Microstructure of Produce Degradation 545
18.4 ARCHITECTURAL DECAY AND
MICROSTRUCTURAL CHANGESFruits that are usually eaten ripe are typically harvested in the unripe or “green”
state to allow desirable changes (i.e., ripening) to occur during the time it takes for
the product to reach the consumer. Ripening usually causes a loss of firmness in
tissue resulting from the depolymerization of cell wall components and the disso-
lution of the middle lamella, except in regions that contain plasmodesmata [27].
Intercellular adhesion decreases with ripening and continues to decrease during aging
[28]. Physiological and chemical changes occur during ripening and have been well
documented [29]. Many papers on ripening have been published, and these provide
insight into some of the physiological changes that continue to occur as the fruit
decays.
Fruit decay can be accelerated by dehydration or water loss. Water loss (weight
loss) is an important consideration in the preservation of structure and texture.
Stomata and the cuticle play roles in vapor diffusion or water loss [30].
Loss of water during storage was found to be associated with an increase in the
volume of intercellular space and the breakdown of the middle lamella, resulting in
separation between cells that correlated well with mealy texture in apples [31]. Water
loss generally occurs through cracks in the cuticle; in ‘Braeburn’ apples, water loss
was positively correlated with microcracks in the cuticle [32]. Cracks also occurred
in the cuticle of ‘Golden Delicious’ apples and grew larger during maturation and
continued to increase during postharvest storage [33]. Thus, the presence of cracks
in the cuticle may decrease firmness of fruit.
Fruit firmness is also related to cellular characteristics such as the strength of
the cell walls, intracellular turgor pressure, the number and sizes of intercellular
spaces, and cell-to-cell adhesion [34]. Adhesion between cells, a function of the
middle lamella, plays a significant role in the texture of produce. Firm apple texture
is highly dependent on an intact middle lamella with good adhesive properties and
firm cell walls. Mealy texture in apple fruit is attributed to the disintegration of the
middle lamella such that, when a force is applied, breaks occur between instead of
through cells [35].
Differences in cell-to-cell cohesiveness was also noted in calcium-treated vs.
untreated ‘Golden Delicious’ apples [36], as well as in ‘Granny Smith’ (a hard, long-
term-storage cultivar) vs. ‘Rubinette’ (a soft-textured, mealy cultivar) apples [37],
in which the calcium-treated and the hard cultivar fractured intracellularly and the
untreated and the softer cultivar fractured intercellularly. Calcium has been shown
to have positive effects on maintaining cell wall structure [38]. Calcium chloride
treatments firm fruit tissue by reacting with pectic acid in the cell wall to form
calcium pectate, which strengthens molecular bonding between constituents of the
cell wall [39]. Transmission electron microscope studies of apple mesocarp tissue
during storage revealed that most of the changes due to calcium treatment involved
the middle lamella [40]. Calcium treatment of ‘Golden Delicious’ apples is common
practice and is known to have positive effects during long-term storage and has been
the subject of numerous investigations [41–43].^