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Postharvest Biology and Technology 26 (2002) 305–312
Cracking of cherry tomatoes in solution
Amnon Lichter
a,
*, Orit Dvir
a
, Elazar Fallik
a
, Shabtai Cohen
b
,
Rami Golan
b
, Zion Shemer
b
, Moshe Sagi
c
a
Department of Posthar6est Science of Fresh Produce,Institute for Technology and Storage of Agricultural Products,ARO,
The Volcani Center,P.O.Box
6
,Bet Dagan
50250
,Israel
b
Ramat Nege6Desert Agroresearch Center,D.N.Chalutza
85515
,Beer-She6a,P.O.Box
653
,Israel
c
The Institute of Applied Research,Ben Gurion Uni6ersity,Beer-She6a,P.O.Box
653
,Israel
Received 5 October 2001; accepted 18 April 2002
Abstract
Tomato fruit cracking occurs both during ripening and after harvest. Cracked fruits cannot be marketed and the
cracks form sites for fungal penetration and infection. An assay based on immersion of the fruit in water was
developed to study factors involved in fruit cracking. Adding calcium to the water reduced cracking whereas chelating
agents increased cracking. Mineral analysis of the fruit following calcium treatment demonstrated an increase in
bound calcium, while CDTA reduced the amount of soluble calcium. Decrease in fruit weight associated with water
loss during storage was correlated with a decrease in the cracking potential of the fruit. Conversely, ripening during
storage resulted in an increase in the cracking potential. Immersion of the fruit in acidic phosphate or citrate buffers
promoted cracking whereas neutral or basic buffers prevented cracking. The cracking potential of cherry tomatoes
was high after morning harvest, and it declined at noon and was low after evening harvest. It is anticipated that this
study will assist to evaluate positive or negative practices which may influence cracking of cherry tomatoes after
harvest. © 2002 Elsevier Science B.V. All rights reserved.
Keywords
:
Cherry tomatoes; Calcium; Cracking; Lycopersicon esculentum
www.elsevier.com/locate/postharvbio
1. Introduction
In nature, fruit cracking can be considered part
of the final developmental stage after ripening and
before seed dispersal. In commercial production,
cracked fruit cannot be marketed and the cracks
become sites for fungal penetration and infection.
One of the features of cracking after harvest is
that its incidence is erratic and monitoring and
control means are unsatisfactory. Thus, develop-
ment of methods to monitor fruit cracking would
serve to evaluate the cracking potential of various
genotypes and the effect of different horticultural
practices.
Fruit cracking in tomatoes (Peet, 1992) and
other fruits (Opara et al., 1997) suggests that
cracking is caused by several factors, mainly asso-
ciated with the water balance of the fruit. While
* Corresponding author. Tel.: +972-3-968-3684; fax: +
972-3-968-3622
E-mail address
:
vtlicht@volcani.agri.gov.il (A. Lichter).
0925-5214/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0925-5214(02)00061-3
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Posthar6est Biology and Technology
26 (2002) 305–312
306
the water potential of the fruit is assumed to be
the driving force for cracking, cell, tissue and
organ structures confer resistance to pressure
(Voisey et al., 1970; Emmons and Scott, 1998).
One major factor in this complex system is the
content of extracellular calcium that crosslinks
pectin polymers to form the packed tissue (De-
marty et al., 1984). The pericarp tissue can be
regarded as a complex gel, which can swell or
shrink in proportion to the hydration status of the
gel and its calcium content (Tibbits et al., 1998).
A number of reports have addressed the effect of
calcium infiltration on apple ripening and firm-
ness, e.g. Sams and Conway (1984), or the capa-
bility of calcium sprays to reduce rain-induced
cracking of sweet cherries (Glenn and Poovaiah,
1989; Yamamoto et al., 1992). Bangerth (1973)
reported that calcium inhibited cracking in toma-
toes immersed in solution.
The small size of cherry tomatoes allows labo-
ratory experiments to be conducted on a scale
that was not feasible for regular size tomatoes.
Our objective was to study factors such as calcium
and water content of the fruit, storage duration,
pH and harvest time, which may correlate to fruit
cracking in cherry tomatoes.
2. Materials and methods
2
.
1
.Fruit source and assay conditions
Tomatoes (Lycopersicon esculentum) cv. 819
and 139 (Hazera Genetics, Israel) were obtained
from commercial plots. The calyx was removed
from the fruit, the fruit were washed in distilled
water to remove dirt and were immersed in dis-
tilled water or one of the various solutions, as
indicated. The standard immersion period was 18
h at room temperature and the cracking index
was scored immediately.
2
.
2
.Materials
Chemicals were purchased from Sigma (St.
Lois, MO). Water for preparation of all solutions
and fruit immersion was purified by a single step
standard ion exchanger column (Zelion, Israel).
The pH of CDTA (1,2-cyclohexylenedinitrilote-
traacetic acid) or EDTA (ethylenediaminete-
traacetic acid) was adjusted to 7 with NaOH.
Sodium carbonate and sodium hydrogen bicar-
bonate (100 mM, 65:35, v:v) were used for prepa-
ration of a buffer with pH 10.3. Potassium
phosphate buffer at pH 7.2 was prepared by
mixing stock solutions of the monobasic and
dibasic salts (100 mM, 72:28, v:v). Acidic phos-
phate buffer (pH 3.0) was prepared by adding 1
ml of phosphoric acid to 100 mM of monobasic
potassium phosphate. A sodium acetate buffer at
pH 4.5 was prepared by mixing stock solutions of
the salt and glacial acetic acid (100 mM, 40:60,
v:v). These buffers were diluted 1:4 prior to ob-
taining the final solution for immersion of tomato
fruit. Citric acid (0.1M) and disodium phosphate
(0.2 M) were mixed in the following volumetric
ratios (in ml) to obtain the desired pH: 4–62:38;
5–49:51; 6–37:63; 7 –19:81; 8 –1:99. They were
diluted with water at a ratio of 1:10 and the pH
was corrected after dilution.
2
.
3
.Cracking index
Cracking of individual fruit was evaluated ac-
cording to a severity index of 0–5. Cracks were
identified as: small, shorter than half the diameter
of the fruit and 1 mm or less in width; medium-
longer than half the diameter of the fruit and 2–4
mm wide; large, longer than half the diameter and
5–10 mm wide. The index was: 0, no cracks; 1, 1
small crack; 2- 2 small cracks or 1 medium sized
crack; 3-a complex of small and medium cracks.
4-a large crack alone or in combination with
smaller cracks; 5-fruit shape completely distorted
by cracks. In spite of the apparent complexity,
inexperienced personnel could generate repro-
ducible results after short training.
The cumulative severity index (I
s
) for each
replication was multiplied by two if the replica-
tion size was ten fruit, yielding a cracking index
on a scale of 0–100. For example, for a replica-
tion of ten fruit that were assigned an I
s
of five the
calculated index would be 100. Each treatment
consisted of 20 fruit in four replications. Tabu-
lated data or histograms (except for mineral anal-
ysis) were statistically analysis using Duncan’s
A.Lichter et al.
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Posthar6est Biology and Technology
26 (2002) 305–312
307
multiple range test (DMRT) at P]0.95. Line
graphs are assigned with standard deviations.
Each type of experiment was performed at least
three times, except for these presented in Fig. 3,
which were performed once.
2
.
4
.The mineral composition of the fruit or
solutions
The mineral composition of tomato fruit was
determined by atomic emission spectrophotome-
try (ICP-inductively coupled plasma, at the Fac-
ulty of Agriculture, Rehovot, Israel). The mineral
composition of each solution in which fruit were
immersed was determined using 1 ml of solution
for each replicate. The soluble mineral content of
fruit was determined for 5 g of tomato pericarp
(including the peel and excluding the gel and
seeds). Five ml of water were added to the tissue
in a 50-ml polypropylene centrifuge tube. The
tissue was homogenized for 30 s with a Polytron
homogenizer (10 mm wide), the homogenate was
centrifuged for 10 min at 10 000×gand 1 ml of
the supernatant was used for analysis. The total
mineral content of the fruit was measured by
drying 5–8 g of the tissue in an oven at 90 °Cfor
24 h and determining the recovery of dry matter,
of which 100 mg were used for mineral analysis.
The concentrations of the minerals in the solution
were calculated by multiplying the measured con-
centration (mg/l) by the dilution factor (52) di-
vided by the molecular weight of the mineral. The
soluble mineral content was calculated by multi-
plying the measured concentration (mg/l) by the
dilution factor (26) divided by the molecular
weight of the mineral. Total mineral content, on a
gram fresh weight basis, was calculated by multi-
plying the measured concentration of the mineral
(mg/l) by a factor of 2.5 which accounts for the
percentage dry matter divided by the dry weight
(0.1 g) and 100, divided by the molecular weight
of the mineral.
3. Results
Study of fruit cracking in cherry tomatoes
would benefit from an assay that would induce
cracking and facilitate evaluation of the effect of
different treatments. It was assumed that immer-
sion of tomato fruit in water would lead to crack-
ing. When fruit that had been stored at 12 °C for
6 days were immersed in distilled water for 20 h,
77% of the fruit cracked, compared with 5%
cracking in tap water. These results indicated that
tap water contained minerals that reduce the
cracking potential, with calcium being the promi-
nent factor. The results shown in Fig. 1 support
this hypothesis: immersion of the fruit in 25 mM
calcium chloride reduced the cracking index to
one-third of that of the control (Fig. 1a). Similar
results were obtained for calcium nitrate (not
shown). Furthermore, CDTA and EDTA, both of
which are chelators of divalent cations, induced
Fig. 1. The effect of calcium and calcium chelators on cracking of tomato fruit (A) and the effect of different solutes (B). Tomato
fruit (cv. 819) were immersed for 18 h in solutions containing the specified compounds in two separate experiments. Cracking was
quantified by measuring the number of cracked fruit multiplied by the cracking severity index. Each treatment was performed in four
replicates with 20 fruit per replicate. Cracking index was calculated and was analyzed using DMRT at P]0.95.
A.Lichter et al.
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Posthar6est Biology and Technology
26 (2002) 305–312
308
Fig. 2. Calcium content of cherry tomatoes after treatments.
The mineral compositions of the fruit was determined by ICP
analysis. Soluble minerals were obtained by homogenization of
tissue in water followed by clearing cellular debris by centrifu-
gation. Total mineral content was obtained from dried matter.
The solutions in which the fruit were immersed were also
assayed (except control fruit). Four replicates per treatment
were assayed and standard deviations were added for each bar.
fruit immersed in water. At both stages calcium
was very effective in eliminating cracking. Thus,
cracking of fruit of cv. 139 in solution was also
responsive to calcium. In spite of the pronounced
effect of calcium demonstrated in Figs. 1 and 3, in
some experiments calcium did not suppress crack-
ing (data not shown), suggesting the involvement
of other dominant factors.
The effect of fruit maturity was further evalu-
ated on fruit of cv. 819 harvested as bunches,
which were separated into green, red and interme-
diate fruit color. Fruit of the three groups were
immersed in water and the cracking index of red
fruit was 6195 compared with a value of 15 915
for fruit that did not develop full color, or no
cracking for green fruit. In this experiment, whole
bunches containing red fruit were dipped in water,
resulting in no cracking compared with the high
cracking of the detached fruit.
Cracking of fruit in solution started within
several hours of immersion and it was faster in
solution containing 2.5 mM CDTA (data not
shown). Cracking in solution was also affected by
fruit and solution temperature: for example, a
cracking index of 6.994.5 was recorded after
immersion of fruit with internal temperature of
12 °C in a solution with a temperature of 24 °C,
compared with a score of 24.399.7 when the
temperature of the fruit was 24 °C and the solu-
tion was at 4 °C.
cracking (Fig. 1a). Another divalent cation, mag-
nesium chloride (Fig. 1b) applied at a similar
concentration as calcium chloride (25 mM) did
not reduce cracking significantly. Similarly,
sodium chloride, manitol (Fig. 1b) and ferrous
chloride (data not shown) did not affect cracking,
demonstrating that the prevention of cracking
was not mediated by an osmotic effect.
The mineral composition of fruit treated with
calcium or CDTA was analyzed to examine the
amount of calcium in fruit following treatments
(Fig. 2). Calcium analysis revealed that CDTA
reduced the amount of calcium, mainly from the
soluble fraction and calcium leakage to the solu-
tion was apparent. The calcium solution increased
both the soluble and total calcium concentration
of the fruit.
The effects of calcium and CDTA were also
tested on cv. 139, the major commercial cherry
tomato cultivar in Israel. This cultivar has smaller
fruit with thicker peel compared with fruit of cv.
819. Fruit of cv. 139 were obtained at the breaker
and orange maturity stages and they were stored
at 12 °C. Assays were performed after 4 or 13
days, and fruit ripened during this period. The
assay performed after 4 days showed that calcium
suppressed cracking to zero and CDTA induced
cracking to an index of ten (Fig. 3). After 13 days
at 12 °C cracking was higher in the three treat-
ments and the greatest increase was recorded for
Fig. 3. Cracking at two ripening stages of cherry tomato cv.
139. Fruit were obtained at the breaker stage and assayed 4 or
13 days after harvest. After 13 days at 12 °C, the fruit became
red and ripe. Fruits were immersed in distilled water, CDTA
(2.5 mM) or CaCl
2
(25 mM). Cracking index was calculated
and was analyzed using DMRT at P]0.95.
A.Lichter et al.
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Posthar6est Biology and Technology
26 (2002) 305–312
309
Fig. 4. The kinetics of cracking and weight loss during storage
at 12 or 24 °C. Cherry tomato fruit were weighed and stored
in punnets at 12 or 24 °C for 6 days. Ten fruit in each of the
four replications were weighed and immersed in distilled water
at the specified times. A. The calculated cracking index for
both treatments. B. The average weight loss of the fruit (%)
and the calculated ratio between the cracking indices at 12 and
24 °C (dotted line). Error bars represent standard deviation.
cracking potential was annulled. Fruit stored at
12 °C lost only 2% of their weight within that
period and had a similar cracking score at the
initial and final time points. The ratio of crack-
ingat12and24°C created a linear curve (Fig.
4b, dotted line).
The pathway by which the solution enters the
fruit can be either the cuticle, or the scar of the
pedicel or both. However, sealing the scar with
silicone grease did not reduce cracking (data not
shown). Wounding of fruit stored for one week
at 12 °C with a needle prior to brief dipping in
water and passive drying resulted in cracking of
the wounded fruit, which was apparent after 3 h
and increased by 20 h. This result was not re-
producible in two other experiments in which
freshly harvested fruits were used, suggesting
that the sensitivity to wound-induced cracking
depended on additional factors.
The pH of the solution used for dipping the
fruits was tested because evidence in the litera-
ture suggested that phosphoric acid promoted
cracking (Bangerth, 1973). The acidic phosphate
buffer described in Table 1 promoted cracking
while acetate at pH 4.5 did not have a marked
effect. In contrast, neutral phosphate or basic
carbonate buffers eliminated cracking. To fur-
ther study the effect of pH on cracking, solu-
tions made of citrate-phosphate buffer were
used. The results in Fig. 5 confirm that pH
Table 1
The effect of different buffers on cracking of cherry tomatoes
cv. 819
DMRT
a
Buffers
a
ph Cracking index
0.390.4NaHCO
3
+Na
2
CO
3
d10.4
KH
2
PO
4
+K
2
HPO
4
7.2 0 d
Sodium acetate
b
4.5 13.9912.1 c
a71.397.6KH
2
PO
4
c
3.0
27.793.3H
2
Ob
a
DMRT was performed on four replications at P]0.95.
b
The pH of sodium acetate was adjusted to 4.5 with acetic
acid.
c
ThepHofKH
2
PO
4
was adjusted to 3.0 with phosphoric
acid.
The changes of the cracking potential during
storage were evaluated by storing the fruit at
either 12 or 24 °C (80–90% and 50 –60% rela-
tive humidity, respectively) and monitoring
weight loss and the cracking index. After 1 day
of incubation at 12 or 24 °C, the differences in
the cracking index or in weight loss became ap-
parent (Fig. 4). Fruit stored at 24 °C lost al-
most 8% of their weight over 6 days and their
A.Lichter et al.
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Posthar6est Biology and Technology
26 (2002) 305–312
310
Fig. 5. The effect of the pH of the immersion solution on
cracking of cherry tomatoes. Fruit (cv. 819) were immersed in
solutions made of citrate-phosphate buffer adjusted to the
specified pH. Experiments were conducted with four replica-
tions of 20 fruit per replicate. Error bars represent standard
deviation.
4. Discussion
The natural level of cracking in tomatoes after
harvest is not high or consistent enough to permit
continuous investigation. Induction of cracking
by immersion has been described previously for
tomatoes (Bangerth, 1973), sweet cherries (Glenn
and Poovaiah, 1989), and bell peppers (Aloni et
al., 1999). To assay induced cracking, one could
either use the percentage of cracked fruit or an
index that will take into account the severity of
cracking. The later possibility was chosen, mainly
because calcium not only reduced the number of
cracked fruit but had also a strong positive im-
pact on the severity of cracking, whereas CDTA
had a negative effect.
Prolonged incubation of tomato in calcium so-
lution resulted in an increase in the calcium con-
tent of the apoplast, leading to a decrease in
cracking. In apples, the major effect of calcium
dips was increased firmness, e.g. Bangerth et al.,
1972). CDTA induced fruit cracking by depleting
calcium from the apoplast, resulting in pectin
disassembly (Bangerth, 1973; Glenn and Poova-
iah, 1989). Occasionally, calcium did not suppress
cracking in solution, suggesting the counter 3–4-
fold affect of a dominant factor. Short exposure
to high doses of calcium did not affect cracking,
suggesting that the process depended on diffusion
of the ion into the fruit apoplast.
Another factor that influences the structure of
of pectin is acidity. Lowering of the pH of the
tissue can dissociate calcium from pectin due to
cation exchange with protons. As a result the
pectic gel can swell (Tibbits et al. 1998). Another
mechanism that can contribute to gel swelling is
the increased activity of polygalacturonases at an
acidic pH, activity which is suppressed by calcium
and stimulated by EDTA (Chun and Huber,
1998). The positive effect of neutral or basic
buffers in prevention of cracking was dominant
over the effect of calcium and the combined effect
of calcium in neutral or basic buffers was domi-
nant in occasions that cracking was not annulled
by either of the components. It should be noted in
this respect that calcium chloride solution is acidic
and the CDTA solution was neutralized prior to
application.
plays a central role in determining cracking in
solution.
It was assumed that water accumulation in the
fruit during the night will increase the cracking
potential of the fruit. This assumption was cor-
roborated in experiments in which fruit of cv. 496
were harvested early in the morning, at noon or in
the evening (Fig. 6). It is shown that the cracking
potential was higher with the morning or the
noon harvests, respectively, compared with the
evening harvest.
Fig. 6. The cracking potential of cherry tomatoes after morn-
ing, noon or evening harvests. Fruit of cv. 496 were harvested
in a commercial plot grown under net. Fruit were harvested at
the specified times and were immersed in water in the morning
after the harvest. Results are the average of four replications
of 25 fruit per replicate. Cracking index was calculated and
was analyzed using DMRT at P]0.95
A.Lichter et al.
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Posthar6est Biology and Technology
26 (2002) 305–312
311
Two contradictory processes are taking part
during fruit maturation and storage of cherry
tomatoes. Water loss results in reduced cracking
potential, because the turgor pressure within the
fruit is the major driving force for cracking (Peet,
1992). On the other hand, the progress of ripening
before or during storage increases the cracking
potential, most likely due to enzymatic cell wall
metabolism.
Combinations of conditions, such as early
morning harvest on colder and cloudy days, un-
sorted bruises in the skin and high humidity dur-
ing storage, may result in erratic fruit cracking
and subsequent decay. The increased cracking
potential of tomato fruit harvested early in the
morning is likely to result from the same mecha-
nism that causes apples to be more sensitive to
subsequent bruising in the morning compared to a
with noon harvest (Viljoen et al., 1996).
It was argued that the model of dipping fruits
in solution might not represent natural cracking
(Peet, 1992). This is because evapotranspiration
disposes most of the water inflow from fruit at-
tached to the vine. On the other hand, no sub-
stantial water inflows occur in detached fruit,
which are stored at optimal conditions. In con-
trast, induced cracking in solution results from
osmotic inflow of water through the skin. Al-
though immersion of cherry tomatoes in aqueous
solutions may not be a perfect model to study
fruit cracking, it is justified because it is respon-
sive to some of the physiological parameters that
may influence natural cracking. These factors in-
clude the water content of the fruit prior to
harvest, storage conditions, calcium content and
harvesting time. Water inflow through the skin
should increase if fruit are bruised or suffer from
micro-cracks. Water inflow should also increase if
the calcium content in the pericarp is such that it
will allow for swelling to occur. Similarly, in-
creased water content in the fruit prior to immer-
sion, would require less osmotic pressure to cause
the peel to crack. Situations of water inflow
through the skin may be encountered during
morning condensation on cold fruit prior to har-
vest, postharvest washing procedures, extreme hu-
midity during storage (e.g. sea transportation)
and disruption of the cold-chain. Finally, artificial
water accumulation in the fruit during immersion
can help to identify structural defects in the fruit,
such as thin or non-elastic peel and non-symmet-
ric pericarp swelling.
If a correlation between induced cracking in
solution and postharvest cracking is established,
this assay can be developed as a monitoring tool
to assess the cracking potential of specific fruit
lots prior to storage or export. Once high cracking
is encountered, fruit pallets may be withheld at
ambient temperature before entering cold storage.
Further use of this technique may be in breeding
programs for selection of cracking-tolerant vari-
eties or in identification of preventive horticul-
tural treatments, such as avoidance of harvest in
the morning under sensitive conditions, or cal-
cium sprays that will prevent cracking.
Acknowledgements
This paper is a contribution from the Agricul-
tural Research Organization, the Volcani Center,
Bet Dagan, Israel, No. 411/01. This research was
supported by grant No. 402-0271 from the Chief
Scientist, the Ministry of Agriculture & Rural
Development.
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