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Applied Engineering in Agriculture
Vol. 26(3): 509‐517 2010 American Society of Agricultural and Biological Engineers ISSN 0883-8542 509
BRUISING PROFILE OF FRESH APPLES ASSOCIATED
WITH TISSUE TYPE AND STRUCTURE
K. Mitsuhashi‐Gonzalez, M. J. Pitts, J. K. Fellman, E. A. Curry, C. D. Clary
ABSTRACT. Bruising of apples results in millions of dollars in loss annually. To reduce economic loss, a greater understanding
of the mechanisms involved is necessary. Our objective was to visualize damage due to bruising, using different methods of
microscopy, as an aid to substantiating which mechanisms were most viable. `Golden Delicious' apples at different maturity
levels were similarly bruised with an artificial silicon finger attached to an Instron machine. Bruising was induced on freshly
harvested fruit and examined after 48 h at room temperature. We used fluorescence microscopy with Calcofluor (to identify
cell walls) and CDFA (to identify intact cell membranes) in the bruised and discolored tissue. Together with scanning electron
microscopy (SEM), different breakage mechanisms were observed in the bruised volume. These techniques revealed that
bruised tissue was comprised of both live cells, and dead cells that appeared burst, crushed or without apparent damage. The
greater the amount of intercellular space present in the tissue, the more tissue damage from bruising occurred. Because
airspaces weakened the tissue, damage initiated close to these sites. As apples matured, there was an increase in damaged
cells surrounding larger intercellular spaces.
Keywords. Apples, Bruising, Harvesting date.
nderstanding the mechanisms of bruising at a cel‐
lular level in apples is important. If the mecha‐
nisms are well‐known and understood, damage to
apples during harvest could be reduced, benefit‐
ing this multi‐billion dollar global industry. This requires an
understanding of apple tissue morphology, mechanical dam‐
age, biochemical reactions in cell fluids released to intercel‐
lular spaces, and the biology of bruise healing mechanisms.
APPLE MORPHOLOGY
Apple epidermis is comprised of epidermal cells (the
outermost layer of cells), followed by subjacent collenchyma
tissue known as the hypodermis. Beneath this are the
parenchyma cells (Esau, 1965). The hypodermis may contain
up to six layers of collenchyma cells, which contribute to the
strength of the epidermis. The outer cells transmit force to the
inner parenchyma cells (Knee and Miller, 2002).
The characteristics of each cell type are key to developing
an understanding of bruising in apples. Collenchyma tissue
Submitted for review in February 2009 as manuscript number BE 7926;
approved for publication by the Biological Engineering Division of
ASABE in February 2010.
The authors are Kay Mitsuhashi‐Gonzalez, Graduate Student,
Department of Horticulture and Landscape Architecture, Washington State
University, Pullman, Washington; Marvin J. Pitts, ASABE Member,
Associate Professor, Department of Biological Systems Engineering,
Washington, John K. Fellman, Professor, Department of Horticulture and
Landscape Architecture, Washington State University, Pullman,
Washington; Eric A. Curry, ASABE Member, Research Plant
Physiologist, U.S. Department of Agriculture, Agriculture Research
Service, Tree Fruit Research Laboratory, Wenatchee, Washington; and
Carter D. Clary, ASABE Member, Assistant Professor, Department of
Horticulture and Landscape Architecture, Washington State University,
Pullman, Washington. Corresponding author: Carter D. Clary,
Department of Horticulture and Landscape Architecture, P.O. Box 646120,
Washington State University, Pullman, Washington 99164‐6414; phone:
509‐335‐6647; fax: 509‐335‐8690; e‐mail: cclary@wsu.edu.
consists of cells which are smaller with thick cell walls and
more extensive middle lamella structure. Thick walls and
close packing make collenchyma tissue strong and resistant
to compression deformation. Collenchyma tissue is able to
absorb a large amount of mechanical energy because of the
combination of high tensile strength along with flexibility
and plasticity to resist deformation‐caused damage due to
external forces. Generally, it is interpreted to function as
structurally specialized supporting tissue (Esau, 1965).
Collenchyma cells have specialized, irregularly thickened,
pectin‐rich, primary cell walls that function in support of
growing parts (Taiz and Zeiger, 2002). In contrast, parenchy‐
ma tissue is metabolically active tissue, comprised of
thin‐walled cells with air‐filled spaces at the cell corners
(Taiz and Zeiger, 2002). These intercellular airspaces may be
relatively large, which reduces the amount of cell‐to‐cell
contact (Hulbary, 1944, in Esau, 1965), thereby weakening
the tissue (Alvarez et al., 2000a) and resulting in areas
vulnerable to damage by external force.
Previous research (Toivonen et al., 2007) showed there
were differences in bruise sensitivity by measuring the
damage imposed by an external load at different maturity
stages. Observing what occurs at a cellular level at different
stages with the use of different microcopy techniques could
help explain why such differences occur.
APPLE TISSUE MECHANICAL PROPERTIES
Apple cortex cells are anisotropic, (i.e., not homogeneous
in all directions) with a radially oriented network of airspaces
(Khan and Vincent, 1990). Airspaces are thought to be the
weakest areas in apple fruit tissue (Vincent et al., 1991).
Alvarez et al. (2000b) emphasize the basic principle of
fracture mechanics in all solids is inhomogeneity. Irregulari‐
ties in tissue structure essentially weaken the cellular matrix
and make it more vulnerable to damage. The strength of a
material is, therefore, directly related to the magnitude and
distribution of these irregularities. Fractures begin at the site
U
510 APPLIED ENGINEERING IN AGRICULTURE
of such irregularities and grow while traversing the solid, thus
creating a new fracture surface.
Alvarez et al. (2000a) tested tissues such as carrots, celery,
cucumber, and apples under the same external load and found
that tissues with smaller cells sustained less damage. Carrots
had the toughest tissue due to their small parenchyma cells
and lack of intercellular spaces. Cells in direct contact with
neighboring cells have greater adhesion, resulting in fewer
flaws or inhomogeneities. Such flaws can compound as stress
concentrates and promote premature failure. In cucumber
and apples, and less in celery, the cellular structure contains
more intercellular spaces where cells are not in intimate
contact. Moreover, the shape of the cells sometimes prevents
adjacent cell walls from touching. Later in fruit development,
depolymerisation of the pectic polysaccharides occurs as a
result of ripening, further reducing cell adhesion.
Harker et al. (2006) observed that firmer fruit showed
fractured and ruptured cells, while softer fruit showed
cellular debonding when the tissue was externally damaged.
In stored fruit, the failure pattern depended on the firmness
of the fruit. Fruit that softened during storage showed both a
fracture surface and intact cells, due to cell‐to‐cell debond‐
ing; however, stored, firm apples had the same characteristic
as firm apples examined after only one day of storage.
Harker et al. (1997b) showed that cell separation without
rupture may be common in ripening soft fruits such as peach,
kiwi, and strawberry. That is, whereas in pears, where cell
injury was due to cell wall failure and cell fracture, in ripened
soft fruit, tissue failure was due to intercellular debonding
(De Belie et al., 2000). The main component of intercellular
adhesion is the extent of intercellular contact, which is
determined by cell shape and packing, water loss, and size or
absence of intercellular spaces. These factors change as fruit
ripens, leading to larger air spaces and reduced intercellular
contact (Glenn and Poovaiah, 1990; Hallett et al., 1992;
Harker and Sutherland, 1993), allowing increased tissue
deformation under stress. Pierzynowska‐Korniak et al.
(2002) showed that different apple cultivars have different
cells shapes which contribute to their unique mechanical
properties.
BIOCHEMISTRY
A third factor to consider in apple bruising is oxidative
browning, a result of the release of enzymes during
breakdown of cell membranes (Toivonen and Stan, 2004).
This occurs when physical stress or deteriorative processes,
such as a wound response or senescence, are initiated and
compartmentalization of cells begins to fail (Marangoni et
al., 1996). The result is the mixing of polyphenol substrates
like catechin or polyphenols with polyphenol oxidase and/or
phenol peroxidase (Degl'Innocenti et al., 2005). Toivonen
and Brummell (2008) point out that membrane stability is a
major factor controlling the rate of browning. Ascorbate in
the tissues can work to inhibit browning because it is a
universal antioxidant (Noctor and Foyer, 1998) and can
quench lipid alkoxyl and peroxyl radicals involved in
membrane deterioration (Espin et al., 2000)
OBSERVATIONS REGARDING BRUISING
More bruise damage has been observed in riper fruit.
Brusewitz et al. (1991) found an increase in bruise volume
with ripeness. Kvaale et al. (1968) separated `Golden
Delicious' apples into two groups based on skin color (green
vs. yellow) which represented different stages of maturity for
the fresh market. Saltveit (1984) and Klein (1987) also
reported that delayed harvest enhanced fruit sensitivity to
bruising.
Apple bruise research has been done mainly with stored
fruit, which are physiologically older. Because of degrada‐
tive changes that occur in the tissue, older fruit contain more
air space (Kays, 1997). Moreover, the dissolution of large
pectin molecules found in the middle lamella makes the
tissue less brittle, which may impart a different bruise
mechanism than fresh apples that have no middle lamella
dissolution (Kays, 1997). According to Pitt (1982), Pitt and
Chen (1983) Holt and Schoorl (1982, 1983, 1984), Van
Woensel and De Baerdemaeker (1983), Lin and Pitt (1986),
Vincent (1990), Gao and Pitt (1991) Roudot et al. (1991),
Khan and Vincent (1990, 1991, 1993a,b) Abbott and Lu
(1996), Loodts et al. (2006) bruising is a consequence of
breakage of intercellular bonds, propagation of cell wall
ruptures and/or cell deflation as a result of loss and diffusion
of cell fluid. These conclusions resulted from analyses at the
cellular level, and from modeling cell response to external
loads.
Holt and Schoorl (1977) proposed a failure mechanism for
apple bruising at the cellular level. Their model shows burst
cells in the affected area and distorted cells under the burst
cells, followed by a layer of unaffected cells some distance
away from the compressed surface. Their model was based
on the assumption that all apple cells are parenchymatic. Holt
and Schoorl (1982) suggested bruising is a mode of failure
associated with damaged tissue due to cell bursting. When
cells burst, they retain little mechanical strength. Diehl et al.
(1979) observed that cells change shape when an external
load is applied, and the cell walls stretch because cellular
content is relatively incompressible. Therefore, cell break‐
age occurs when the cell walls fracture. Bruising may also be
considered a distortion or shear phenomenon according to
Holt and Schoorl (1982). Shearing, unlike cracking, results
from slippage. Peleg et al. (1976) showed that slip failure
occurred in unripe mango and papaya, while bruising
occurred in ripened fruit.
According to Harker et al. (1997a), the actual mechanism
of breakage is characterized by three modes of failure: cell
fracture, rupture, and cell‐to‐cell debonding. Fracturing is
characterized by cells cleaving across the equator whereas
rupture is characterized by cell bursting and collapse.
Cell‐to‐cell debonding on the other hand, is characterized by
the separation of cells with no damage, or only minor
deflation or distortion, while other cells remain intact. It may
be that the weakened middle lamella in the tissue is less rigid
with polymers that are able to move relative to each other, or
that the weakened middle lamella results in tissue that is still
rigid, but not as strong. Extrapolating from cell‐to‐cell bonds
to tissue‐level mechanics, the second alternative is more
likely to result in brittle tissue behavior.
Bruising is an important problem for the fruit and
vegetable industry in general, and for the fresh market apple
industry in particular. In the U.S. in 2001, apple bruising costs
an estimated $113 million in loss (Varith, 2001). Reduction
of bruising in the fruit and vegetable industry could provide
an annual payback of millions of dollars (Barietelle and
Hyde, 2001). Because bruising is a complicated process
many theories have been suggested but few verified because
511Vol. 26(3): 509‐517
of the inability to observe and measure what is happening to
the cells. Our objectives in using different microscopy
techniques were to determine: 1) the location of damaged
cells within the imposed bruised tissue, and 2) the percentage
of living cells versus dead cells in the bruised tissue volume.
MATERIALS AND METHODS
EXPERIMENTAL DESIGN AND FRUIT SELECTION
Apples used in this study (Malus domestica Borkh cv.
`Golden Delicious') came from a commercial organic
orchard located in Orondo, Washington, in both 2006 and
2007. Tree vigor and cropping were uniform and irrigation,
fertilizer, and spraying practices were the same across all
treatments. Fruit were harvested at different maturity levels
based on skin color. Green apples were harvested 143 days
after full bloom (dafb) in 2007 and 132 dafb in 2008; creamy
colored apples 150 dafb in 2007 and 139 dafb in 2008; and
yellow apples 158 dafb in 2007 and 146 dafb in 2008.
Typically, `Golden Delicious' are harvested based on peel
ground color change as described by Mitcham et al. (2008).
In this work we added the intermediate creamy/white color
as a treatment.
Six fruit from three different trees were harvested at the
three color intervals during the commercial harvest season,
as detailed above, for a total of three treatments based on
maturity index/skin color. Harvested fruit, free of apparent
damage, were transported in tri‐layered European apple
boxes, nested in cylindrical holes cut in low‐density foam
with an additional foam sheet on the bottom of the box to
protect fruit from damage.
BRUISE INDUCTION
An Instron Model 1350 (Instron Industrial Products,
Grove City, Pa.) was used to apply an external 10‐mm
deformation on the apples using a silicon cylinder similar
with the shape and rigidity to a human finger called `creepy
finger 13‐0053' (Loftus International, Salt Lake City, Utah).
Its dimensions are: length: 10 cm and diameter 3 cm. The
imposed force was applied at harvest to the same general
location on each fruit (fig. 1). Deformation rate of the
artificial finger was 0.425 mm/s rate (25.5 mm/min or
1 in./min), and a 10 mm deflection into the surface of the
fruit. Bruising was evaluated and tissue samples prepared for
observation using confocal and scanning electron microsco‐
py (SEM) 48 hours after treatment.
PREPARATION OF TISSUE FOR SEM
Tissue from bruised apples was cut into approximately
1‐cm3 thick sections and observed using a Quanta 200F SEM
(FEI Company, Hillsboro, Oreg.), under low vacuum mode.
Depending on the bruise size, the whole discolored area was
cut in half and observed under the microscope (fig. 1). Since
the experimental methods for studying in situ deformation
are limited and that the tissue is not studied in its natural state
due to the procedures used to preserve the tissue, this method
was selected to analyze the tissue under its natural state. With
this technique, we expected to see the natural state of the
damaged tissue within the bruised volume without being
altered by the fixing or dehydration procedures of the other
techniques.
Figure 1. Area on the apple were bruising was imposed.
FLUORESCENCE MICROSCOPY
The LSM 510 meta laser scanning microscope (Carl Zeiss
MicroImaging Inc., Thornwood, N.Y.) was used to observed
live and dead tissue using the fluorescent dyes Calcofluor
white (1 drop mixed with 1 drop of distilled water) and
5(6)‐CFDA, SE; DFSE (5‐(and‐) 6‐carboxyflourescein suc‐
cinimidyl ester, mixed isomers (CDFA, C1157) from Invitro‐
gen (Life Technologies, Carlsbad, Calif.). Four mL of DMSO
was mixed with 25 mg of CFDA and 1 mL of this solution
mixed with 1 mL of distilled water. The bruise was cut and
0.5‐mm thick slices of tissue were mounted onto white snow
coat micro slides (1 in. × 3 in. × 1.00 mm) and dyed with
CDFA for 10 min. A drop of the calcofluor solution was
added and covered with a cover slip.
512 APPLIED ENGINEERING IN AGRICULTURE
RESULTS
BRUISE FORMATION
All apples in the study formed a bruise under the point of
10‐mm deflection nominal force applied in the 10‐mm
deflection was 50N. Bruise volume varied by apple maturity,
ranging from 583.0 mm3 in green to 1024.2 mm3 in white to
3565.4 mm3 in yellow stages.
The formation of the bruise was in a conical shape, larger
on the top layers and narrower at the lower layers. The tissue
that was underneath the area where the external damage
occurred turned into a brownish color (fig. 2).
Figure 2. Discolored (bruised) area of compressed `Golden Delicious'
apple.
SEM
Using SEM, it was observed that bruise damage starts
under the collenchyma cell layer, approximately six layers
under the epidermis. Where larger intercellular spaces were
located, there was a large amount of dead, burst, or crushed
cells. More cells were damaged as harvest dates progressed,
and the damage was observed to be adjacent to the larger
airspaces and transmitted to the neighboring cells (fig. 3). In
all treatments the hypodermis was intact. In some cases,
some compactness of the hypodermis layer was observed
(fig. 4); however, there was no apparent damage (fig. 5 ‐
images of tissue from green stage maturity). At an angle of
45° relative to the direction of the applied load (fig. 6), we
observed a crack (large intercellular space), in addition to a
crack in a parallel plane to the applied load within the fruit
tissue. The crack parallel to the load was not surrounded by
discolored tissue.
Air space
Healthy cells
beside air space
a
Air space
area
Increased Damaged
cells beside
air space
Dead cells
Live cells
b
Figure 3. SEM images of control (a) and bruised (b) apple tissue.
513Vol. 26(3): 509‐517
a b
Figure 4. Compressed area of the hypodermis in unbruised tissue (a) and bruised tissue (b).
In our study, cell bursting and detachment were observed
under SEM (fig. 3) but this was not observed in detailed under
fluorescence microscopy (data not shown).
FLUORESCENCE MICROSCOPY
Use of CFDA and Calcofluor dyes indicated in the bruised
area cells were not all dead (damaged cell membrane), but
rather a mixture of dead and live cells. Cells adjacent to larger
airspaces were dead and the amount of dead cells varied
depending on the maturity of the apple. The more mature the
fruit was, the greater the number of dead cells surrounding the
airspaces. The hypodermis layer of cells was generally intact,
beneath which areas of dead cells were visible (fig. 5).
It appeared that the collenchyma cells transmitted the
applied force to the underlying parenchyma cells, sustaining
no apparent damage themselves (fig. 7). We also observed
greater damage with increasing apple maturity due to larger
airspaces and cell detachment, causing a larger area of
discolored, damaged tissue (figs. 8 and 9).
DISCUSSION
One of our primary assumptions in this work was that the
imposed deflection of 10 mm using the silicon finger is
representative of that imposed by a picker's finger. We used
this deflection distance because it consistently gave us a
reproducible bruise. Keeping the deflection constant was
probably unrealistic; however, we elected to keep this
variable constant rather than increasing variation due to
reducing the applied force or deflection distance. In this
regard, it is important to note, first, that the deflection of
10 mm represents both the compression of the finger pad
touching the apple and the distance it was pushed below the
plane of the fruit surface. On a green, firm apple this force of
roughly 50 N likely represents a “worst case scenario” since
1) not all pickers grasp the fruit identically, 2) not all fingers
exert the same amount of force when removing the fruit, and
3) not every fruit is bruised because the fruit removal force
and, therefore, the force required to “pull” the fruit off the tree
varies due to varying degrees of pedicel abscission layer
formation. Second, as the apple increases in maturity on the
tree, both flesh firmness and fruit removal force decrease
These are somewhat opposing in their effects on bruising
since softer fruit could make bruising easier, but decreasing
fruit removal force allows the fruit to be removed from the
tree more easily. Indeed, sometimes, just a slight movement
of the fruit is all that is required to break the well‐formed
pedicel abscission layer. Other cultural and physiological
issues may also be involved and we refer the reader to
Toivonen et al. (2007) who addressed numerous factors
affecting bruising and bruise recovery.
Apple parenchyma tissue was observed to suffer varying
mechanical failures depending on the force applied. Contrary
to what Upchurch (1985) found, the presence of larger
intercellular spaces in undamaged tissue, our findings show
that tissue with larger airspaces is more vulnerable to
bruising. As Tu et al. (1996) observed, larger intercellular
spaces are due to degradation of the middle lamella and a
reduction of cell adhesion or part of the apple's tissue
properties. Zamorskyi (2007) observed that `Golden Deli‐
cious' apples have denser intercellular spaces and uneven
cell size and structure which also confirms what Alvarez
et al. (2000b) reported regarding inhomogeneities in struc‐
ture that weakens tissue and makes it more vulnerable to
damage. Our finding that collenchyma cells transmit the
applied force to the underlying parenchyma cells agreed with
those of Knee and Miller (2002). Similarly, we observed that
collenchyma cells were not damaged, while the parenchyma
cells absorbed all the energy and therefore burst or were
crushed, resulting in cell death. All our observations are
consistent with those of other investigators. Mohsenin (1970)
showed that in a bruised area, layers of cells remain
unbroken, and even among the damaged layers, groups of
cells survived. Holt and Schoorl (1983) estimated that only
0.03% of cells are fractured in bruised areas. Using TEM, we
did not observe cell membrane damage (data not shown),
however, such damage has been reported by Rodriguez et al.
(1990).
514 APPLIED ENGINEERING IN AGRICULTURE
a
b
Figure 5. Confocal imaging of bruised vs. undamaged apple tissue (blue
dye showing cell walls of dead cells and green dye showing intact live cell
membranes). (a) Damaged cells (parenchyma cells) under undamaged hy‐
podermis cells (collenchymas cells, (b) healthy undamaged cells.
As reported by Alvarez et al. (2000b) and Vincent et al.
(1991) we observed also that generally, irregularities (air‐
space) in tissue structure weakens the tissue and makes it
more vulnerable to damage. Where tissue was cracking
parallel to the applied load but not discolored, we suggest this
may be due to minor damage in the tissue such as Holt and
Schoorl (1983) observed in potato tissue. As Donald et al.
(2003) explained, experimental methodologies for studying
in situ deformation are limited and it is uncertain whether the
procedures used to preserve the tissue, such as vacuum for
dehydration and coating in SEM, fixing and embedding for
light microscopy alter tissue structure.
Numerous and varied definitions and symptoms of
bruising can be found in the literature. A bruise can be due to
cell injury that results in flesh browning (Mohsenin et al.,
1962); a discoloration or fracture of the tissue (Chen et al.,
1987); cell bursting (Holt and Schoorl, 1977); cell rupture
Figure 6. A 455 crack from the applied force.
(Diehl et al.,1979; Pitt, 1982) and flesh browning (Robitaille
and Janick 1973); an injured area which is flat, soft and brown
with a high respiration rate (Ericsson and Tahir, 1996); a dark
spot (Blahovec, 1999); or just an indentation of the surface
and a slight discoloration (Crisosto et al., 1993) where tissue
is compressed or impacted. Based on our observations, we
would define a bruise as an area of discolored tissue
comprising an array of undamaged and burst and crushed
cells (fig. 8), along with cells that have not been physically
damaged. The discoloration is only an indication of where the
damage can be found. This can occur inside the cell if the cell
membrane is damaged or outside if the cell contents are
released to the intercellular spaces when the cell ruptures.
CONCLUSIONS
Based mainly on tissue properties, our major findings
regarding the mechanism of bruising of freshly harvested
`Golden Delicious' apples are: 1) cell wall rupture or damage
is not required for bruise induction; 2) parenchyma cells are
involved in the discoloration of bruised tissue in `Golden
Delicious' apples because they are more fragile than the
collenchyma cells; 3) cracking as well as rupture appear to be
present in the damaged tissue; 4) damaged tissue is made up
of a mixture of dead and live cells; 5) cell rupture appears to
Figure 7. Proposed bruising mechanism for `Golden Delicious' apples.
515Vol. 26(3): 509‐517
White bruised
crushed cells
burst cells
intact cells
Figure 8. SEM and confocal images of cell wall and membrane damaged due to imposed bruising.
ab
Figure 9. Difference in intercellular spaces of fresh and stored apple cortex tissue. (a) Fresh unbruised tissue, (b) Stored unbruised tissue.
be facilitated by intercellular spaces which are the weakest
point in the tissue; and 6) cells adjacent to intercellular spaces
are more vulnerable to damage presumably due to the lack of
cell‐to‐cell contact, thereby allowing them to expand and
rupture more easily than cells that are tightly packed.
Although there is much yet to do, these data add to our
understanding of the mechanisms underlying bruising of
apple flesh at harvest.
ACKNOWLEDGEME NTS
The authors recognize the support of the Agricultural
Research Center, Washington State University, the Consejo
Nacional de Ciencia y Technologia, Government of Mexico,
Stemilt growers and Mr. & Mrs Ohrazda, Bud and Karen
(Lucky Badger Orchards, Orondo, Wash.).
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