18 Vegetables and Fruits(part 1): 18 Vegetables and Fruits (part 1)


Slide2: 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.


GENERAL PROPERTIES: 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. 根菜類 變莖類 洋薊 芹菜 藤蔓果實 嫩芽 葉柄 球莖 塊莖 大黃 茄子 南瓜 萵苣 麵包果 鱷梨 花椰菜 蘆筍、龍鬚菜


Slide4: 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.


GROSS COMPOSITON: 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%.


Slide6: 帶莢青豆 豌豆 蘿蔔


Slide7: 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.


STRUCTURAL FEATURES: 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.


Fig. 18.1 Diagram of a parenchyma cell.: Fig. 18.1 Diagram of a parenchyma cell. 液泡 原生質膜 細胞質 液泡膜 主壁 胞間層 胞間隙 質體 核仁 細胞核 薄壁組織


Slide10: 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. 胞間層


Slide11: 液泡 原生質體 胞間層 胞間連絲 液泡膜 原生質膜 植酸 中質 葉綠體 葉綠素 微粒體 糊粉 有色體 貯藏蛋白質 惰性


Slide12: 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. 氣門


Turgor and Texture: Turgor and Texture 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. 飽滿度


Slide14: 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.


Slide15: 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.


Slide16: 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.


Slide17: 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.


Slide18: 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.


Slide19: 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.


Color and Color Changes: 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.


Slide21: 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.


Slide22: 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.


Slide23: 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.


Slide24: 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.


Slide25: 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.