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During storage, the fleshy tissues of fruits and vegetables normally undergo ripening after maturation and then continue to senescence. Senescence occurs quite rapidly with an accompanying loss of palatability. Certain types of biochemical activities occur in all fruits and vegetables, including respiration, protein synthesis and changes in some constituents of cell walls.
Respiration rate varies with the stage of maturity and ripening in many fruits, with the rate increasing to a maximum just prior to full ripening. This phase is called the climacteric. Those fruits that exhibit this increase in respiratory rate just prior to senescence are termed climacteric fruits. They are distinguished by their ability to continue to ripen when they are harvested at the time that they are horticulturally mature but not yet ripe. Citrus fruits and grapes are non- climacteric. Their respiration rate does not accelerate after harvesting. Non climacteric fruits are best when ripened before harvesting. Classification of climacteric and non-climacteric fruits is given in table.
Table: Classification of climacteric and non- climacteric fruits
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Climacteric
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Non- climacteric fruits
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Apple
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Cherry
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Apricot
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Citrus fruits
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Banana
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Figs
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Peach
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Grapes
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Plum
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Melons
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Papaya
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Pineapple
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Mango
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Strawberry
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Tomato
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Pectic substances: Pectic substances are derivatives of carbohydrates. They are methylated polymer of galacturonic acid. They are present in the primary cell wall and the middle lamella of fruits and vegetables.
Protopectin is the water-insoluble form of pectic substances occurring in immature fruits and to a less extent in vegetables. Protopectin gives firm texture to unripe fruits. As fruit ripens, some demethylation and hydrolysis occur along the protopectin molecules due to the enzymes pectinesterases .
The transition takes place from a methylated water-insoluble polymer-protopectin-to a shorter methylated compound capable of dispersing easily in water-pectin. Pectin forms gel on heating with acid and sugar.
Pectic substance and gel forming ability is given in Table.
Table: Pectic substances and gel formation
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Pectic substance
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Occurrence
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Chemical characteristic
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Capacity to form gel
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Protopectin
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Raw fruits and vegetables
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Insoluble methylated polymer of galacturonic acid
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Cannot form a gel
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Pectinic acid
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Slightly ripened fruits
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Polygalacturonic acids containing more than a negligible proportion of methyl ester groups
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Form a gel with very little sugar
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Pectin
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Optimum ripe fruits
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Water-soluble shorter methylated compound
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Forms firm gel with acid and sugar
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Pectic acid
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Over ripe fruits
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Polygalacturonic acids free from methyl esters
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Forms firm gel with acid and sugar cannot form gel
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As the degradation of pectin continues the molecules gradually become shorter and lose all of their methoxyl groups. These shorter polymers of galacturonic acid are designated as pectic acid. Pectic acid is found in over ripe, very soft fruits and vegetables. This type of pectic substance has lost the gel forming ability characteristic of the longer methyl esters of galacturonic acid polymers.
Cell wall components undergo changes after harvest as a consequence of the action various enzymes. The pectic substance in cell walls and the middle lamella undergo degradation as a result of the increasing levels of two types of enzymes, pectinesterases and polygalacturonases. Other enzymes include hemicellulase and cellulase. As a consequence of the reactions catalysed by these enzymes, some sugars are released from the complex polysaccharides constituting the cell walls. The result is ripened fruits increase in sweetness despite the fact that they may have little or no starch to serve as a potential source of sugar.
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