4. Discussion
4
.
1
. Optimum conditioning treatments for heat injury reduction
Conditioning ‘Kensington’ mango fruit at 40°C for 8 h provided a ‘window of opportunity’ for
reduction, and in some instances, elimination of, hot water injuries in both experiments. In Experi-
ment 2, this conditioning treatment prior to HWT alleviated external and internal injuries Tables 5
and 6, despite the duration of the HWT being longer through standardisation of the immersion
time. These results demonstrated that the condi- tioning temperature is a more important factor in
injury alleviation than the duration of the HWT. It was shown that conditioning fruit at 22 or 30°C
was not as effective as the 40°C temperature in reducing injuries Tables 2, 3, 5 – 7 and this corre-
lates with the findings of Jacobi et al. 1995, 1996. Joyce and Shorter 1994 conditioned
‘Kensington’ mangoes for 7 – 19 h at 37°C before a HWT of fruit core temperature of 47°C held for
25 min. In this study, conditioning was found to reduce heat injuries, but did not completely elimi-
nate them. A possible reason for the different responses obtained with the same mango variety
is a difference in the method of application of the conditioning treatments. In the experiments in
this paper, the fruit core was elevated from 22 to 40°C within a 2 h period. The procedure outlined
by Joyce and Shorter 1994 indicated that the elevation of fruit core temperature to 37°C oc-
curred over a 7 h period with fruit individually wrapped in plastic cling wrap for the conditioning
period. Therefore, for ‘Kensington’ fruit, increas- ing the fruit core temperature rapidly was more
effective in reducing heat injuries than a gradual increase in temperature over a number of hours.
4
.
2
. Effect of conditioning on fruit ripening Conditioning fruit at 40°C accelerated the
ripening of ‘Kensington’ mango fruit in both ex- periments, as reflected in the increased weight loss
and the higher total soluble solids and lower titratable acidity levels within the mesocarp of the
fruit compared to other untreated and heat- treated fruit Tables 1 and 4. Generally, the more
ripe fruit had the lowest levels of injuries. There-
Table 3 The severity of skin scalding, lenticel spotting, starch layer and starch spot injuries of ‘Kensington’ fruit either given an 8 h
conditioning treatment and then a HWT 47°C fruit core temperature held for 15 min before storage at 22°C, or placed directly at 22°C after harvest untreated
External Treatment
a
External Internal
Internal Skin scalding
b
Starch layer
c
Starch spot
c
Lenticel spotting
b
0 0 100 no starch spot
0 0 Untreated
100 no starch layer 3.8 1
49.4 4 22°C for 8 h+HWT
25, severe 3–6; 25 moderate 25 severe; 38 moderate; 37 no
1–3; 50, no starch layer starch spot
38.8 4 76, moderate 1–3; 24 no starch
10 2 30°C for 8 h+HWT
25 severe; 63 moderate; 12 no starch spot
layer 38°C for 8 h+HWT
0 0 33.1 4
100 no starch layer 12.5 mild; 87.5 no starch spot
0 0 16.9 3
100 no starch layer 100 no starch spot
40°C for 8 h+HWT 25 mild; 75 no starch spot
0 0 100 no starch layer
42°C for 8 h+HWT 5.6 1
a
All measurements were made after 4 days of storage at 22°C.
b
For skin scalding and lenticel spotting, the values presented are the mean percentage skin area affected of eight fruit, as well as the severity rating in parentheses.
c
For starch layer and starch spot, the values presented are the percentages of fruit showing a particular severity rating from a total of eight fruit. Starch layer thickness is presented in parentheses and expressed in mm.
Table 4 Weight loss, firmness, mesocarp brix and titratable acidity levels of ‘Kensington’ fruit either given an 8 h conditioning treatment and
then a HWT 47°C fruit core temperature held for 15 min before storage at 22°C, or placed directly at 22°C after harvest untreated
a
Fruit firmness N Treatment
b
Brix° Weight loss
Titratable acidity citric acid 14.21
b
Untreated 13.10
ab
2.78
b
1.05
a
22°C for 8 h +HWT 6.22
a
11.18
abc
12.07
b
1.00
a
30°C for 8 h +HWT 12.10
ab
6.32
a
12.07
b
1.05
a
11.36
abc
13.38
ab
5.78
a
1.10
a
38°C for 8 h +HWT 5.51
a
40°C for 8 h +HWT 9.31
bc
13.92
a
0.96
a
7.86
c
14.44
a
42°C for 8 h +HWT 0.94
a
5.68
a
1.34 0.54
0.20 0.09
SEM
c a
Means within a column not followed by a common bold superscript letter are significantly differentPB0.05.
b
All measurements were made after 4 days of storage at 22°C; values presented are the means of ten fruit.
c
Standard error of mean.
fore, the advancement in ripening may play a key role in the protective mechanism established
within fruit cells to heat stress. Weight loss of all fruit conditioned prior to
HWT was significantly higher than that of un- treated fruit in both experiments. However, the
temperature of the conditioning treatment in the range 22 – 42°C did not influence weight loss Ta-
bles 1 and 4. Increased weight loss following conditioning of fruit has been reported to occur in
mango fruit and other fruit species. ‘Keitt’ and ‘Tommy Atkins’ mangoes intermittently warmed
at 34°C for 1 or 2 days prior to HWT of 46 to 46.5°C for up to 12 min lost more weight than
fruit not warmed prior to HWT Nyanjage et al., 1998. Apples conditioned for 4 days at 38°C
Klein and Lurie, 1990; Lurie et al., 1996 lost about 0.5 of their weight, as water, each day.
Similarly, an increase in weight loss occurs in ‘Hass’ avocados heated in air at 25 – 46°C for up
to 24 h Woolf et al., 1996, and in ‘Vibelco’ tomato fruit heated at either 35, 37 or 39°C for 72
h Hakim et al., 1996. Fruit lost weight linearly with increasing temperature and duration of the
heat treatment.
The increased weight loss associated with con- ditioning could be due to either increased respira-
tion associated with accelerated ripening or increased transpiration, or both processes. In-
creased fruit respiration with a heat treatment at 38°C for 3 or 4 days has been recorded for apples
Lurie and Klein, 1990 and tomatoes Lurie and Klein, 1992. However, for ‘Keitt’ mango fruit,
the respiration rates of unheated and fruit condi- tioned at 38°C for 24 – 48 h were similar McCol-
lum et al., 1993. In contrast, the respiration rates of tomatoes heated at 33°C for 12 days Ogura et
al., 1976, or 35°C for 3 days Inaba and Chachin, 1988 or 37°C for 3 days Cheng et al., 1988 and
papayas heated at 49°C for 70 min Paull and Chen, 1990 declined after these respective treat-
ments. The reason for the disparity in respiration response between fruit types may be related to a
combination of factors, as suggested by Lurie 1998, including the time and temperature of the
heat exposure, the preharvest environmental con- ditions and fruit physiological age. Therefore, the
possibility that conditioning at 40°C increased the respiration rate of ‘Kensington’ mangoes requires
further verification.
Fruit conditioned at 40°C prior to HWT were softer than untreated fruit in both experiments
Tables 1 and 4. Fruit conditioned at tempera- tures lower than 40°C tended to be firmer after 4
days in storage at 22°C than fruit conditioned at 40°C. A trend of increasing fruit softness with
heat treatment has also been reported for ‘Keitt’ mango fruit conditioned at 38°C for 24 or 48 h
McCollum et al., 1993. Heated ‘Keitt’ mango were softer than non-heated fruit after 3 and 6
days of ripening at 21°C McCollum et al., 1993. Similarly, ‘Keitt’ and ‘Tommy Atkins’ mangoes
intermittently warmed at 34°C for 1 or 2 days prior to a HWT of 46 – 46.5°C for 5 – 12 min were
Table 5 The incidence of external and internal injuries of ‘Kensington’ fruit either given an 8 h conditioning treatment and then a HWT
47°C fruit core temperature held for 15 min before storage at 22°C, or placed directly at 22°C after harvest untreated Internal
External Treatment
a
External cavities Lenticel spotting
Starch layer Starch spot
Skin scalding 100
Untreated 22°C for 8 h+HWT
100 20
70 100
100 30°C for 8 h+HWT
100 80
100 90
100 50
100 60
90 38°C for 8 h+HWT
30 100
70 40°C for 8 h+HWT
70 50
100 90
10 40
42°C for 8 h+HWT
a
All measurements were made after 4 days of storage at 22°C; values presented are the percentages of fruit showing injury from a total of ten fruit.
softer than fruit not warmed prior to HWT Nyanjage et al., 1998. These results are in con-
trast to reports of retarded rates of softening of fruit following heat treatment, as papaya Paull
and Chen, 1990, tomatoes Ogura et al., 1976; Picton and Grierson, 1988, apples Porritt and
Lidster, 1978; Lurie and Klein, 1990; Sams et al., 1993; Conway et al., 1994, plums Tsuji et al.,
1984 and avocados Eaks, 1978 softened more slowly when heated at temperatures ranging from
30 to 49°C for up to 4 days. It is likely that the increase in mango fruit softness associated with
conditioning treatments is related to the duration and temperature of the hot water immersion,
which leads to an increase in the activity of cell wall enzymes.
TSS brix and titratable acidity provide indica- tors of ripening within the mango mesocarp and
reflect the conversion of starch reserves to sugars and organic acids which are used in respiration.
In Experiment 2, fruit conditioned at tempera- tures of 38°C and above, particularly at 40°C, had
higher brix and lower titratable acidity levels 4 days after treatment with HWT Table 4. These
results indicate that fruit conditioned at tempera- tures of 38°C or higher were riper than either
untreated fruit or fruit conditioned at lower tem- peratures. In addition to conditioning tempera-
tures of 40°C and higher promoting an increase in brix levels, the duration of the conditioning treat-
ment at 40°C was important. The longer the conditioning treatment the higher the brix level.
Increased TSS and decreased titratable acidity levels within heat-conditioned fruit have been re-
ported for a range of fruit types including ‘Keitt’ mango following a treatment of 38 9 2°C for 48 h
McCollum et al., 1993; Burdon et al., 1994; ‘Keitt’ and ‘Tommy Atkins’ mangoes intermit-
Table 6 The severity of skin scalding, lenticel spotting and disease for
‘Kensington’ fruit either given an 8 h conditioning treatment and then a HWT 47°C fruit core temperature held for 15 min
before storage at 22°C, or placed directly at 22°C after harvest untreated
a
Lenticel Disease
Skin scalding Treatment
b
spotting Untreated
0.00 0
b
12.20 2
c
0.00 0
b
9.90 2
a
38.50 4
a
22°C for 8 27.00 3
b
h+HWT 32.50 4
a
43.00 4
a
6.40 2
a
30°C for 8 h+HWT
8.90 2
b
46.00 4
a
0.00 0
b
38°C for 8 h+HWT
40°C for 8 0.00 0
b
9.20 2 b 41.50 4
a
h+HWT 2.00 1
b
38.50 4
ab
0.50 1
b
42°C for 8 h+HWT
3.81 SEM
c
5.03 1.65
a
All measurements were made after 4 days of storage at 22°C; values presented are the mean percentage skin area
affected of 10 fruit, as well as the severity rating in parenthe- ses.
b
Means within a column not followed by a common letter are significantly different PB0.05.
c
Standard error of mean.
Table 7 The severity of starch layer and starch spot injuries in ‘Kens-
ington’ fruit either given an 8 h conditioning treatment and then a HWT 47°C fruit core temperature held for 15 min
before storage at 22°C, or placed directly at 22°C after harvest untreated
Internal injury Treatment
a
Internal injury Starch Layer
b
Starch Spot Untreated
100 no starch spot 100 no starch layer
100 severe 2–25 40 severe; 60
22°C for 8 h+HWT
moderate 90 severe 2–20;
30°C for 8 20 severe; 40
10 no starch layer moderate; 40 mild
h+HWT 90 moderate
38°C for 8 100 no starch spot
h+HWT 2–15; 10 no
starch layer 40°C for 8
70 moderate 2–8; 100 no starch spot
30 no starch layer h+HWT
42°C for 8 90 moderate
10 mild; 90 no starch spot
h+HWT 2–15; 10 no
starch layer
a
All measurements were made after 4 days of storage at 22°C; values presented are the percentages of fruit showing a
particular severity rating from a total of ten fruit.
b
Starch layer thickness is presented in parentheses and expressed in mm.
duced injury was the treatment that accelerated the greatest level of fruit ripening.
4
.
3
. Protecti6e effect of sugars against heat stress The probable role of soluble sugars in protect-
ing cellular membranes from temperature ex- tremes and desiccation has been studied by
Leprince et al. 1992 and Ingram and Bartels 1996 and a mechanism for the action of sugars
has been proposed by Back et al. 1979. It is proposed that sugars stabilise proteins from heat
denaturation through effects on water structure and the degree of hydrophobic interactions ex-
erted between molecules. An alternative theory is that sugars tend to form a metastable state char-
acterised by high viscosity, which provides greater stability to chemical reactions, and biomembranes
under stress Leopold, 1986; Roos, 1993. There- fore, from what is currently known about the
probable roles of soluble sugars, it is possible that conditioning treatments applied to ‘Kensington’
fruit, which accelerated sugar accumulation in fruit, may have caused cellular changes resulting
in the protection of membrane integrity during HWT. This was measured as a reduced level of
heat-induced injuries.
4
.
4
. Effect of conditioning temperature on injury incidence and se6erity and disease
Conditioning at 40°C was the most effective treatment in eliminating skin scalding in both
experiments Tables 2 and 3, and these results are in agreement with the findings of Jacobi et al.
1995, 1996. Conditioning at 42°C was also effec- tive in reducing the severity of skin scalding in
both experiments Tables 2, 3, 5 and 6. Fruit conditioned at 22 and 30°C had the highest inci-
dence and severity of skin scalding of all treated fruit. Therefore, the temperature of the condition-
ing treatment is critical in increasing the heat tolerance of the fruit to skin injury.
Preharvest factors affected the occurrence of lenticel spotting, since this injury was pre-existing
in the untreated fruit of Experiment 2. HWT increased the severity of lenticel spotting, and this
result correlates with the findings of Spalding et al. 1988 who reported lenticel darkening on
tently warmed at 34°C for 1 or 2 days prior to HWT of 46 – 46.5°C for 5 – 12 min Nyanjage et
al., 1998; and tomato fruit following treatments of 35, 37 or 39°C for 72 h Hakim et al., 1996, or
38°C for 3 days Lurie and Klein, 1992. In apple fruit, titratable acidity was consistently lower in
fruit conditioned at 38 or 40°C for durations ranging from 8 h to 4 days Lurie and Klein,
1990; Lay-Yee et al., 1997. A connection between enhanced respiration and a decrease in titratable
acidity has been suggested by Lurie and Klein 1990 to be due to the use of organic acids as
respiratory substrates in the respiratory cycle in fruits. Therefore, in ‘Kensington’ mango, the
measured accelerated mesocarp ripening was most probably due to increases in activities of enzymes
involved in starch degradation and respiration. It appears then that the most effective conditioning
treatment for protection against hot water-in-
‘Tommy Atkins’ and ‘Keitt’ mango fruit as a result of immersion in 46°C water for 90 – 120
min. In Experiment 1, conditioning fruit at a temperature of 40°C or higher alleviated severity
of lenticel spotting severity Table 3, but did not eliminate the lenticel damage. However, conflict-
ing trends arose between the experiments of this study with respect to the influence of conditioning
temperature on the severity of lenticel spotting. In Experiment 1, the incidence and severity of
lenticel spotting decreased with increasing condi- tioning temperature Tables 2 and 3, while in
Experiment 2, the fruit treated at the lower tem- perature of 22°C were less severely injured Table
6. One difference between the fruit used in these two experiments is that fruit for Experiment 1 had
no lenticel spotting before HWT, whereas fruit in Experiment 2 had 100 incidence and a severity
level of 2 lenticel spotting. Therefore, a preharvest level of lenticel spotting complicated the ranking
of fruit response to the conditioning treatment.
The appearance of cavities within fruit was influenced by conditioning temperature in both
experiments. Fruit conditioned at 40°C for 8 h showed no cavitation in Experiment 1 Table 2.
However, in Experiment 2, all conditioned fruit contained cavities, with the fruit conditioned at
40°C having the lowest incidence of cavitation Table 5. Therefore, 40°C was still the most
effective temperature in reducing the occurrence of cavitation.
Conditioning at 40°C was the most effective treatment in eliminating both starch layer and
starch spot injuries in both experiments. The con- ditioning temperature was critical for increasing
fruit heat tolerance, since the lower temperatures of 22 and 30°C were not as effective in preventing
occurrence of starch layer and starch spot Tables 2, 3, 5 and 6 as were the higher temperatures.
Conditioning the fruit at 42°C appeared to exceed a physiological limit, since in Experiment 1, starch
spot injury appeared within fruit treated at this temperature, but not within fruit treated at 40°C
Tables 2 and 3. These results are in agreement with those of Joyce and Shorter 1994 who found
that conditioning ‘Kensington’ fruit at 37°C for 7 h prior to HWT reduced the inhibition of starch
breakdown injury in the mesocarp. However, these authors did not identify the temperature
that optimises fruit heat tolerance. The effective- ness of a 38 – 40°C conditioning treatment in re-
ducing the appearance of starch layers and starch spots in heat-treated fruit may be directly corre-
lated with the synthesis of HSPs in the mango mesocarp. Certain HSPs have been proposed to
protect proteins through a chaperone-like func- tion or by removing heat-denatured proteins in
the cell Howarth and Ougham, 1993. In ‘Kens- ington’ fruit conditioned at 40°C, synthesis of
HSPs may have protected the enzymes involved in starch
degradation, such
as alpha-amylase,
against heat-induced inactivation. Starch degrada- tion would then have proceeded normally and to
completion in all cells in conditioned fruit follow- ing HWT.
The severity of postharvest disease was low in Experiment 2. However, differences occurred in
disease severity between fruit conditioned at dif- ferent temperatures. The highest disease severity
was found on fruit conditioned at the lower tem- peratures of 22 and 30°C Table 6. Fruit condi-
tioned at these temperatures also had high skin scalding ratings Table 6, and it is possible that
the HWT-induced skin damage contributed to the increased fruit susceptibility to the fungi produc-
ing anthracnose and stem end rot diseases. Condi- tioning the fruit at 38, 40° and 42°C increased the
level of heat tolerance of fruit. This was evident by the lower severity of external heat injuries on
these fruit. Increased fruit heat tolerance was correlated with increased defence mechanisms
against fungal invasion. The use of conditioning treatments in other mango varieties and fruit spe-
cies has also been reported to reduce disease development. Intermittent warming of ‘Keitt’ and
‘Tommy Atkins’ mangoes at 34°C for 1 or 2 days prior to a HWT of 46 – 46.5°C for 5 – 12 min
suppressed the development of anthracnose Col- letotrichum gloeosporioides and stem end rot
Nyanjage et al., 1998. Reduced postharvest de- cay in tomatoes occurs when fruit are conditioned
at either 35, 37 or 39°C for 72 h Hakim et al., 1996. Similarly, treatment of tomato fruit at
38°C for 3 days prevented Botrytis cinerea infec- tion Lurie et al., 1997a, and a range of hot air
conditioning treatments from 38°C for 48 h and
hot water dips of 30 min at 40°C or 2 min at 46, 48 or 50°C also reduced decay in ‘Daniella’ toma-
toes Lurie et al., 1997b. There may be several reasons for the success of the 40°C conditioning
treatment in reducing disease severity in ‘Kensing- ton’ fruit. Firstly, the conditioning treatment may
have reduced the number of viable spores on the fruit surface. For anthracnose, the appressoria
become more melanized at temperatures above 30°C Nyanjage et al., 1998. Secondly, the num-
ber of viable spores may have been reduced. A reduction in the numbers of viable Colletotrichum
gloeosporioides spores on Valencia oranges oc- curred when fruit were conditioned at 30°C for 24
h Williams et al., 1994. Additionally, the en- hanced rate of water loss from fruit during the
conditioning treatment may have been a further obstruction to the development of anthracnose in
storage, because germination of the fungi and appressorial formation increase at relative humid-
ity around 100 Dodd et al., 1997. Therefore, for ‘Kensington’ mango, the conditioning temper-
ature of 40°C, which was optimum for reducing heat injury, was also very effective in conferring
fruit resistance to postharvest fungal invasions.
5. Final conclusions
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