28 Produce Degradation: Reaction Pathways and their Prevention
easily identified by polarized light microscopy. The results indicated that the lenticels
were important pathways of calcium penetration. Further support for these conclu-
sions involved experiments comparing calcium flux through cuticles with or without
lenticels exposed to a calcium donor solution. The cuticles were mounted on a
diffusion cell and the calcium flux was determined to be several times higher for
the cuticles containing exposed lenticels.
Other long-season fruits also have pathways of penetration not unlike those of
apples. Dietz [57] studied the structure and function of cuticles and lenticels in
mango cultivars. The cuticle thickness ranged from 15.8 to 22.4 μm. In addition,
epicuticular wax structures developed on the cuticle surface and it differed among
the cultivars studied. Also, the number of lenticels ranged from 19 to 35 lenticels
per square centimeter. Moreover, the stomates in young fruits were functional up to
21 days in some varieties and 36 days in others.
Cracks in the epicuticular layer provide another preferential pathway of cuticle
penetration. Cuticular cracks have been reported in apple fruit near maturity and
have been shown to increase cuticular penetration of calcium [48]. Maguire et al.
[58] studied cuticular cracking in apple fruit using scanning electron, confocal, and
light microscopy. The cracks observed on the cuticle surface formed a reticulate
network and did not appear to completely traverse the cuticle. Also, the cracks did
not appear to coincide with the blush area of the fruit. Fruit with cuticular cracks
had greater water vapor permeance than fruit without any cracks. The results indicate
that cuticular cracks play a major role in the water vapor permeance of apple fruit.
The importance of cuticular cracks on other aspects of fruit quality is discussed in
further detail elsewhere in this chapter.
Another pathway of cuticular penetration may be through small pores that
traverse the cuticle. There has been some debate on whether cuticular pores really
exist and whether they can facilitate cuticular penetration [25,59]. One model of the
cuticle membrane describes the cuticle as a porous membrane with pores that vary
in size depending on the degree of hydration [6]. Support for this model comes from
data showing greater cuticular penetration of herbicides at higher relative humidity
[37]. Harker and Ferguson [56] used a diffusion cell to determine cuticle permeability
using enzyme-isolated cuticles. They found that cuticular permeability might be
altered by using different aqueous chemical solutions. Luque et al. [60,61] studied
the water permeability of isolated tomato cuticles and found that the homoionic
form of the cuticle and pH affected water permeability. They also observed the
cuticle swell in the presence of water and noted that permeability depended on the
water content of the cuticle. Some claim that pores present in the cuticle have polar
regions composed of cutin and weakly polar regions due to hydroxyl groups, unes-
terified carboxyl groups, acidic pectin, or cellulosic moieties. Riederer and Schreiber
[25,62] reported that the bulk of the water passes through the cuticle via diffusion
rather than via pathways of mass flow or polar pores present in the cutin matrix.
Cuticular penetration through stomates, lenticels, cracks, or other structures may
provide sites through which mass flow of solution can occur [26,52]. Maguire et al.
[63] noted that lenticular transpiration accounted for 8 to 20% of the total flow in
apple fruit. However, the bulk of the cuticular penetration probably occurs by
diffusion through the cuticle membrane itself, even though it is quite impermeable