18 Vegetables and Fruits part 1

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18 Vegetables and Fruits(part 1)
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• Vegetables and fruits have many similarities with
  respect to their compositions, methods of
  cultivation and harvesting, storage properties,
  and processing. In fact many vegetables may be
  considered fruits in the true botanical sense.
  Botanically, fruits are those portions of a plant
  that house seeds. Therefore tomatoes, cucumbers,
  eggplant, peppers, okra, sweet corn, and other
  vegetables would be classified as fruits
  according to this definition.
• However, the important distinction between fruits
  and vegetables has come to be made on a usage
  basis those plant items that are generally eaten
  with the main course of a meal are considered to
  be vegetables those that commonly are eaten as
  dessert are considered fruits. This is the
  distinction made by food processors, certain
  marketing laws, and the consuming public, and
  this distinction will be followed in this
  discussion.

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GENERAL PROPERTIES

• Because vegetables are derived from various parts
  of plants, it is sometimes helpful to classify
  vegetables according to

the plant part from which they are derived. See
Table 18.1.
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• Fruits are the mature ovaries of plants with
  their seeds. The edible portion of most fruits is
  the fleshy part of the pericarp or vessel
  surrounding the seeds. Fruits in general are
  acidic and sugary. They commonly are grouped into
  several major divisions, depending principally on
  botanical structure, chemical composition, and
  climatic requirements.
• Thus, berries are generally small and quite
  fragile, although cranberries are rather tough.
  Grapes are also berries, which grow in clusters.
  Melons, on the other hand, are large and have a
  tough outer rind.
• Drupes contain single pits and include such items
  as apricots, cherries, peaches, and plums.
• Pomes contain many pits and are represented by
  apples, quince, and pears.
• Citrus fruits, characteristically high in citric
  acid, include oranges, grapefruit, and lemons.
• Tropical and subtropical fruits include bananas,
  dates, figs, pineapples, papayas, mangos, and
  others but not the separate group of citrus
  fruits these all require warm climates for
  growth.

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GROSS COMPOSITON

• The compositions of representative vegetables and
  fruits in comparison with a few of the cereal
  grains are shown in Table 18.2. The composition
  of vegetables and fruits depends not only on
  botanical variety, cultivation practices, and
  weather, but also on the degree of maturity prior
  to harvest and the condition of ripeness, which
  continues after harvest and is influenced by
  storage conditions. Nevertheless, some
  generalizations can be made.
• Most fresh vegetables and fruits are high in
  water, low in protein, and low in fat. The water
  content is generally greater than 70 and
  frequently greater than 85. Commonly, protein
  content is no greater than 3.5 and fat content
  no greater than 0.5.

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• Exceptions exist to these typical values dates
  and raisins are substantially lower in moisture
  but cannot be considered fresh in the above
  sense legumes such as peas and certain beans are
  higher in protein a few vegetables such as sweet
  corn are slightly higher in fat and avocados are
  substantially higher in fat.
• On the other hand, vegetables and fruits are
  important sources of both digestible and
  indigestible carbohydrates. The digestible
  carbohydrates are present largely as sugars and
  starches, and the indigestible cellulosic and
  pectic materials provide fiber, which is
  important to normal digestion.
• Fruits and vegetables also are important sources
  of minerals and certain vitamins, especially
  vitamins A and C. The precursors of vitamin A,
  including b-carotene and certain other
  carotenoids, are present particularly in the
  yellow-orange fruits and vegetables and in the
  green, leafy vegetables. Citrus fruits are
  excellent sources of vitamin C, but green, leafy
  vegetables and tomatoes are also good sources.
  Potatoes also are an important source of vitamin
  C in many countries, not so much because of the
  level of vitamin C in potatoes, which is not
  especially high, but rather because of the large
  quantities of potatoes consumed.

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STRUCTURAL FEATURES

• The structural unit of the edible portion of most
  fruits and vegetables is the parenchyma cell
  (Fig. 18.1). Although parenchyma cells of
  different fruits and vegetables differ somewhat
  in gross size and appearance, all have
  essentially the same fundamental structure.
  Parenchyma cells of plants differ from animal
  cells in that the actively metabolizing
  protoplast portion of plant cells represents only
  a small fraction (about 5) of the total cell
  volume. This protoplast is rather filmlike and is
  pressed against the cell wall by the large
  water-filled central vacuole. The protoplast has
  inner and outer semipermeable membrane layers
  between which are confined the cytoplasm and its
  nucleus. The cytoplasm contains various
  inclusions, among them starch granules and
  plastids such as the chloroplasts and other
  pigment-containing chromoplasts. The cell wall,
  cellulosic in nature, contributes rigidity to the
  parenchyma cell and confines the outer
  protoplasmic membrane. It also is the structure
  against which other parenchyma cells are cemented
  to form extensive three-dimensional tissue
  masses.

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Fig. 18.1 Diagram of a parenchyma cell.
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• The layer between cell walls of adjacent
  parenchyma cells, referred to as the middle
  lamella, is composed largely of pectic and
  polysaccharide cement-like materials. Air spaces
  also exist, especially at the angles formed where
  several cells come together.
• The relationships between these structures and
  their chemical compositions are further indicated
  in Table 18.3. Parenchyma cells vary in size from
  plant to plant but are quite large when compared
  to bacterial or yeast cells. The larger
  parenchyma cells may have volumes many thousand
  times greater than a typical bacterial cell.

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• Several types of cell other than parenchyma cells
  contribute to the familiar structures of fruits
  and vegetables. These include various types of
  tubelike conducting cells, which distribute water
  and salts throughout the plant. Such cells
  produce fibrous structures toughened by the
  presence of cellulose and the woodlike substance
  lignin. Cellulose, lignin, and pectic substances
  also occur in specialized supporting cells, which
  increase in importance as plants become older.
• An important structural feature of all plants,
  including fruits and vegetables, is protective
  tissue. This can take many forms but usually is
  made up of specialized parenchyma cells that are
  pressed compactly together to form a skin, peel,
  or rind. Surface cells of these protective
  structures on leaves, stems, or fruits secrete
  waxy cutin and form a water impermeable cuticle.
  These surface tissues, especially on leaves and
  young stems, also contain numerous valvelike
  cellular structures (stomata) through which
  moisture and gases can pass.

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Turgor and Texture
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• The range of textures encountered in fresh and
  cooked vegetables and fruits is indeed great, and
  to a large extent can be explained in terms of
  changes in specific cellular components. Since
  plant tissues generally contain more than
  two-thirds water, the relationships between these
  components and water further determine textural
  differences.
• Cell Turgor. Quite apart from other contributing
  factors, the state of turgor, which depends on
  osmotic forces, plays a paramount role in
  determining the texture of fruits and vegetables.
  The cell walls of plant tissues have varying
  degrees of elasticity and are largely permeable
  to water and ions as well as to small molecules.
  The membranes of the living protoplast are
  semipermeable, that is they allow passage of
  water but selectively transfer dissolved and
  suspended materials. The cell vacuoles contain
  most of the water of plant cells within this
  water are dissolved sugars, acids, salts, amino
  acids, some water-soluble pigments and vitamins,
  and other low molecular weight constituents.

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• Other Factors Affecting Texture. Whether a high
  degree of turgor exists in live fruits and
  vegetables or a relative state of flabbiness
  develops from loss of osmotic pressure, final
  texture is further influenced by several cell
  constituents.
• Cellulose, Hemicellulose, and Lignin. Cell walls
  in young plants are very thin and are composed
  largely of cellulose. As the plant ages, cell
  walls tend to thicken and become higher in
  hemicellulose and in lignin. These materials are
  fibrous and tough and are not significantly
  softened by cooking.

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• Pectic Substances. The complex polymers of sugar
  acid derivatives include pectin and closely
  related substances. The cement-like substance
  found especially in the middle lamella, which
  helps hold plant cells to one another, is a
  water-insoluble pectic substance. Upon mild
  hydrolysis, this substance yields water-soluble
  pectin, which can form gels or viscous colloidal
  suspensions with sugar and acid. Certain
  water-soluble pectic substances also react with
  metal ions, particularly calcium, to form
  water-insoluble salts such as calcium pectates.
  The various pectic substances may influence
  texture of vegetables and fruits in several ways.
  When vegetables or fruits are cooked some of the
  water-insoluble pectic substance is hydrolyzed
  into water-soluble pectin. This results in a
  degree of cell separation in the tissues and
  contributes to tenderness. Since many fruits and
  vegetables are somewhat acidic and contain
  sugars, the soluble pectin also tends to form
  colloidal suspensions which thicken the juice or
  pulp of these products.

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• Fruits and vegetables also contain a natural
  enzyme that can further hydrolyze pectin to the
  extent that it loses much of its gel-forming
  property. This enzyme is known as pectin methyl
  esterase. Some products (e.g., tomato juice and
  tomato paste) contain both pectin and pectin
  methyl esterase. If freshly prepared tomato juice
  or paste is allowed to stand, the original
  viscosity gradually decreases due to the action
  of pectin methyl esterase on pectin gel. This can
  be prevented if the tomato products are quickly
  heated to a temperature of about 82? to
  inactivate enzyme liberated from broken cells
  before the pectin is hydrolyzed. This treatment,
  known as the hot-break process, is commonly
  practiced in the manufacture of tomato paste and
  tomato juice products to yield products of high
  viscosity. In contrast, when low-viscosity
  products are desired, no heat is used and enzyme
  activity is allowed to proceed. This is the
  cold-break process. After the appropriate
  viscosity is achieved, the product can be heat
  treated, as in canning, to preserve it for
  long-term storage.

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• In the living plant, water taken up by the roots
  passes through the cell walls and membranes into
  the cytoplasm of the protoplasts and into the
  vacuoles to establish a state of osmotic
  equilibrium within the cells. The osmotic
  pressure within the cell vacuoles and within the
  protoplasts pushes the protoplasts against the
  cell walls and causes them to stretch slightly in
  accordance with their elastic properties. These
  processes result in the characteristic appearance
  of live plants and are responsible for the
  desired plumpness, succulence, and much of the
  crispness of harvested live fruits and
  vegetables.
• When plant tissues are damaged or killed by
  storage, freezing, cooking, or other causes,
  denaturation of the proteins of the cell
  membranes occurs, resulting in the loss of
  perm-selectivity. Without perm-selectivity,
  osmotic pressure in cell vacuoles and protoplasts
  cannot be maintained, and water and dissolved
  substances are free to diffuse out of the cells
  and leave the remaining tissue in a soft and
  wilted condition.

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• It often is desirable to firm the texture of
  fruits or vegetables, especially when products
  are normally softened by processing. In this
  case, advantage is taken of the reaction between
  soluble pectic substances and calcium ions to
  form calcium pectates. These calcium pectates are
  water insoluble when they are produced within
  the tissues of fruits and vegetables, they
  increase structural rigidity. Thus, it is common
  commercial practice to add low levels of calcium
  salts to tomatoes, apples, and other vegetables
  and fruits prior to canning or freezing.
• Starch. The occurrence of starch within starch
  granules and the swelling and gelatinization of
  these granules in the presence of moisture and
  heat have previously been mentioned. When starch
  granules absorb water and gelatinize, they
  gradually lose their granular structure and
  produce a pasty, viscous colloidal suspension.
  The swelling of starch granules within the cells
  of plant tissues upon heating causes a
  corresponding swelling of these cells and
  contributes to firm texture and plumpness.

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• On the other hand, starch swelling together with
  osmotic pressure can be so great as to cause
  plant cells to burst. When this happens, the
  viscous colloidal starch suspension oozes from
  the cells and imparts pastiness to the system.
  The same occurs when cells containing much starch
  are ruptured by processing conditions. This is
  particularly important in the case of potato
  products. The desirable texture of mashed
  potatoes and other potato products is a mealiness
  rather than a stickiness or pastiness. Therefore,
  in the production of dehydrated potato granules
  and flakes much of the technology of mixing and
  drying is aimed at minimizing both cell rupture
  and release of free starch. The same is true in
  the cooking and mashing of fresh potatoes, which
  if excessive can produce undesirable pastiness.

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Color and Color Changes

• Much of the appeal of fruits and vegetables in
  our diets is due to their delightful and variable
  colors. The pigments and color precursors found
  in fruits and vegetables occur for the most part
  in the cellular plastid inclusions (e.g.,
  chloroplasts and other chromoplasts) and to a
  lesser extent dissolved in fat droplets or water
  within the cell protoplast and vaculole. These
  pigments are classified into four major groups
  chlorophylls, carotenoids, anthocyanins, and
  anthoxanthins. Pigments belonging to the latter
  two groups also are referred to as flavonoids,
  and include the tannins.
• Chlorophylls. Chlorophylls are largely contained
  within the chloroplasts and have a primary role
  in the photosynthetic production of carbohydrates
  from carbon dioxide and water. The bright green
  color of leaves and other plant parts is due
  largely to oil-soluble chlorophylls, which in
  nature are bound to protein molecules in highly
  organized complexes.

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• When plant cells are killed by ageing,
  processing, or cooking, the protein of these
  complexes is denatured and the chlorophyll may be
  released. Such chlorophyll is highly unstable and
  rapidly changes in color to olive green or brown.
  This color change is believed to be due to the
  conversion of chlorophyll to pheophytin.
• Conversion to pheophytin is favored by acid pH
  and does not occur readily under alkaline
  conditions. For this reason peas, beans, spinach,
  and other green vegetables, which tend to lose
  their bright green colors on heating, can be
  largely protected against such color changes by
  the addition of sodium bicarbonate or other
  alkali to the cooking or canning water. However,
  this practice is not looked upon favorably nor
  used commercially because alkaline pH tends to
  soften cellulose and vegetable texture, and to
  increase the destruction of vitamin C and thiamin
  at cooking temperatures.

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• Carotenoids. Pigments belonging to the carotenoid
  group are fat soluble and range in color from
  yellow through orange to red. They often occur
  along with the chlorophylls in the chloroplasts,
  but also are present in other chromoplasts and
  may occur free in fat droplets. Important
  carotenoids include the orange carotenes of
  carrot, corn, apricot, peach, citrus fruits, and
  squash the red lycopene of tomato, watermelon,
  and apricot the yellow-orange xanthophyll of
  corn, peach, paprika, and squash and the
  yellow-orange crocetin of the spice saffron.
  These and other carotenoids seldom occur singly
  within plant cells.
• Of major importance is the relationship of some
  carotenoids to vitamin A. A molecule of orange
  -carotene is converted into two molecules of
  colorless vitamin A in the animal body. Some
  other carotenoids (e.g., a-carotene, -carotene,
  and cryptoxanthin) also are precursors of vitamin
  A, but because of minor differences in chemical
  structure one molecule of each of these yields
  only one molecule of vitamin A.
• In food processing the carotenoids are fairly
  resistant to heat, changes in pH, and water
  leaching since they are fat soluble. However,
  they are very sensitive to oxidation, which
  results in both color loss and destruction of
  vitamin A activity.

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• Flavonoids. Pigments and color precursors
  belonging to the flavonoids are water soluble and
  commonly are present in the juices of fruits and
  vegetables. The flavonoids include the purple,
  blue, and red anthocyanins of grapes, berries,
  plums, eggplant, and cherry the yellow
  anthoxanthins of light colored fruits and
  vegetables such as apple, onion, potato, and
  cauliflower and the colorless catechins and
  leucoan-thocyanins which are food tannins and are
  found in apples, grapes, tea, and other plant
  tissues. These colorless tannin compounds are
  easily converted to brown pigments upon reaction
  with metal ions.
• The color of anthocyanins depends upon the pH.
  Thus, many of the anthocyanins that are violet or
  blue in alkaline media become red upon addition
  of acid. Cooking of beets with vinegar tends to
  shift the color from a purplish red to a brighter
  red, while in alkaline water the color of red
  fruits and vegetables shifts toward violet and
  gray-blue. Red anthocyanins also tend to become
  violet and blue upon reaction with metal ions,
  which is one reason for lacquering the inside of
  metal cans when the true color of
  anthocyanin-containing fruits and vegetables is
  to be preserved.

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• The water solubility of anthocyanins also results
  in easy leaching of these pigments from cut
  fruits and vegetables during processing and
  cooking.
• Yellow anthoxanthins also are pH sensitive
  tending toward a deeper yellow in alkaline media.
  Thus potatoes or applies become somewhat yellow
  when cooked in water with a pH of 8 or higher,
  which is common in many areas. Acidification of
  the water to pH 6 or lower favors a whiter color.
• The colorless tannin compounds upon reaction with
  metal ions form a range of dark-colored complexes
  which may be red, brown, green, gray, or black.
  The various shades of these colored complexes
  depend upon the particular tannin, the specific
  metal ion, pH, concentration of the complex, and
  other factors not yet fully understood.
• Water-soluble tannins appear in the juices
  squeezed from grapes, apples, and other fruits as
  well as in the brews extracted from tea and
  coffee.

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• The color and clarity of tea are influenced by
  the hardness and pH of the brewing water.
  Alkaline waters that contain calcium and
  magnesium favor the formation of dark brown
  tannin complexes, which precipitate when the tea
  is cooled. If acid in the form of lemon juice is
  added to such tea, its color lightens and the
  precipitate tends to dissolve. Iron from
  equipment or from pitted cans has caused a number
  of unexpected colors to develop in
  tannin-containing products, such as coffee,
  cocoa, and foods flavored with these.
• The tannins also are important because they
  possess astringency which influences flavor and
  contributes body to coffee, tea, wine, apple
  cider, beer, and other beverages. Excessive
  astringency causes a puckery sensation in the
  mouth, which is the condition produced when tea
  becomes high in tannins from overbrewing.