On Food And Cooking Part 34

Broadly speaking, the texture of plant foods is determined by the fullness of the storage vacuole, the strength of the cell walls, and the absence or presence of starch granules. Color is determined by the chloroplasts and chromoplasts, and sometimes by water-soluble pigments in vacuoles. Flavor comes from the contents of the storage vacuoles.

Plant Tissues Tissues are groups of cells organized to perform a common function. Plants have four basic tissues. Tissues are groups of cells organized to perform a common function. Plants have four basic tissues.

Ground tissue is the primary ma.s.s of cells. Its purpose depends on its location in the plant. In leaves the ground tissue performs photosynthesis; elsewhere it stores nutrients and water. Cells in the ground tissue usually have thin cell walls, so the tissue is generally tender. Most of our fruits and vegetables are mainly ground tissue. is the primary ma.s.s of cells. Its purpose depends on its location in the plant. In leaves the ground tissue performs photosynthesis; elsewhere it stores nutrients and water. Cells in the ground tissue usually have thin cell walls, so the tissue is generally tender. Most of our fruits and vegetables are mainly ground tissue.

Vascular tissue runs through the ground tissue, and resembles our veins and arteries. It is the system of microscopic tubes that transport nutrients throughout the plant. The work is divided between two subsystems: runs through the ground tissue, and resembles our veins and arteries. It is the system of microscopic tubes that transport nutrients throughout the plant. The work is divided between two subsystems: xylem, xylem, which takes water and minerals from the roots to the rest of the plant, and which takes water and minerals from the roots to the rest of the plant, and phloem, phloem, which conducts sugars down from the leaves. Vascular tissue usually provides mechanical support as well, and is often tough and fibrous compared to the surrounding tissue. which conducts sugars down from the leaves. Vascular tissue usually provides mechanical support as well, and is often tough and fibrous compared to the surrounding tissue.

Dermal tissue forms the outer surface of the plant, the layer that protects it and helps it retain its moisture. It may take the form of either forms the outer surface of the plant, the layer that protects it and helps it retain its moisture. It may take the form of either epidermis epidermis or or periderm. periderm. The epidermis is usually a single layer of cells that secretes several surface coatings, including a fatty material called cutin, and wax (long molecules made by joining fatty acids with alcohols), which is what makes many fruits naturally take a shine. Periderm is found instead of epidermis on underground organs and older tissues, and has a dull, corky appearance. Our culinary experience of periderm is usually limited to the skins of potatoes, beets, and so on. The epidermis is usually a single layer of cells that secretes several surface coatings, including a fatty material called cutin, and wax (long molecules made by joining fatty acids with alcohols), which is what makes many fruits naturally take a shine. Periderm is found instead of epidermis on underground organs and older tissues, and has a dull, corky appearance. Our culinary experience of periderm is usually limited to the skins of potatoes, beets, and so on.

Secretory tissue usually occurs as isolated cells on the surface or within the plant. These cells correspond to the oil and sweat glands in our skin, and produce and store various aroma compounds, often to attract or repel animals. The large mint family, which includes other common herbs like thyme and basil, is characterized by glandular hairs on stems and leaves that contain aromatic oils. Vegetables in the carrot family concentrate their aromatic substances in inner secretory cells. usually occurs as isolated cells on the surface or within the plant. These cells correspond to the oil and sweat glands in our skin, and produce and store various aroma compounds, often to attract or repel animals. The large mint family, which includes other common herbs like thyme and basil, is characterized by glandular hairs on stems and leaves that contain aromatic oils. Vegetables in the carrot family concentrate their aromatic substances in inner secretory cells.

The three kinds of plant tissue in a stem. Fibrous vascular tissue and thick dermal layers are common causes of toughness in vegetables.

Plant Organs There are six major plant organs: the root, the stem, the leaf, the flower, the fruit, and the seed. We'll take a closer look at seeds in chapter 9. There are six major plant organs: the root, the stem, the leaf, the flower, the fruit, and the seed. We'll take a closer look at seeds in chapter 9.

Roots Roots anchor the plant in the ground, and absorb and conduct moisture and nutrients to the rest of the plant. Most roots are tough, fibrous, and barely edible. The exceptions are roots that swell up with nonfibrous storage cells; they allow plants to survive temperate-zone winter to flower in their second year (carrots, parsnips, radishes) or seasonal dryness in the tropics (sweet potatoes, manioc). Root vegetables develop this storage area in different ways, and so have different anatomies. In the carrot, storage tissue forms around the central vascular core, which is less flavorful. The beet produces concentric layers of storage and vascular tissue, and in some varieties these acc.u.mulate different pigments, so their slices appear striped. Roots anchor the plant in the ground, and absorb and conduct moisture and nutrients to the rest of the plant. Most roots are tough, fibrous, and barely edible. The exceptions are roots that swell up with nonfibrous storage cells; they allow plants to survive temperate-zone winter to flower in their second year (carrots, parsnips, radishes) or seasonal dryness in the tropics (sweet potatoes, manioc). Root vegetables develop this storage area in different ways, and so have different anatomies. In the carrot, storage tissue forms around the central vascular core, which is less flavorful. The beet produces concentric layers of storage and vascular tissue, and in some varieties these acc.u.mulate different pigments, so their slices appear striped.

Stems, Stalks, Tubers, and Rhizomes Stems and stalks have the main function of conducting nutrients between the root and leaves, and providing support for the aboveground organs. They therefore tend to become fibrous, which is why asparagus and broccoli stems often need to be peeled before cooking, celery and cardoon stalks deveined. The junction between stem and root, which is called the Stems and stalks have the main function of conducting nutrients between the root and leaves, and providing support for the aboveground organs. They therefore tend to become fibrous, which is why asparagus and broccoli stems often need to be peeled before cooking, celery and cardoon stalks deveined. The junction between stem and root, which is called the hypocotl, hypocotl, can swell into a storage organ; turnips, celery "root," and beets are actually part stem, part root. And some plants, including the potato, yam, sunchoke, and ginger, have developed special underground stem structures for nons.e.xual reproduction: they "clone" themselves by forming a storage organ that can produce its own roots and stem and become an independent - but genetically identical - plant. The common potato and true yam are such swollen underground stem tips called can swell into a storage organ; turnips, celery "root," and beets are actually part stem, part root. And some plants, including the potato, yam, sunchoke, and ginger, have developed special underground stem structures for nons.e.xual reproduction: they "clone" themselves by forming a storage organ that can produce its own roots and stem and become an independent - but genetically identical - plant. The common potato and true yam are such swollen underground stem tips called tubers, tubers, while the sunchoke and ginger "root" are horizontal underground stems called while the sunchoke and ginger "root" are horizontal underground stems called rhizomes. rhizomes.

Leaves Leaves specialize in the production of high-energy sugar molecules via photosynthesis, a process that requires exposure to sunlight and a good supply of carbon dioxide. They therefore contain very little storage or strengthening tissue that would interfere with access to light or air, and are the most fragile and short-lived parts of the plant. To maximize light capture, the leaf is flattened out into a thin sheet with a large surface area, and the photosynthetic cells are heavily populated with chloroplasts. To promote gas exchange, the leaf interior is filled with thousands of tiny air pockets, which further increase the area of cells exposed to the air. Some leaves are as much as 70% air by volume. This structure helps explain why leafy vegetables shrink so much when cooked: heat collapses the spongy interior. (It also wilts the leaves so that they pack together more compactly.) Leaves specialize in the production of high-energy sugar molecules via photosynthesis, a process that requires exposure to sunlight and a good supply of carbon dioxide. They therefore contain very little storage or strengthening tissue that would interfere with access to light or air, and are the most fragile and short-lived parts of the plant. To maximize light capture, the leaf is flattened out into a thin sheet with a large surface area, and the photosynthetic cells are heavily populated with chloroplasts. To promote gas exchange, the leaf interior is filled with thousands of tiny air pockets, which further increase the area of cells exposed to the air. Some leaves are as much as 70% air by volume. This structure helps explain why leafy vegetables shrink so much when cooked: heat collapses the spongy interior. (It also wilts the leaves so that they pack together more compactly.) Cross section of a leaf. Because photosynthesis requires a continuous supply of carbon dioxide, leaf tissue often has a spongy structure that directly exposes many inner cells to the air.

An exception to the rule against storage tissue in leaves is the onion family (tulips and other bulb ornamentals are exceptions as well). The many layers of the onion (and the single layer of a garlic clove) surrounding the small inner stem are the swollen bases of leaves whose tops die and fall off. The leaf bases store water and carbohydrates during the plant's first year of growth so that they can be used during the second, when it will flower and produce seed.

Flowers Flowers are the plant's reproductive organs. Here the male pollen and female ovules are formed; here too they unite in the chamber that contains the ovules, the ovary, and develop into embryos and seeds. Flowers are often brilliantly colored and aromatic to attract pollinating insects, and can be a striking ingredient. However, some familiar plants protect their flowers from animal predators with toxins, so their edibility should be checked before use (p. 326). We also eat a few flowers or their supporting tissues before they mature; broccoli, cauliflower, and artichokes are examples. Flowers are the plant's reproductive organs. Here the male pollen and female ovules are formed; here too they unite in the chamber that contains the ovules, the ovary, and develop into embryos and seeds. Flowers are often brilliantly colored and aromatic to attract pollinating insects, and can be a striking ingredient. However, some familiar plants protect their flowers from animal predators with toxins, so their edibility should be checked before use (p. 326). We also eat a few flowers or their supporting tissues before they mature; broccoli, cauliflower, and artichokes are examples.

Fruits The fruit is the organ derived from the flower's ovary (or adjacent stem tissue). It contains the seeds, and promotes their dispersal away from the mother plant. Some fruits are inedible - they're designed to catch the wind, or the fur of a pa.s.sing animal - but the fruits that we eat were made by the plant to be eaten, so that an animal would intentionally take it and the seeds away. The fruit has no support, nutrition, or transport responsibilities to the other organs. It therefore consists almost entirely of storage tissue filled with appealing and useful substances for animals. When ready and ripe, it's usually the most flavorful and tenderest part of the plant. The fruit is the organ derived from the flower's ovary (or adjacent stem tissue). It contains the seeds, and promotes their dispersal away from the mother plant. Some fruits are inedible - they're designed to catch the wind, or the fur of a pa.s.sing animal - but the fruits that we eat were made by the plant to be eaten, so that an animal would intentionally take it and the seeds away. The fruit has no support, nutrition, or transport responsibilities to the other organs. It therefore consists almost entirely of storage tissue filled with appealing and useful substances for animals. When ready and ripe, it's usually the most flavorful and tenderest part of the plant.

Texture The texture of raw fruits and vegetables can be crisp and juicy, soft and melting, mealy and dry, or flabby and chewy. These qualities are a reflection of the way the plant tissues break apart as we chew. And their breaking behavior depends on two main factors: the construction of their cell walls, and the amount of water held in by those walls.

The cell walls of our fruits and vegetables have two structural materials: tough fibers of cellulose that act as a kind of framework, and a semisolid, flexible mixture of water, carbohydrates, minerals, and proteins that cross-link the fibers and fill the s.p.a.ce between them. We can think of the semisolid mixture as a kind of cement whose stiffness varies according to the proportions of its ingredients. The cellulose fibers act as reinforcing bars in that cement. Neighboring cells are held together by the cement where their walls meet.

Crisp Tenderness: The Roles of Water Pressure and Temperature Cell walls are thus firm but flexible containers. The cells that they contain are mostly water. When water is abundant and a cell approaches its maximum storage capacity, the vacuole swells and presses the surrounding cytoplasm (p. 261) against the cell membrane, which in turn presses against the cell wall. The flexible wall bulges to accommodate the swollen cell. The pressure exerted against each other by many bulging cells - which can reach 50 times the pressure of the surrounding air - results in a full, firm, turgid fruit or vegetable. But if the cells are low on water, the mutually supporting pressure disappears, the flexible cell walls sag, and the tissue becomes limp and flaccid. Cell walls are thus firm but flexible containers. The cells that they contain are mostly water. When water is abundant and a cell approaches its maximum storage capacity, the vacuole swells and presses the surrounding cytoplasm (p. 261) against the cell membrane, which in turn presses against the cell wall. The flexible wall bulges to accommodate the swollen cell. The pressure exerted against each other by many bulging cells - which can reach 50 times the pressure of the surrounding air - results in a full, firm, turgid fruit or vegetable. But if the cells are low on water, the mutually supporting pressure disappears, the flexible cell walls sag, and the tissue becomes limp and flaccid.

Water and walls determine texture. A vegetable that is fully moist and firm will seem both crisp and more tender than the same vegetable limp from water loss. When we bite down on a vegetable turgid with water, the already-stressed cell walls readily break and the cells burst open; in a limp vegetable, chewing compresses the walls together, and we have to exert much more pressure to break through them. The moist vegetable is crisp and juicy, the limp one chewy and less juicy. Fortunately, water loss is largely reversible: soak a limp vegetable in water for a few hours and its cells will absorb water and reinflate. Crispness can also be enhanced by making sure that the vegetable is icy cold. This makes the cell-wall cement stiff, so that when it breaks under pressure, it seems brittle.

Mealiness and Meltingness: The Role of Cell Walls Fruits and vegetables can sometimes have a mealy, grainy, dry texture. This results when the cement between neighboring cells is weak, so that chewing breaks the cells apart from each other rather than breaking them open, and we end up with lots of tiny separate cells in our mouth. Then there's the soft, melting texture of a ripe peach or melon. This too is a manifestation of weakened cell walls, but here the weakening is so extreme that the walls have practically disintegrated, and the watery cell interior oozes out under the least pressure. The contents of the cells also have an effect: a ripe fruit's vacuole full of sugar solution will give a melting, succulent impression, while a potato's solid starch grains will contribute a firm chalkiness. Because starch absorbs water when heated, cooked starchy tissue becomes moist but mealy or pasty, never juicy. Fruits and vegetables can sometimes have a mealy, grainy, dry texture. This results when the cement between neighboring cells is weak, so that chewing breaks the cells apart from each other rather than breaking them open, and we end up with lots of tiny separate cells in our mouth. Then there's the soft, melting texture of a ripe peach or melon. This too is a manifestation of weakened cell walls, but here the weakening is so extreme that the walls have practically disintegrated, and the watery cell interior oozes out under the least pressure. The contents of the cells also have an effect: a ripe fruit's vacuole full of sugar solution will give a melting, succulent impression, while a potato's solid starch grains will contribute a firm chalkiness. Because starch absorbs water when heated, cooked starchy tissue becomes moist but mealy or pasty, never juicy.

The changes in texture that occur during ripening and cooking result from changes in the cell-wall materials, in particular the cement carbohydrates. One group is the hemicelluloses, which form strengthening cross-links between celluloses. They are built up from glucose and xylose sugars, and can be partly dissolved and removed from cell walls during cooking (p. 282). The other important component is the pectic substances, large branched chains of a sugar-like molecule called galacturonic acid, which bond together into a gel that fills the s.p.a.ces between cellulose fibers. Pectins can be either dissolved or consolidated by cooking, and their gel-like consistency is exploited in the making of fruit jellies and jams (p. 296). When fruits soften during ripening, their enzymes weaken the cell walls by modifying the pectins.

Wilting in vegetables. Plant tissue that is well supplied with water is filled with fluids and mechanically rigid (left) (left) . Loss of water causes cell vacuoles to shrink. The cells become partly empty, the cell walls sag, and the tissue weakens . Loss of water causes cell vacuoles to shrink. The cells become partly empty, the cell walls sag, and the tissue weakens (right). (right).

Tough Cellulose and Lignin Cellulose, the other major cell-wall component, is very resistant to change, and this is one reason that it's the most abundant plant product on earth. Like starch, cellulose consists of a chain of glucose sugar molecules. But a difference in the way they're linked to each other allows neighboring chains to bond tightly together into fibers that are invulnerable to human digestive enzymes and all but extreme heat or chemical treatment. Cellulose becomes most visible to us in the winter as hay, a stubble field, or the fine skeletons of weeds. This remarkable stability makes cellulose valuable to long-lived trees and to the human species as well. Wood is one-third cellulose, and cotton and linen fibers are almost pure cellulose. However, cellulose is a problem for the cook: it simply can't be softened by normal kitchen techniques. Sometimes, as in the gritty "stone cells" of pears, quince, and guava, this is a relatively minor distraction. But when it's concentrated to provide structural support in stems and stalks - in celery and cardoons, for example - cellulose makes vegetables permanently stringy, and the only remedy is to pull the fibers from the tissue. Cellulose, the other major cell-wall component, is very resistant to change, and this is one reason that it's the most abundant plant product on earth. Like starch, cellulose consists of a chain of glucose sugar molecules. But a difference in the way they're linked to each other allows neighboring chains to bond tightly together into fibers that are invulnerable to human digestive enzymes and all but extreme heat or chemical treatment. Cellulose becomes most visible to us in the winter as hay, a stubble field, or the fine skeletons of weeds. This remarkable stability makes cellulose valuable to long-lived trees and to the human species as well. Wood is one-third cellulose, and cotton and linen fibers are almost pure cellulose. However, cellulose is a problem for the cook: it simply can't be softened by normal kitchen techniques. Sometimes, as in the gritty "stone cells" of pears, quince, and guava, this is a relatively minor distraction. But when it's concentrated to provide structural support in stems and stalks - in celery and cardoons, for example - cellulose makes vegetables permanently stringy, and the only remedy is to pull the fibers from the tissue.

One last cell wall component is seldom significant in food. Lignin is also a strengthening agent and very resistant to breakdown; it's the defining ingredient of wood. Most vegetables are harvested well in advance of appreciable lignin formation, but occasionally we do deal with woody asparagus and broccoli stems. The only remedy for this kind of toughness is to peel away the lignified areas.

Color Plant pigments are one of life's glories! The various greens of forest and field, the purples and yellows and reds of fruits and flowers - these colors speak to us of vitality, renewal, and the sheer pleasure of sensation. Some pigments are designed to catch our eye, some actually become part of our eye, and some made possible the very existence of us and our eyes (see box, p. 271). Many turn out to have beneficial effects on our health. The cook's challenge is to preserve the vividness and appeal of these remarkable molecules.

There are four families of plant pigments, each with different functions in the plant's life and different behaviors in the kitchen. All of them are large molecules that appear to be a certain color because they absorb certain wavelengths of light, and thus leave only parts of the spectrum for our eyes to detect. Chlorophylls are green, for example, because they absorb red and blue wavelengths.

The softening of plant cell walls. The walls are made up of a framework of cellulose fibers embedded in a ma.s.s of amorphous materials, including the pectins (left). (left). When cooked in boiling water, the cellulose fibers remain intact, but the amorphous materials are partly extracted into fluids from within the cells, thus weakening the walls When cooked in boiling water, the cellulose fibers remain intact, but the amorphous materials are partly extracted into fluids from within the cells, thus weakening the walls (right) (right) and tenderizing the vegetable or fruit. and tenderizing the vegetable or fruit.

Green Chlorophylls The earth is painted green with chlorophylls, the molecules that harvest solar energy and funnel it into the photosynthetic system that converts it into sugar molecules. Chlorophyll The earth is painted green with chlorophylls, the molecules that harvest solar energy and funnel it into the photosynthetic system that converts it into sugar molecules. Chlorophyll a a is bright blue-green, chlorophyll is bright blue-green, chlorophyll b b a more muted olive color. The a more muted olive color. The a a form dominates the form dominates the b b by 3 to 1 in most leaves, but the balance is evener in plants that grow in the shade, and in aging tissues, where the by 3 to 1 in most leaves, but the balance is evener in plants that grow in the shade, and in aging tissues, where the a a form is degraded faster. The chlorophylls are concentrated in cell bodies called chloroplasts, where they're embedded in the many folds of a membrane along with the other molecules of the photosynthetic system. Each chlorophyll molecule is made up of two parts. One is a ring of carbon and nitrogen atoms with a magnesium atom at the center, quite similar to the heme ring in the meat myoglobin pigment (p. 133). This ring portion is soluble in water, and does the work of absorbing light. The second part is a fat-soluble tail of 16 carbon atoms, which anchors the whole molecule in the chloroplast membrane. This part is colorless. form is degraded faster. The chlorophylls are concentrated in cell bodies called chloroplasts, where they're embedded in the many folds of a membrane along with the other molecules of the photosynthetic system. Each chlorophyll molecule is made up of two parts. One is a ring of carbon and nitrogen atoms with a magnesium atom at the center, quite similar to the heme ring in the meat myoglobin pigment (p. 133). This ring portion is soluble in water, and does the work of absorbing light. The second part is a fat-soluble tail of 16 carbon atoms, which anchors the whole molecule in the chloroplast membrane. This part is colorless.

These complex molecules are readily altered when their membrane home is disrupted during cooking. This is why the bright green of fresh vegetables is fragile. Ironically, prolonged exposure to intense light also damages chlorophylls. Attention to cooking times, temperatures, and acidities are thus essential to serving bright green vegetables (p. 280).

Yellow, Orange, Red Carotenoids Carotenoids Carotenoids are so named because the first member of this large family to be chemically isolated came from carrots. These pigments absorb blue and green wavelengths and are responsible for most of the yellow and orange colors in fruits and vegetables (beta-carotene, xanthophylls, zeaxanthin), as well as the red of tomatoes, watermelons, and chillis (lycopene, capsanthin, and capsorubin; most red colors in plants are caused by anthocyanins). Carotenoids are zigzag chains of around 40 carbon atoms and thus resemble fat molecules (p. 797). They're generally soluble in fats and oils and are relatively stable, so they tend to stay bright and stay put when a food is cooked in water. Carotenoids are found in two different places in plant cells. One is in special pigment bodies, or chromoplasts, which signal animals that a flower is open for business or a fruit is ripe. Their other home is the photosynthetic membranes of chloroplasts, where there is one carotenoid molecule for every five or so chlorophylls. Their main role there is to protect chlorophyll and other parts of the photosynthetic system. They absorb potentially damaging wavelengths in the light spectrum, and act as antioxidants by soaking up the many high-energy chemical by-products generated in photosynthesis. They can do the same in the human body, particularly in the eye (p. 256). Chloroplast carotenoids are usually invisible, their presence masked by green chlorophyll, but it's a good rule of thumb that the darker green the vegetable, the more chloroplasts and chlorophyll it contains, and the more carotenoids as well. are so named because the first member of this large family to be chemically isolated came from carrots. These pigments absorb blue and green wavelengths and are responsible for most of the yellow and orange colors in fruits and vegetables (beta-carotene, xanthophylls, zeaxanthin), as well as the red of tomatoes, watermelons, and chillis (lycopene, capsanthin, and capsorubin; most red colors in plants are caused by anthocyanins). Carotenoids are zigzag chains of around 40 carbon atoms and thus resemble fat molecules (p. 797). They're generally soluble in fats and oils and are relatively stable, so they tend to stay bright and stay put when a food is cooked in water. Carotenoids are found in two different places in plant cells. One is in special pigment bodies, or chromoplasts, which signal animals that a flower is open for business or a fruit is ripe. Their other home is the photosynthetic membranes of chloroplasts, where there is one carotenoid molecule for every five or so chlorophylls. Their main role there is to protect chlorophyll and other parts of the photosynthetic system. They absorb potentially damaging wavelengths in the light spectrum, and act as antioxidants by soaking up the many high-energy chemical by-products generated in photosynthesis. They can do the same in the human body, particularly in the eye (p. 256). Chloroplast carotenoids are usually invisible, their presence masked by green chlorophyll, but it's a good rule of thumb that the darker green the vegetable, the more chloroplasts and chlorophyll it contains, and the more carotenoids as well.

About ten carotenoids have a nutritional as well as aesthetic significance: they are converted to vitamin A in the human intestinal wall. Of these the most common and active is beta-carotene. Strictly speaking, only animals and animal-derived foods contain vitamin A itself; fruits and vegetables contain only its precursors. But without these pigment precursors there would be no vitamin A in animals either. In the eye, vitamin A becomes part of the receptor molecule that detects light and allows us to see. Elsewhere in the body it has a number of other important roles.

Red and Purple Anthocyanins, Pale Yellow Anthoxanthins Anthocyanins (from the Greek for "blue flower") are responsible for most of the red, purple, and blue colors in plants, including many berries, apples, cabbage, radishes, and potatoes. A related group, the anthoxanthins ("yellow flower") are pale yellow compounds found in potatoes, onions, and cauliflower. This third major cla.s.s of plant pigments is a subgroup of the huge phenolic family, which is based on rings of 6 carbon atoms with two-thirds of a water molecule (OH) attached to some of them, which makes phenolics soluble in water. The anthocyanins have 3 rings. There are about 300 known anthocyanins, and a given fruit or vegetable will usually contain a mixture of a dozen or more. Like many other phenolic compounds, they are valuable antioxidants (p. 255). Anthocyanins (from the Greek for "blue flower") are responsible for most of the red, purple, and blue colors in plants, including many berries, apples, cabbage, radishes, and potatoes. A related group, the anthoxanthins ("yellow flower") are pale yellow compounds found in potatoes, onions, and cauliflower. This third major cla.s.s of plant pigments is a subgroup of the huge phenolic family, which is based on rings of 6 carbon atoms with two-thirds of a water molecule (OH) attached to some of them, which makes phenolics soluble in water. The anthocyanins have 3 rings. There are about 300 known anthocyanins, and a given fruit or vegetable will usually contain a mixture of a dozen or more. Like many other phenolic compounds, they are valuable antioxidants (p. 255).

Anthocyanins and anthoxanthins reside in the storage vacuole of plant cells, and readily bleed into surrounding tissues and ingredients when cell structures are damaged by cooking. This is why the lovely color of purple-tinted asparagus, beans, and other vegetables often disappears with cooking: the pigment is stored in just the outer layers of cells, and gets diluted to invisibility when the cooked cells break open. The main function of the anthocyanins is to provide signaling colors in flowers and fruits, though they may have begun their career as light-absorbing protection for the photosynthetic systems in young leaves (see box, p. 271). Anthocyanins are very sensitive to the acid-alkaline balance of foods - alkalinity shifts their color to the blue - and they're altered by traces of metals, so they are often the source of strange off-colors in cooked foods (p. 281).

Red and Yellow Betains A fourth group of plant pigments is the betains, which are only found in a handful of distantly related species. However, these include three popular and vividly colored vegetables: beets and chard (both varieties of the same species), amaranth, and the p.r.i.c.kly pear, the fruit of a cactus. The betains (sometimes called betalains) are complex nitrogen-containing molecules that are otherwise similar to anthocyanins: they are water-soluble, sensitive to heat and light, and tend toward the blue in alkaline conditions. There are about 50 red betains and 20 yellow betaxanthins, combinations of which produce the almost fluorescent-looking stem and vein colors of novelty chards. The human body has a limited ability to metabolize these molecules, so a large dose of red beets or p.r.i.c.kly pears can give a startling but harmless tinge to the urine. The red betains contain a phenolic group and are good antioxidants; yellow betaxanthins don't and aren't. A fourth group of plant pigments is the betains, which are only found in a handful of distantly related species. However, these include three popular and vividly colored vegetables: beets and chard (both varieties of the same species), amaranth, and the p.r.i.c.kly pear, the fruit of a cactus. The betains (sometimes called betalains) are complex nitrogen-containing molecules that are otherwise similar to anthocyanins: they are water-soluble, sensitive to heat and light, and tend toward the blue in alkaline conditions. There are about 50 red betains and 20 yellow betaxanthins, combinations of which produce the almost fluorescent-looking stem and vein colors of novelty chards. The human body has a limited ability to metabolize these molecules, so a large dose of red beets or p.r.i.c.kly pears can give a startling but harmless tinge to the urine. The red betains contain a phenolic group and are good antioxidants; yellow betaxanthins don't and aren't.

The three major kinds of plant pigments. For the sake of clarity, most hydrogen atoms are left unlabeled; dots indicate carbon atoms. Top: Top: Beta-carotene, the most common carotenoid pigment, and the source of the orange color of carrots. The long fat-like carbon chain makes these pigments much more soluble in fats and oils than in water. Beta-carotene, the most common carotenoid pigment, and the source of the orange color of carrots. The long fat-like carbon chain makes these pigments much more soluble in fats and oils than in water. Bottom left: Bottom left: Chlorophyll Chlorophyll a, a, the main source of the green in vegetables and fruits, with a heme-like region (p. 133) and a long carbon tail that makes chlorophyll more soluble in fats and oils than in water. the main source of the green in vegetables and fruits, with a heme-like region (p. 133) and a long carbon tail that makes chlorophyll more soluble in fats and oils than in water. Bottom right: Bottom right: Cyanidin, a blue pigment in the anthocyanin family. Thanks to their several hydroxyl (OH) groups, anthocyanins are water-soluble, and readily leak out of boiled vegetables. Cyanidin, a blue pigment in the anthocyanin family. Thanks to their several hydroxyl (OH) groups, anthocyanins are water-soluble, and readily leak out of boiled vegetables.

Discoloration: Enzymatic Browning Many fruits and vegetables - for example apples, bananas, mushrooms, potatoes - quickly develop a brown, red, or gray discoloration when cut or bruised. This discoloration is caused by three chemical ingredients: 1- and 2-ring phenolic compounds, certain plant enzymes, and oxygen. In the intact fruit or vegetable, the phenolic compounds are kept in the storage vacuole, the enzymes in the surrounding cytoplasm. When the cell structure is damaged and phenolics are mixed with enzymes and oxygen, the enzymes oxidize the phenolics, forming molecules that eventually react with each other and bond together into light-absorbing cl.u.s.ters. This system is one of the plant's chemical defenses: when insects or microbes damage its cells, the plant releases reactive phenolics that attack the invaders' own enzymes and membranes. The brown pigments that we see are essentially ma.s.ses of spent weapons. (A similar kind of enzyme acting on a similar compound is responsible for the "browning" of humans in the sun; here the pigment itself is the protective agent.) Many fruits and vegetables - for example apples, bananas, mushrooms, potatoes - quickly develop a brown, red, or gray discoloration when cut or bruised. This discoloration is caused by three chemical ingredients: 1- and 2-ring phenolic compounds, certain plant enzymes, and oxygen. In the intact fruit or vegetable, the phenolic compounds are kept in the storage vacuole, the enzymes in the surrounding cytoplasm. When the cell structure is damaged and phenolics are mixed with enzymes and oxygen, the enzymes oxidize the phenolics, forming molecules that eventually react with each other and bond together into light-absorbing cl.u.s.ters. This system is one of the plant's chemical defenses: when insects or microbes damage its cells, the plant releases reactive phenolics that attack the invaders' own enzymes and membranes. The brown pigments that we see are essentially ma.s.ses of spent weapons. (A similar kind of enzyme acting on a similar compound is responsible for the "browning" of humans in the sun; here the pigment itself is the protective agent.) Minimizing Brown Discoloration Enzymatic browning can be discouraged by several means. The single handiest method for the cook is to coat cut surfaces with lemon juice: the browning enzymes work very slowly in acidic conditions. Chilling the food below about 40F/4C will also slow the enzymes down somewhat, as will immersing the cut pieces in cold water, which limits the availability of oxygen. In the case of precut lettuce for salads, enzyme activity and browning can be reduced by immersing the freshly cut leaves in a pot of water at 115F/47C for three minutes before chilling and bagging them. Boiling temperatures will destroy the enzyme, so cooking will eliminate the problem. However, high temperatures can encourage phenolic oxidation in the absence of enzymes: this is why the water in which vegetables have been cooked sometimes turns brown on standing. Various sulfur compounds will combine with the phenolic substances and block their reaction with the enzyme, and these are often applied commercially to dried fruits. Sulfured apples and apricots retain their natural color and flavor, while unsulfured dried fruits turn brown and develop a more cooked flavor. Enzymatic browning can be discouraged by several means. The single handiest method for the cook is to coat cut surfaces with lemon juice: the browning enzymes work very slowly in acidic conditions. Chilling the food below about 40F/4C will also slow the enzymes down somewhat, as will immersing the cut pieces in cold water, which limits the availability of oxygen. In the case of precut lettuce for salads, enzyme activity and browning can be reduced by immersing the freshly cut leaves in a pot of water at 115F/47C for three minutes before chilling and bagging them. Boiling temperatures will destroy the enzyme, so cooking will eliminate the problem. However, high temperatures can encourage phenolic oxidation in the absence of enzymes: this is why the water in which vegetables have been cooked sometimes turns brown on standing. Various sulfur compounds will combine with the phenolic substances and block their reaction with the enzyme, and these are often applied commercially to dried fruits. Sulfured apples and apricots retain their natural color and flavor, while unsulfured dried fruits turn brown and develop a more cooked flavor.

Brown discoloration caused by plant enzymes. When the cells in certain fruits and vegetables are damaged by cutting, bruising, or biting, browning enzymes in the cell cytoplasm come into contact with small, colorless phenolic molecules from the storage vacuole. With the help of oxygen from the air, the enzymes bind the phenolic molecules together into large, colored a.s.semblies that turn the damaged area brown.

Another acid that inhibits browning by virtue of its antioxidant properties is as...o...b..c acid, or vitamin C. It was first identified around 1925 when the Hungarian biochemist Albert Szent-Gyorgyi found that the juice of some nonbrowning plants, including the chillis grown for paprika, could delay the discoloration of plants that do brown, and he isolated the responsible substance.

Flavor The overall flavor of a fruit or vegetable is a composite of several distinct sensations. From the taste buds on our tongues, we register salts, sweet sugars, sour acids, savory amino acids, and bitter alkaloids. From the cells in our mouth sensitive to touch, we notice the presence of astringent, puckery tannins. A variety of cells in and near the mouth are irritated by the pungent compounds in peppers, mustard, and members of the onion family. Finally, the olfactory receptors in our nasal pa.s.sages can detect many hundreds of volatile molecules that are small and chemically repelled by water, and therefore fly out of the food and into the air in our mouth. The sensations from our mouth give us an idea of a food's basic composition and qualities, while our sense of smell allows us to make much finer discriminations.

Taste: Salty, Sweet, Sour, Savory, Bitter Of the five generally recognized tastes, three are especially prominent in fruits and vegetables. Sugar is the main product of photosynthesis, and its sweetness is the main attraction provided by fruits for their animal seed dispersers. The average sugar content of ripe fruit is 10 to 15% by weight. Often the unripe fruit stores its sugar as tasteless starch, which is then converted back into sugar during ripening to make the fruit more appealing. At the same time, the fruit's acid content usually drops, a development that makes the fruit seem even sweeter. There are several organic acids - citric, malic, tartaric, oxalic - that plants can acc.u.mulate in their vacuoles and variously use as alternative energy stores, chemical defenses, or metabolic wastes, and that account for the acidity of most fruits and vegetables (all are acid to some degree). The sweet-sour balance is especially important in fruits. Of the five generally recognized tastes, three are especially prominent in fruits and vegetables. Sugar is the main product of photosynthesis, and its sweetness is the main attraction provided by fruits for their animal seed dispersers. The average sugar content of ripe fruit is 10 to 15% by weight. Often the unripe fruit stores its sugar as tasteless starch, which is then converted back into sugar during ripening to make the fruit more appealing. At the same time, the fruit's acid content usually drops, a development that makes the fruit seem even sweeter. There are several organic acids - citric, malic, tartaric, oxalic - that plants can acc.u.mulate in their vacuoles and variously use as alternative energy stores, chemical defenses, or metabolic wastes, and that account for the acidity of most fruits and vegetables (all are acid to some degree). The sweet-sour balance is especially important in fruits.

Most vegetables contain only moderate amounts of sugar and acid, and these are quickly used up by the plant cells after harvest. This is why vegetables picked just before cooking are more full-flavored than store-bought produce, which is usually days to weeks from the field.

Browning Enzymes, Breath Fresheners, and the Order of the MealThe browning enzymes are normally considered a nuisance, because they discolor foods as we prepare them. Recently a group of j.a.panese scientists found a constructive use for their oxidizing activities: they can help clear our breath of persistent garlic, onion, and other sulfurous odors! The reactive phenolic chemicals produced by the enzymes combine with sulfhydryl groups to form new and odorless molecules. (Phenolic catechins in green tea do the same.) Many raw fruits and vegetables are effective at this, notably pome and stone fruits, grapes, blueberries, mushrooms, lettuces, burdock, basil, and peppermint. This may be one of the benefits of ending a meal with fruit, and one of the reasons that some cultures serve a salad after the main course, not before.

Bitter tastes are generally encountered only among vegetables and seeds (for example, coffee and cocoa beans), which contain alkaloids and other chemical defenses meant to discourage animals from eating them. Farmers and plant breeders have worked for thousands of years to reduce the bitterness of such crops as lettuce, cuc.u.mbers, eggplants, and cabbage, but chicory and radicchio, various cabbage relatives, and the Asian bitter gourd are actually prized for their bitterness. In many cultures, bitterness is thought to be a manifestation of medicinal value and therefore of healthfulness, and there may be some truth to this a.s.sociation (p. 334).

Though savory, mouth-filling amino acids are more characteristic of protein-rich animal foods, some fruits and vegetables do contain significant quant.i.ties of glutamic acid, the active portion of MSG. Notable among them are tomatoes, oranges, and many seaweeds. The glutamic acid in tomatoes, together with its balanced sweetness and acidity, may help explain why this fruit is so successfully used as a vegetable, both with meats and without.

Touch: Astringency Astringency is neither a taste nor an aroma, but a tactile sensation: that dry, puckery, rough feeling that follows a sip of strong tea or red wine, or a bite into an unripe banana or peach. It is caused by a group of phenolic compounds consisting of 3 to 5 carbon rings, which are just the right size to span two or more normally separate protein molecules, bond to them, and hold them together. These phenolics are called Astringency is neither a taste nor an aroma, but a tactile sensation: that dry, puckery, rough feeling that follows a sip of strong tea or red wine, or a bite into an unripe banana or peach. It is caused by a group of phenolic compounds consisting of 3 to 5 carbon rings, which are just the right size to span two or more normally separate protein molecules, bond to them, and hold them together. These phenolics are called tannins tannins because they have been used since prehistory to tan animal hides into tough leather by bonding with the skin proteins. The sensation of astringency is caused when tannins bond to proteins in our saliva, which normally provide lubrication and help food particles slide smoothly along the mouth surfaces. Tannins cause the proteins to clump together and stick to particles and surfaces, increasing the friction between them. Tannins are another of the plant kingdom's chemical defenses. They counteract bacteria and fungi by interfering with their surface proteins, and deter plant-eating animals by their astringency and by interfering with digestive enzymes. Tannins are most often found in immature fruit (to prevent their consumption before the seeds are viable), in the skins of nuts, and in plant parts strongly pigmented with anthocyanins, phenolic molecules that turn out to be the right size to cross-link proteins. Red-leaf lettuces, for example, are noticeably more astringent than green. because they have been used since prehistory to tan animal hides into tough leather by bonding with the skin proteins. The sensation of astringency is caused when tannins bond to proteins in our saliva, which normally provide lubrication and help food particles slide smoothly along the mouth surfaces. Tannins cause the proteins to clump together and stick to particles and surfaces, increasing the friction between them. Tannins are another of the plant kingdom's chemical defenses. They counteract bacteria and fungi by interfering with their surface proteins, and deter plant-eating animals by their astringency and by interfering with digestive enzymes. Tannins are most often found in immature fruit (to prevent their consumption before the seeds are viable), in the skins of nuts, and in plant parts strongly pigmented with anthocyanins, phenolic molecules that turn out to be the right size to cross-link proteins. Red-leaf lettuces, for example, are noticeably more astringent than green.

Leaves and Fruits Shaped Our VisionWe can distinguish and enjoy the many hues of anthocyanin- and carotenoid-rich plants - as well as the same hues in paintings and clothing, makeup and warning signs - because our eyes are designed to see well in this color range of yellow to orange to red. It now looks as though we owe this ability to leaves and fruits! It turns out that we are among a small handful of animal species with eyes that can distinguish red from green. The other species are tropical forestdwelling primates like our probable ancestors, and they have in common a need to detect their foods against the green of the forest canopy. The young leaves of many tropical plants are red with anthocyanins, which apparently absorb excess solar energy during the momentary shafts of direct sunlight in an otherwise shaded life; and young leaves are more tender than the older, green, fibrous leaves, more easily digested and nutritious, and more sought after by monkeys. Without good red vision, it would be hard to find them - or carotenoid-colored fruits - among the green leaves. So leaves and fruits shaped our vision. The pleasure we take today in their colors was made possible by our ancestors' hunger, and the sustenance they found in red-tinged leaves and yellow-orange fruits.

Though a degree of astringency can be desirable in a dish or drink - it contributes a feeling of substantialness - it often becomes tiresome. The problem is that the sensation becomes stronger with each dose of tannins (whereas most flavors become less prominent), and it lingers, with the duration also increasing with each exposure. So it's worth knowing how to control astringency (p. 284).

Irritation: Pungency The sensations caused by "hot" spices and vegetables - chillis, black pepper, ginger, mustard, horse-radish, onions, and garlic - are most accurately described as irritation and pain (for why we can enjoy such sensations, seep. 394). The active ingredients in all of these are chemical defenses that are meant to annoy and repel animal attackers. Very reactive sulfur compounds in the mustard and onion families apparently do mild damage to the unprotected cell membranes in our mouth and nasal pa.s.sages, and thus cause pain. The pungent principles of the peppers and ginger, and some of the mustard compounds, work differently; they bind to a specific receptor on the cell membranes, and the receptor then triggers reactions in the cell that cause it to send a pain signal to the brain. The mustard and onion defenses are created only when tissue damage mixes together normally separate enzymes and their targets. Because enzymes are inactivated by cooking temperatures, cooking will moderate the pungency of these foods. By contrast, the peppers and ginger stockpile their defenses ahead of time, and cooking doesn't reduce their pungency. The sensations caused by "hot" spices and vegetables - chillis, black pepper, ginger, mustard, horse-radish, onions, and garlic - are most accurately described as irritation and pain (for why we can enjoy such sensations, seep. 394). The active ingredients in all of these are chemical defenses that are meant to annoy and repel animal attackers. Very reactive sulfur compounds in the mustard and onion families apparently do mild damage to the unprotected cell membranes in our mouth and nasal pa.s.sages, and thus cause pain. The pungent principles of the peppers and ginger, and some of the mustard compounds, work differently; they bind to a specific receptor on the cell membranes, and the receptor then triggers reactions in the cell that cause it to send a pain signal to the brain. The mustard and onion defenses are created only when tissue damage mixes together normally separate enzymes and their targets. Because enzymes are inactivated by cooking temperatures, cooking will moderate the pungency of these foods. By contrast, the peppers and ginger stockpile their defenses ahead of time, and cooking doesn't reduce their pungency.

The nature and use of pungent ingredients are described in greater detail in the next several chapters, in entries on particular vegetables and spices.

Aroma: Variety and Complexity The subject of aroma is both daunting and endlessly fascinating! Daunting because it involves many hundreds of different chemicals and sensations for which we don't have a good everyday vocabulary; fascinating because it helps us perceive more, and find more to enjoy, in the most familiar foods. There are two basic facts to keep in mind when thinking about the aroma of any food. First, the distinctive aromas of particular foods are created by specific volatile chemicals that are characteristic of those foods. And second, nearly all food aromas are composites of many different volatile molecules. In the case of vegetables, herbs, and spices, the number may be a dozen or two, while fruits typically emit several hundred volatile molecules. Usually just a handful create the dominant element of an aroma, while the others supply background, supporting, enriching notes. This combination of specificity and complexity helps explain why we find echoes of one food in another, or find that two foods go well together. Some affinities result when the foods happen to share some of the same aroma molecules. The subject of aroma is both daunting and endlessly fascinating! Daunting because it involves many hundreds of different chemicals and sensations for which we don't have a good everyday vocabulary; fascinating because it helps us perceive more, and find more to enjoy, in the most familiar foods. There are two basic facts to keep in mind when thinking about the aroma of any food. First, the distinctive aromas of particular foods are created by specific volatile chemicals that are characteristic of those foods. And second, nearly all food aromas are composites of many different volatile molecules. In the case of vegetables, herbs, and spices, the number may be a dozen or two, while fruits typically emit several hundred volatile molecules. Usually just a handful create the dominant element of an aroma, while the others supply background, supporting, enriching notes. This combination of specificity and complexity helps explain why we find echoes of one food in another, or find that two foods go well together. Some affinities result when the foods happen to share some of the same aroma molecules.

One way to approach the richness of plant flavors is to taste actively and with other people. Rather than simply recognizing a familiar flavor as what you expect, try to dissect that flavor into some of its component sensations, just as a musical chord can be broken down into its component notes. Run through a checklist of the possibilities, and ask: Is there a green-gra.s.s note in this aroma? A fruity note? A spicy or nutty or earthy note? If so, which kind of fruit or spice or nut? Chapters 68 give interesting facts about the aromas of particular fruits, vegetables, herbs, and spices.

Aroma Families The box on pp. 274275 identifies some of the more prominent aromas to be found in plant foods. Though I've divided them by type of food, this division is arbitrary. Fruits may have green-leaf aromas; vegetables may contain chemicals more characteristic of fruits or spices; spices and herbs share many aromatics with fruits. Some examples: cherries and bananas contain the dominant element of cloves; coriander contains aromatics that are prominent in citrus flowers and fruits; carrots share piney aromatics with Mediterranean herbs. While a given plant does usually specialize in the production of a certain kind of aromatic, plants in general are biochemical virtuosos, and may operate a number of different aromatic production lines at once. Some of the most important production lines are these: The box on pp. 274275 identifies some of the more prominent aromas to be found in plant foods. Though I've divided them by type of food, this division is arbitrary. Fruits may have green-leaf aromas; vegetables may contain chemicals more characteristic of fruits or spices; spices and herbs share many aromatics with fruits. Some examples: cherries and bananas contain the dominant element of cloves; coriander contains aromatics that are prominent in citrus flowers and fruits; carrots share piney aromatics with Mediterranean herbs. While a given plant does usually specialize in the production of a certain kind of aromatic, plants in general are biochemical virtuosos, and may operate a number of different aromatic production lines at once. Some of the most important production lines are these: "Green," cuc.u.mber/melon, and mushroom aromas, produced from unsaturated fatty acids in cell membranes when tissue damage mixes an oxidizing enzyme (lipoxygenase) with unsaturated fatty acids in cell membranes. This enzyme breaks the long fatty acid chains into small, volatile pieces, and other enzymes then modify the pieces.

"Fruity" aromas, produced when enzymes in the intact fruit combine an acid molecule with an alcohol molecule to produce an ester. ester.

"Terpene" aromas, produced by a long series of enzymes from small building blocks that also get turned into carotenoid pigments and other important molecules. They range from flowery to citrusy, minty, herbaceous, and piney (p. 391).

"Phenolic" aromas, produced by a series of enzymes from an amino acid with a 6-carbon ring. These are offshoots of the biochemical pathway that makes woody lignin (p. 266), and include many spicy, warming, and pungent molecules (p. 391).

"Sulfur" aromatics, usually produced when tissue damage mixes enzymes with nonaromatic aroma precursors. Most sulfur aromatics are pungent chemical defenses, though some give a more subtle depth to a number of fruits and vegetables.

Fascinating and useful as it is to a.n.a.lyze the flavors of the plant world, the greatest pleasure still comes from savoring them whole. This is one of the great gifts of life in the natural world, as Henry David Th.o.r.eau reminded us: Some gnarly apple which I pick up in the road reminds by its fragrance of all the wealth of Pomona. There is thus about all natural products a certain volatile and ethereal quality which represents their highest value.... For nectar and ambrosia are only those fine flavors of every earthly fruit which our coa.r.s.e palates fail to perceive,- just as we occupy the heaven of the G.o.ds without knowing it.

Handling and Storing Fruits and Vegetables Post-Harvest Deterioration There's no match for the flavor of a vegetable picked one minute and cooked the next. Once a vegetable is harvested it begins to change, and that change is almost always for the worse. (Exceptions include plant parts designed to hibernate, for example onions and potatoes.) Plant cells are hardier than animal cells, and may survive for weeks or even months. But cut off from their source of renovating nutrients, they consume themselves and acc.u.mulate waste products, and their flavor and texture suffer. Many varieties of corn and peas lose half their sugar in a few hours at room temperature, either by converting it to starch or using it for energy to stay alive. Bean pods, asparagus, and broccoli begin to use their sugar to make tough lignified fibers. As crisp, crunchy lettuce and celery use up their water, their cells lose turgor pressure and they become limp and chewy (p. 265).

Some of the Aromas in Foods from PlantsThis table provides a quick overview of the kinds of aromas found in plant foods, where they come from, and how they behave when the food is cooked.

Vegetables

Aroma

Examples Examples

Chemicals responsible Chemicals responsible

"Green leaf": fresh-cut leaves, gra.s.s

Most green vegetables; also tomatoes, apples, other fruits Most green vegetables; also tomatoes, apples, other fruits

Alcohols, aldehydes (6-carbon) Alcohols, aldehydes (6-carbon)

Cuc.u.mber

Cuc.u.mbers, melons Cuc.u.mbers, melons

Alcohols, aldehydes (9-carbon) Alcohols, aldehydes (9-carbon)

"Green vegetable"

Bell peppers, fresh peas Bell peppers, fresh peas

Pyrazines Pyrazines

Earthy

Potatoes, beets Potatoes, beets

Pyrazines, geosmin Pyrazines, geosmin

Fresh Mushroom

Mushrooms Mushrooms

Alcohols, aldehydes (8-carbon) Alcohols, aldehydes (8-carbon)

Cabbage-like

Cabbage family Cabbage family