Size Characterization of Stress Cracks
in Corn Kernels
S. Gunasekaran, S. S. Deshpande, M. R. Paulsen, G. C. Shove
ASSOC. MEMBER
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ABSTRACT
CANNING electron microscopic pictures were taken
of corn kernels from four different varieties, all dried
at a high temperature. Stress cracks were observed to
originate at the inner core of the floury endosperm and
propagate radially outward along the boundary of starch
granules. Many cracks did not advance far enough to
open up at the surface underneath the pericarp. An
average stress crack was measured to be 58 ± 14 /^m in
width at its widest part.
S
INTRODUCTION
Stress cracks are fine fissures in grain kernel
endosperm. In general, the formation of stress cracks is
associated with rapid drying of grain at high
temperatures. When handled and transported, the
kernels with stress cracks break more readily than sound
kernels leading to considerable amounts of brokens and
fines. Excessive broken material, included as BCFM
(Broken Corn and Foreign Material) in the U.S. Grading
Standards, reduces the market value of the grain.
Therefore, evaluation of a grain lot for the presence of
stress-cracked kernels would help assess its quality. Such
an index of quality would be helpful in assessing not only
the end-use value of the grain but also the drying method
used and the appropriateness of subsequent handling
procedures. Accordingly, improved drying, tempering
and handling systems could be developed to reduce the
incidence of stress cracking.
At present a candling procedure, shining light through
the kernel holding the germ side toward the light source,
is used to manually inspect corn kernels for presence of
stress cracks. If large samples are involved, this process
would take a considerable amount of time and effort and
tend to be less accurate owing to fatigue of the human
eye. Hence, there is a definite need for a faster, more
accurate and automatic system of stress crack
evaluation. One of the first steps toward developing such
a system is to know the surface nature and size
characteristics of stress cracks to more clearly
understand their occurence.
Article was submitted for publication in September, 1984; reviewed
and approved for publication by the Electric Power and Processing Div.
of ASAE in April, 1985. Presented as ASAE Paper No. 84-3014.
The authors are: S. GUNASEKARAN, Assistant Professor,
Agricultural Engineering Dept., University of Delaware, Newark, S. S.
DESHPANDE, Research Assistant, Food Science Dept., M. R.
PAULSEN, Associate Professor, and G. C. SHOVE, Professor,
Agricultural Engineering Dept., University of Illinois, Urbana.
Acknowledgments: This study was a part of Project No. 10-0359 of
the Agricultural Experiment Station, College of Agriculture, University
of Illinois at Urbana-Champaign. Appreciation is expressed to the
Center for Electron Microscopy, University of Illinois, for the use of its
equipment and facilities.
1668
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Theory of Stress Cracking
Stress cracks are generally associated with rapid
drying of corn with high temperature drying air. The first
indication of drying stress is a single crack usually
extending from the tip toward the crown of the kernel
and visible on the side of the kernel opposite to the germ.
As stresses increase, multiple cracks develop, giving
some kernels a checked or crazed appearance
(Thompson and Foster, 1963). Gustafson et al. (1979)
performed a finite element analysis of a corn kernel
subjected to thermal stresses and concluded that the
location of the maximum tensile stresses correlated well
with laboratory observations of stress crack locations.
Kunze and Choudhury (1972) working with rice
hypothesized that, as the moisture gradient declines after
drying, moisture from the central portion of the kernel
diffuses to the surface causing the surface to expand
while the internal portions contract due to moisture loss
resulting in compression at the surface and tension in the
central portions of the grain. When compressive stresses
at the grain surface exceed the tensile strength of its
interior the kernel pulls apart and fissures.
Balastreire et al. (1982), through optical microscopy,
observed that the crack narrowed as it approached the
kernel surface. Based on this they concluded that the
stress cracks initiated at the center of the kernels and
propagated toward the outside. Also, the similarity
between surfaces that ruptured due to external loading
and those that ruptured due to drying stresses indicated
that stress cracks propagated through the cells and
around the starch granules.
Factors Influencing Stress Crack Development
Thompson and Foster (1963) noted that the drying
speed, expressed in terms of moisture loss in percentage
points per hour, is the most significant factor in stress
crack development. However, researchers working with
rice have observed that grain fissures do not necessarily
develop in the grain during drying (Sharma et al., 1979).
Kunze and Choudhury (1972) reported that the moisture
gradient created during the drying period provides the
potential for later damage. Substantiating this, White et
al. (1982), in their experiments with popcorn, observed
that very few kernels had developed stress cracks at the
time the grain was removed from the dryer. Instead
stress cracks developed after the grains were stored for a
period of time.
Ekstrom et al. (1966) reported that stress cracks
probably are not caused exclusively by temperature
gradients in the kernel; but that moisture gradient
stresses or a combination of moisture and thermal
stresses are most likely to induce stress cracks. In a
simple model study of the corn kernel they observed that
if no moisture gradient is present, a temperature
ic) 1985 American Society of Agricultural Engineers 0001-2351/85/2805-1668$02.00
TRANSACTIONS of the ASAE
difference of at least 97°C (175°F) must exist between
the center and the outer surface of the kernel for the
cracking to occur due to temperature effects alone.
Other factors which contribute to stress cracking are
initial and final moisture content of drying, drying air
temperature and airflow rate, drying procedures and
type of corn. Thompson and Foster (1963) reported that
corn dried from near 30% initial moisture content had
33% multiple stress cracked kernels, while that dried
from near 20% moisture content had only 23% kernels
of that category. Also, drying air temperatures of 60, 87,
and 115°C resulted in 20, 30 and 34% stress-cracked
kernels respectively. Yellow corn is more susceptible to
multiple stress cracks than white (White and Ross,
1972); and large and rounded kernels stress crack more
readily than flat kernels (Thompson and Foster, 1963).
Equally important as moisture and thermal stresses
are the mechanical impact stresses. Roberts (1972)
observed that 26% of combine shelled corn had stress
cracks before the corn was artificially dried. Corn shelled
by the conventional combine (rasp-bar cylinder and
concave) would receive external damage to the sound
pericarp and internal damage, (such as stress cracks and
fissures) to the inside of the kernel. Kernel moisture
content, cylinder speed and location of impact with the
concave were also found to have a significant effect on
formation of stress cracks in corn. This mechanically
inflicted damage could be further aggravated during
high speed drying (Chowdhury and Kline, 1978a).
In an artificial loading experiment quasi-static loading
was reported to initiate stress cracks, fissures or faults in
the endosperm portion of the kernel. When this
deformation exceeded the rupture point of the kernel,
external cracks were formed depending on the kernel
loading position and moisture content (Chowdhury and
Kline, 1978b). Stress cracking also reportedly increased
during shipping and handling of corn (Paulsen and Hill,
1977).
Stress Cracking—Problem and Prevention
The most common problem associated with stress
cracking is breakage susceptibility of the kernels. As the
number of stress cracks in the corn increases the
susceptibility to breakage also increases (Litchfield et al.,
1982). Shelled corn dried with heated air was two to three
times more susceptible to breakage than the same corn
dried with unheated air. The drying air temperature and
airflow rate appears to have a direct relationship with
kernel breakage susceptibility (Thompson and Foster,
1963). Dry millers expect a lower yield of the more
valuable large grits in corn dried with a high percentage
of stress cracks. During wet milling, kernels with stress
cracks and subsequent broken kernels steep more
rapidly and lose part of their starch in the steep water
before the sound kernels steep adequately. Also, the
protein in the endosperm of severely stress-cracked
kernels, due to high kernel temperatures during drying,
is case-hardened which makes separation of protein and
starch more difficult (Freeman, 1972). Thus, highly
stress cracked corn will likely to yield less recoverable
starch than sound kernels.
A high percentage of checked or crazed kernels in a
corn sample always causes low germination. Also, stresscracked grain has a reduced market value due to likely
insect and microbial attack (Thompson and Foster,
1963). In case of popcorn, internal stress cracks may
Vol. 28(5):September-October, 1985
reduce the expansion volume resulting in poor corn
popping quality (White et al., 1980).
Hammerle (1972) presented a theoretical analysis of
failure in a viscoelastic slab subjected to temperature and
moisture gradients. He concluded that timetemperature-moisture relationships could be suitably
selected to prevent stress cracking if the stress states due
to thermal and moisture gradients and the necessary
material properties could be determined.
Since most kernels are not fissured immediately after
drying some post drying treatment or procedure can be
developed that will prevent subsequent stress cracking.
The most beneficial post-drying treatment has been
tempering the grain for a certain period of time before
cooling (Thompson and Foster, 1963; Gustafson et al.,
1982). Also, allowing the kernel to cool slowly after
removal from the drier results in a dramatic reduction in
the number of stress-cracked kernels (White and Ross,
1972). Short term tempering and multipass, multistage
drying have been shown to be effective as well (Emam et
al., 1979).
Based on model prediction, that during the abrupt
transition from drying to tempering the kernel stresses
increase quite rapidly due to rapid moisture
redistribution, Litchfield et al. (1982) suggested a
smooth transition from drying to tempering, that is
gradually decreasing the drying rate before tempering.
Ross and White (1972) observed a general decrease in
stress cracking of corn when it was dried to a lower
moisture content started at lower moisture content,
supposedly due to unexplained physical and chemical
changes during overdrying which would make the kernel
more resistant to cracking during cooling. In addition,
reducing the drying air temperature and airflow rate also
help minimize stress cracking (Thompson and Foster,
1963; Ross and White, 1972).
OBJECTIVES
The objectives of this investigation were:
1. to discuss the microscopic structure of a corn
kernel with respect to formation and propagation of
stress cracks,
2. to study the external and internal characteristics
of stress cracks, and
3. to determine average dimenisons of stress cracks.
MICROSCOPIC STRUCTURE OF A CORN KERNEL
Careful study of the microscopic structure of a corn
kernel (Fig. 1) will provide some useful insights in
understanding the formation and propagation of stress
cracks within the kernel. Several researchers have
investigated various aspects of kernel structure in detail
(MacMasters, 1962; Wolf et al. 1952 a, b, c and d).
Major parts of the kernal structure, pericarp, endosperm
and germ, are here in discussed with particular reference
to stress cracking.
The pericarp, the outermost covering of the corn
kernel, is composed of four layers. The outer layers,
epidermis and mesocarp, are composed of closely
adherent, long and fiberous, thick-walled cells with no
intercellular spaces (MacMasters, 1962). The
characteristic overlapping, interlocking arrangement of
the cells provides much crack and breakage resistant
strength to corn kernels. Thermal and moisture stresses
that induce stress cracks within the kernels are generally
1669
ftt
EPIDERMIS
MESOCARP
CROSS CELLS
TUBE CELLS
SEED COAT
Fig. 2—Categories showing none, single,
double, and multiple stress cracks in corn
kernels from left to right, respectively.
ALEURONE LAYER
HORNY ENDOSPERM
FLOURY ENDOSPERM
CELLS FILLED WITH
STARCH GRANULES
IN PROTEIN MATRIX
WALLS OF CELLS
SCUTELLUM
PLUMULE OR
RUDIMENTARY
SHOOT a LEAVES
TIP CAP
RADICLE OR
PRIMARY ROOT
Fig. 1—Microscopic structure of a corn kernel (Brooker et al., 1974).
not sufficient to rupture the pericarp. Thus, the cracks
occur internally.
The endosperm of a corn kernel has two distinct
regions, the horny endosperm and the floury endosperm.
The horny endosperm is hard and translucent whereas
the floury endospery is soft, friable and relatively
opaque. Horny endosperm cells have thinner cell walls
but a thicker proteinaceous matrix. The starch granules
within the floury endosperm cells are large and loosely
organized in contrast to small and tightly packed
granules in horny endosperm cells (Wolf et al., 1952c).
Because of the thicker proteinaceous matrix, more force
is required to break the cell walls of horny endosperm
compared to that of floury endosperm (MacMasters,
1962). According to Schultze and MacMasters (1962),
endosperm breakage occurs across cell walls, not
between the walls of adjacent cells. Therefore the horny
endosperm is more resistant to stress cracking. This
explains the narrowing of crack front as the crack
approached the kernel periphery as reported by
Balastreire et al. (1982), and substantiated in the latter
sections of this paper. The aleurone layer, the outermost
layer of the endosperm, is even stronger than the horny
endosperm. The aleurone layer contains no intercellular
spaces and it contains protein and oil but no starch (Wolf
et al., 1952c). This layer provides added resistance to the
propagation of stress cracks. Hence most cracks do not
advance through the aleurone layer and therefore cannot
be seen at the surface of the endosperm as reported in the
following sections.
The germ or embryo, embedded in the endosperm
tissue, consists of oil and protein and very little starch.
Though the outer portion of the germ, the scutellum, is
in close contact with the endosperm, it is separate and
discontinuous from the endosperm (Wolf et al., 1952a;
MacMasters, 1962). Also, over the upper half of the
germ, a thin hyaline band of non-cellular material
occupies space between scutellum and endosperm (Wolf
et al., 1952c). This structural discontinuity probably
prevents the propagation of stress cracks in the germ
region of the kernel.
EXPERIMENTAL PROCEDURE
Sample Preparation
Corn samples were prepared to examine the kernel
stress cracks using a scanning electron microscope. Four
corn genotypes (FR4A x FR4C) x Mol7, Mol7 x H100,
FRB73, x Mol7, and B73 x FR16 were used. They were
grown on the University of Illinois Agricultural
Engineering Research Farm, hand picked, hand shelled,
and dried with 60 °C air from an initial moisture content
of about 25.0% to about 14.0% final moisture.
Additional moisture loss of about 2 to 3% was observed
when the kernels were allowed to cool overnight at room
temperature. The kernels were free of stress cracks at the
beginning of the drying process; but the stress cracks
developed during drying and shortly after.
Test weight and moisture content of the corn samples
were determined using a quart cup and a Dickey-John
moisture meter respectively. Two hundred kernels of
each genotype were randomly selected and examined for
stress cracks. The kernels were grouped into none,
single, double and multiple stress-cracked categories
(Fig. 2). Table 1 summarizes the test weight, moisture
and percentage stress cracks in the corn samples used.
Since the stress cracks are internal fissures in the
endosperm, following techniques were used for easy
examination with the scanning electron microscope. The
kernels were drawn at random for all the tests. Initially
the kernels were soaked in water at room temperature for
2 to 3 min to aid in peeling off the pericap. After pericap
removal, the kernels were quickly surface blotted and
allowed to dry to original moisture content. Soaking the
kernels in water for a brief period of time did not cause
the stress cracks to develop further. The SEM picture of
the crack as it appeared under the pericarp is shown in
Fig. 3. To study the stress crack characteristics inside the
kernel, selected kernels were cut perpendicular to the
longitudinal axis at about 5 mm from the kernel tip cap
(Fig. 4). A belt sander with fine sand paper at low speed
was used for this to avoid inducing any artificial flaws in
TABLE 1. MOISTURE CONTENT, TEST WEIGHT AND STRESS CRACK DATA FOR FOUR
CORN GENOTYPES
Corn genotype
(FR4AxFR4C)xMol7
FRB73xMol7
B73xFR16
Mol7xH100
1670
Moisture content, Test weight,
kg/m3
% w.b.
10.8
11.1
12.2
11.3
782.5
776.1
794.1
745.2
Single
21
20
38
35
Stress cracks,%
Double Multiple
13
13
19
14
59
51
40
22
Total
93
84
97
71
TRANSACTIONS of the ASAE
Fig. 3—Scanning electron microscope
photograph with the pericarp removed
showing stress crack at the surface just
beneath the pericarp.
Fig. 4—Kernels that have the crown removed
by sanding to study internal characteristics of
stress cracks.
the kernel. Although it appeared that sanding did not
alter stress crack characteristics, the internal structure of
starch granules was greatly disturbed (Figs. 5 and 6). To
observe stress cracks while retaining the original
structure of the endosperm, samples were obtained by
splitting the kernels manually across the longitudinal
axis with a sharp razor blade. Kernels that split with
relatively even faces were used in the experiments. The
SEM pictures taken with this type of kernel sample are
presented in Figs. 7 and 8.
Scanning Electron Microscopy
The kernel samples were glued onto aluminum stubs
covered with a coat of silver conducting paint at the point
of attachment to facilitate easy grounding of electrical
charge. They were then sputter coated to about 100 nm
thickness with vaporized gold:palladium (60:40) under
vacuum to obtain an electrical conductive surface. A
JSM JEOL-U3 model scanning electron microscope was
used for viewing and photographing the samples. An
accelerating potential of 15 kV was maintained for all
exposures.
RESULTS AND DISCUSSION
An SEM picture of the stress cracked kernel surface is
presented in Fig. 4. The stress crack appears to be an
opening at the kernel surface under the pericap.
However, in a large number of kernels it was not possible
to observe the stress cracks with the SEM that were
Fig. 6—Scanning electron microscope
photograph of the endosperm of a kernel with
its crown sanded showing the narrowing of the
crack front as the crack approached the
periphery (upper left) of the kernel.
Vol. 28(5):September-October, 1985
Fig. 5—Scanning electron microscope
photograph with the pericarp removed
showing stress crack at the surface just
underneath the pericarp revealing the internal
starch granules.
observable visually. This suggests that stress cracks are
strictly an internal flaw, some of which do not extend far
enough to open up at the pericarp. This also implies that
the cracks initiate inside of the kernels and propagate
toward the periphary. Balastreire et al. (1982) made a
similar observation. When the cracks reach the surface,
it is possible with high magnification to see the internal
starch granules, Fig. 5. This further substantiates the
idea that the stress cracks originate at the inner core of
the endosperm. The strands of material on the surface of
the kernel in Fig. 5 are parts of pericarp adhering to the
endosperm.
Stress crack characteristics were more easily observed
in the SEM pictures of the sanded kernels. Fig. 6 shows
the crack front narrowing toward the surface as the crack
propagates to the otuside. It is also clear from this
picture that the stress cracks may not extend to the outer
periphery of the kernel surface, presumably due to
greater resistance offered by the aleurone layer and the
outer layers of endosperm. The variations in crack width
in the interior part of the kernel could be caused by
differences in the hydrostatic expansion (or contraction)
coefficient of the floury endosperm as compared to the
horny endosperm.
The stress crack in Fig. 7 can be observed as being
surrounded by numerous tiny starch granules. This SEM
picture of a split kernel surface indicates that a stress
crack propagates across the cell walls, but along the
boundary of starch granules. Here the crack is seen to
branch off indicating the possibility of stress cracking in
a totally unpredictable manner. Also, several minor
Fig. 7—Scanning electron microscope
photograph of the split surface of kernel
endosperm showing the stress crack being
surrounded by starch granules.
Fig. 8—Magnified view of the region marked
by the arrow in Fig. 7 to show how the stress
crack propagates along the boundary of starch
granules.
1671
TABLE 2. STRESS CRACK WIDTHS FOR FIVE KERNELS OF
EACH CORN GENOTYPE
Corn
genotype
(FR4AxFR4C)xMol7
FRB73xMol7
B73xFR16
Mol7xH100
1
2
50
45
40
35
66
49
44
47
Stress crack width, /dm
Kernel
Average Standard
deviation
5
3
4
74
53
51
54
70
58
55
59
75
95
60
75
Overall
67
60
50
54
57.75
10.15
20.15
8.09
14.80
13.5
flaws were observed within the kernel endosperm under
the scanning electron microscope. It is possible that
these minature internal flaws grow in size and propagate
with increase in thermal and moisture stresses and are
eventually recognized as stress cracks. Fig. 8 is a
magnified view of the region marked by an arrow in Fig.
7. This clearly shows how stress cracks propagate along
the boundary of starch granules.
Table 2 summarizes the average stress crack
dimensions in the four different corn gentoypes studied.
The width of stress cracks was measured at the widest
part of the crack and averaged for five kernels. The
average crack width varied from 50 to 67 jwm. The
average crack width for all the samples was 58 /urn. The
individual measurements ranged from 35 jum to 90 pan.
Even greater variability in crack dimensions is possible
due to individual kernel differences. Therefore, the
values obtained in this investigation are only an
indication of the size of a typical stress crack. Analysis of
variance statistics indicated that the stress crack
dimensions are independent of the kernel gentoype at 95
percent level of significance. And there was no
correlation (r = 0.12) between the crack width and the
percentage of stress cracks in the samples. Since the
cracks originate at the center of the kernel and propagate
radially outward, average depth of the cracks was
estimated to be one half of the kernel thickness which
would be 1.9 to 2.4 mm for the corn samples considered
in this investigation.
CONCLUSIONS
1. Stress cracks are internal fissures in the kernel
endosperm which may not be open at the surface
underneath the pericarp.
2. The stress cracks originate at the center of the
floury endosperm and propagate radially outward,
toward the kernel periphery along the boundary of starch
granules.
3. A typical corn kernel stress crack has a maximum
width of about 5 8 + 1 4 jLtm.
4. For the four corn genotypes tested, stress crack
dimensions were independent of kernel genotype and the
percentage of stress-cracked kernels in the sample.
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