Texture

Changes in texture, perceived as crispness, juiciness, and hardness, occur during fruit ripening, but the type and extent of change varies greatly by species and cultivar. Temperate fruit can be divided into two groups—those which soften considerably to melting texture, such as the pear, plum, and peach, and those which soften only moderately and retain a crisp fracturable texture, such as the apple and nashi pear. Within both groups, variations occur. These include the peach, in which freestone types soften to a greater extent than clingstone types. Also, apple cultivars such as 'Cox's Orange Pippin' and 'McIntosh' lose texture rapidly after harvest in contrast to 'Honey-crisp' and 'Fuji' that maintain texture for extended periods.

Fruit flesh consists mainly of thin-walled parenchyma cells with considerable intercellular space between them. The primary cell wall consists of cellulose microfibrils embedded in a matrix of other poly-saccharides and proteins. A pectin-rich middle lamella region connects the adjacent cell walls. Wall-to-wall adhesion, which is a function of the strength of the middle lamella, the area of cell-to-cell contact, and the extent of plasmodesmatal connections, is considered to be the major factor affecting fruit texture.

Loss of texture can be caused by loss of turgor and degradation of starch, but the primary cause is disassembly of the primary wall and middle lamella. However, the mechanisms of this disassembly during ripening are still unclear, despite intensive research at the chemical, microscopic, enzymatic, and molecular levels. Much less research attention has been given to temperate fruit compared with the tomato, often used as a model system. Nevertheless, those fruit which soften to melting textures are characterized by pronounced cell wall swelling and pectin solubilization; cell-to-cell adhesion is poor and minimal cell disruption occurs. In contrast, no cell wall swelling and little pectin solubilization occurs in fruit that remain crisp; cell-to-cell adhesion is strong and cell walls rupture.

Cell wall hydrolases that have been studied include pectin methyl-esterase (PME), endo- -1,4-gluconase (cellulase), -galactosidase, and xyloglucan endotransferase (XET), but especially polygalacturonase (PG) because of the pronounced pectin solubilization that occurs during ripening. Moreover, initiation of fruit ripening is often associated with expression of genes encoding PG and its increased activity. Although recent transgenic studies with tomato have not supported a straightforward relationship between PG activity and softening, close correlations between the factors have been repeatedly documented. Examples include the apple and cherry where endoPG activity is low or undetectable, and little pectin depoly-merization occurs. Freestone peaches have high activities of both endo- and exoPG and pronounced pectin solubilization, whereas clingstone peaches that soften to a lesser extent than freestone peaches have less pectin solubilization and lower endoPG activity. Increased pectin solubilization and PG activity occurs in winter pears only after the chilling requirement during storage has been met.

Failure to soften properly has been related to impaired cell wall metabolism. Development of mealiness in apples and pears, and woolliness in peaches, is associated with separation of parenchyma cells, an absence of cell fracturing, and an absence of free juice on the cell surfaces. It has been proposed that development of woolliness results from enhancement of PME activity and inhibition of endoPG activity in fruit kept at low storage temperatures.

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