because the liquid is so thin that it easily runs out of the bubble walls back into a large pool. Egg white, by contrast, is very viscous and slow-moving, and some of its proteins readily denature and coagulate when foamed.
The fact that cream can succeed where milk fails dearly has to do with the greater concentration of fat globules. For one thing, that concentration has a noticeable effect on the viscosity of the liquid: cream flows less rapidly than milk. More important is the globules' activity. They apparently cluster together in the bubble walls, where surface forces rupture some of their membranes. The exposed spheres of soft fat then stick to each other and form a rigid but delicate network that the milk proteins alone cannot provide. In milk, the globules are too few and far between to do the lob, while in cream, beginning at about 20% fat content, their number is adequate to support the foam. Milk also has the disadvantage of being homogenized. Whey proteins leave the solution to fill in the gaps of the globules' suddenly larger surface area, and this leaves less to stabilize the interface between water and air. In addition, the initial clustering of globules is probably disturbed by the change in their covering membranes. For precisely these reasons, whipping cream is not homogenized. Pasteurization, which is generally required for creams as well as milk, has only a slightly detrimental effect on whipping.
Cookbooks commonly advise the cook to store bowl and beaters in the freezer before trying to whip cream, while egg whites are said to whip best at room temperature or even slightly above it. There is nothing mysterious about this divergence, and too high a temperature is frequently the cause of failure to whip cream into a good foam. We all know that the properties of milk fat change drastically within the range of ordinary kitchen temperatures. Butter is stiff, even brittle, right out of the refrigerator, but spreads easily at room temperature, and liquefies completely on hot summer afternoons. Milk fat actually melts at around 90°F (32°C), but it gets soft long before that point. Now, if the fat globules lining the bubble walls are too soft, they will be deformed by the weight of the foam, and the whole structure will be weakened. And if even a small amount of fat escapes as liquid from a globule (the equivalent of a drop of egg yolk in whipped whites), it will interfere with the ordered system of water, unfolded protein, and air, and a stable foam will not form in the first place. It also appears that globules cluster more readily at low temperatures. And the fluid as a whole is much more viscous when cold than when warm, so that it is slower to drain from the foam. All in all, then, the colder the cream and beating utensils, the better, especially considering the fact that both the activity of beating and the incorporation of room-temperature air will heat the cream up.
Cooling the cream in the freezer before whipping can be a help, especially in the summer, but be careful not to let it begin to freeze. The water leaves the solution to form ice, and this segregation of phases makes an even redispersion of fat, and so a good foam, difficult to achieve afterward. A temperature of 45°F (7°C) or lower is recommended; above about 70°F (21°C), even heavy cream is too thin and its globules too soft to make a stable foam. Light (80%) whipping cream is considered ideal for making foams; heavy cream more readily turns lumpy and buttery. It also appears that larger globules produce stiffer foams than small ones; if you happen to have a choice of cows, be advised that Jerseys and Guernseys give milk with the largest average globule size (the other common dairy breed, the Holstein, is the more copious producer). Vegetable gums or gelatin are sometimes added to cream to improve its foaming properties (they make it more viscous and help stabilize the bubble walls); if you feel that such aid would compromise your achievements, check carton labels carefully. Sugar will decrease the final volume and stability of whipped cream when added at the beginning of the process, probably by interfering with the clumping proteins on the globule membranes. These effects are diminished by sweetening the cream after the whipping is mostly done. As is the case with egg whites, the point at which a cream foam is most stable does not coincide with its greatest volume. If you want whipped cream that will last for a while, stop beating it when it seems the stiffest, before it begins to turn soft and glossy.
Butter and Margarine
A sure sign of failure in whipping cream is the appearance of small lumps in the liquid. These lumps are butter, and once they have formed there is no longer any chance of getting a good volume of stable foam, even if everything is put back into the refrigerator to cool down. In fact, butter has always been made by a process much like whipping, though generally it is less delicate, and churning is usually the word applied. Cream is removed from the whole milk, then agitated, and eventually butter granules form, grow larger, and coalesce. In the end, there are two phases left: a semisolid mass of butter, and the liquid left over, which is the buttermilk.
Exactly how churning works is still unknown. Current theory runs along these lines: just as happens in whipped cream, some air is incorporated into the liquid, bubbles form, and the fat globules collect in the bubble walls. But where whipping cream is kept cold, and the agitation stopped when a a stable, airy foam is produced, churned cream is warmed to the point that the globules soften and to some degree liquify. The ideal temperature range is said to be 55° to 65°F (12° to 18°C). Persistent agitation knocks the softened globules into each other enough to break through the protective membrane, and liquid fat cements the exposed droplets together. The foam structure is broken both by the free fat and the released membrane materials, which include emulsifiers like lecithin. These materials disrupt thin water layers and so burst bubble walls, and once enough of them have been freed in the process of whipping or churning cream, the foam will never be stable again. As churning continues, then, the foam gradually subsides, and the butter granules are worked together into larger and larger masses.
The process of butter making can be described as an inversion of the original cream emulsion. The system of fat droplets dispersed in water is converted into a continuous phase of fat that contains water droplets. The final product is about 80% milk fat, 18% water, and 2% milk solids, mainly proteins and salts carried in the water. The physical structure of butter is, however, a bit more complicated. The continuous, amorphous phase of solid fat surrounds not only the water droplets, but also air bubbles, intact fat globules, and highly ordered crystals of milk fat that have grown during the cooling process. The proportion of continuous or "free" fat can vary from 50% of the total to nearly 100%, and it has a direct influence on the behavior of butter. The more fat there is in discrete globules or crystals, the harder and more crumbly the butter, even to the point of brittleness. A preponderance of free fat, on the other hand, makes for a malleable butter that softens readily and may even weep some liquid fat in the process. The difference is a matter of both large-scale and molecular arrangements. In a mass where the free fat merely fills the small interstices between globules and crystals, the texture will be largely that of the separate particles. And it takes more energy to separate the molecules ordered in a crystal than it does to disrupt an already disordered phase of the same molecules. Mostly crystalline butter, then, will be relatively stiff and not as smooth as mostly amorphous butter. The ideal, of course, lies somewhere between the two extremes, and is attained by manipulating the cooling process (much as one controls the texture of candy).
Butter is frequently heated in a saucepan and then used either as one of several ingredients in a dish, or as a medium in which to fry other foods. When the temperature reaches the boiling point of water, the dispersed water droplets vaporize and bubble off the melted fat; this is what causes the butter to sizzle. And as the temperature continues to rise, the small amount of initially white sediment -- the milk proteins and salts -- will turn brown and eventually burn, thereby imparting a harsh flavor to the butter and to delicate foods that may be cooked in it. To avoid this, or simply to improve the appearance of a melted butter sauce, the cook "clarifies" butter by skimming off the froth, which contains whey proteins, and then carefully pouring the melted fat off the white sediment of casein and salts. In the Indian version of clarified butter, ghee, the solids are allowed to brown before being removed, and this imparts a nutty flavor to the fat.
These days, any deterioration in the flavor of butter is probably due not to bacterial spoilage, but to oxidation of the fats and the liberation of odorous short-chain fatty acids. In very small concentrations, these molecules give milk and butter characteristic notes of flavor, but in larger amounts, they are disgusting. This chemical process is slowed down at low temperatures, and butter will keep for months in the freezer, provided it is wrapped air-tight and cannot pick up odors from the appliance. Before the advent of refrigeration, bacteria were also a problem, and this is where the practice of salting butter originated: salt was added in sufficient amounts to act as a preservative. Today, it is used primarily as a flavoring.
Margarine: A French Pearl
Margarine, a butter substitute made originally from other animal fats, but nowadays exclusively from vegetable oils, is, like homogenization and pasteurization, a French innovation. It was developed in 1869 by a pharmacist and chemist, Hippolyte Mège-Mouriés, after Napoleon III offered a prize for the formulation of a synthetic edible fat. Western Europe was running low on fats and oils; petroleum hydrocarbons were as yet unexploited, and the growing industrial need for lubricants and the popular demand for soaps (caused by a rising standard of living and interest in hygiene) were cutting into vegetable sources.
The name margarine comes from a minor scientific error. Michel Chevreul, a chemist whose investigations of color influenced the painter Seurat, also worked on natural fats early in his career, isolating and naming many fatty acids and establishing a model of analytic research in the heretofore rather casual field of organic chemistry. In 1818, Chevreul isolated a substance from animal fat that formed pearly drops, and, thinking it to be a new fatty acid, he named it margaric acid, from the Greek for "pearl" (margaron, also the root of "Margaret"). As it later turned out, there was no such thing as margaric acid (a synthetic fatty acid has since been given that name), but Mège-Mouriés used an extract of animal fat that supposedly contained a great deal of it, and so gave his concoction the name margarine. He was looking for a butter substitute, and so of course had to use animal fats, which are semisolid at room temperature. Mège-Mouriés was not the first to give suet a buttery texture, but he was the first to make it palatable by flavoring it with a small amount of milk. It was not until 1905, after French and German chemists had developed the process of hydrogenation for hardening normally liquid vegetable oils (see page 604), that these oils could be made into a butter substitute.
Margarine caught on quickly in both Europe and the United States, where patents began pouring out in 1871, and large-scale production was under way by 1880. At the turn of the century, Mark Twain overheard a conversation between two businessmen aboard the Cincinnati riverboat, and recorded it in Life on the Mississippi.
Why, we are turning out oleomargarine now, by the thousands of tons. And we can sell it so dirt-cheap that the whole country has got to take it -- can't get around it, you see. Butter don't stand any show -- there ain't any chance for competition.
Little did this enthusiast suspect what resistance margarine would meet from the dairy industry and from government. First it was defined as a "harmful drug" and its sale restricted. Then it was heavily taxed, stores had to be licensed to sell it, and, like alcohol and tobacco, it was bootlegged. The government refused to purchase it for use in the armed forces. And, in an attempt to hold it to its true colors, some states did not allow margarine to be dyed yellow (animal fats and vegetable oils are much paler than butter); the dye was sold separately and mixed in by the consumer. Two world wars, which brought butter rationing, probably did the most to establish margarine's respectability. But it was not until 1950 that the federal taxes on margarine were abolished, and not until 1967 that yellow margarine could be sold in Wisconsin. Today, we consume nearly three times as much margarine as we do butter. Both price and the current concern about cardiovascular disease are responsible for this differential. Margarine, once far cheaper than butter, is still marginally so, and contains none of the cholesterol and less of the saturated fats that have been implicated in heart disease. (A fat's hardness at a given temperature is an index of its saturation; the proportion of saturated fats in liquid oil, tub margarine, stick margarine, and butter increases in that order. See page 603 for an explanation of saturation.)
Like its model, margarine is about 80% fat, 20% water and solids. It is flavored, colored, and fortified with vitamin A and sometimes D to match butter's nutritional contribution. A single oil or a blend may be used. During World War I, coconut oil was favored; in the thirties, it was cottonseed, and in the fifties, soy. Today, soy and corn oils predominate. The raw oil is pressed from the seeds, purified, hydrogenated, and then fortified and colored, either with a synthetic carotene or with annatto, a pigment extracted from a tropical seed. The water phase is usually reconstituted or skim milk that is cultured with lactic bacteria to produce a stronger flavor, although pure diacetyl, the compound most responsible for the flavor of butter, is also used. Emulsifiers such as lecithin help disperse the water phase evenly throughout the oil, and salt and preservatives are also commonly added. The mixture of oil and water is then heated, blended, and cooled. The softer tub margarines are made with less hydrogenated, more liquid oils than go into stick margarines.
Though ice cream seems to be one of those quintessentially American foods, it is at least a century older than this country. Exactly where and how it got its start is unclear. It may have begun as a way of preserving milk, but pleasure soon became the dominant motive. Some historians give credit to Catherine de Medici and her Florentine cooks, who arrived at the French court in the middle of the 16th century. Fruit ices were enjoyed there soon after, and Charles I, who reigned early in the 17th century, is the first on record to have served "cream ice." A hundred years later, ice cream was a standard item in middle-class English cookbooks. Here, for example, is the popular Hannah Glasse's recipe, from her Compleat Confectioner, about 1760. (She plays loose with pronouns.)
To make ice cream. Take two pewter basons, one larger than the other; the inward one must have a close cover, into which you put your cream, and mix it with what you think proper, to give it a flavour and colour, as raspberries, etc. then sweeten it to your palate, cover it close, and set it in the larger bason; fill it with ice, and a large handful of salt under and over and round about; let it stand in the ice three quarters of an hour, uncover, and stir it and the cream well together, then cover it again; let it stand half an hour longer, and then turn it into your plate....
In the American colonies at about the same time, newspaper ads gave notice of ice cream for sale, and it was frequently served at dinner by prominent citizens.
If America did not invent ice cream, it certainly did pioneer in its refinement, and -- no surprise -- in its commercial development. Hannah Glasse's ice cream, like modern "freezer" versions, would have been relatively dense, coarse, and crystalline because it did not involve the continuous mixing of ingredients as they froze. Other early directions called for the inner bowl to be rocked or shaken while in the brine, but this would have been an awkward, messy business. The problem received its classic solution in 1846, when one Nancy Johnson, an American about whom little else is known, invented the hand-cranked freezer that is still used in many homes today. This design, which was patented two years later by William G. Young, employed a simple and Steady mechanical action to keep the mix moving, thereby cooling it evenly, preventing the growth of large ice crystals, and incorporating some air. The second fateful advance came in 1851 when Jacob Fussell, a Baltimore milk dealer, decided to use up his surplus milk by freezing it into ice cream, and thereby became its first large-scale manufacturer.
From this point on, the country's considerable creative forces went into action. The following is just a partial honor roll of milestones in the progress of ice cream. In 1874, it was substituted for cream in sodas, a concoction that had been around since the thirties. In the nineties, Midwestern laws against Sunday sales of ice-cream sodas incited the invention of the sodaless, legal "sundae." At the St. Louis World Exposition of 1904, an ice cream vendor ran out
Excerpted from On Food and Cooking by Harold J. McGee Copyright © 1984 by Harold J. McGee. Excerpted by permission.
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Harold McGee is a world-renowned authority on the chemistry of foods and cooking. He studied science and literature at Caltech and Yale, and has written two prize-winning books, On Food and Cooking and The Curious Cook, as well as many articles and reviews. He lives near San Francisco.
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