The phenomenon of Tomato bruising: Where biomechanics and biochemistry meet

Abstract

Tomatoes are a commercially important vegetable worldwide. Tomato fruit quality is substantially reduced by bruise (i.e. impact) damage. The occurrence of bruising depends on two main factors: the direct mechanical damage of the tomato, and the presence and subsequent action of unregulated cell wall-modifying enzymes. Bruising is considered to be a two-step process, in which mechanical damage occurs first and then enzymatic degradation of the affected tissue, including cell walls, takes place. This could result in a rapid enzymatic breakdown of the cell wall polysaccharides, observed as soft spots (bruises) on the fruit. To discover the bruising mechanism in tomatoes, the mechanical and biochemical properties of the pericarp tissue were investigated. At first, a logistic regression was established between impact energy and the resulting bruise damage. Results suggested that the fruit's characteristics could not be neglected. A new logistic regression was established, expressing the probability to develop bruises as a function of the energy absorbed and the specific fruit properties. In addition, a comparative biochemical analysis of the bruised and intact tissue of green, pink, light red and red tomatoes, showed that a mechanical impact results in an immediate loss of cell wall material for the riper fruit only. This study revealed that, in the case of mechanically damaged fruit, polysaccharide-digesting enzymes are responsible for the rapid breakdown of the cell wall. This action results in soft spots on the fruit. No bruises will form without the enzymatic digestion of the cell wall.

Full text

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Effect of Bruise Damage in Combination with Storage Temperature
on Cell Wall Breakdown of Fresh Market Tomatoes
Van linden V 1, Labavitch JM 2, De Baerdemaeker J 1
1Laboratory of Agro-Machinery and -Processing
Katholieke Universiteit Leuven
B- 3000 Leuven, Belgium
2Pomology Department
University California (Davis)
Davis CA 95616-8683, USA
Correspondence to: Veerle.Vanlinden@agr.kuleuven.ac.be
Abstract
Tomatoes are commercially important vegetables worldwide. With the mechanization of
post-harvest handling, damage to the fruits contributes to significant annual losses.
Tomato bruises are one example. The bruising mechanism is not well understood and
the main responsible parameters for bruise damage are not yet entirely characterized.
Fresh market tomatoes were subjected to a mechanical impact by means of a pendulum.
Fruits were sorted into classes representing four ripening stages ranging from “mature
green” to “red-ripe”. After pendulum impact, each class of fruits was divided, with one
half stored at room temperature and the other half at 12°C. After an incubation period of
3 hours, bruised areas and non-bruised control areas of the same fruit were collected.
Cell walls were extracted and assayed for pectin and hemicellulose content.
The objectives were to investigate the effect of mechanical damage to cells on cell wall
breakdown and the role of the ripening stage and storage temperature on bruise
development and fruit deterioration.
Introduction
Tomatoes are commercially important vegetables worldwide. In Belgium, tomato is the
most important greenhouse crop with an annual production of about 300 000 tons
representing 169 million euro (numbers for 2000). In 2000, the tomato export amounted
to 148 570 tons (Belgian Ministry of Small Enterprises, Traders and Agriculture).
With the mechanization of post-harvest handling, damage to the fruits contributes to
significant annual losses (Mohsenin, 1986). Tomato bruises are one example. The
bruising mechanism is not well understood and the main responsible parameters for
bruise damage are not yet entirely characterized.

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Bruise susceptibility of fruits and vegetables depends upon a number of elements: the
produce variety, the cultivar, the texture of the produce, the maturity, the water status,
the firmness, the temperature, the size, the shape and other factors (Mohsenin, 1986;
Studman, 1997). Most of these parameters are measured at the entire-fruit level.
Nevertheless, fruit texture might vary considerably within the fruit (Khan & Vincent,
1990 and 1993; Kerstens et al., 2000). The texture of tomato tissue is largely determined
by the mechanical properties of the pericarp tissue. Changes in its texture will alter the
fruit mechanical properties and therefore will affect the quality and the bruise
susceptibility of the commodity. Fischer and Bennett (1991) and also Rose and Bennett
(1999) attributed textural changes to alterations in primary cell wall metabolism, a
common ripening-related process in all flowering plants. Shackel et al. (1991) pointed
out that a decrease in cellular turgor associated with ripening also contributes to textural
changes.
The molecular components of primary walls are modified during fruit ripening by the
temporally and spatially regulated action of endogenous enzymes (Fischer & Bennett,
1991). The type of alterations depends on the presence and the activity of specific
enzymes in the tissue. Rose et al. (Rose et al., 1998; Rose & Bennett, 1999) have
proposed and tested a general model of sequential cell wall disassembly during
ripening, in which the hemicellulose network is affected before the pectin network.
More specifically, at least 2 distinct and sequential stages are known to control the
softening process: (i) xyloglucans (especially those bound to the cellulose surface) are
affected in early ripening with expansin playing a critical and perhaps regulatory role,
and (ii) pectins are degraded in the later stages of ripening by action of exo-
polygalacturonase (PG), endo-PG, polymethylesterase (PME), pectate lyase and beta-
galactosidase. Even very low levels of PG cause significant ripening-associated pectin
depolymerization. However, it is possible that an interaction exists between the two
networks so that the disassociation of the one influences that of the other. In contrast to
hemicellulose and pectin there is no disassembly of cellulose during fruit development
and ripening (Bennett, 2002).
The role of individual enzymes, more specifically of polygalacturonase (PG) in tomato
fruit softening, has been studied by modification of PG gene expression in transgenic
plants (Hadfield and Bennett, 1998; Brummell and Labavitch, 1997). Suppression or
enhancement of PG enzyme activity elucidated the physiological function of the
enzyme (Hadfield and Bennett, 1998). Similar studies have been carried out to identify
the role of proteins that act on the hemicellulose network (Brummell et al., 1999a).
Nevertheless, the process of fruit softening is extremely complex and new insights are
rapidly developed.
Since tomato bruises appear as soft spots on the surface, enzymatically mediated cell
wall disassembly might be involved in the bruising mechanism. This preliminary study
aims to identify the enzymatic contribution to mechanically induced tissue deterioration.
The effect of bruising on cell wall disassembly was investigated for tomatoes at
different stages of ripeness and incubated at cold or room temperature.
Materials and Methods
Fruit and impact characteristics
40 uniformly grown fresh-market tomatoes Lycopersicon esculentum Mill. cv. 'Tradiro'
were hand-picked in the greenhouse. Four times ten fruits with equal colour

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development were chosen to form 4 distinct classes according to the ripening stage of
the tomatoes: mature green (MG), turning (TU), orange (OR) and red ripe (RR) fruit.
Turning fruit appeared to be pink (PI) the next day at measurement, and will from here
on be referred to as pink. Of each tomato, the L*a*b* colour coordinates (CIELAB
Colour Space, 1976), the mass (g), the stiffness (106 Hz2g2/3) and the impact properties
were measured. Impact parameters are: energy of impact (J), absorbed energy (J),
maximum force at impact (N), restitution coefficient, which is the ratio of the rebound
energy over the impact energy (-), and duration of impact (ms). With increasing
ripeness, the fruits had decreasing stiffness values. Fig. 1 shows the average values of
the stiffness at harvest per ripening group.
0
2
4
6
8
10
12
14
green
turning
orange
red
Stiffness (106 Hz2g2/3 )
Fig.1: Average fruit stiffness with standard deviation Fig. 2: Pendulum with spherical
impactor
at day of harvest
Fruit bruising and incubation
Tomatoes were bruised in the equatorial region at the locular tissue by means of a
pendulum with spherical impactor. The pendulum is designed to apply controlled
impact energy (Fig. 2). An average impact force of 90N ( 13N) was used. After
bruising, fruits of each class were divided in two groups of 5 fruits each. One group was
incubated for 3 hours at cold temperature (12°C) whereas the other group was incubated
for 3 hours at room temperature (20°C) under steady state conditions. In total, 8 sets of
unique treatments could be distinguished, namely the four ripening stages at incubation
temperatures of 12°C and 20°C respectively. All sets will be separately discussed.
Preparation and extraction of cell wall material
After incubation, tissue discs were cut out of the fruit at two locations: the impacted or
bruised area and a non-impacted control area at an angle of 90° from the bruised spot.
This resulted in 2 times 5 tissue discs per ripening stage*temperature set. Per set, the
bruised discs were pooled for cell wall extraction. The sound control discs of each set

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were similarly pooled. Cell walls were extracted and assayed for pectin and
hemicellulose content.
Cell wall extraction comprised four phases: (1) pericarp tissue was boiled in 250 ml of
96% ethanol for 15 minutes; (2) the boiled tissue was then homogenized in a blender (3)
and reboiled for 15 minutes; (4) finally, the homogenized tissue in ethanol suspension
was run through a glass filter (pore size 1.6 µm) and the crude cell wall was washed
sequentially with 125 ml of 96% ethanol (3x), 125 ml of methanol: chloroform (1:1,
v/v) (soaking for 5 minutes), 125 ml of 96% ethanol and finally 125 ml of acetone.
Material was carefully scraped off the filter and allowed to dry overnight under the hood
until acetone evaporation was complete. The air-dried residue was weighed and kept in
glass jars until further analysis.
Cell wall analysis
Pectin content was measured according to the colorimetric assay method proposed by
Ahmed and Labavitch (1978). Assay reagents were those described by Blumenkrantz
and Asboe-Hansen (1973). Cell walls were dissolved in concentrated sulphuric acid and
diluted with distilled water. Aliquots of the preparation were mixed with 12.5 mM
sodium borate and heated in a boiling water bath for 5 minutes. Finally, 0.15% w/v
meta-phenyl phenol was added to the aliquots. A standard of D-(+)-galacturonic acid
(200µg.ml-1) was similarly prepared. The absorbance was read at 520nm against the
blank.
Neutral sugar content was measured according to the colorimetric assay method
proposed by Dische (1962). Cell walls were dissolved in concentrated sulphuric acid
and diluted with distilled water, as above. Aliquots of the preparation were mixed with
anthrone in concentrated sulphuric acid (2 mg.ml-1) and heated for 5 minutes. A
standard of D-(+)-glucose (200µg.ml-1) was similarly prepared. The absorbance was
read at 620nm against the blank.
The percentage of uronide, representing the amount of pectin components in the cell
wall, was calculated, as well as the percentage of neutral sugars, representing the
amount of hemicellulose and cellulose components and parts of the complex pectins in
the cell wall. Per set, the values of the bruised and the intact tissue were compared.
Results and Discussion
Bruising is defined as 'damage to plant tissue by external forces causing physical change
in texture and/or eventual chemical alteration of colour, flavour and texture; bruising
does not break the skin' (Mohsenin, 1986). Upon impact, cell walls are broken and cell
content might leak into the neighbouring cells. The cell wall, which is a complex
network of celluloses, hemicelluloses and pectins, becomes more exposed and the wall
polymers might be partially digested as access of endogenous wall-degrading enzymes
to the polymers is rapidly increased.
Hence, the wall is more easily accessible for enzymes that act on one of the wall
components. Consequently, cell wall polymers might be rapidly broken down after
impact. This research aimed to investigate the biochemical alterations of the texture as
resulting from changes in the cell wall components.
The molecular composition and arrangement of the cell wall polymers differ among
species, among tissues of a single species, among individual cells and even among
regions of the cell wall around single protoplasts (Carpita & McCann, 2000). From the
biomechanical point of view, Gao and Pitt (1991) provided evidence for the variability
in single cells. They computed cell wall stretch ratios in single inflated cells and

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concluded that cell wall stiffnesses are not uniform in a single cell. They suggested that
this inequality might be accomplished in real cells by deposition of more cell wall
material in those particular wall regions.
The total amount of measured cell wall components is represented in figure 3 for each
sample.
0
10
20
30
40
50
60
70
80
90
100
R1T1B
R1T1nB
R1T2B
R1T2nB
R2T1B
R2T1nB
R2T2B
R2T2nB
R3T1B
R3T1nB
R3T2B
R3T2nB
R4T1B
R4T1nB
R4T2B
R4T2nB
% in cell wall
UrA NS
Fig. 3: Cell wall pectin (or uronic acid, UrA) and hemicellulose (or neutral sugars, NS) content
for the four ripening stages R1, R2, R3 and R4 representing MG, PI, OR and RR respectively.
Per ripening stage, values for the bruised (B) and the non-bruised (nB) tissue are depicted for
both incubation temperatures T1 (12°C) and T2 (20°C).
Data show increasing pectin content with ripening, as we found. Also, there is a clear
loss of pectin and maybe hemicellulose after bruising at R3 and R4.
The above-mentioned natural variation in cell wall composition could be one
explanation. Anyway, the information that is of real importance, is captured in the
comparison between the bruised and sound samples of each ripening*temperature set.
The values for MG and PI fruit do not differ a lot, apart from the sound sample of PI
fruit, incubated at room temperature (T2). For the OR and RR fruit, the sum of the
pectic and hemicellulosic components is lower in the bruised tissue compared to the
sound tissue for the both temperatures. This supports the idea of an enhanced
disassembly of the cell wall after impact.
Figure 4 illustrates the differences in uronide content and the differences in neutral
sugar content in more detail for each ripening group.

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0
10
20
30
40
50
60
T1B
T1nB
T2B
T2nB
% in cell wall
UrA NS
green
0
10
20
30
40
50
60
T1B
T1nB
T2B
T2nB
% in cell wall
UrA NS
pink
0
10
20
30
40
50
60
T1B
T1nB
T2B
T2nB
% in cell wall
UrA NS
orange
0
10
20
30
40
50
60
T1B
T1nB
T2B
T2nB
% in cell wall
UrA NS
red
Fig. 4: Differences in uronic acid content (UrA) and neutral sugars content (NS) in the cell walls
of the bruised (B) and the non-bruised (nB) samples of each ripening group (clockwise from
upper left: MG, PI, RR and OR). In each graph, data for the fruits, incubated at 12°C (T1) and
the fruits, incubated at 20°C (T2) are depicted.
Mature green fruit had an equal amount of pectins and an equal amount of neutral
sugars in the bruised and the intact tissue. The same holds true for pink fruit incubated
at 12°C. However, when incubated at room temperature, a decrease in polymer content
was observed in the intact tissue. A possible reason would be the spatial variation in
cell wall composition in the pericarp tissue. Also, because the fruit could not be tagged
at pollination, it was difficult to obtain exactly matching fruit at the respective ripening
stages. The variation within each ripening*temperature set might have caused a bias.
For the orange and the red ripe fruit, both pectin content and hemicellulose content are
remarkably lower in the bruised tissue. Differences in the pectin content range from
5.3% to 11.1% whereas differences in hemicellulose content range from 6.7% to 19.7%.
However, the experiment is too preliminary to draw conclusions about the effect of the
incubation temperature on the wall disassembly or about the extent of wall polymer
degradation. If, as expected, the cause of the decreased cell wall component content
after bruising is the increased activity of endogenous enzymes acting on cell wall
polymers, elevated temperature would be expected to accentuate the effect of bruising
on wall component recovery.
Nevertheless, it is remarkable that, in the early stages of fruit ripening, no immediate
enzymatic wall disassembly takes place. What happens later on, more than three hours
past impact, has not been investigated. It is likely that the enzymes, necessary for
polymer digestion, are not yet expressed in those fruits or expressed only to a limited
extent. In contrast to the early ripening stages, tomatoes of the orange and red ripe
stages clearly show an immediate depolymerization of the pectic and hemicellulosic

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components. One could conclude that this enhanced tissue softening is induced by
mechanical damage, caused by an external force under dynamic conditions.
Four components are said to control fruit softening. One component is the relaxation of
the cell wall directly mediated by expansin protein Exp1. Two other components are
polyuronide and hemicellulose depolymerization (Brummell et al., 1999b; Cosgrove et
al., 2002). Many enzymes and proteins become abundantly expressed during the
ripening process. Some of these enzymes hydrolyse the cellulose, the pectin and the
hemicellulose components of the cell wall (Hadfield & Bennett, 1998). Others may
stretch or loosen the cell wall to provide easier access to the other enzymes e.g.
expansins (Brummell et al., 1999b). A last component that contributes to fruit softening
is the cell turgor, which decreases during ripening (Shackel et al., 1991).
A wide range of genes is known to catalyse aspects of pectin modification and
disassembly (Hadfield & Bennett, 1998). Polygalacturonase (PG) and
pectylmethylesterase (PME) are main contributors to the solubilization of cell wall
pectins in ripening tomato fruit. Although previous studies have linked their activity to
tissue softening (Crookes and Grierson, 1983), it has now become clear that neither PG
nor PME alone is responsible for tomato fruit softening. Other enzymes or factors must
also be involved (Gross et al., 1995; Simons and Tucker, 1999). Gross et al. (1995)
detected activities of both, - and - galactosidase, as well as rhamnogalacturonase, all
acting on the pectin network, and considered these enzymes as potential contributors in
tomato fruit softening next to PG and PME. Carey et al. (1995) characterized the
expression and activity of exo-(14)- -D-galactanase in tomato fruit. Thus, PG
activity may be one of multiple, redundant pectin-solubilizing activities (Hadfield &
Bennett, 1998). However, in late stages of tissue ripening, PG is likely to contribute
significantly to overripe tissue softening and deterioration. Possible side effects of
extensive pectin disassembly are, besides tissue deterioration, increased pore size of the
pectin network resulting in cell wall swelling, or increased accessibility of the substrate
to enzymatic action (Hadfield & Bennett, 1998).
Brummell et al. (1999b) gave evidence for hemicellulose depolymerization during
ripening. Especially xyloglucans showed substantially depolymerization with the
greatest change in molecular mass profile between the pink and the red ripe ripening
stages. Nevertheless, they said that ripening related changes in other hemicellulosic
polysaccharides must also occur.
Expansins differ from the above-mentioned enzymes in that way that they don’t show
any hydrolysing activity. They act on the cellulose-hemicellulose interface and it has
been proposed that they disrupt non-covalent interactions, causing the cell wall to
loosen. Brummell et al. (1999b) showed a relationship between Exp1 action and
hemicellulose depolymerization. The exact character of this relationship is not yet
clear. Either, Exp1 and hemicellulose breakdown are independent, or Exp1 action is a
trigger for the hemicellulose breakdown during ripening. It is likely that Exp1 exposes
previously unavailable structural hemicellulose molecules to other degradative
enzymes.
Besides ripening-related enzymes, there are enzymes such as endo-1,4--glucanase
(Egase) and xyloglucans endotransglycolase (XET) that may appear in a non-ripening
related form and nevertheless act on the cell wall (Maclachlan and Brady, 1994).
Mechanical impact might bring about similar changes in the cell wall exposure to
enzymes. If the wall is loosened or disrupted by mechanical impact in the presence of
cell wall related enzymes, disassembly of cell wall polysaccharides as observed in this

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study, might occur. Nevertheless, the presence of non-ripening related enzymes in e.g.
the MG stage could not bring about the same changes in wall composition.
This might signify that enzymes present in the MG and PI stages are not able to digest
the wall, at least not to such extent, or, that more and abundant enzymes are required to
significantly break down the hemicelluloses and pectins of the wall.
Conclusion
Measurement of the cell wall pectin content and the cell wall hemicellulose content of
tomatoes cv. ‘Tradiro’ after mechanical impact suggested that immediate enzymatic
wall disassembly takes place in orange and red ripe fruit, either incubated at room
temperature or cold temperature. Hence, there is an enzymatic contribution to
mechanically induced tissue deterioration. Moreover, this contribution seems to depend
on the ripening stage of the fruit.
Assuming that the cell wall is loosened or disrupted by mechanical impact in the
presence of cell wall related enzymes, a rapid breakdown of cell wall polysaccharides as
observed in this study, might occur. Nevertheless, the presence of cell-wall related
enzymes in e.g. the MG stage could not bring about the same changes. Further, detailed
research about the type of cell wall disassembly due to mechanical impact is necessary
to understand the process of enzymatic bruise formation.
Acknowledgement
We thank D. Brummell for his help and advice on the cell wall analysis. This research
was financed with a specialisation bursary of the Flemish Institute for the Promotion of
the Scientific-Technological Research in Industry (IWT). The authors also wish to
express their gratitude to the Pomology Department of UC Davis for additional
financial support.
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