On Food And Cooking Part 36

"Fruity"

Preformed Preformed Delicate, altered by cooking Delicate, altered by cooking

Citrus Preformed Preformed Persistent Persistent

"Fatty," "creamy"

Preformed Preformed Persistent Persistent

Caramel, nutty Preformed Preformed Persistent Persistent

Tropical fruit, "exotic," musky Preformed Preformed Persistent Persistent

Herbs & Spices Pine-like, mint-like, herbaceous

Sage, thyme, rosemary, mint, nutmeg Sage, thyme, rosemary, mint, nutmeg

Terpenes Terpenes

Spicy, warming

Cinnamon, clove, anise, basil, vanilla Cinnamon, clove, anise, basil, vanilla

Phenolic compounds Phenolic compounds

Pine-like, mint-like, herbaceous

Preformed Preformed

Strong, persistent Strong, persistent

Spicy, warming

Preformed Preformed

Strong, persistent Strong, persistent

Fruits are a different story. Some fruits may actually get better after harvest because they continue to ripen. But ripening soon runs its course, and then fruits also deteriorate. Eventually fruit and vegetable cells alike run out of energy and die, their complex biochemical organization and machinery break down, their enzymes act at random, and the tissue eats itself away.

The spoilage of fruits and vegetables is hastened by microbes, which are always present on their surfaces and in the air. Bacteria, molds, and yeasts all attack weakened or damaged plant tissue, break down its cell walls, consume the cell contents, and leave behind their distinctive and often unpleasant waste products. Vegetables are mainly attacked by bacteria, which grow faster than other microbes. Species of Erwinia Erwinia and and Pseudomonas Pseudomonas cause familiar "soft rot." Fruits are more acidic than vegetables, so they're resistant to many bacteria but more readily attacked by yeasts and molds ( cause familiar "soft rot." Fruits are more acidic than vegetables, so they're resistant to many bacteria but more readily attacked by yeasts and molds (Penicillium, Botrytis).

Precut fruits and vegetables are convenient but especially susceptible to deterioration and spoilage. Cutting has two important effects. The tissue damage induces nearby cells to boost their defensive activity, which depletes their remaining nutrients and may cause such changes as toughening, browning, and the development of bitter and astringent flavors. And it exposes the normally protected nutrient-rich interior to infection by microbes. So precut produce requires special care.

Handling Fresh Produce The aim in storing fruits and vegetables is to slow their inevitable deterioration. This begins with choosing and handling the produce. Mushrooms as well as some ripe fruits - berries, apricots, figs, avocados, papayas - have a naturally high metabolism and deteriorate faster than lethargic apples, pears, kiwi fruits, cabbages, carrots, and other good keepers. "One rotten apple spoils the barrel": moldy fruit or vegetables should be discarded and refrigerator drawers and fruit bowls should be cleaned regularly to reduce the microbial population. Produce shouldn't be subjected to physical stress, whether dropping apples on the floor or packing tomatoes tightly into a confined s.p.a.ce. Even rinsing in water can make delicate berries more susceptible to infection by abrading their protective epidermal layer with clinging dirt particles. On the other hand, soil harbors large numbers of microbes, and should be removed from the surfaces of st.u.r.dier fruits and vegetables before storing them.

The Storage Atmosphere The storage life of fresh produce is strongly affected by the atmosphere that surrounds it. All plant tissues are mostly water, and require a humid atmosphere to avoid drying out, losing turgidity, and damaging their internal systems. Practically, this means it's best to keep plant foods in restricted s.p.a.ces - plastic bags, or drawers within a refrigerator - to slow down moisture loss to the compartment as a whole and to the outside. At the same time, living produce exhales carbon dioxide and water, so moisture can acc.u.mulate and condense on the food surfaces, which encourages microbial attack. Lining the container with an absorbent material - a paper towel or bag - will delay condensation.

The metabolic activity of the cells can also be slowed by limiting their access to oxygen. Commercial packers fill their bags of produce with a well-defined mixture of nitrogen, carbon dioxide, and just enough oxygen (8% or less) to keep the plant cells functioning normally; and they use bags whose gas permeability matches the respiration rate of the produce. (Too little oxygen and fruits and vegetables switch to anaerobic metabolism, which generates alcohol and other odorous molecules characteristic of fermentation, and causes internal tissue damage and browning.) Home and restaurant cooks can approximate such a controlled atmosphere by packing their produce in closed plastic bags with most of the air squeezed out of them. The plant cells consume oxygen and create carbon dioxide, so the oxygen levels in the bags slowly decline. However, a major disadvantage of a closed plastic bag is that it traps the gas ethylene, a plant hormone that advances ripening in fruits and induces defensive activity and accelerated aging in other tissues. This means that bagged fruits may pa.s.s from ripe to overripe too quickly, and one damaged lettuce leaf can speed the decline of a whole head. Recently, manufacturers have introduced produce containers with inserts that destroy ethylene and extend storage life (the inserts contain permanganate).

A very common commercial treatment that slows both water loss and oxygen uptake in whole fruits and fruit-vegetables - apples, oranges, cuc.u.mbers, tomatoes - is to coat them at the packing facility with a layer of edible wax or oil. A number of different materials are used, including natural beeswax and carnauba, candellila, and rice-bran waxes and vegetable oils, and such petrochemical by-products as paraffin, polyethylene waxes, and mineral oil. These treatments are harmless, but they can make produce surfaces unpleasantly waxy or hard.

Temperature Control: Refrigeration The most effective way to prolong the storage life of fresh produce is to control its temperature. Cooling slows chemical reactions in general, so it slows the metabolic activity of the plant cells themselves, and the growth of the microbes that attack them. A reduction of just 10F/5C can nearly double storage life. However, the ideal storage temperature is different for different fruits and vegetables. Those native to temperate climates are best kept at or near the freezing point, and apples may keep for nearly a year if the storage atmosphere is also controlled. But fruits and vegetables native to warmer regions are actually injured by temperatures that low. Their cells begin to malfunction, and uncontrolled enzyme action causes damage to cell walls, the development of off-flavors, and discoloration. Chilling injury may become apparent during storage, or only after the produce is brought back to room temperature. Banana skins turn black in the refrigerator; avocados darken and fail to soften further; citrus fruits develop spotted skins. Foods of tropical and subtropical origin keep best at the relatively high temperature of 50F/10C, and are often better off at room temperature than in the refrigerator. Among them are melons, eggplants, squash, tomatoes, cuc.u.mbers, peppers, and beans.

Temperature Control: Freezing The most drastic form of temperature control is freezing, which stops cold the overall metabolism of fruits, vegetables, and spoilage microbes. It causes most of the water in the cells to crystallize, thus immobilizing other molecules and suspending most chemical activity. The microbes are hardy, and most of them revive on warming. But freezing kills plant tissues, which suffer two kinds of damage. One is chemical: as the water crystallizes, enzymes and other reactive molecules become unusually concentrated and react abnormally. The other damage is physical disruption caused by the water crystals, whose edges puncture cell walls and membranes. When the food is thawed, the cell fluids leak out of the cells, and the food loses crispness and becomes limp and wet. Producers of frozen foods minimize the size of the ice crystals, and so the amount of damage done, by freezing the food as quickly as possible to as low a temperature as possible, often 40F/40C. Under these conditions, many small ice crystals form; at higher temperatures fewer and larger crystals form, and do more damage. Home and restaurant freezers are warmer than commercial freezers and their temperatures fluctuate, so during storage some water melts and refreezes into larger crystals, and the food's texture suffers.

Although freezing temperatures generally reduce enzymatic and other chemical activity, some reactions are actually enhanced by the concentrating effects of ice formation, including enzymatic breakdown of vitamins and pigments. The solution to this problem is blanching. blanching. In this process the food is immersed in rapidly boiling water for a minute or two, just enough time to inactivate the enzymes, and then just as rapidly immersed in cold water to stop further cooking and softening of the cell walls. If vegetables are to be frozen for more than a few days, they should be blanched first. Fruits are less commonly blanched because their cooked flavor and texture are less appealing. Enzymatic browning in frozen fruit can be prevented by packing it in a sugar syrup supplemented with as...o...b..c acid (between and teaspoon per quart, 7502,250 mg per liter, depending on the fruit's susceptibility to browning). Sugar syrup (usually around 40%, or 1.5 lb sugar per quart water, 680 gm per liter) can also improve the texture of frozen fruit by being absorbed into the cell-wall cement, which becomes stiffer. Frozen produce should be wrapped as air- and watertight as possible. Surfaces left exposed to the relatively dry atmosphere of the freezer will develop freezer burn, the slow, patchy drying out caused by the evaporation of frozen water molecules directly into vapor (this is called "sublimation"). Freezer-burned patches develop a tough texture and stale flavor. In this process the food is immersed in rapidly boiling water for a minute or two, just enough time to inactivate the enzymes, and then just as rapidly immersed in cold water to stop further cooking and softening of the cell walls. If vegetables are to be frozen for more than a few days, they should be blanched first. Fruits are less commonly blanched because their cooked flavor and texture are less appealing. Enzymatic browning in frozen fruit can be prevented by packing it in a sugar syrup supplemented with as...o...b..c acid (between and teaspoon per quart, 7502,250 mg per liter, depending on the fruit's susceptibility to browning). Sugar syrup (usually around 40%, or 1.5 lb sugar per quart water, 680 gm per liter) can also improve the texture of frozen fruit by being absorbed into the cell-wall cement, which becomes stiffer. Frozen produce should be wrapped as air- and watertight as possible. Surfaces left exposed to the relatively dry atmosphere of the freezer will develop freezer burn, the slow, patchy drying out caused by the evaporation of frozen water molecules directly into vapor (this is called "sublimation"). Freezer-burned patches develop a tough texture and stale flavor.

Cooking Fresh Fruits and Vegetables Compared to meats, eggs, and dairy products, vegetables and fruits are easy to cook. Animal tissues and secretions are mainly protein, and proteins are sensitive molecules; moderate heat (140F/60C) causes them to cling tightly to each other and expel water, and they quickly become hard and dry. Vegetables and fruits are mainly carbohydrates, and carbohydrates are robust molecules; even boiling temperatures simply disperse them more evenly in the tissue moisture, so the texture becomes soft and succulent. However, the cooking of vegetables and fruits does have its fine points. Plant pigments, flavor compounds, and nutrients are sensitive to heat and to the chemical environment. And even carbohydrates sometimes behave curiously! The challenge of cooking vegetables and fruits is to create an appealing texture without compromising color, flavor, and nutrition.

How Heat Affects the Qualities of Fruits and Vegetables Color Many plant pigments are altered by cooking, which is why we can often judge by their color how carefully vegetables have been prepared. The one partial exception to this rule is the yellow-orange-red carotenoid group, which is more soluble in fat than in water, so the colors don't readily leak out of the tissue, and are fairly stable. However, even carotenoids are changed by cooking. When we heat carrots, their beta-carotene shifts structure and hue, from red-orange toward the yellow. Apricots and tomato paste dried in the sun lose much of their intact carotenoids unless they're treated with antioxidant sulfur dioxide (p. 291). But compared to the green chlorophylls and multihued anthocyanins, the carotenoids are the model of steadfastness. Many plant pigments are altered by cooking, which is why we can often judge by their color how carefully vegetables have been prepared. The one partial exception to this rule is the yellow-orange-red carotenoid group, which is more soluble in fat than in water, so the colors don't readily leak out of the tissue, and are fairly stable. However, even carotenoids are changed by cooking. When we heat carrots, their beta-carotene shifts structure and hue, from red-orange toward the yellow. Apricots and tomato paste dried in the sun lose much of their intact carotenoids unless they're treated with antioxidant sulfur dioxide (p. 291). But compared to the green chlorophylls and multihued anthocyanins, the carotenoids are the model of steadfastness.

Aromas from Altered Carotenoid PigmentsBoth drying and cooking break some of the pigment molecules in carotenoid-rich fruits and vegetables into small, volatile fragments that contribute to their characteristic aromas. These fragments provide notes reminiscent of black tea, hay, honey, and violets.

Green Chlorophyll One change in the color of green vegetables as they are cooked has nothing to do with the pigment itself. That wonderfully intense, bright green that develops within a few seconds of throwing vegetables into boiling water is a result of the sudden expansion and escape of gases trapped in the s.p.a.ces between cells. Ordinarily, these microscopic air pockets cloud the color of the chloroplasts. When they collapse, we can see the pigments much more directly. One change in the color of green vegetables as they are cooked has nothing to do with the pigment itself. That wonderfully intense, bright green that develops within a few seconds of throwing vegetables into boiling water is a result of the sudden expansion and escape of gases trapped in the s.p.a.ces between cells. Ordinarily, these microscopic air pockets cloud the color of the chloroplasts. When they collapse, we can see the pigments much more directly.

The Enemy of Green: Acids Green chlorophyll is susceptible to two chemical changes during cooking. One is the loss of its long carbon-hydrogen tail, which leaves the pigment water-soluble - so that it leaks out into the cooking liquid - and more susceptible to further change. This loss is encouraged by both acid and alkaline conditions and by an enzyme called chlorophyllase, which is most active between 150170F/6677C and only destroyed near the boiling point. The second and more noticeable change in chlorophyll is the dulling of its color, which is caused when either heat or an enzyme nudge the magnesium atom from the center of the molecule. The replacement of magnesium by hydrogen is by far the most common cause of color change in cooked vegetables. In even slightly acidic water, the plentiful hydrogen ions displace the magnesium, a change that turns chlorophyll Green chlorophyll is susceptible to two chemical changes during cooking. One is the loss of its long carbon-hydrogen tail, which leaves the pigment water-soluble - so that it leaks out into the cooking liquid - and more susceptible to further change. This loss is encouraged by both acid and alkaline conditions and by an enzyme called chlorophyllase, which is most active between 150170F/6677C and only destroyed near the boiling point. The second and more noticeable change in chlorophyll is the dulling of its color, which is caused when either heat or an enzyme nudge the magnesium atom from the center of the molecule. The replacement of magnesium by hydrogen is by far the most common cause of color change in cooked vegetables. In even slightly acidic water, the plentiful hydrogen ions displace the magnesium, a change that turns chlorophyll a a into grayish-green pheophytin into grayish-green pheophytin a, a, chlorophyll chlorophyll b b into yellowish pheophytin into yellowish pheophytin b. b. Cooking vegetables without water - stir-frying, for example - will also cause a color change, because when the temperature of the plant tissue rises above 140F/60C, the organizing membranes in and around the chloroplast are damaged, and chlorophyll is exposed to the plant's own natural acids. Freezing, pickling, dehydration, and simple aging also damage chloroplasts and chlorophyll. This is why dull, olive-green vegetables are so common. Cooking vegetables without water - stir-frying, for example - will also cause a color change, because when the temperature of the plant tissue rises above 140F/60C, the organizing membranes in and around the chloroplast are damaged, and chlorophyll is exposed to the plant's own natural acids. Freezing, pickling, dehydration, and simple aging also damage chloroplasts and chlorophyll. This is why dull, olive-green vegetables are so common.

Changes in chlorophyll during cooking. Left: Left: The normal chlorophyll molecule is bright green and has a fat-like tail that makes it soluble in fats and oils. The normal chlorophyll molecule is bright green and has a fat-like tail that makes it soluble in fats and oils. Center: Center: Enzymes in the plant cells can remove the fat-like tail, producing a tailless form that is water-soluble and readily leaks into cooking liquids. Enzymes in the plant cells can remove the fat-like tail, producing a tailless form that is water-soluble and readily leaks into cooking liquids. Right: Right: In acid conditions, the central magnesium atom is replaced by hydrogens, and the resulting chlorophyll molecule is a dull olive green. In acid conditions, the central magnesium atom is replaced by hydrogens, and the resulting chlorophyll molecule is a dull olive green.

Traditional Aids: Soda and Metals There are two chemical tricks that can help keep green vegetables bright, and cooks have known about them for hundreds and even thousands of years. One is to cook them in alkaline water, which has very few hydrogen ions that are free to displace the magnesium in chlorophyll. The great 19th-century French chef Antonin Careme de-acidified his cooking water with wood ash; today baking soda (sodium bicarbonate) is the easiest. The other chemical trick is to add to the cooking water other metals - copper and zinc - that can replace magnesium in the chlorophyll molecule, and resist displacement by hydrogen. However, both tricks have disadvantages. Copper and zinc are essential trace nutrients, but in doses of more than a few milligrams they can be toxic. And while there's nothing toxic about sodium bicarbonate, excessively alkaline conditions can turn vegetable texture to mush (p. 282), speed the destruction of vitamins, and leave a soapy off-taste. There are two chemical tricks that can help keep green vegetables bright, and cooks have known about them for hundreds and even thousands of years. One is to cook them in alkaline water, which has very few hydrogen ions that are free to displace the magnesium in chlorophyll. The great 19th-century French chef Antonin Careme de-acidified his cooking water with wood ash; today baking soda (sodium bicarbonate) is the easiest. The other chemical trick is to add to the cooking water other metals - copper and zinc - that can replace magnesium in the chlorophyll molecule, and resist displacement by hydrogen. However, both tricks have disadvantages. Copper and zinc are essential trace nutrients, but in doses of more than a few milligrams they can be toxic. And while there's nothing toxic about sodium bicarbonate, excessively alkaline conditions can turn vegetable texture to mush (p. 282), speed the destruction of vitamins, and leave a soapy off-taste.

Watch the Water, Time, and Sauce Dulling of the greens can be minimized by keeping cooking times short, between five and seven minutes, and protecting chlorophyll from acid conditions. Stir-frying and microwaving can be very quick, but they expose chlorophyll fully to the cells' own acids. Ordinary boiling in copious water has the advantage of diluting the cells' acids. Most city tap water is kept slightly alkaline to minimize pipe corrosion, and slightly alkaline water is ideal for preserving chlorophyll's color. Check the pH of your water: if it's acid, its pH below 7, then experiment with adding small amounts of baking soda (start with a small pinch per gallon/4 liters) to adjust it to neutral or slightly alkaline. Once the vegetables are cooked, either serve them immediately or plunge them briefly in ice water so that they don't continue to cook and get dull. Don't dress the vegetables with acidic ingredients like lemon juice until the last minute, and consider protecting them first with a thin layer of oil (as in a vinaigrette) or b.u.t.ter. Dulling of the greens can be minimized by keeping cooking times short, between five and seven minutes, and protecting chlorophyll from acid conditions. Stir-frying and microwaving can be very quick, but they expose chlorophyll fully to the cells' own acids. Ordinary boiling in copious water has the advantage of diluting the cells' acids. Most city tap water is kept slightly alkaline to minimize pipe corrosion, and slightly alkaline water is ideal for preserving chlorophyll's color. Check the pH of your water: if it's acid, its pH below 7, then experiment with adding small amounts of baking soda (start with a small pinch per gallon/4 liters) to adjust it to neutral or slightly alkaline. Once the vegetables are cooked, either serve them immediately or plunge them briefly in ice water so that they don't continue to cook and get dull. Don't dress the vegetables with acidic ingredients like lemon juice until the last minute, and consider protecting them first with a thin layer of oil (as in a vinaigrette) or b.u.t.ter.

Old Tricks for Green VegetablesCooks had worked out the practical chemistry of chlorophyll long before it had a name. The Roman recipe collection of Apicius advises, "omne holus smaragdinum fit, si c.u.m nitro coquatur." "All green vegetables will be made emerald colored, if they are cooked with nitrum." Nitrum was a natural soda, and alkaline like our baking soda. In her English cookbook of 1751, Hannah Gla.s.se directed readers to "Boil all your Greens in a Copper Sauce-pan by themselves, with a great Quant.i.ty of Water. Use no iron pans, etc., for they are not proper; but let them be Copper, Bra.s.s, or Silver." Cookbooks of the early 19th century suggest cooking vegetables and making cuc.u.mber pickles with a copper ha'penny coin thrown in to improve the color. All of these practices survived in some form until the beginning of the 20th century, though Sweden outlawed the use of copper cooking pots in its armed services in the 18th century due to the toxicity of copper in large, c.u.mulative doses. And "Tabitha Tickletooth" wrote in The Dinner Question The Dinner Question (1860): "Never, under any circ.u.mstances, unless you wish entirely to destroy all flavor, and reduce your peas to pulp, boil them with soda. This favorite atrocity of the English kitchen cannot be too strongly condemned." (1860): "Never, under any circ.u.mstances, unless you wish entirely to destroy all flavor, and reduce your peas to pulp, boil them with soda. This favorite atrocity of the English kitchen cannot be too strongly condemned."

Red-Purple Anthocyanins and Pale Anthoxanthins The usually reddish anthocyanins and their pale yellow cousins, the anthoxanthins, are chlorophyll's opposites. They're naturally water-soluble, so they always bleed into the cooking water. They too are sensitive to pH and to the presence of metal ions, but acidity is good for them, metals bad. And where chlorophyll just gets duller or brighter according to these conditions, the anthocyanins change color completely! This is why we occasionally see red cabbage turn blue when braised, blueberries turn green in pancakes and m.u.f.fins, and garlic turn green or blue when pickled. (The betacyanins and betaxanthins in beets and chard are different compounds and somewhat more stable.) The usually reddish anthocyanins and their pale yellow cousins, the anthoxanthins, are chlorophyll's opposites. They're naturally water-soluble, so they always bleed into the cooking water. They too are sensitive to pH and to the presence of metal ions, but acidity is good for them, metals bad. And where chlorophyll just gets duller or brighter according to these conditions, the anthocyanins change color completely! This is why we occasionally see red cabbage turn blue when braised, blueberries turn green in pancakes and m.u.f.fins, and garlic turn green or blue when pickled. (The betacyanins and betaxanthins in beets and chard are different compounds and somewhat more stable.) The Enemies: Dilution, Alkalinity, and Metals Anthocyanins and anthoxanthins are concentrated in cell vacuoles, and sometimes (as in purple beans and asparagus) just in a superficial layer of cells. So when the food is cooked and the vacuoles damaged, the pigments escape and can become so diluted that their color fades or disappears - especially if they're cooked in a pot of water. The pigments that remain are affected by the new chemical environment of the cooked plant tissue. The vacuoles in which anthocyanins are stored are generally acid, while the rest of the cell fluids are less so. Cooking water is often somewhat alkaline, and quick breads include distinctly alkaline baking soda. In acid conditions, anthocyanins tend toward the red; around neutral pH, they're colorless or light violet; and in alkaline conditions, bluish. And pale anthoxanthins become more deeply yellow as alkalinity rises. So red fruits and vegetables can fade and even turn blue when cooked, while pale yellow ones darken. And traces of metals in the cooking liquid can generate very peculiar colors: some anthocyanins and anthoxanthins form grayish, green, blue, red, or brown complexes with iron, aluminum, and tin. Anthocyanins and anthoxanthins are concentrated in cell vacuoles, and sometimes (as in purple beans and asparagus) just in a superficial layer of cells. So when the food is cooked and the vacuoles damaged, the pigments escape and can become so diluted that their color fades or disappears - especially if they're cooked in a pot of water. The pigments that remain are affected by the new chemical environment of the cooked plant tissue. The vacuoles in which anthocyanins are stored are generally acid, while the rest of the cell fluids are less so. Cooking water is often somewhat alkaline, and quick breads include distinctly alkaline baking soda. In acid conditions, anthocyanins tend toward the red; around neutral pH, they're colorless or light violet; and in alkaline conditions, bluish. And pale anthoxanthins become more deeply yellow as alkalinity rises. So red fruits and vegetables can fade and even turn blue when cooked, while pale yellow ones darken. And traces of metals in the cooking liquid can generate very peculiar colors: some anthocyanins and anthoxanthins form grayish, green, blue, red, or brown complexes with iron, aluminum, and tin.

The Aid: Acids The key to maintaining natural anthocyanin coloration is to keep fruits and vegetables sufficiently acidic, and avoid supplying trace metals. Lemon juice in the cooking water or sprinkled on the food can help with both aims: its citric acid binds up metal ions. Cooking red cabbage with acidic apples or vinegar keeps it from turning purple; dispersing baking soda evenly in batters, and using as little as possible to keep the batter slightly acidic, will keep blueberries from turning green. The key to maintaining natural anthocyanin coloration is to keep fruits and vegetables sufficiently acidic, and avoid supplying trace metals. Lemon juice in the cooking water or sprinkled on the food can help with both aims: its citric acid binds up metal ions. Cooking red cabbage with acidic apples or vinegar keeps it from turning purple; dispersing baking soda evenly in batters, and using as little as possible to keep the batter slightly acidic, will keep blueberries from turning green.

Creating Color from Tannins On rare and wonderful occasions, cooking can actually create anthocyanins: in fact, it transforms touch into color! Colorless quince slices cooked in a sugar syrup lose their astringency and develop a ruby-like color and translucency. Quinces and certain varieties of pear are especially rich in phenolic chemicals, including aggregates (proanthocyanidins) of from 2 to 20 anthocyanin-like subunits. The aggregates are the right size to cross-link and coagulate proteins, so they feel astringent in our mouth. When these fruits are cooked for a long time, the combination of heat and acidity causes the subunits to break off one by one; and then oxygen from the air reacts with the subunits to form true anthocyanins: so the tannic, pale fruits become more gentle-tasting and anything from pale pink to deep red. (Interestingly, the similar development of pinkness in canned pears is considered discoloration. It's accentuated by tin in unenameled cans.) On rare and wonderful occasions, cooking can actually create anthocyanins: in fact, it transforms touch into color! Colorless quince slices cooked in a sugar syrup lose their astringency and develop a ruby-like color and translucency. Quinces and certain varieties of pear are especially rich in phenolic chemicals, including aggregates (proanthocyanidins) of from 2 to 20 anthocyanin-like subunits. The aggregates are the right size to cross-link and coagulate proteins, so they feel astringent in our mouth. When these fruits are cooked for a long time, the combination of heat and acidity causes the subunits to break off one by one; and then oxygen from the air reacts with the subunits to form true anthocyanins: so the tannic, pale fruits become more gentle-tasting and anything from pale pink to deep red. (Interestingly, the similar development of pinkness in canned pears is considered discoloration. It's accentuated by tin in unenameled cans.) Turning Red Wine into WhiteThe sensitivity of anthocyanin pigments to pH is the basis for a remarkable recipe in the late Roman collection attributed to Apicius:To make white wine out of red wine. Put bean-meal or three egg whites into the flask and stir for a very long time. The next day the wine will be white. The ashes of white grape vines have the same effect.Both vine ashes and egg whites are alkaline substances and do transform the wine's color - though when I've tried this with eggs, the result is not so much a white wine as a gray one.

Texture We've seen that the texture of vegetables and fruits is determined by two factors: the inner water pressure of the tissue's cells, and the structure of the cell walls (p. 265). Cooking softens plant tissues by releasing the water pressure and dismantling the cell walls. When the tissue reaches 140F/60C, the cell membranes are damaged, the cells lose water and deflate, and the tissue as a whole goes from firm and crisp to limp and flabby. (Even vegetables surrounded by boiling water lose water during cooking, as weighings before and after will prove.) At this stage, vegetables often squeak against the teeth: they've lost the crunch of turgid tissue, but the cell walls are still strong and resist chewing. Then as the tissue temperature approaches the boiling point, the cell walls begin to weaken. The cellulose framework remains mostly unchanged, but the pectin and hemicellulose "cement" softens, gradually breaks down into shorter chains, and dissolves. Teeth now easily push adjacent cells apart from each other, and the texture becomes tender. Prolonged boiling will remove nearly all of the cell-wall cement and cause the tissue to disintegrate, thus transforming it into a puree. We've seen that the texture of vegetables and fruits is determined by two factors: the inner water pressure of the tissue's cells, and the structure of the cell walls (p. 265). Cooking softens plant tissues by releasing the water pressure and dismantling the cell walls. When the tissue reaches 140F/60C, the cell membranes are damaged, the cells lose water and deflate, and the tissue as a whole goes from firm and crisp to limp and flabby. (Even vegetables surrounded by boiling water lose water during cooking, as weighings before and after will prove.) At this stage, vegetables often squeak against the teeth: they've lost the crunch of turgid tissue, but the cell walls are still strong and resist chewing. Then as the tissue temperature approaches the boiling point, the cell walls begin to weaken. The cellulose framework remains mostly unchanged, but the pectin and hemicellulose "cement" softens, gradually breaks down into shorter chains, and dissolves. Teeth now easily push adjacent cells apart from each other, and the texture becomes tender. Prolonged boiling will remove nearly all of the cell-wall cement and cause the tissue to disintegrate, thus transforming it into a puree.

Acid and Hard Water Maintain Firmness; Salt and Alkalinity Speed Softening The wall-dissolving, tenderizing phase of fruit and vegetable cooking is strongly influenced by the cooking environment. Hemicelluloses are not very soluble in acid conditions, and readily soluble in alkaline conditions. This means that fruits and vegetables cooked in an acid liquid - a tomato sauce for example, or other fruit juices and purees - may remain firm during hours of cooking, while in neutral boiling water, neither acid nor alkaline, the same vegetables soften in 10 or 15 minutes. In distinctly alkaline water, fruits and vegetables quickly become mushy. Table salt in neutral cooking water speeds vegetable softening, apparently because its sodium ions displace the calcium ions that cross-link and anchor the cement molecules in the fruit and vegetable cell walls, thus breaking the cross-links and helping to dissolve the hemicelluloses. On the other hand, the dissolved calcium in hard tap water slows softening by reinforcing the cement cross-links. When vegetables are cooked without immersion in water - when they're steamed or fried or baked - the cell walls are exposed only to the more or less acid cell fluids (steam itself is also a somewhat acidic pH 6), and a given cooking time often produces a firmer result than boiling. The wall-dissolving, tenderizing phase of fruit and vegetable cooking is strongly influenced by the cooking environment. Hemicelluloses are not very soluble in acid conditions, and readily soluble in alkaline conditions. This means that fruits and vegetables cooked in an acid liquid - a tomato sauce for example, or other fruit juices and purees - may remain firm during hours of cooking, while in neutral boiling water, neither acid nor alkaline, the same vegetables soften in 10 or 15 minutes. In distinctly alkaline water, fruits and vegetables quickly become mushy. Table salt in neutral cooking water speeds vegetable softening, apparently because its sodium ions displace the calcium ions that cross-link and anchor the cement molecules in the fruit and vegetable cell walls, thus breaking the cross-links and helping to dissolve the hemicelluloses. On the other hand, the dissolved calcium in hard tap water slows softening by reinforcing the cement cross-links. When vegetables are cooked without immersion in water - when they're steamed or fried or baked - the cell walls are exposed only to the more or less acid cell fluids (steam itself is also a somewhat acidic pH 6), and a given cooking time often produces a firmer result than boiling.

Cooking starchy vegetables. Left: Left: Before cooking, the plant cells are intact, the starch granules compact and hard. Before cooking, the plant cells are intact, the starch granules compact and hard. Right: Right: Cooking causes the starch granules to absorb water from the cell fluids, swell, and soften. Cooking causes the starch granules to absorb water from the cell fluids, swell, and soften.

The cook can make use of these influences to diagnose the cause of excessively rapid or slow softening and adjust the preparation - for example, precooking vegetables in plain water before adding them to a tomato sauce, or compensating for hard water with a softening pinch of alkaline baking soda. In the case of green vegetables, shortening the softening time with the help of salt and a discreet dose of baking soda helps preserve the bright green of the chlorophyll (p. 280).

Starchy Vegetables Potatoes, sweet potatoes, winter squashes, and other starchy vegetables owe their distinctive cooked texture to their starch granules. In the raw vegetables, starch granules are hard, closely packed, microscopic agglomerations of starch molecules, and give a chalky feeling when chewed out of the cells. They begin to soften at about the same temperature at which the membrane proteins denature, the "gelation range," which in the potato is from 137150F/5866C (it varies from plant to plant). In this range the starch granules begin to absorb water molecules, which disrupt their compact structure, and the granules swell up to many times their original size, forming a soft gel, or sponge-like network of long chains holding water in the pockets between chains. The overall result is a tender but somewhat dry texture, because the tissue moisture has been soaked up into the starch. (Think of the textural difference between cooked high-starch potatoes and low-starch carrots.) In starchy vegetables with relatively weak cell walls, the gel-filled cells may be cohesive enough to pull away from each other as separate little particles, giving a mealy impression. This water absorption and the large surface area of separate cells are the reasons that mashed potatoes and other cooked starchy purees benefit from and accommodate large amounts of lubricating fat. Potatoes, sweet potatoes, winter squashes, and other starchy vegetables owe their distinctive cooked texture to their starch granules. In the raw vegetables, starch granules are hard, closely packed, microscopic agglomerations of starch molecules, and give a chalky feeling when chewed out of the cells. They begin to soften at about the same temperature at which the membrane proteins denature, the "gelation range," which in the potato is from 137150F/5866C (it varies from plant to plant). In this range the starch granules begin to absorb water molecules, which disrupt their compact structure, and the granules swell up to many times their original size, forming a soft gel, or sponge-like network of long chains holding water in the pockets between chains. The overall result is a tender but somewhat dry texture, because the tissue moisture has been soaked up into the starch. (Think of the textural difference between cooked high-starch potatoes and low-starch carrots.) In starchy vegetables with relatively weak cell walls, the gel-filled cells may be cohesive enough to pull away from each other as separate little particles, giving a mealy impression. This water absorption and the large surface area of separate cells are the reasons that mashed potatoes and other cooked starchy purees benefit from and accommodate large amounts of lubricating fat.

Precooking Can Give a Persistent Firmness to Some Vegetables and Fruits It turns out that in certain vegetables and fruits - including potatoes, sweet potatoes, beets, carrots, beans, cauliflower, tomatoes, cherries, apples - the usual softening during cooking can be reduced by a low-temperature precooking step. If preheated to 130140F/5560C for 2030 minutes, these foods develop a persistent firmness that survives prolonged final cooking. This can be valuable for vegetables meant to hold their shape in a long-cooked meat dish, or potatoes in a potato salad, or for foods to be preserved by canning. It's also valuable for boiled whole potatoes and beets, whose outer regions are inevitably over-softened and may begin to disintegrate while the centers cook through. These and other long-cooked root vegetables are usually started in cold water, so that the outer regions will firm up during the slow temperature rise. Firm-able vegetables and fruits have an enzyme in their cell walls that becomes activated at around 120F/50C (and inactivated above 160F/70C), and alters the cell-wall pectins so that they're more easily cross-linked by calcium ions. At the same time, calcium ions are being released as the cell contents leak through damaged membranes, and they cross-link the pectin so that it will be much more resistant to removal or breakdown at boiling temperatures. It turns out that in certain vegetables and fruits - including potatoes, sweet potatoes, beets, carrots, beans, cauliflower, tomatoes, cherries, apples - the usual softening during cooking can be reduced by a low-temperature precooking step. If preheated to 130140F/5560C for 2030 minutes, these foods develop a persistent firmness that survives prolonged final cooking. This can be valuable for vegetables meant to hold their shape in a long-cooked meat dish, or potatoes in a potato salad, or for foods to be preserved by canning. It's also valuable for boiled whole potatoes and beets, whose outer regions are inevitably over-softened and may begin to disintegrate while the centers cook through. These and other long-cooked root vegetables are usually started in cold water, so that the outer regions will firm up during the slow temperature rise. Firm-able vegetables and fruits have an enzyme in their cell walls that becomes activated at around 120F/50C (and inactivated above 160F/70C), and alters the cell-wall pectins so that they're more easily cross-linked by calcium ions. At the same time, calcium ions are being released as the cell contents leak through damaged membranes, and they cross-link the pectin so that it will be much more resistant to removal or breakdown at boiling temperatures.

Persistently Crisp Vegetables A few underground stem vegetables are notable for retaining some crunchiness after prolonged cooking and even canning. These include the Chinese water chestnut, lotus root, bamboo shoots, and beets. Their textural robustness comes from particular phenolic compounds in their cell walls (ferulic acids) that form bonds with the cell-wall carbohydrates and prevent them from being dissolved away during cooking. A few underground stem vegetables are notable for retaining some crunchiness after prolonged cooking and even canning. These include the Chinese water chestnut, lotus root, bamboo shoots, and beets. Their textural robustness comes from particular phenolic compounds in their cell walls (ferulic acids) that form bonds with the cell-wall carbohydrates and prevent them from being dissolved away during cooking.

Flavor The relatively mild flavor of most vegetables and fruits is intensified by cooking. Heating makes taste molecules - sweet sugars, sour acids - more prominent by breaking down cell walls and making it easier for the cell contents to escape and reach our taste buds. Carrots, for example, taste far sweeter when cooked. Heat also makes the food's aromatic molecules more volatile and so more noticeable, and it creates new molecules by causing increased enzyme activity, mixing of cell contents, and general chemical reactivity. The more prolonged or intense the heating, the more the food's original aroma molecules are modified and supplemented, and so the more complex and "cooked" the flavor. If the cooking temperature exceeds the boiling point - in frying and baking, for example - then these carbohydrate-rich materials will begin to undergo browning reactions, which produce characteristic roasted and caramelized flavors. Cooks can create several layers of flavor in a dish by combining well-cooked, lightly cooked, and even raw batches of the same vegetables or herbs. The relatively mild flavor of most vegetables and fruits is intensified by cooking. Heating makes taste molecules - sweet sugars, sour acids - more prominent by breaking down cell walls and making it easier for the cell contents to escape and reach our taste buds. Carrots, for example, taste far sweeter when cooked. Heat also makes the food's aromatic molecules more volatile and so more noticeable, and it creates new molecules by causing increased enzyme activity, mixing of cell contents, and general chemical reactivity. The more prolonged or intense the heating, the more the food's original aroma molecules are modified and supplemented, and so the more complex and "cooked" the flavor. If the cooking temperature exceeds the boiling point - in frying and baking, for example - then these carbohydrate-rich materials will begin to undergo browning reactions, which produce characteristic roasted and caramelized flavors. Cooks can create several layers of flavor in a dish by combining well-cooked, lightly cooked, and even raw batches of the same vegetables or herbs.

One sensory quality unique to plants is astringency (p. 271), and it can make such foods as artichokes, unripe fruits, and nuts less than entirely pleasant to eat. There are ways to control the influence of tannins in these foods. Acids and salt increase the perception of astringency, while sugar reduces it. Adding milk, gelatin, or other proteins to a dish will reduce its astringency by inducing the tannins to bind to food proteins before they can affect salivary proteins. Ingredients rich in pectin or gums will also take some tannins out of circulation, and fats and oils will slow the initial binding of tannins and proteins.

Nutritional Value Cooking destroys some of the nutrients in food, but makes many nutrients more easily absorbed. It's a good idea to include both raw and cooked fruits and vegetables in our daily diet. Cooking destroys some of the nutrients in food, but makes many nutrients more easily absorbed. It's a good idea to include both raw and cooked fruits and vegetables in our daily diet.

Some Diminishment of Nutritional Value... Cooking generally reduces the nutritional content of fruits and vegetables. There are some important exceptions to this rule, but the levels of most vitamins, antioxidants, and other beneficial substances are diminished by the combination of high temperatures, uncontrolled enzyme activity, and exposure to oxygen and to light. They and minerals can also be drawn out of plant tissues by cooking water. These losses can be minimized by rapid and brief cooking. Baked potatoes, for example, heat up relatively slowly and lose much more vitamin C to enzyme action than do boiled potatoes. However, some techniques that speed cooking - cutting vegetables into small pieces, and boiling in a large volume of water, which maintains its temperature - can result in increased leaching of water-soluble nutrients, including minerals and the B and C vitamins. To maximize the retention of vitamins and minerals, cook small batches of vegetables and fruits in the microwave oven, in a minimal amount of added water. Cooking generally reduces the nutritional content of fruits and vegetables. There are some important exceptions to this rule, but the levels of most vitamins, antioxidants, and other beneficial substances are diminished by the combination of high temperatures, uncontrolled enzyme activity, and exposure to oxygen and to light. They and minerals can also be drawn out of plant tissues by cooking water. These losses can be minimized by rapid and brief cooking. Baked potatoes, for example, heat up relatively slowly and lose much more vitamin C to enzyme action than do boiled potatoes. However, some techniques that speed cooking - cutting vegetables into small pieces, and boiling in a large volume of water, which maintains its temperature - can result in increased leaching of water-soluble nutrients, including minerals and the B and C vitamins. To maximize the retention of vitamins and minerals, cook small batches of vegetables and fruits in the microwave oven, in a minimal amount of added water.

...And Some Enhancement Cooking has several general nutritional benefits. It eliminates potentially harmful microbes. By softening and concentrating foods, it also makes them easier to eat in significant quant.i.ties. And it actually improves the availability of some nutrients. Two of the most important are starch and the carotenoid pigments. Starch consists of long chains of sugar molecules crammed into ma.s.ses called granules. Our digestive enzymes can't penetrate past the outer layer of raw starch granules, but cooking unpacks the starch chains and lets our enzymes break them down. Then there are beta-carotene, the precursor to vitamin A, its chemical relative lycopene, an important antioxidant, and other valuable carotenoid pigments. Because they're not very soluble in water, we simply don't extract these chemicals very efficiently by just chewing and swallowing. Cooking disrupts the plant tissues more thoroughly and allows us to extract much more of them. (Added fat also significantly improves our absorption of fat-soluble nutrients.) Cooking has several general nutritional benefits. It eliminates potentially harmful microbes. By softening and concentrating foods, it also makes them easier to eat in significant quant.i.ties. And it actually improves the availability of some nutrients. Two of the most important are starch and the carotenoid pigments. Starch consists of long chains of sugar molecules crammed into ma.s.ses called granules. Our digestive enzymes can't penetrate past the outer layer of raw starch granules, but cooking unpacks the starch chains and lets our enzymes break them down. Then there are beta-carotene, the precursor to vitamin A, its chemical relative lycopene, an important antioxidant, and other valuable carotenoid pigments. Because they're not very soluble in water, we simply don't extract these chemicals very efficiently by just chewing and swallowing. Cooking disrupts the plant tissues more thoroughly and allows us to extract much more of them. (Added fat also significantly improves our absorption of fat-soluble nutrients.) There are many different ways of cooking vegetables and fruits. What follows is a brief outline of the most common methods and their general effects. They can be divided into three groups: moist methods that transfer heat by means of water; dry methods that transfer heat by means of air, oil, or infrared radiation; and a more miscellaneous group that includes ways of restructuring the food, either turning it into a fluid version of itself, or extracting the essence of its flavor or color.

Hot Water: Boiling, Steaming, Pressure-Cooking Boiling and steaming are the simplest methods for cooking vegetables, because they require no judgment of cooking temperature: whether water is boiling on a high flame or low, its temperature is 212F/100C (near sea level, with predictably lower temperatures at higher elevations). And because hot water and steam are excellent carriers of heat, these are efficient methods as well, ideal for the rapid cooking of green vegetables that minimizes their loss of color (p. 280). One important difference is that hot water dissolves and extracts some pectin and calcium from cell walls, while steaming leaves them in place: so boiling will soften vegetables faster and more thoroughly.

Boiling In the case of boiling green vegetables, it's good to know the pH and dissolved mineral content of your cooking water. Ideally it should be neutral or just slightly alkaline (pH 78), and not too hard, because acidity dulls chlorophyll, and acidity and calcium both slow softening and so prolong the cooking. A large volume of rapidly boiling water will maintain a boil even after the cold vegetables are added, cut into pieces small enough to cook through in about five minutes. Salt in the cooking water at about the concentration of seawater (3%, or 2 tablespoons/30 gm per quart/liter) will speed softening (p. 282) and also minimize the loss of cell contents to the water (cooking water without its own dissolved salt will draw salts and sugars from the plant cells). When just tender enough, the vegetables should be removed and either served immediately or scooped briefly into ice water to stop the cooking and prevent further dulling of the color. In the case of boiling green vegetables, it's good to know the pH and dissolved mineral content of your cooking water. Ideally it should be neutral or just slightly alkaline (pH 78), and not too hard, because acidity dulls chlorophyll, and acidity and calcium both slow softening and so prolong the cooking. A large volume of rapidly boiling water will maintain a boil even after the cold vegetables are added, cut into pieces small enough to cook through in about five minutes. Salt in the cooking water at about the concentration of seawater (3%, or 2 tablespoons/30 gm per quart/liter) will speed softening (p. 282) and also minimize the loss of cell contents to the water (cooking water without its own dissolved salt will draw salts and sugars from the plant cells). When just tender enough, the vegetables should be removed and either served immediately or scooped briefly into ice water to stop the cooking and prevent further dulling of the color.

Starchy vegetables, especially potatoes cooked whole or in large pieces, benefit from a different treatment. Their vulnerability is a tendency for the outer portions to soften excessively and fall apart while the interiors cook through. Hard and slightly acid water can help them maintain their surface firmness, as will starting them in cold water and raising the temperature only gradually to reinforce their cell walls (p. 283). Salt is best omitted from the water, since it encourages early softening of the vulnerable exterior. Nor is it necessarily best to cook them at the boiling point: 180190F/8085C is sufficient to soften starch and cell walls and won't overcook the exterior as badly, though the cooking through will take longer.

When vegetables are included in a meat braise or stew and are expected to have a tender integrity, their cooking needs as much attention as the meat's. A very low cooking temperature that keeps the meat tender may leave the vegetables hard, while repeated bouts of simmering to dissolve a tough cut's connective tissue may turn them to mush. The vegetables can be precooked separately, either to soften them for a low-temperature braise or firm them for long simmering; or they can be removed from a long-simmered dish when they reach the desired texture and added back when the meat is done.

Steaming Steaming is a good method for cooking vegetables at the boiling point, but without the necessity of heating a whole pot of water, exposing the food directly to turbulent water, and leaching out flavor or color or nutrients. It doesn't allow the cook to control saltiness, calcium cross-linking, or acidity (steam itself is a slightly acid pH 6, and plant cells and vacuoles are also more acid than is ideal for chlorophyll); and evenness of cooking requires that the pieces be arranged in a single layer, or that the pile be very loose to allow the steam access to all food surfaces. Steaming leaves the food tasting exclusively of its cooked self, though the steam can also be aromatized by the inclusion of herbs and spices. Steaming is a good method for cooking vegetables at the boiling point, but without the necessity of heating a whole pot of water, exposing the food directly to turbulent water, and leaching out flavor or color or nutrients. It doesn't allow the cook to control saltiness, calcium cross-linking, or acidity (steam itself is a slightly acid pH 6, and plant cells and vacuoles are also more acid than is ideal for chlorophyll); and evenness of cooking requires that the pieces be arranged in a single layer, or that the pile be very loose to allow the steam access to all food surfaces. Steaming leaves the food tasting exclusively of its cooked self, though the steam can also be aromatized by the inclusion of herbs and spices.

Pressure Cooking Pressure cooking is sometimes applied to vegetables, especially in the canning of low-acid foods. It is essentially cooking by a combination of boiling water and steam, except that both are at about 250F/120C rather than 212F/100C. (Enclosing the water in an airtight container traps the water vapor, which in turn raises the boiling point of the water.) Pressure cooking heats foods very rapidly, which means that it's also very easy to overcook fresh vegetables. It's best to follow specialized recipes closely. Pressure cooking is sometimes applied to vegetables, especially in the canning of low-acid foods. It is essentially cooking by a combination of boiling water and steam, except that both are at about 250F/120C rather than 212F/100C. (Enclosing the water in an airtight container traps the water vapor, which in turn raises the boiling point of the water.) Pressure cooking heats foods very rapidly, which means that it's also very easy to overcook fresh vegetables. It's best to follow specialized recipes closely.

Hot Air, Oil, and Radiation: Baking, Frying, and Grilling These "dry" cooking methods remove moisture from the food surface, thus concentrating and intensifying flavor, and can heat it above the boiling point, to temperatures that generate the typical flavors and colors of the browning reactions (p. 777).

Baking The hot air in an oven cooks vegetables and fruits relatively slowly, for several reasons. First, air is not as dense a medium as water or oil, so air molecules collide with the food less often, and take longer to impart energy to it. Second, a cool object in a hot oven develops a stagnant "boundary layer" of air molecules and water vapor that slows the collision rate even further. (A convection fan speeds cooking by circulating the air more rapidly and disrupting the boundary layer.) Third, in a dry atmosphere the food's moisture evaporates from the surface, and this evaporation absorbs most of the incoming energy, only a fraction of which gets to the center. So baking is much less efficient than boiling or frying. The hot air in an oven cooks vegetables and fruits relatively slowly, for several reasons. First, air is not as dense a medium as water or oil, so air molecules collide with the food less often, and take longer to impart energy to it. Second, a cool object in a hot oven develops a stagnant "boundary layer" of air molecules and water vapor that slows the collision rate even further. (A convection fan speeds cooking by circulating the air more rapidly and disrupting the boundary layer.) Third, in a dry atmosphere the food's moisture evaporates from the surface, and this evaporation absorbs most of the incoming energy, only a fraction of which gets to the center. So baking is much less efficient than boiling or frying.

Of course, the oven's thin medium is why the oven is a good means for drying foods, either partly - for example, to concentrate the flavor of watery tomatoes - or almost fully, to preserve and create a chewy or crisp texture. And once the surface has dried and its temperature rises close to the oven's, then carbohydrates and proteins can undergo the browning reactions, which generate hundreds of new taste and aroma molecules and so a greater depth of flavor.

Often vegetables are coated with oil before baking, and this simple pretreatment has two important consequences. The thin surface layer of oil doesn't evaporate the way the food moisture does, so all the heat the oil absorbs from the oven air goes to raising its and the food's temperature. The surface therefore gets hotter than it would without the oil, and the food is significantly quicker both to brown and to cook through. Second, some of the oil molecules partic.i.p.ate in the surface browning reactions and change the balance of reaction products that are formed; they create a distinctly richer flavor.

Frying and Sauteing Baking oiled vegetables is sometimes called "oven frying," and indeed true frying in oil also desiccates the food surface, browns it, and enriches the flavor with the characteristic notes contributed by the oil itself. A food may be fried partly or fully immersed in oil, or just well lubricated with it (sauteing); and typical oil temperatures range from 325375F/160190C. True frying is faster than oven frying because oil is much denser than air, so energetic oil molecules collide with the food much more frequently. The key to successful frying is getting the piece size and frying temperature right, so that the pieces cook through in the time that the surfaces require to be properly browned. Starchy vegetables are the most commonly fried plant foods, and I describe the important example of potatoes in detail in chapter 6 (p. 303). Many more delicate vegetables and even fruits are fried with a protective surface coating of batter (p. 553) or breading, which browns and crisps while the food inside is insulated from direct contact with the high heat. Baking oiled vegetables is sometimes called "oven frying," and indeed true frying in oil also desiccates the food surface, browns it, and enriches the flavor with the characteristic notes contributed by the oil itself. A food may be fried partly or fully immersed in oil, or just well lubricated with it (sauteing); and typical oil temperatures range from 325375F/160190C. True frying is faster than oven frying because oil is much denser than air, so energetic oil molecules collide with the food much more frequently. The key to successful frying is getting the piece size and frying temperature right, so that the pieces cook through in the time that the surfaces require to be properly browned. Starchy vegetables are the most commonly fried plant foods, and I describe the important example of potatoes in detail in chapter 6 (p. 303). Many more delicate vegetables and even fruits are fried with a protective surface coating of batter (p. 553) or breading, which browns and crisps while the food inside is insulated from direct contact with the high heat.

Stir-Frying and Sweating Two important variations on frying exploit opposite ends of the temperature scale. One is high-temperature stir-frying. The vegetables are cut into pieces sufficiently small that they heat through in about a minute, and they're cooked on a smoking-hot metal surface with just enough oil to lubricate them, and with constant stirring to ensure even heating and prevent burning. In stir-frying it's important to preheat the pan alone and add the oil just a few seconds before the vegetables; otherwise the high heat will damage the oil and make it unpalatable, viscous, and sticky. The rapidity of stir-frying makes it a good method for retaining pigments and nutrients. At the other extreme is a technique sometimes called "sweating" (Italian Two important variations on frying exploit opposite ends of the temperature scale. One is high-temperature stir-frying. The vegetables are cut into pieces sufficiently small that they heat through in about a minute, and they're cooked on a smoking-hot metal surface with just enough oil to lubricate them, and with constant stirring to ensure even heating and prevent burning. In stir-frying it's important to preheat the pan alone and add the oil just a few seconds before the vegetables; otherwise the high heat will damage the oil and make it unpalatable, viscous, and sticky. The rapidity of stir-frying makes it a good method for retaining pigments and nutrients. At the other extreme is a technique sometimes called "sweating" (Italian soffrito soffrito or Catalan or Catalan soffregit, soffregit, both meaning "underfrying"): the very slow cooking over low heat of finely chopped vegetables coated with oil, to develop a flavor base for a dish featuring other ingredients. Often the cook wants to avoid browning, or to minimize it; here the low heat and oil function to soften the vegetables, develop and concentrate their flavors, and blend those flavors together. Vegetables cooked in a version of the both meaning "underfrying"): the very slow cooking over low heat of finely chopped vegetables coated with oil, to develop a flavor base for a dis