Microstructure of Produce Degradation 543
of hemicellulose were also involved in ripening and fruit softening in papaya [24].
The swollen cell walls in kiwifruit were resistant to staining with Ruthenium red,
which is characteristic of pectic material, compared with those in unripe fruit.
Morphometric studies showed that the volume of intercellular air spaces
increased with ripening and correlated with the disintegration of the middle lamella,
suggesting that the middle lamella was replaced by air. However, intercellular spaces
decreased as fruit became very ripe probably due to the loss of tissue integrity.
Changes in cellular structure are tissue-dependent. Intercellular air spaces did not
increase in the locular region in the inner pericarp. Similarly, cell walls show less change
in the locular region than those of other tissues, and thickening of cell walls occur
minimally. In the locular wall, however, intercellular space did increase [20].
The swelling of the cell walls and weakening of the middle lamella produced a
desirable soft texture in the kiwifruit [20]. The changes in kiwifruit are similar to
those in other fruit; however, the changes in kiwifruit occurred prior to the climacteric
(or commencement of the ethylene production), contrary to that in other climacteric
fruits. Structural studies using tensile strength measurements were applied to kiwi-
fruit at different stages of ripeness and the rupture locations were determined by
scanning electron microscope observation. Cell walls ruptured in freshly harvested,
unripe kiwifruit. As the tissue got softer, the stress breaks occurred around cells,
indicating that the adhesive substance between cells, the middle lamella, was weaker
than the cell walls [25].
The skin of kiwifruit is covered with numerous hairs or trichomes of two size
classifications. The large trichomes are roughly 2.5 mm in length and the shorter
ones are twisted and are about 200 μm long (Figure 18.10a–h). In addition to the
elongated trichomes, platelets occur on the surface of the skin. The platelets do not
really look like wax, since they are quite thick, but look more like part of the
epidermis. They might, therefore, be flattened trichomes or outgrowths of epidermal
cells [1]. In fresh kiwifruit, the platelets appear to be closely appressed to the surface
of the fruit (Figure 18.10c and e), whereas those in the aged fruit have started peeling
up from the fruit surface (Figure 18.10d and f), allowing the fungal hyphae, which are
now covering the surface, to penetrate into the fruit and cause decay of the fleshy fruit.
The outer pericarp tissue of fresh kiwifruit (Figure 18.11a) contains a mixture
of large and small cells. The outer cells, just beneath the skin, are radially compressed
in the fresh (Figure 18.11a) as well as in the aged (Figure 18.11b) fruits. Cells further
into the outer pericarp tissue of the aged fruit are compressed slightly (Figure
18.11b). Closer examination reveals that the cells in the outer region of the outer
pericarp in the fresh kiwifruit are rounded just below the radially compressed cells
(Figure 18.11c and e). Cells in aged tissue have begun to lose their shapes (Figure
18.11d and f) and cell walls have been damaged (Figure 18.11f).
Starch granules apparent in the fresh (Figure 18.11e and g) kiwifruit have
virtually disappeared in the aged (Figure 18.11f and h) fruit, in agreement with
Hallett et al. [26] in their observations of starch degradation. Cell walls in the outer
pericarp in the aged fruit (Figure 18.11f) were much thicker than those in the fresh
(Figure 18.11e) kiwifruit.