The making of milk. Cells in the cow's mammary gland synthesize the components of milk, including proteins and globules of milk fat, and release them into many thousands of small compartments that drain toward the teat. The fat globules pa.s.s through the cells' outer membranes, and carry parts of the cell membrane on their surface.

Milk owes its milky opalescence to microscopic fat globules and protein bundles, which are just large enough to deflect light rays as they pa.s.s through the liquid. Dissolved salts and milk sugar, vitamins, other proteins, and traces of many other compounds also swim in the water that accounts for the bulk of the fluid. The sugar, fat, and proteins are by far the most important components, and we'll look at them in detail in a moment.

First a few words about the remaining components. Milk is slightly acidic, with a pH between 6.5 and 6.7, and both acidity and salt concentrations strongly affect the behavior of the proteins, as we'll see. The fat globules carry colorless vitamin A and its yellow-orange precursors the carotenes, which are found in green feed and give milk and undyed b.u.t.ter whatever color they have. Breeds differ in the amount of carotene they convert into vitamin A; Guernsey and Jersey cows convert little and give especially golden milk, while at the other extreme sheep, goats, and water buffalo process nearly all of their carotene, so their milk and b.u.t.ter are nutritious but white. Riboflavin, which has a greenish color, can sometimes be seen in skim milk or in the watery translucent whey that drains from the curdled proteins of yogurt.

Milk Sugar: Lactose The only carbohydrate found in any quant.i.ty in milk is also peculiar to milk (and a handful of plants), and so was named lactose, lactose, or "milk sugar." ( or "milk sugar." (Lac- is a prefix based on the Greek word for "milk"; we'll encounter it again in the names of milk proteins, acids, and bacteria.) Lactose is a composite of the two simple sugars glucose and galactose, which are joined together in the secretory cell of the mammary gland, and nowhere else in the animal body. It provides nearly half of the calories in human milk, and 40% in cow's milk, and gives milk its sweet taste. is a prefix based on the Greek word for "milk"; we'll encounter it again in the names of milk proteins, acids, and bacteria.) Lactose is a composite of the two simple sugars glucose and galactose, which are joined together in the secretory cell of the mammary gland, and nowhere else in the animal body. It provides nearly half of the calories in human milk, and 40% in cow's milk, and gives milk its sweet taste.

The uniqueness of lactose has two major practical consequences. First, we need a special enzyme to digest lactose; and many adults lack that enzyme and have to be careful about what dairy products they consume (p. 14). Second, most microbes take some time to make their own lactose-digesting enzyme before they can grow well in milk, but one group has enzymes at the ready and can get a head start on all the others. The bacteria known as Lactobacilli Lactobacilli and and Lactococci Lactococci not only grow on lactose immediately, they also convert it into lactic acid ("milk acid"). They thus acidify the milk, and in so doing, make it less habitable by other microbes, including many that would make the milk unpalatable or cause disease. Lactose and the lactic-acid bacteria therefore turn milk sour, but help prevent it from spoiling, or becoming undrinkable. not only grow on lactose immediately, they also convert it into lactic acid ("milk acid"). They thus acidify the milk, and in so doing, make it less habitable by other microbes, including many that would make the milk unpalatable or cause disease. Lactose and the lactic-acid bacteria therefore turn milk sour, but help prevent it from spoiling, or becoming undrinkable.

Lactose is one-fifth as sweet as table sugar, and only one-tenth as soluble in water (200 vs. 2,000 gm/l), so lactose crystals readily form in such products as condensed milk and ice cream and can give them a sandy texture.

Milk Fat Milk fat accounts for much of the body, nutritional value, and economic value of milk. The milk-fat globules carry the fat-soluble vitamins (A, D, E, K), and about half the calories of whole milk. The higher the fat content of milk, the more cream or b.u.t.ter can be made from it, and so the higher the price it will bring. Most cows secrete more fat in winter, due mainly to concentrated winter feed and the approaching end of their lactation period. Certain breeds, notably Guernseys and Jerseys from the Channel Islands between Britain and France, produce especially rich milk and large fat globules. Sheep and buffalo milks contain up to twice the b.u.t.terfat of whole cow's milk (p. 13).

The way the fat is packaged into globules accounts for much of milk's behavior in the kitchen. The membrane that surrounds each fat globule is made up of phospholipids (fatty acid emulsifiers, p. 802) and proteins, and plays two major roles. It separates the droplets of fat from each other and prevents them from pooling together into one large ma.s.s; and it protects the fat molecules from fat-digesting enzymes in the milk that would otherwise attack them and break them down into rancid-smelling and bitter fatty acids.

Creaming When milk fresh from the udder is allowed to stand and cool for some hours, many of its fat globules rise and form a fat-rich layer at the top of the container. This phenomenon is called When milk fresh from the udder is allowed to stand and cool for some hours, many of its fat globules rise and form a fat-rich layer at the top of the container. This phenomenon is called creaming, creaming, and for millennia it was the natural first step toward obtaining fat-enriched cream and b.u.t.ter from milk. In the 19th century, centrifuges were developed to concentrate the fat globules more rapidly and thoroughly, and h.o.m.ogenization was invented to prevent whole milk from separating in this way (p. 23). The globules rise because their fat is lighter than water, but they rise much faster than their buoyancy alone can account for. It turns out that a number of minor milk proteins attach themselves loosely to the fat globules and knit together cl.u.s.ters of about a million globules that have a stronger lift than single globules do. Heat denatures these proteins and prevents the globule cl.u.s.tering, so that the fat globules in unh.o.m.ogenized but pasteurized milk rise more slowly into a shallower, less distinct layer. Because of their small globules and low cl.u.s.tering activity, the milks of goats, sheep, and water buffalo are very slow to separate. and for millennia it was the natural first step toward obtaining fat-enriched cream and b.u.t.ter from milk. In the 19th century, centrifuges were developed to concentrate the fat globules more rapidly and thoroughly, and h.o.m.ogenization was invented to prevent whole milk from separating in this way (p. 23). The globules rise because their fat is lighter than water, but they rise much faster than their buoyancy alone can account for. It turns out that a number of minor milk proteins attach themselves loosely to the fat globules and knit together cl.u.s.ters of about a million globules that have a stronger lift than single globules do. Heat denatures these proteins and prevents the globule cl.u.s.tering, so that the fat globules in unh.o.m.ogenized but pasteurized milk rise more slowly into a shallower, less distinct layer. Because of their small globules and low cl.u.s.tering activity, the milks of goats, sheep, and water buffalo are very slow to separate.

Milk Fat Globules Tolerate Heat... Interactions between fat globules and milk proteins are also responsible for the remarkable tolerance of milk and cream to heat. Milk and cream can be boiled and reduced for hours, until they're nearly dry, without breaching the globule membranes enough to release their fat. The globule membranes are robust to begin with, and it turns out that heating unfolds many of the milk proteins and makes them more p.r.o.ne to stick to the globule surface and to each other - so the globule armor actually gets progressively thicker as heating proceeds. Without this stability to heat, it would be impossible to make many cream-enriched sauces and reduced-milk sauces and sweets. Interactions between fat globules and milk proteins are also responsible for the remarkable tolerance of milk and cream to heat. Milk and cream can be boiled and reduced for hours, until they're nearly dry, without breaching the globule membranes enough to release their fat. The globule membranes are robust to begin with, and it turns out that heating unfolds many of the milk proteins and makes them more p.r.o.ne to stick to the globule surface and to each other - so the globule armor actually gets progressively thicker as heating proceeds. Without this stability to heat, it would be impossible to make many cream-enriched sauces and reduced-milk sauces and sweets.

...But Are Sensitive to Cold Freezing is a different story. It is fatal to the fat globule membrane. Cold milk fat and freezing water both form large, solid, jagged crystals that pierce, crush, and rend the thin veil of phospholipids and proteins around the globule, just a few molecules thick. If you freeze milk or cream and then thaw it, much of the membrane material ends up floating free in the liquid, and many of the fat globules get stuck to each other in grains of b.u.t.ter. Make the mistake of heating thawed milk or cream, and the b.u.t.ter grains melt into puddles of oil. Freezing is a different story. It is fatal to the fat globule membrane. Cold milk fat and freezing water both form large, solid, jagged crystals that pierce, crush, and rend the thin veil of phospholipids and proteins around the globule, just a few molecules thick. If you freeze milk or cream and then thaw it, much of the membrane material ends up floating free in the liquid, and many of the fat globules get stuck to each other in grains of b.u.t.ter. Make the mistake of heating thawed milk or cream, and the b.u.t.ter grains melt into puddles of oil.

Milk Proteins: Coagulation by Acid and Enzymes Two Protein Cla.s.ses: Curd and Whey There are dozens of different proteins floating around in milk. When it comes to cooking behavior, fortunately, we can reduce the protein population to two basic groups: Little Miss m.u.f.fet's curds and whey. The two groups are distinguished by their reaction to acids. The handful of curd proteins, the There are dozens of different proteins floating around in milk. When it comes to cooking behavior, fortunately, we can reduce the protein population to two basic groups: Little Miss m.u.f.fet's curds and whey. The two groups are distinguished by their reaction to acids. The handful of curd proteins, the caseins, caseins, clump together in acid conditions and form a solid ma.s.s, or clump together in acid conditions and form a solid ma.s.s, or coagulate, coagulate, while all the rest, the whey proteins, remain suspended in the liquid. It's the clumping nature of the caseins that makes possible most thickened milk products, from yogurt to cheese. The whey proteins play a more minor role; they influence the texture of casein curds, and stabilize the milk foams on specialty coffees. The caseins usually outweigh the whey proteins, as they do in cow's milk by 4 to 1. while all the rest, the whey proteins, remain suspended in the liquid. It's the clumping nature of the caseins that makes possible most thickened milk products, from yogurt to cheese. The whey proteins play a more minor role; they influence the texture of casein curds, and stabilize the milk foams on specialty coffees. The caseins usually outweigh the whey proteins, as they do in cow's milk by 4 to 1.

Both caseins and whey proteins are unusual among food proteins in being largely tolerant of heat. Where cooking coagulates the proteins in eggs and meat into solid ma.s.ses, it does not coagulate the proteins in milk and cream - unless the milk or cream has become acidic. Fresh milk and cream can be boiled down to a fraction of their volume without curdling.

The Caseins The casein family includes four different kinds of proteins that gather together into microscopic family units called The casein family includes four different kinds of proteins that gather together into microscopic family units called micelles. micelles. Each casein micelle contains a few thousand individual protein molecules, and measures about a ten thousandth of a millimeter across, about one-fiftieth the size of a fat globule. Around a tenth of the volume of milk is taken up by casein micelles. Much of the calcium in milk is in the micelles, where it acts as a kind of glue holding the protein molecules together. One portion of calcium binds individual protein molecules together into small cl.u.s.ters of 15 to 25. Another portion then helps pull several hundred of the cl.u.s.ters together to form the micelle (which is also held together by the water-avoiding hydrophobic portions of the proteins bonding to each other). Each casein micelle contains a few thousand individual protein molecules, and measures about a ten thousandth of a millimeter across, about one-fiftieth the size of a fat globule. Around a tenth of the volume of milk is taken up by casein micelles. Much of the calcium in milk is in the micelles, where it acts as a kind of glue holding the protein molecules together. One portion of calcium binds individual protein molecules together into small cl.u.s.ters of 15 to 25. Another portion then helps pull several hundred of the cl.u.s.ters together to form the micelle (which is also held together by the water-avoiding hydrophobic portions of the proteins bonding to each other).

Keeping Micelles Separate... One member of the casein family is especially influential in these gatherings. That is kappa-casein, which caps the micelles once they reach a certain size, prevents them from growing larger, and keeps them dispersed and separate. One end of the capping-casein molecule extends from the micelle out into the surrounding liquid, and forms a "hairy layer" with a negative electrical charge that repels other micelles. One member of the casein family is especially influential in these gatherings. That is kappa-casein, which caps the micelles once they reach a certain size, prevents them from growing larger, and keeps them dispersed and separate. One end of the capping-casein molecule extends from the micelle out into the surrounding liquid, and forms a "hairy layer" with a negative electrical charge that repels other micelles.

A close-up view of milk. Fat globules are suspended in a fluid made up of water, individual molecules of whey protein, bundles of casein protein molecules, and dissolved sugars and minerals.

...And Knitting Them Together in Curds The intricate structure of casein micelles can be disturbed in several ways that cause the micelles to flock together and the milk to curdle. One way is souring. Milk's normal pH is about 6.5, or just slightly acidic. If it gets acid enough to approach pH 5.5, the capping-casein's negative charge is neutralized, the micelles no longer repel each other, and they therefore gather in loose cl.u.s.ters. At the same acidity, the calcium glue that holds the micelles together dissolves, the micelles begin to fall apart, and their individual proteins scatter. Beginning around pH 4.7, the scattered casein proteins lose their negative charge, bond to each other again and form a continuous, fine network: and the milk solidifies, or curdles. This is what happens when milk gets old and sour, or when it's intentionally cultured with acid-producing bacteria to make yogurt or sour cream. The intricate structure of casein micelles can be disturbed in several ways that cause the micelles to flock together and the milk to curdle. One way is souring. Milk's normal pH is about 6.5, or just slightly acidic. If it gets acid enough to approach pH 5.5, the capping-casein's negative charge is neutralized, the micelles no longer repel each other, and they therefore gather in loose cl.u.s.ters. At the same acidity, the calcium glue that holds the micelles together dissolves, the micelles begin to fall apart, and their individual proteins scatter. Beginning around pH 4.7, the scattered casein proteins lose their negative charge, bond to each other again and form a continuous, fine network: and the milk solidifies, or curdles. This is what happens when milk gets old and sour, or when it's intentionally cultured with acid-producing bacteria to make yogurt or sour cream.

Another way to cause the caseins to curdle is the basis of cheese making. Chymosin, a digestive enzyme from the stomach of a milk-fed calf, is exquisitely designed to give the casein micelles a haircut (p. 57). It clips off just the part of the capping-casein that extends into the surrounding liquid and shields the micelles from each other. Shorn of their hairy layer, the micelles all clump together - without the milk being noticeably sour.

The Whey Proteins Subtract the four caseins from the milk proteins, and the remainder, numbering in the dozens, are the whey proteins. Where the caseins are mainly nutritive, supplying amino acids and calcium for the calf, the whey proteins include defensive proteins, molecules that bind to and transport other nutrients, and enzymes. The most abundant one by far is lactoglobulin, whose biological function remains a mystery. It's a highly structured protein that is readily denatured by cooking. It unfolds at 172F/78C, when its sulfur atoms are exposed to the surrounding liquid and react with hydrogen ions to form hydrogen sulfide gas, whose powerful aroma contributes to the characteristic flavor of cooked milk (and many other animal foods). Subtract the four caseins from the milk proteins, and the remainder, numbering in the dozens, are the whey proteins. Where the caseins are mainly nutritive, supplying amino acids and calcium for the calf, the whey proteins include defensive proteins, molecules that bind to and transport other nutrients, and enzymes. The most abundant one by far is lactoglobulin, whose biological function remains a mystery. It's a highly structured protein that is readily denatured by cooking. It unfolds at 172F/78C, when its sulfur atoms are exposed to the surrounding liquid and react with hydrogen ions to form hydrogen sulfide gas, whose powerful aroma contributes to the characteristic flavor of cooked milk (and many other animal foods).

In boiling milk, unfolded lactoglobulin binds not to itself but to the capping-casein on the casein micelles, which remain separate; so denatured lactoglobulin doesn't coagulate. When denatured in acid conditions with relatively little casein around, as in cheese whey, lactoglobulin molecules do bind to each other and coagulate into little clots, which can be made into whey cheeses like true ricotta. Heat-denatured whey proteins are better than their native forms at stabilizing air bubbles in milk foams and ice crystals in ice creams; this is why milks and creams are usually cooked for these preparations (pp. 26, 43).

A model of the milk protein casein, which occurs in micelles, or small bundles a fraction of the size of a fat globule. A single micelle consists of many individual protein molecules (lines) (lines) held together by particles of calcium phosphate held together by particles of calcium phosphate (small spheres). (small spheres).

Milk Flavor The flavor of fresh milk is balanced and subtle. It's distinctly sweet from the lactose, slightly salty from its complement of minerals, and very slightly acid. Its mild, pleasant aroma is due in large measure to short-chain fatty acids (including butyric and capric acids), which help keep highly saturated milk fat fluid at body temperature, and which are small enough that they can evaporate into the air and reach our nose. Normally, free fatty acids give an undesirable, soapy flavor to foods. But in sparing quant.i.ties, the 4- to 12-carbon rumen fatty acids, branched versions of these, and acid-alcohol combinations called esters, provide milk with its fundamental blend of animal and fruity notes. The distinctive smells of goat and sheep milks are due to two particular branched 8-carbon fatty acids (4-ethyl-octanoic, 4-methyl-octanoic) that are absent in cow's milk. Buffalo milk, from which traditional mozzarella cheese is made, has a characteristic blend of modified fatty acids reminiscent of mushrooms and freshly cut gra.s.s, together with a barnyardy nitrogen compound (indole).

The basic flavor of fresh milk is affected by the animals' feed. Dry hay and silage are relatively poor in fat and protein and produce a less complicated, mildly cheesy aroma, while lush pasturage provides raw material for sweet, raspberry-like notes (derivatives of unsaturated long-chain fatty acids), as well as barnyardy indoles.

Flavors from Cooking Low-temperature pasteurization (p. 22) slightly modifies milk flavor by driving off some of the more delicate aromas, but stabilizes it by inactivating enzymes and bacteria, and adds slightly sulfury and green-leaf notes (dimethyl sulfide, hexa.n.a.l). High-temperature pasteurization or brief cooking - heating milk above 170F/76C - generates traces of many flavorful substances, including those characteristic of vanilla, almonds, and cultured b.u.t.ter, as well as eggy hydrogen sulfide. Prolonged boiling encourages browning or Maillard reactions between lactose and milk proteins, and generates molecules that combine to give the flavor of b.u.t.terscotch. Low-temperature pasteurization (p. 22) slightly modifies milk flavor by driving off some of the more delicate aromas, but stabilizes it by inactivating enzymes and bacteria, and adds slightly sulfury and green-leaf notes (dimethyl sulfide, hexa.n.a.l). High-temperature pasteurization or brief cooking - heating milk above 170F/76C - generates traces of many flavorful substances, including those characteristic of vanilla, almonds, and cultured b.u.t.ter, as well as eggy hydrogen sulfide. Prolonged boiling encourages browning or Maillard reactions between lactose and milk proteins, and generates molecules that combine to give the flavor of b.u.t.terscotch.

The Development of Off-Flavors The flavor of good fresh milk can deteriorate in several different ways. Simple contact with oxygen or exposure to strong light will cause the oxidation of phospholipids in the globule membrane and a chain of reactions that slowly generate stale cardboard, metallic, fishy, paint-like aromas. If milk is kept long enough to sour, it also typically develops fruity, vinegary, malty, and more unpleasant notes. The flavor of good fresh milk can deteriorate in several different ways. Simple contact with oxygen or exposure to strong light will cause the oxidation of phospholipids in the globule membrane and a chain of reactions that slowly generate stale cardboard, metallic, fishy, paint-like aromas. If milk is kept long enough to sour, it also typically develops fruity, vinegary, malty, and more unpleasant notes.

Exposure to sunlight or fluorescent lights also generates a distinctive cabbage-like, burnt odor, which appears to result from a reaction between the vitamin riboflavin and the sulfur-containing amino acid methionine. Clear gla.s.s and plastic containers and supermarket lighting cause this problem; opaque cartons prevent it.

Unfermented Dairy Products Fresh milk, cream, and b.u.t.ter may not be as prominent in European and American cooking as they once were, but they are still essential ingredients. Milk has bubbled up to new prominence atop the coffee craze of the 1980s and '90s.

Milks Milk has become the most standardized of our basic foods. Once upon a time, people lucky enough to live near a farm could taste the pasture and the seasons in milk fresh from the cow. City life, ma.s.s production, and stricter notions of hygiene have now put that experience out of reach. Today nearly all of our milk comes from cows of one breed, the black-and-white Holstein, kept in sheds and fed year-round on a uniform diet. Large dairies pool the milk of hundreds, even thousands of cows, then pasteurize it to eliminate microbes and h.o.m.ogenize it to prevent the fat from separating. The result is processed milk of no particular animal or farm or season, and therefore of no particular character. Some small dairies persist in milking other breeds, allowing their herds out to pasture, pasteurizing mildly, and not h.o.m.ogenizing. Their milk can have a more distinctive flavor, a rare reminder of what milk used to taste like.

Raw Milk Careful milking of healthy cows yields sound raw milk, which has its own fresh taste and physical behavior. But if it's contaminated by a diseased cow or careless handling - the udder hangs right next to the tail - this nutritious fluid soon teems with potentially dangerous microbes. The importance of strict hygiene in the dairy has been understood at least since the Middle Ages, but life far from the farms made contamination and even adulteration all too common in cities of the 18th and 19th centuries, where many children were killed by tuberculosis, undulant fever, and simple food poisoning contracted from tainted milk. In the 1820s, long before anyone knew about microbes, some books on domestic economy advocated boiling all milk before use. Early in the 20th century, national and local governments began to regulate the dairy industry and require that it heat milk to kill disease microbes. Careful milking of healthy cows yields sound raw milk, which has its own fresh taste and physical behavior. But if it's contaminated by a diseased cow or careless handling - the udder hangs right next to the tail - this nutritious fluid soon teems with potentially dangerous microbes. The importance of strict hygiene in the dairy has been understood at least since the Middle Ages, but life far from the farms made contamination and even adulteration all too common in cities of the 18th and 19th centuries, where many children were killed by tuberculosis, undulant fever, and simple food poisoning contracted from tainted milk. In the 1820s, long before anyone knew about microbes, some books on domestic economy advocated boiling all milk before use. Early in the 20th century, national and local governments began to regulate the dairy industry and require that it heat milk to kill disease microbes.

Today very few U.S. dairies sell raw milk. They must be certified by the state and inspected frequently, and the milk carries a warning label. Raw milk is also rare in Europe.

Pasteurization and UHT Treatments In the 1860s, the French chemist Louis Pasteur studied the spoilage of wine and beer and developed a moderate heat treatment that preserved them while minimizing changes in their flavor. It took several decades for pasteurization to catch on in the dairy. Nowadays, in industrial-scale production, it's a practical necessity. Collecting and pooling milk from many different farms increases the risk that a given batch will be contaminated; and the plumbing and machinery required for the various stages of processing afford many more opportunities for contamination. Pasteurization extends the shelf life of milk by killing pathogenic and spoilage microbes and by inactivating milk enzymes, especially the fat splitters, whose slow but steady activity can make it unpalatable. Pasteurized milk stored below 40F/5C should remain drinkable for 10 to 18 days. In the 1860s, the French chemist Louis Pasteur studied the spoilage of wine and beer and developed a moderate heat treatment that preserved them while minimizing changes in their flavor. It took several decades for pasteurization to catch on in the dairy. Nowadays, in industrial-scale production, it's a practical necessity. Collecting and pooling milk from many different farms increases the risk that a given batch will be contaminated; and the plumbing and machinery required for the various stages of processing afford many more opportunities for contamination. Pasteurization extends the shelf life of milk by killing pathogenic and spoilage microbes and by inactivating milk enzymes, especially the fat splitters, whose slow but steady activity can make it unpalatable. Pasteurized milk stored below 40F/5C should remain drinkable for 10 to 18 days.

There are three basic methods for pasteurizing milk. The simplest is batch batch pasteurization, in which a fixed volume of milk, perhaps a few hundred gallons, is slowly agitated in a heated vat at a minimum of 145F/62C for 30 to 35 minutes. Industrial-scale operations use the pasteurization, in which a fixed volume of milk, perhaps a few hundred gallons, is slowly agitated in a heated vat at a minimum of 145F/62C for 30 to 35 minutes. Industrial-scale operations use the high-temperature, short-time high-temperature, short-time (HTST) method, in which milk is pumped continuously through a heat exchanger and held at a minimum of 162F/72C for 15 seconds. The batch process has a relatively mild effect on flavor, while the HTST method is hot enough to denature around 10% of the whey proteins and generate the strongly aromatic gas hydrogen sulfide (p. 87). Though this "cooked" flavor was considered a defect in the early days, U.S. consumers have come to expect it, and dairies now often intensify it by pasteurizing at well above the minimum temperature; 171F/77C is commonly used. (HTST) method, in which milk is pumped continuously through a heat exchanger and held at a minimum of 162F/72C for 15 seconds. The batch process has a relatively mild effect on flavor, while the HTST method is hot enough to denature around 10% of the whey proteins and generate the strongly aromatic gas hydrogen sulfide (p. 87). Though this "cooked" flavor was considered a defect in the early days, U.S. consumers have come to expect it, and dairies now often intensify it by pasteurizing at well above the minimum temperature; 171F/77C is commonly used.

The third method of pasteurizing milk is the ultra-high temperature ultra-high temperature (UHT) method, which involves heating milk at 265300F/ 130150C either instantaneously or for 1 to 3 seconds, and produces milk that, if packaged under strictly sterile conditions, can be stored for months without refrigeration. The longer UHT treatment imparts a cooked flavor and slightly brown color to milk; cream contains less lactose and protein, so its color and flavor are less affected. (UHT) method, which involves heating milk at 265300F/ 130150C either instantaneously or for 1 to 3 seconds, and produces milk that, if packaged under strictly sterile conditions, can be stored for months without refrigeration. The longer UHT treatment imparts a cooked flavor and slightly brown color to milk; cream contains less lactose and protein, so its color and flavor are less affected.

Sterilized milk has been heated at 230250F/110121C for 8 to 30 minutes; it is even darker and stronger in flavor, and keeps indefinitely at room temperature.

h.o.m.ogenization Left to itself, fresh whole milk naturally separates into two phases: fat globules clump together and rise to form the cream layer, leaving a fat-depleted phase below (p. 18). The treatment called Left to itself, fresh whole milk naturally separates into two phases: fat globules clump together and rise to form the cream layer, leaving a fat-depleted phase below (p. 18). The treatment called h.o.m.ogenization h.o.m.ogenization was developed in France around 1900 to prevent creaming and keep the milk fat evenly - h.o.m.ogeneously - dispersed. It involves pumping hot milk at high pressure through very small nozzles, where the turbulence tears the fat globules apart into smaller ones; their average diameter falls from 4 micrometers to about 1. The sudden increase in globule numbers causes a proportional increase in their surface area, which the original globule membranes are insufficient to cover. The naked fat surface attracts casein particles, which stick and create an artificial coat (nearly a third of the milk's casein ends up on the globules). The casein particles both weigh the fat globules down and interfere with their usual clumping: and so the fat remains evenly dispersed in the milk. Milk is always pasteurized just before or simultaneously with h.o.m.ogenization to prevent its enzymes from attacking the momentarily unprotected fat globules and producing rancid flavors. was developed in France around 1900 to prevent creaming and keep the milk fat evenly - h.o.m.ogeneously - dispersed. It involves pumping hot milk at high pressure through very small nozzles, where the turbulence tears the fat globules apart into smaller ones; their average diameter falls from 4 micrometers to about 1. The sudden increase in globule numbers causes a proportional increase in their surface area, which the original globule membranes are insufficient to cover. The naked fat surface attracts casein particles, which stick and create an artificial coat (nearly a third of the milk's casein ends up on the globules). The casein particles both weigh the fat globules down and interfere with their usual clumping: and so the fat remains evenly dispersed in the milk. Milk is always pasteurized just before or simultaneously with h.o.m.ogenization to prevent its enzymes from attacking the momentarily unprotected fat globules and producing rancid flavors.

h.o.m.ogenization affects milk's flavor and appearance. Though it makes milk taste blander - probably because flavor molecules get stuck to the new fat-globule surfaces - it also makes it more resistant to developing most off-flavors. h.o.m.ogenized milk feels creamier in the mouth thanks to its increased population (around sixty-fold) of fat globules, and it's whiter, because the carotenoid pigments in the fat are scattered into smaller and more numerous particles.

Nutritional Alteration; Low-Fat Milks One nutritional alteration of milk is as old as dairying itself: skimming off the cream layer substantially reduces the fat content of the remaining milk. Today, low-fat milks are made more efficiently by centrifuging off some of the globules before h.o.m.ogenization. Whole milk is about 3.5% fat, low-fat milks usually 2% or 1%, and skim milks can range between 0.1 and 0.5%. One nutritional alteration of milk is as old as dairying itself: skimming off the cream layer substantially reduces the fat content of the remaining milk. Today, low-fat milks are made more efficiently by centrifuging off some of the globules before h.o.m.ogenization. Whole milk is about 3.5% fat, low-fat milks usually 2% or 1%, and skim milks can range between 0.1 and 0.5%.

More recent is the practice of supplementing milk with various substances. Nearly all milks are fortified with the fat-soluble vitamins A and D. Low-fat milks have a thin body and appearance and are usually filled out with dried milk proteins, which can lend them a slightly stale flavor. "Acidophilus" milk contains Lactobacillus acidophilus, Lactobacillus acidophilus, a bacterium that metabolizes lactose to lactic acid and that can take up residence in the intestine (p. 47). More helpful to milk lovers who can't digest lactose is milk treated with the purified digestive enzyme lactase, which breaks lactose down into simple, absorbable sugars. a bacterium that metabolizes lactose to lactic acid and that can take up residence in the intestine (p. 47). More helpful to milk lovers who can't digest lactose is milk treated with the purified digestive enzyme lactase, which breaks lactose down into simple, absorbable sugars.

Powdered Milk in 13th Century Asia[The Tartar armies] 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 b.u.t.ter; 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.- Marco Polo, Travels Travels Storage Milk is a highly perishable food. Even Grade A pasteurized milk contains millions of bacteria in every gla.s.sful, and will spoil quickly unless refrigerated. Freezing is a bad idea because it disrupts milk fat globules and protein particles, which clump and separate when thawed. Milk is a highly perishable food. Even Grade A pasteurized milk contains millions of bacteria in every gla.s.sful, and will spoil quickly unless refrigerated. Freezing is a bad idea because it disrupts milk fat globules and protein particles, which clump and separate when thawed.

Concentrated Milks A number of cultures have traditionally cooked milk down for long keeping and ease of transport. According to business legend, the American Gail Borden reinvented evaporated milk around 1853 after a rough transatlantic crossing that sickened the ship's cows. Borden added large amounts of sugar to keep his concentrated milk from spoiling. The idea of sterilizing unsweetened milk in the can came in 1884 from John Meyenberg, whose Swiss company merged with Nestle around the turn of the century. Dried milk didn't appear until around the turn of the 20th century. Today, concentrated milk products are valued because they keep for months and supply milk's characteristic contribution to the texture and flavor of baked goods and confectionery, but without milk's water. A number of cultures have traditionally cooked milk down for long keeping and ease of transport. According to business legend, the American Gail Borden reinvented evaporated milk around 1853 after a rough transatlantic crossing that sickened the ship's cows. Borden added large amounts of sugar to keep his concentrated milk from spoiling. The idea of sterilizing unsweetened milk in the can came in 1884 from John Meyenberg, whose Swiss company merged with Nestle around the turn of the century. Dried milk didn't appear until around the turn of the 20th century. Today, concentrated milk products are valued because they keep for months and supply milk's characteristic contribution to the texture and flavor of baked goods and confectionery, but without milk's water.

Condensed or or evaporated milk evaporated milk is made by heating raw milk under reduced pressure (a partial vacuum), so that it boils between 110 and 140F/4360C, until it has lost about half its water. The resulting creamy, mild-flavored liquid is h.o.m.ogenized, then canned and sterilized. The cooking and concentration of lactose and protein cause some browning, and this gives evaporated milk its characteristic tan color and note of caramel. Browning continues slowly during storage, and in old cans can produce a dark, acidic, tired-tasting fluid. is made by heating raw milk under reduced pressure (a partial vacuum), so that it boils between 110 and 140F/4360C, until it has lost about half its water. The resulting creamy, mild-flavored liquid is h.o.m.ogenized, then canned and sterilized. The cooking and concentration of lactose and protein cause some browning, and this gives evaporated milk its characteristic tan color and note of caramel. Browning continues slowly during storage, and in old cans can produce a dark, acidic, tired-tasting fluid.

For sweetened condensed milk, sweetened condensed milk, the milk is first concentrated by evaporation, and then table sugar is added to give a total sugar concentration of about 55%. Microbes can't grow at this osmotic pressure, so sterilization is unnecessary. The high concentration of sugars causes the milk's lactose to crystallize, and this is controlled by seeding the milk with preformed lactose crystals to keep the crystals small and inconspicuous on the tongue (large, sandy lactose crystals are sometimes encountered as a quality defect). Sweetened condensed milk has a milder, less "cooked" flavor than evaporated milk, a lighter color, and the consistency of a thick syrup. the milk is first concentrated by evaporation, and then table sugar is added to give a total sugar concentration of about 55%. Microbes can't grow at this osmotic pressure, so sterilization is unnecessary. The high concentration of sugars causes the milk's lactose to crystallize, and this is controlled by seeding the milk with preformed lactose crystals to keep the crystals small and inconspicuous on the tongue (large, sandy lactose crystals are sometimes encountered as a quality defect). Sweetened condensed milk has a milder, less "cooked" flavor than evaporated milk, a lighter color, and the consistency of a thick syrup.

Powdered or or dry milk dry milk is the result of taking evaporation to the extreme. Milk is pasteurized at a high temperature; then about 90% of its water is removed by vacuum evaporation, and the remaining 10% in a spray drier (the concentrated milk is misted into a chamber of hot air, where the milk droplets quickly dry into tiny particles of milk solids). Some milk is also freeze-dried. With most of its water removed, powdered milk is safe from microbial attack. Most powdered milk is made from low-fat milk because milk fat quickly goes rancid when exposed to concentrated milk salts and atmospheric oxygen, and because it tends to coat the particles of protein and makes subsequent remixing with water difficult. Powdered milk will keep for several months in dry, cool conditions. is the result of taking evaporation to the extreme. Milk is pasteurized at a high temperature; then about 90% of its water is removed by vacuum evaporation, and the remaining 10% in a spray drier (the concentrated milk is misted into a chamber of hot air, where the milk droplets quickly dry into tiny particles of milk solids). Some milk is also freeze-dried. With most of its water removed, powdered milk is safe from microbial attack. Most powdered milk is made from low-fat milk because milk fat quickly goes rancid when exposed to concentrated milk salts and atmospheric oxygen, and because it tends to coat the particles of protein and makes subsequent remixing with water difficult. Powdered milk will keep for several months in dry, cool conditions.

The Composition of Concentrated MilksThe figures are the percentages of each milk's weight accounted for by its major components.

Kind of Milk Protein Protein Fat Fat Sugar Sugar

Evaporated milk 7 7.

8 8.

10 10.

Evaporated skim milk 8 8.

0.3 0.3.

11 11.

Sweetened condensed milk 8 8.

9 9.

55 55.

Dry milk, full fat 26 26.

27 27.

38 38.

Dry milk, nonfat 36 36.

1 1.

52 52.

Fresh milk 3.4 3.4.

3.7 3.7.

4.8 4.8.

Kind of Milk Minerals Minerals Water Water

Evaporated milk 1.4 1.4.

73 73.

Evaporated skim milk 1.5 1.5.

79 79.

Sweetened condensed milk 2 2.

27 27.

Dry milk, full fat 6 6.

2.5 2.5.

Dry milk, nonfat 8 8.

3 3.

Fresh milk 1 1.

87 87.

Cooking with Milk Much of the milk that we use in the kitchen disappears into a mixture - a batter or dough, a custard mix or a pudding - whose behavior is largely determined by the other ingredients. The milk serves primarily as a source of moisture, but also contributes flavor, body, sugar that encourages browning, and salts that encourage protein coagulation. Much of the milk that we use in the kitchen disappears into a mixture - a batter or dough, a custard mix or a pudding - whose behavior is largely determined by the other ingredients. The milk serves primarily as a source of moisture, but also contributes flavor, body, sugar that encourages browning, and salts that encourage protein coagulation.

When milk itself is a prominent ingredient - in cream soups, sauces, and scalloped potatoes, or added to hot chocolate, coffee, and tea - it most often calls attention to itself when its proteins coagulate. The skin that forms on the surface of scalded milk, soups, and sauces is a complex of casein, calcium, whey proteins, and trapped fat globules, and results from evaporation of water at the surface and the progressive concentration of proteins there. Skin formation can be minimized by covering the pan or whipping up some foam, both of which minimize evaporation. Meanwhile, at the bottom of the pan, the high, dehydrating temperature transmitted from the burner causes a similar concentration of proteins, which stick to the metal and eventually scorch. Wetting the pan with water before adding milk will reduce protein adhesion to the metal; a heavy, evenly conducting pan and a moderate flame help minimize scorching, and a double boiler will prevent it (though it's more trouble).

Between the pan bottom and the surface, particles of other ingredients can cause curdling by providing surfaces to which the milk proteins can stick and clump together. And acid in the juices of all fruits and vegetables and in coffee, and astringent tannins in potatoes, coffee, and tea, make milk proteins especially sensitive to coagulation and curdling. Because bacteria slowly sour milk, old milk may be acidic enough to curdle instantly when added to hot coffee or tea. The best insurance against curdling is fresh milk and careful control of the burner.

Cooking Sweetened Condensed Milk Because it contains concentrated protein and sugar, sweetened condensed milk will "caramelize" (actually, undergo the Maillard browning reaction, p. 778) at temperatures as low as the boiling point of water. This has made cans of sweetened condensed milk a favorite shortcut to a creamy caramel sauce: many people simply put the can in a pot of boiling water or a warm oven and let it brown inside. While this does work, it is potentially dangerous, since any trapped air will expand on heating and may cause the can to burst open. It's safer to empty the can into an open utensil and then heat it on the stovetop, in the oven, or in the microwave. Because it contains concentrated protein and sugar, sweetened condensed milk will "caramelize" (actually, undergo the Maillard browning reaction, p. 778) at temperatures as low as the boiling point of water. This has made cans of sweetened condensed milk a favorite shortcut to a creamy caramel sauce: many people simply put the can in a pot of boiling water or a warm oven and let it brown inside. While this does work, it is potentially dangerous, since any trapped air will expand on heating and may cause the can to burst open. It's safer to empty the can into an open utensil and then heat it on the stovetop, in the oven, or in the microwave.

Intentionally Curdled MilkFor most cooks most of the time, curdled milk betokens crisis: the dish has lost its smoothness. But there are plenty of dishes in which the cook intentionally causes the milk proteins to clot precisely for the textural interest this creates. The English syllabub syllabub was sometimes made by squirting warm milk directly from the udder into acidic wine or juice; and in the 17th century, the French writer Pierre de Lune described a reduced milk "marbled" by the addition of currant juice. More contemporary examples include roast pork braised in milk, which reduces to moist brown nuggets; the Kashmiri practice of cooking milk down to resemble browned ground meat; and eastern European summertime cold milk soups like the Polish was sometimes made by squirting warm milk directly from the udder into acidic wine or juice; and in the 17th century, the French writer Pierre de Lune described a reduced milk "marbled" by the addition of currant juice. More contemporary examples include roast pork braised in milk, which reduces to moist brown nuggets; the Kashmiri practice of cooking milk down to resemble browned ground meat; and eastern European summertime cold milk soups like the Polish chlodnik chlodnik, thickened by the addition of "sour salt," or citric acid.

Milk Foams A foam is a portion of liquid filled with air bubbles, a moist, light ma.s.s that holds its shape. A meringue is a foam of egg whites, and whipped cream is a foam of cream. Milk foams are more fragile than egg foams and whipped cream, and are generally made immediately before serving, usually as a topping for coffee drinks. They prevent a skin from forming on the drink, and keep it hot by insulating it and preventing evaporative cooling. A foam is a portion of liquid filled with air bubbles, a moist, light ma.s.s that holds its shape. A meringue is a foam of egg whites, and whipped cream is a foam of cream. Milk foams are more fragile than egg foams and whipped cream, and are generally made immediately before serving, usually as a topping for coffee drinks. They prevent a skin from forming on the drink, and keep it hot by insulating it and preventing evaporative cooling.

Milk owes its foaming power to its proteins, which collect in a thin layer around the pockets of air, isolate them, and prevent the water's strong cohesive forces from popping the bubbles. Egg foams are also stabilized by proteins (p. 101), while the foam formed by whipping cream is stabilized by fat (below, p. 31). Milk foams are more fragile and short-lived than egg foams because milk's proteins are spa.r.s.e - just 3% of the milk's weight, where egg white is 10% protein - and two-thirds of the milk proteins are resistant to being unfolded and coagulated into a solid network, while most of the egg proteins readily do so. However, heat around 160F/70C does unfold the whey proteins (barely 1% of milk's weight). And if they unfold at the air-water boundary of a bubble wall, then the force imbalance does cause the proteins to bond to each other and briefly stabilize the foam.

Milks and Their Foams Some milks are better suited to foaming than others. Because the whey proteins are the critical stabilizers, milks that are fortified with added protein - usually reduced-fat and skim milks - are most easily foamed. Full-fat foams, on the other hand, are fuller in texture and flavor. Milk should always be as fresh as possible, since milk that has begun to sour can curdle when heated. Some milks are better suited to foaming than others. Because the whey proteins are the critical stabilizers, milks that are fortified with added protein - usually reduced-fat and skim milks - are most easily foamed. Full-fat foams, on the other hand, are fuller in texture and flavor. Milk should always be as fresh as possible, since milk that has begun to sour can curdle when heated.