Agriculture Reference
In-Depth Information
treatment also increased the percentage of ripe fruit with
acceptable flesh quality (less than 5% of the flesh with rots
or disorders) from 0% to 16-20%, due mainly to reduced
body rots. These results indicate the commercial potential
of HWTs of about 41°C for 25-30 min, or 42°C for 25 min
to improve avocado external and internal fruit quality fol-
lowing cold disinfestations. The effects of a transient
(warming) temperature spike on efficacy of an APHIS
approved quarantine cold treatment, T107 (a), against
Mediterranean fruit fly,
Ceratitis capitata
, was tested on
Hawaii grown 'Sharwil' avocados (Jang
et al
. 2001).
Avocados infested with late stage eggs were subjected to a
warming temperature spike (
ca
. 4.2°C for 1 h) at 6-9 days
into the treatment and subsequently allowed to resume the
treatment until conclusion (12 days at <1.1°C, 14 days at
<1.67°C or 16 days at <2.2°C). Insertion of a
ca
. 4.2°C
temperature spike into the treatment at 6-9 days had no
effect on the efficacy of the quarantine cold treatment when
fruit were allowed to resume the treatment to completion.
Infested fruit which did not receive a 'heat shock' treat-
ment (recommended to improve fruit quality) and sub-
jected to cold treatment for 6-16 days at fruit centre
temperature of <1.1, <1.67 or <2.2°C had no survivors in
the fruit by the ninth day of cold treatment. Infested avoca-
dos subjected to a 'heat shock' treatment for 10-12 h at
38°C prior to cold treatment had no survivors in the fruit by
the 8th day of cold treatment. Therefore, the T107 (a) cold
treatment (as stated in the APHIS treatment manual) seems
to be effective against Mediterranean fruit fly eggs in
'Sharwil' avocados, and that use of a 'heat shock' to pre-
vent CI during the cold treatment do not extend survivor-
ship of fruit fly eggs. The potential for low temperature
conditioning (LTC) treatments, either alone or in combina-
tion with HWT, to improve the quality of 'Hass' avocado
fruit following cold disinfestations of Queensland fruit
flies, was investigated by Hoffman
et al
. (2003). LTC at
4°C for 4 days or at 6-8°C for 3-4 days increased the per-
centage of fruit with acceptable external appearance (less
than 5% of the fruit with discrete dark patches on the skin,
and skin spotting combined) after disinfestations from 0%
to 100% due to the effective elimination of discrete patches
on the skin. Disinfestations alone increased body rots and
diffuse flesh discolouration severity in ripe fruit, while
LTC before disinfestations reduced severity of these disor-
ders to similar levels as those in non-disinfested, non-
stored fruit. LTC before disinfestations reduced discrete
patches severity and improved flesh quality more than
HWT. Combined treatments of HWT and LTC before
disinfestations were no more beneficial compared with
LTC alone. Conditioning of fruit at 6°C for 3 days followed
by disinfestations resulted in no survivors from 50 748
third instars of Queensland fruit fly (
Bactrocera tryoni
Froggatt). LTC efficacy was verified commercially by con-
ditioning fruit at 6°C for 3 days followed by disinfestations
and airfreight to New Zealand. External fruit appearance
and internal ripe fruit quality after disinfestations were
found to be high. Therefore, LTC before cold disinfesta-
tions could effectively eliminate skin damage and improve
flesh quality of ripe 'Hass' avocado fruit, with no negative
effect on Queensland fruit fly disinfestations efficacy.
'Hass' avocados have been reported to be sensitive to hot
air treatments (Yahia 1997a, 1997b, 2001).
Pre-treatment at moderate temperatures applied
immediately prior to the high or low temperature
treatments can reduce skin damage to avocados. A hot-air
pre-treatment (38 °C for 6 h) applied prior to storage at
0°C for 3 weeks with intervening delays of 1-4 days at
20°C, showed a large reduction in CI as a result of the
pre-treatment but that this was progressively lost with
increasing delay to storage (Woolf
et
al
. 2004). Hot water
pre-treatment (38 °C for 0, 5, 20 and 60 min) increasingly
reduced chilling damage at 0°C and heat damage from a
HWT at 50°C for 10 min. With delays of up to 3-24 h
prior to the HWT, heat damage was reduced for the 5
and 20 min pre-treatments. However, delays up to 5
days between pre-treatment and HWT, loss of heat toler-
ance was observed. For delays of between 1 and 5 days
there was a clear loss of chilling tolerance which was
more rapid than the increase in CI in control treatments
for the same delays. However, the effect of delays <24 h
was less clear for the 5 and 20 min treatments. Heat shock
proteins (hsp) 17 and 70 homologous RNA levels were
induced by heat pre-treatment and delays lead to first an
increase in RNA levels (maximum induction at 6 h),
which paralleled the induced tolerance, and then a decline
which was less closely associated with loss in tolerance.
Thus, delayed time between thermo tolerance inducing
pre-treatment and high or low temperatures may lead to a
general reduction in tolerance, and can be exploited in the
application of temperature treatments.
MATURITY AND HARVESTING INDICES
Avocado differs from most other fruits in that ripening
does not normally take place on the tree, but only after
picking (Schroeder 1953). Avocado is characterized by
great natural variability. The question of when to start
harvesting avocado fruit is of great commercial importance.
Harvesting of the fruit before reaching an optimal stage
can lead to deficient ripening and quality. On the other
hand, when the harvest of the fruit is carried out after the
