Avocado - Crop Post-Harvest: Science and Technology - page 162

climacteric peak, but depressed evolution if applied after
the peak, concluding that ABA accelerates ageing. Gazit
and Blumenfeld (1972) reported little change in ABA level
during fruit development, but once ripening started, a
considerable increase occurred. They have also reported
that the increase in ABA closely followed the ethylene
curve with peaks at the same time. Bower et al . (1986)
found that the free ABA content in ripening avocados
increased with fruit softening, with the peak at
approximately the same degree of softness at which the
maximum ethylene peak occurred, and declined thereafter.
Factors other than water stress can also affect both ABA
levels and ripening rate.

to be the fruit most representative of those in which
cellulases are of primary importance. Pesis et al . (1978)
found a rapid increase in this enzyme accompanying
softening, which was closely correlated with the respiratory
climacteric and ethylene. Application of ethylene in the air
surrounding fruit for 48 hours after harvest also caused an
increase in cellulase activity, which led Pesis et al . (1978)
to conclude that ethylene plays a role in controlling
cellulase activity. Tucker and Laties (1984) showed the
possibility of ethylene causing gene transcription of
cellulase. The early stages of softening in the avocado
appear to be due to cellulase, controlled at least in part by
ethylene, with polygalacturonase responsible for final
softening (Bower & Cutting 1988).

Effect of minerals on fruit development
Yields of avocado on a tree-by-tree basis are extremely
variable (Jones et al . 1957), which could be due to the
alternate bearing nature of the avocado and its susceptibility
to root rot. Nitrogen, phosphorus and potassium are largely
mobilized from the leaf prior to leaf abscission. In contrast
calcium, magnesium and iron are not mobilized from the
leaf and it is estimated that up to 60% of the total tree
calcium can be lost each year (Cameron et al . 1952).
Addition of phosphorus and potassium in a 3-year trial did
not significantly increase yield (Lynch & Goldweber
1956). In contrast nitrogen, and calcium levels markedly
affect yield, fruit size and post-harvest quality, and it
appears that there is considerable interaction between these
two elements in controlling vegetative reproductive balance
in the avocado tree (Bower & Cutting 1988). Witney et al .
(1986) found that 'Fuerte' and 'Hass' fruit from non
vigorous trees accumulated more calcium during
development than the fruit from vigorous trees. Calcium is
known to affect fruit ripening to a considerable extent
(Tingwa & Young, 1974). Infiltration of calcium can delay
the overall softening process during ripening, reducing the
ethylene peak and respiratory rise (Davenport 1984;
Wills & Tirmanzi 1982). Tingwa and Young (1974) found
that avocados with higher endogenous levels of calcium
had slower rates of ripening.

Respiration and ethylene production
The avocado is a climacteric fruit, with a marked rise in
respiration rate at the onset of ripening, followed by a
decline (Zauberman & Schffman-Nadel 1972). Respiration
rate of avocado fruit is relatively high compared to many
other fruits; about 20 to 50 mg CO 2 kg −1 h −1 at 5°C, 50 to
160 at 10°C, and 80 to 300 at 20°C (Kader & Arpaia 2001).
Rates of ethylene production are generally low for unripe
avocados, <0.1 μL kg −1 h −1 at 20°C, but increase rapidly
after harvest up to levels >100 μL kg −1 h −1 at 20°C when
fully ripe. Therefore, ripe avocados should not be stored
with commodities that are sensitive to ethylene damage.
Unripe avocados are sensitive to ethylene, and therefore
should not be stored near ripe fruit or other fresh produce
that produce more than trace ethylene. Ethylene exposure
during storage accelerates ripening/softening and can
increase incidence and severity of internal chilling injury
and decay. Exogenous applications of ethylene after
harvest cause an earlier climacteric with consequent
ripening (Eaks 1978). Although, other factors initiate the
respiratory rise, alter sensitivity to ethylene, or control its
increase, the autocatalytic production of ethylene is of vital
importance to normal avocado ripening. Any factors
affecting this process could be expected to alter the fruit
ripening pattern.

Fruit softening during ripening
During avocado fruit ripening, it was found that the middle
lamella of cell walls begins to disappear, with pectin
removal from the matrix of the cell walls. Later, a loss of
organization and density in the walls occurs and during the
post climacteric the walls almost completely disappeared
(Platt-Aloia et al . 1980; Platt-Aloia & Thomson 1981).
Scott et al . (1963) reported cellulose as the major constituent
of avocado cell walls. Huber (1983) considers the avocado

COMPOSITIONAL CHANGES DURING FRUIT
DEVELOPMENT, AND THE NUTRITIONAL
AND HEALTH VALUE OF AVOCADO FRUIT
Avocado fruit is a high fat fruit, contains rare sugars of high
carbon number and is relatively rich in certain vitamins,
dietary fibre, minerals and nitrogenous substances
(Tables 8.3, 8.4 and 8.5). It has a high oil content with a wide
range (3-30%) and low sugar (about 1%); hence it is recom-
mended as a high energy food for diabetics (Salunkhe &