Food Structure
Volume 11 | Number 2
Article 9
1992
Microstructural Differences Among Adzuki Bean
(Vigna Angularis) Cultivars
Anup Engquist
Barry G. Swanson
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Engquist, Anup and Swanson, Barry G. (1992) "Microstructural Differences Among Adzuki Bean (Vigna Angularis) Cultivars," Food
Structure: Vol. 11: No. 2, Article 9.
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FOOD STRUCTURE, Vol. II (1992), pp. 171-179
Scanning Microscopy International , Chicago (AMF O' Hare), IL 60666 USA
MICROSTRUCTURAL DIFFERENCES AMONG
ADZUKI BEAN (Vigna angularis) CULTIVARS
An up Engquist and Barry G. Swanson
Department of Food Science and Human Nutrition
Washington State University, Pullman, WA 99164-6376
Abstract
Introduction
Scanning electron microscopy (SEM) was used to
study microstructural differences among five adzuki bean
Adzuki beans are one of the oldest cultivated beans
in the Orient, often used for human food, prepared as a
bean paste used in soups and confections (Tjahjadi and
Breene, 1984). The starch content of adzuki beans is
about 50 %, while the protein content ranges between
20%-25% (Tjahjadi and Breene, 1984) .
cultivars: Erimo, Express, Hatsune, Takara and VBSC .
Seed coat surfaces showed different patterns of cracks,
pits and deposits . Cross-sections of the seed coats re-
vealed well organized layers of elongated palisade cells
followed by many layers of amorphous parenchyma
cells . Typical sub-epidermal layers of organized colum-
Adzuki beans are a majo r crop consumed in Japan,
China, and other oriental countries (Sacks, 1977). In
Japan most adzuki beans are used in the form of sweet-
nar, ho ur-glass cells were characteristica11 y absent in the
five culti vars of adzuki beans. The cross-sections of the
coty ledons revea led intercellular materials between storage cells. Storage cell s in Hatsune cultivar were loosely
ened paste called "An" in many confectio ns , candies,
cakes and ice cream (McClary eta/., 1989) . Between
1980 and 1989 about 27% of the adzuki beans consumed
in Japan was provided by imports (McClary et a/.,
1989). Thus, a very promising future market exists in
packed with many intercellular spaces as compared to
the other four cultivars. Cross-sections of storage cells
revealed starch granules embedded in protein matrices.
The protein matrix appeared more granular in cultivars
Erimo and VBSC and less granular in cuhivars Express,
Hatsun e and Takara. Flours of the five cultivars were
Japan for Washington grown adzuki beans.
Consumer acceptance is an important aspect of
marketing adzuki beans. Japanese consumer acceptance
examined under SEM to photograph individual starch
for adzuk.i beans and substitutes depends upon the
g ranul es and protein matrices . Starch granule shape was
characteristic to each cultivar: spherical in Erimo
culti var, s pherical to irregular in Express and Hatsune
di stinct color and shape of the bean and the end use in
various foods (McClary eta/., 1989). The size, shape,
color and nutritional quality of adzuki beans vary among
different cultivars of adzuki beans (Hsieh, 1991).
cultivars, highly irregular to slightly spherical or oval in
Takara cultivar, and oval to spherical in VBSC cultivar.
No grooves o r fi ssures were observed in Erimo or
Factors such as appearance, functi onal and textural
V BSC starch granules. Shallow grooves were observed
properties, organoleptic and nutritional qualities, and the
on the starch granules of Express and Hatsune cultivars.
A characteri stic deep fissure was observed in starch
g ranules of the Takara cultivar. The characteristic deep
fissure on starch granules may be used as an identifying
feature for Takara cultivar.
price of a particular cultivar of ad zuki beans determine
the potential as a marketable crop in the Japanese
market.
Microstructural variations can be related to tex tural,
chemical and physical variations among bean culti vars
Key words: Adzuki beans, cultivars, seed microstructure , scanning electro n microscope, seed coat, sto rage
cell s, starch granules, protein matrix .
(Sefa-Dedeh and Stanley, 1979). Microstructural variations were used to identify different culti vars of soybeans (Wolf et a/., 1981). Scanning electron microscopy (SEM) is a technique recently employed in various
studies of food microstructure (Swanson et a/., 1985).
The objective of this study was to survey microstructural
variations among five different cultivars of adzuki beans
us ing SEM .
Initial paper received October 31 , 1991
Manuscript received April 28, 1992
Direct inquiries to B. G . Swanson
Telephone number: 509-335-3793
Fax number: 509-335-4815
171
A. Engquist and B. G. Swanson
Table I - Microstructural Variations Among Adzuki Bean Cultivars
Cultivar
Seed coat surface features
deposits
Erimo
pits
irregular shape,
mostly clustered
few scattered
few, small
pits
compressed,
flower-shaped,
clustered or
scattered
not common
irregular shape,
small, scattered,
some very large
deep , long
pits
irregular shape.
some small
rod-shaped ,
scattered
not common
VBSC
irregular shape,
small. mostly
scattered, some
very large
deep, large
pits
• 1dent1fymg
feature
Express
Hat sune
Takara
Storage cells
Starch granules
cracks
shape
no cracks
tightly packed,
many
mostly
spherical
inter-cellular
deposits
distinct, •
convoluted
pattern of
cracks
no cracks
no cracks
no cracks
tightly packed ,
many
intercellular
materials
loosely packed, •
few
intercellular
materials
tightly packed ,
many
intercellular
materials
tightly packed ,
many
intercellular
materials
surface
pitted.
no grooves.
no fi ssures
spherical
to
irregular
spherical
to
irregular
highly
irregular to
slightly
spherical
oval to
slightly
spherical
pitted,
with shallow
grooves
pitted,
with shallow
grooves
pitted •
with deep
fissure
pitted.
no grooves
no fissure
the adzuki bean cultivars observed in this study are
summarized in Table 1.
Materials and Methods
Five cultivars of adzuki beans (Vigna angularis),
Erimo, Express, Hatsune, Takara and VBSC used in this
study were obtained from Washington State University
Irrigated Agriculture Research Extension Center ,
Prosser, WA.
Adzuki beans were surface cleaned with dry
Kleenex 13 facial tissue to remove any dirt and other particles sticking to the surface. Cleaned beans were freeze
fractured by freezing in liquid nitrogen and fracturing
with a razor blade. Freeze fractured beans were attached to standard SEM mounts using double sided adhesive tape and dried overnight in a desiccator. To prepare adzuki bean flours 100 g of cleaned beans of each
cultivar were ground to flour for 2 minutes in a Stein
mill to pass through a sieve of 0.5 nun pore size. A
tiny amount of flour for each bean cultivar was spread
on double sided adhesive tape attached to a standard
SEM mount and dried in a desiccator overnight. Dried
specimens were sputter-coated with 300 A gold (Hummer Technics). The gold-coated specimens were examined and photographed with a Hitachi S570 SEM operating at 20 kV.
Seed coat surface
Observations of seed coat surfaces of adzuki
cultivars revealed distinct differences among the five
cultivars. Seed coat surfaces of adzuki beans were
characterized by the presence of deposits, pits and
cracks. On the surface of the Erimo culti var numerous
deposits and few pits were observed (Figure 1). The
surface deposits of Erimo cultivar were large and mostly
clustered (Figure 2). The seed coat surface of the
Express cultivar was characterized by convoluted pattern
of cracks (Figure 3). The surface deposits of Express
cultivars were large, compressed and flower-shaped
(Figure 4) Pits and small scattered surface deposits
were observed on the seed coat surfaces of Hatsune
(Figure 5) and VBSC (Figure 6) cultivars. At greater
magnification the largest surface deposits were observed
on Hatsune (Figure 7) and VBSC (Figure 8) cultivars.
Large surface deposits on Hatsune and VBSC cultivars
exhibited shapes not observed in the other three cultivars
examined. Surface cracks were not observed on surfaces of Hatsune and VBSC cultivars. In Takara cultivar, the surface of the seed coat was characterized by
many small scattered irregular and rod shaped deposits
(Figure 9). Rod-shaped deposits were the size of a
Results and Discussion
The significant microstructural differences among
172
Mic rostructural Di fferences Among Adzuki Bean Cultivars
Figure I. Adzuki bean cv. Erimo. Seed coat surface. SD - surface deposit.
Figure 2. Adzuki bean cv. Erimo. Seed coat surface deposits at higher magnification. SD -surface deposit.
Figure 3. Adzuki bean cv. Express. Seed coat surface pattern . C indicates convoluted pattern of cracks.
Figure 4. Adzuki bean cv. Express. Seed coat surface deposits at higher magnification.
compressed surface deposits.
173
SD - flower shaped ,
A. Engquist and B. G. Swanson
Figure 5. Adzuki bean cv. Hatsune. Seed coat surface. P indicate surface pits.
Figure 6. Adzuki bean cv. VBSC. Seed coat surface. P indicates surface pit.
Figure 7. Adzuki bean cv. Hatsune. Seed coat surface deposits (SD) at higher magnification.
Figure 8. Adzuki bean cv. VBSC. Seed coat surface deposit (SD) at higher magnification.
174
Microstructural Differences Among Adzuki Bean Culti vars
Figure 9. Adzuki bean cv. Takara. Seed coat surface. SO -surface deposit.
Figure 10. Adzuki bean cv. Takara. Seed coat surface at higher magnification. Arrow bead indicates small rod-shaped
surface deposits. SD - irregular surface deposit.
Figure II . Adzuki bean cv . Hatsune. Cross-section of the seed coat. PL - palisade cells, PR - parenchyma cells.
Figure 12. Adzuki bean cv. Express. Cross-section of the bean cotyledon showing arrangement of storage cells.
Arrow head indicates intercellular materials.
175
A. Engquist and B. G. Swanson
Figure 13. Adzuki bean cv. Hatsune. Cross-section of bean cotyledon showing loosely packed storage cells. Arrow
head indicates intercellular materials.
Figure 14. Adzuki bean cv. Erimo. Starch granules (S) embedded in protein matrix (PM).
Figure 15. Adzuki bean cv. VBSC. Starch granules (S) embedded in protein matrix (PM).
Figure 16. Adzuki bean cv. Erimo . Protein matrix (PM) formed aggregates of loose granular nature in flour sample.
176
Microstructural Differences Among Adzuki Bean Cultivars
Figure 17. Adzuki bean cv. Express. Protein matrix (PM) formed aggregates of loose granular nature in fl our sample.
Figure 18. Adzuki bean cv . Express. Individual starch granule from flour sample. Spherical to irregular shape.
S - starch granule, G - groove.
Figure 19. Adzuki bean cv. VBSC. Individual starch granule from flour sample. Oval shape. S -starch granule.
Figure 20. Adzuki bean cv. Takara. Individual starch granule from flour sample. Highly irregular to slightly spherical
shape. S - starch granule, F - fissure.
177
A. Engquist and B. G. Swanson
bacteria, while the irregularly shaped deposits were
larger (Figure 10). Pits and cracks were not observed
on the seed-coat surface of Takara cultivar.
Seed coat surface characteristics are important in
evaluating the differences among legume cultivars.
Seeds of many plants have species-specific seed coat
patterns (Hughes and Swanson, 1985). Obvious microstructural differences between lentils and other food
legumes were observed by examining seed coat surfaces
(Hughes and Swanson, 1986). Size, shape and frequency of papillae on lentil seed coats vary among lentil
cultivars (Trivedi and Gupta, 1988). Numerous pits or
pore-like indentations were observed on intact soybeans
by Wolf et al. (1981). Observations of maturing seed
coats of common bean (Phaseolus vulgaris L.) seeds revealed a characteristic pattern which developed as the
seeds matured (Hughes and Swanson, 1985).
The significance of seed coat surface deposits to
food processing is unknown, but large quantities of deposits on the surface can affect the luster and acceptability of the seed (Swanson et al., 1985). Pits and cracks
on the seed coat may be natura11y present or inflicted
during harvesting and handling. Pits or cracks may influence the moisture retention capacity of the beans during storage, which in tum effects the market value of
beans.
Cross-sections or the cotyledons
Cross-sections of adzuki bean cotyledons revealed
storage cells containing starch granules and protein
matrices.
The arrangement of storage cells varied
among the five cultivars of adzuki beans. Storage cells
were compactly packed with few intercellular spaces in
Erimo, Express, Takara and VBSC cultivars (Figure
12), with many intercellular materials. Contrary to the
other four cultivars, Hatsune cultivar revealed loosely
packed storage cells with large intercellular spaces and
few intercellular materials (Figure 13). The intercellular
materials between the cell walls of the adjacent cells
may provide mechanical strength and finnness to the
plant tissue.
Spherical starch granules were embedded in a granular protein matrix in the five cultivars of adzuki beans.
Granules of the protein matrix were more distinct in
Erimo (Figure 14) and VBSC (Figure 15) cultivars, than
in Express Hatsune and Takara cultivars. Micrographs
of the bean flour indicated that the starch granules remained intact upon milling, whereas the protein matrices
formed aggregates of a loose granular nature (Figures
16, 17).
Bean cotyledons are the storehouses of common reserve starch, which is used for the nourishment of the
growing embryo of the germinating seed. Common
bean (Phaseolus vulgaris L.) cotyledons contain large
(10-50 ;<m) spherical starch granules and small (5-10
Jlffi) round protein bodies, embedded in a protein matrix
(Hughes and Swanson, 1985).
Cross-sections or seed coats
The cross-sections of seed coats of adzuki bean cultivars exhibited an organized layer of elongated palisade
cells and multiple layers of amorphous parenchyma cells
(Figure II). A sub-epidermal layer of hour-glass cells
was not observed in the adzuki bean cultivars examined
in this study. The thickness of the palisade layer varied
in five cultivars of the adzuki beans, from 40-44 ;<m in
Hatsune, 50-56 ;<min Erimo, VBSC, and Takara to 5860 ;<m in Express. The differences in thickness of seed
coat may account for differences in rate of water imbibition of the adzuki bean cultivars.
Cross-sections of seed coats of legumes are generally composed of three distinctive layers of cells: an epidermal layer of organized, elongated palisade cells, a
sub-epidermal layer of shorter, columnar hour-glass cells
supported by many layers of amorphous parenchyma
cells (Comer, 1951). Three distinct layers of cells were
observed in common beans (Phaseolus vulgaris L.) by
Hughes and Swanson (1985). A sub-epidermal layer of
hour-glass cells was observed in common black beans
and Mexican red beans but not in adzuki beans
(Chilukuri and Swanson, 1991 ).
Sefa-Dedeh and
Stanley ( 1979) evaluated the microstructure of the seed
coats of adzuki, blackeye, soy, white and pinto beans
and observed an initial decrease in water absorption rate
with increasing seed coat thickness .
Starch granules
The shapes of starch granules varied in the flours of
five cultivars observed. Starch granules of Erimo cultivar were spherical, Express and Hatsune cultivars were
spherical to irregular (Figure 18), Takara cultivar were
highly irregular to slightly spherical and VBSC were
oval or spherical to slightly irregular (Figure 19). The
shape and size of the starch granules is dependent on the
developmental stage of the granule, hence it is difficult
to attribute a particular shape and size to a particular
cultivar.
No grooves or fissures were observed on the surface
of the starch granules of Erimo and VBSC cultivars.
Shallow grooves were present on the surface of the
starch granules of Express and Hatsune cultivars. The
presence of a characteristic deep fissure was unique to
Takara cultivar among adzuki beans (Figure 20).
Grooves and fissures on the starch granules may be
fonned after progressive deposition of starch layers in a
particular fashion around a nucleation point. Specific
morphology and composition of starch granule may be
unique to a particular cultivar, and assist in identification
or differentiation of cultivars of adzuki beans.
178
Microstructural Differences Among Adzuki Bean Cultivars
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The five cultivars of adzuki beans examined in this
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of cracks o n the seed coat surface was unique to Express
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Seed coat surface characteristics such as deposits, pits
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Thus a knowledge of microstructural variations among
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Acknowledgements
Editor's Note: All of the reviewer's concerns were appropriately addressed by text changes, hence there is no
Discussion with Reviewers.
The authors acknowledge the use of the facilities of
Electron Microscopy Center, Washington State University, Pullman, Washington, and partial financial support
from USAJD Title XII Bean/Cowpea CRSP.
179