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Journal of Food Engineering 64 (2004) 119–127
www.elsevier.com/locate/jfoodeng
Characteristics of the different corn types and their grain fractions:
physicochemical, thermal, morphological, and rheological
properties of starches
Kawaljit Singh Sandhu, Narpinder Singh
*
, Maninder Kaur
Department of Food Science and Technology, Guru Nanak Dev University, Amritsar 143005, Punjab, India
Received 26 May 2003; accepted 13 September 2003
Abstract
Pop corn, dent corn and baby corn (dent type) grains were fractionated into different fractions on the basis of their size. The
starches were separated from these fractions and evaluated for physicochemical, morphological, thermal and rheological properties.
Significant difference was observed in various properties among different fractions of dent corn and pop corn. Mean granule dia-
meter of the starches separated from different fractions ranged between 6.33 and 13.64 lm. The shape of starch granules varied from
oval to polyhedral. Baby corn starch showed the presence of oval shape granules whereas polyhedral shape granules were observed
in starches from other corn types. Amylose content of starches from different corn types ranged between 15.3% and 25.1%. Baby
corn starch showed lowest swelling power, solubility, amylose content and mean granule diameter. The transition temperatures (T
o
,
T
p
and T
c
) and enthalpy of gelatinization (DH
gel
) of starches were determined using differential scanning calorimetry. T
o
, T
p
, T
c
and
DH
gel
varied from 66.3 to 69.3, 71.5 to 73.1, 76.5 to 78 C and 8.9 to 10.9 J/g, respectively. Baby corn starch showed lowest T
o
, T
p
,
DH
gel
and PHI. The values of these parameters were highest for both the fractions of dent corn and large grain fraction of pop corn
starch. The rheological properties of the starches from different fractions of pop corn and dent corn measured using a dynamic
rheometer, showed significant variation in the peak G
0
, G
00
and peak tan d values. Pop corn large grain fraction showed highest
values for peak G
0
, G
00
and breakdown in G
0
, whereas pop corn small grain fraction showed lower values of these parameters. The
turbidity of the gelatinized aqueous starch pastes from all the corn types increased with increase in storage period. Baby corn starch
showed the lowest and pop corn large grain fraction showed highest retrogradation values during storage.
2003 Elsevier Ltd. All rights reserved.
Keywords: Corn starch; Morphological; Particle size; Thermal; Rheological; Amylose content
1. Introduction
production of corn in India was 12,000,000 Mt against
the total world production of 602,026,822 Mt (FAO,
2002).
Starch is the major carbohydrate of corn, making up
72–73% of the kernel (Watson & Ramstad, 1991). Corn
wet milling produces starch for food, textile, paper and
the pharmaceutical industry. Singh and Eckhoff (1996)
compared laboratory and pilot scale corn wet milling
procedures used in assessing millability and component
yields. There are five general classes of corn––flint, pop,
flour, dent and sweet corn which differ significantly in
physicochemical characteristics and horny to floury en-
dosperm ratio. These variations in corn characteristics
have been attributed to hereditary and environmental
factors (Hamilton, Hamilton, Johnson, & Mitchell,
1951). Starches from different corn types differ widely
with respect to the morphological, rheological, func-
tional and thermal properties. Li, Berke, and Glover
Starch is the most abundant reserve carbohydrate in
plants. Amylose and amylopectin are two macromole-
cular components of starch granules. Normal maize
starch consists of 75% branched amylopectin; the re-
maining 25% is linear amylose. Starch is a valuable in-
gredient to the food industry, being widely used as a
thickener, gelling agent, bulking agent and water re-
tention agent (Singh, Singh, Kaur, Sodhi, & Gill, 2003).
In India, corn has become the third important food
grain after wheat and rice. The demand for corn is
growing up in India with the setting up of food pro-
cessing units involved in the processing of corn. The
*
Corresponding author.
E-mail address:
narpinders@yahoo.com(N. Singh).
0260-8774/$ - see front matter 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2003.09.023
120
K.S. Sandhu et al. / Journal of Food Engineering 64 (2004) 119–127
(1994) characterized thermal properties of several exotic
maize populations and suggested that thermal properties
can be used as criteria for selection of desirable lines for
breeding purposes to obtain starch with specific prop-
erties useful to industry. DSC has been used to study
thermal properties associated with starch gelatinization.
Gelatinization, the process by which the internal struc-
ture of the granule is broken down and the whole
granule disintegrates releasing the polysaccharide into
the surrounding medium, is accompanied by a variety of
changes. Being semi-crystalline, the granule exhibits bi-
refringence when viewed between crossed polar. This
birefringence is lost as the starch granules gelatinize and
their structure is disrupted. DSC monitors changes in
the physical and chemical properties of starches, offering
a thermodynamic approach to the study of starch ge-
latinization (Donovan, Lorenz, & Kulp, 1983). Liu,
Lelievre, and Chee-Ayoung (1991) quantitatively cor-
related crystallinity loss with thermal events as measured
by DSC. Cooke and Gidley (1992) observed that crys-
talline and molecular order is lost concurrently during
gelatinization. White, Abbas, Pollak, and Johnson
(1990) studied intra- and interpopulation variability in
thermal properties of starches from normal southern
dent and exotic corn populations and found significant
differences among plants of the same populations as well
as differences between populations.
Starch granules swell when heated in excess of water,
and their volume fraction and morphology play im-
portant roles in rheological behavior of the starch dis-
persions (Bagley & Christianson, 1982; Da Silva,
Oliveira, & Rao, 1997; Evans & Haisman, 1979). The
surface characteristics of starch granules from different
type of starch sources have been determined by using
scanning electron microscopy (SEM). Hall and Sayre
(1970) observed the presence of pinholes on the surface
of starch granules of yellow dent corn. Fannon and
BeMiller (1992) also observed the presence of surface
pores in starch granules from corn, sorghum and millet.
Campbell, Li, Berke, and Glover (1996) examined
granule size distribution of 35 tropical and subtropical
corn populations and reported significant differences in
the average granule size within and among the popula-
tions. The present investigation was undertaken to study
characteristics of different corn types, their grain frac-
tions and physicochemical, thermal, morphological, and
rheological properties of their starches.
according to their size and shape. Two fractions of dent
corn i.e. bold and long grains were separated having
average length of 12 and 13 mm and width of 11 and 9
mm, respectively. Pop corn grains was separated into
three fractions i.e. small, medium and large size with
average length of 7, 9 and 9 mm, respectively and av-
erage width of 5, 6 and 8 mm, respectively. Baby corn
(dent type) had fairly uniform size grains having average
length and width of 9 and 7 mm, respectively.
2.2. Starch isolation
Starch was isolated from different corn fractions
using a combination of alkali extraction method of
Mistry and Eckhoff (1992) and corn wet milling method
by Watson (1964) with following modifications. Differ-
ent corn types (300 g) were steeped in water containing
0.16% sodium hydrogen sulphite for 12 h at 50 C. The
steep water was drained off, and grains were ground in a
laboratory blender. The ground slurry was screened
through nylon cloth (100 mesh). The slurry was allowed
to stand for 2 h. The cloudy supernatant was drained
off, and the sediment was then steeped in 5–6 volumes of
NaOH solution (0.2%) at 25 C for 12 h. The slurry was
then passed through 325-mesh sieve. The material left
over the sieve was discarded and the filtrate was resus-
pended in distilled water. The supernatant was then re-
moved by suction. Starch was given repeated washings
with distilled water to remove all the alkali until the
supernatant no longer showed any pink color with the
phenolphthalein. The starch was then collected and
dried in an oven at 40 C for 24 h.
2.3. Physicochemical properties of starch
2.3.1. Amylose content
Amylose content of the isolated starch was deter-
mined by using the method of Williams, Kuzina, and
Hlynka (1970).
2.3.2. Swelling power (g/g) and solubility (%)
Swelling power and solubility were determined in
triplicate, using the method of Leach, McCowen, and
Schoch (1959).
2.3.3. Turbidity
Turbidity of starch paste from different corn types
was measured as described by Perera and Hoover
(1999). A 1% aqueous suspension of starch from each
corn type was heated in a water bath at 90 C for 1 h
with constant stirring. The starch paste was cooled for 1
hat30C. The samples were stored for five days at 4 C
in a refrigerator and turbidity was determined every 24 h
by measuring absorbance at 640 nm against a water
blank with a Shimadzu UV-1601 spectrophotometer
(Shimadzu Corporation, Kyoto, Japan).
2. Materials and methods
2.1. Materials
Dent corn, pop corn and baby corn (dent type) were
procured from local market, Amritsar, India from the
2002 harvest. The grains of each type were separated
K.S. Sandhu et al. / Journal of Food Engineering 64 (2004) 119–127
121
2.3.4. Water binding capacity (WBC)
WBC of the starches from different corn types was
determined using the method described by Yamazaki
(1953) as modified by Medcalf and Gilles (1965). A
suspension of 5 g starch (dry weight) in 75 ml distilled
water was agitated for 1 h and centrifuged (3000g) for 10
min. The free water was removed from wet starch,
drained for 10 min and wet starch was weighed.
2.7. Particle size analysis
Particle size analysis of starches was done using
coulter small volume module model LS 230 laser light
scattering particle size analyzer. 0.25 g of corn starch
was combined with 3 ml of distilled water in a small
glass vial and vortexed followed by sonication for 1 h.
Dried sample was completely deagglomerated after ap-
proximately 10 min of sonication at 40 C. The sample
was vortexed and approximately 10 drops were added
into the sample port until the instrument read 45%
polarization intensity differential scattering (PIDS) or
10–14% obscuration. Isopropanol was used as the sus-
pension fluid within the instrument. The sample was
allowed to equilibrate the isopropanol for 15 min before
starting the analysis.
2.4. Retrogradation (%)
Starch suspension (2%, w/v) was heated at 85 C for
30 min in a temperature controlled water bath, followed
by rapid cooling in an ice water bath to room temper-
ature. The starch sample was stored for 24, 48 and 120 h
at 4 C. Syneresis was measured as percentage amount
of water released after centrifugation at 3200 rpm for
15 min.
2.8. Rheological properties
2.5. Thermal properties
A small amplitude oscillatory rheological measure-
ment was made for different corn starches with a
dynamic rheometer (Carri-Med CSL
2
-100, TA Instru-
ments Ltd., Surrey, England) equipped with parallel
plate system (4 cm diameter). The gap size was set at
1000 lm. The strain and frequency were set at 0.5% and
1 Hz, respectively, for all determinations. The dynamic
rheological properties, such as storage modulus (G
0
),
loss modulus (G
00
) and loss factor (tan d) were deter-
mined for starches from different corn types. Starch
suspensions of 20% (w/w) concentration were loaded
onto the ram of the rheometer and covered with a thin
layer of low-density silicon oil (to minimize evaporation
losses). The starch samples were subjected to tempera-
ture sweep testing and were heated from 45 to 90 Cat
the rate of 2 C/min.
Thermal characteristics of isolated starches were
studied by using a DSC––821
e
(Mettler Toledo, Swit-
zerland) equipped with a thermal analysis data station.
Starch (3.5 mg, dry weight) was loaded into a 40-ll
capacity aluminium pan (Mettler, ME-27331) and dis-
tilled water was added with the help of Hamilton
microsyringe to achieve a starch–water suspension
containing 70% water. Samples were hermetically sealed
and allowed to stand for 1 h at room temperature before
heating in the DSC. The DSC analyzer was calibrated
using indium and an empty aluminium pan was used as
reference. Sample pans were heated at a rate of 10 C/
min from 20 to 100 C. Thermal transitions of starch
samples were defined as T
o
(onset), T
p
(peak of gelati-
nization) and T
c
(conclusion), and DH
gel
was referred to
enthalpy of gelatinization. These were calculated auto-
matically. The gelatinization temperature range (R) was
computed as 2
ð
T
p
T
o
Þ
as described by Krueger,
Knutson, Inglett, and Walker (1987). Enthalpies were
calculated on starch dry basis. The peak height index
(PHI) was calculated by the ratio DH

T
p
T
o
Þ
as
described by Krueger et al. (1987).
2.9. Statistical analysis
The data reported in all the tables are an average
of triplicate observations. The data were subjected to
statistical analysis using Minitab Statistical Software
(Minitab Inc., USA).
3. Results and discussion
2.6. Morphological properties
3.1. Physicochemical characteristics of corn starches
SEMs were taken by a Jeol JSM-6100 scanning
electron microscope (Jeol Ltd., Tokyo, Japan). Starch
samples were suspended in ethanol to obtain a 1% sus-
pension. One drop of the starch–ethanol suspension was
applied on an aluminium stub using double-sided ad-
hesive tape and the starch was coated with gold–palla-
dium (60:40). An accelerating potential of 20 kV was
used during micrography.
Amylose content of starches separated from different
corn types ranged between 15.3% and 25.1% (Table 1).
Pop corn small grain fraction starch had the highest
whereas baby corn starch had the lowest amylose con-
tent. An amylose content of 22.1% in corn starches has
been reported earlier by Singh and Singh (2003). Pop
corn small grain fraction showed higher amylose content
than its counterpart medium and large grain fractions.
122
K.S. Sandhu et al. / Journal of Food Engineering 64 (2004) 119–127
Table 1
Swelling power, solubility, water binding capacity, amylose content, mean diameter and turbidity of starches separated from different corn types and
their fractions
Corn type Fraction Swelling
power
(g/g)
Solubility
(%)
Water
binding
capacity
(%)
Amylose
content
(%)
Mean
diameter
(lm)
Turbidity (nm)
0 h
24 h 48 h 72 h 120 h
Dent corn Bold
17.9
18.9
96
20.7
13.35
0.89
1.12
1.33
1.44
1.56
Dent corn Long
17.7
20.3
104
23.4
13.13
0.93
1.22
1.38
1.50
1.58
Pop corn
Small
16.7
18.3
102
25.1
12.77
0.93
1.44
1.56
1.60
1.66
Pop corn Medium 17.3
18.8
107
22.4
13.64
0.91
1.44
1.52
1.60
1.64
Pop corn Large
16.4
18.2
105
24.4
13.42
0.95
1.50
1.56
1.62
1.64
Baby corn –
14.9
12.5
107
15.3
6.33
1.32
1.40
1.48
1.56
1.66
Dent corn long grain fraction had higher amylose con-
tent than its counterpart bold grain fraction. Cluskey,
Knutson, and Inglett (1980) observed that amylose
content of dent corn starch granules, fractionated ac-
cording to size ranged from 24% for the largest to 22%
for the smallest granules. The ability of the starches
from different corn types to swell in excess of water and
their solubility is presented in Table 1. The swelling
power and solubility of starches from different corn
types ranged from 14.9 to 17.9 g/g and 12.5 to 20.3%,
respectively. Highest swelling power was observed for
dent corn bold grain starch. The swelling power of
starch has been reported to depend on water holding
capacity of starch molecules by hydrogen bonding (Lee
& Osman, 1991). Hydrogen bonds stabilizing the
structure of the double helices in crystallites are broken
during gelatinization and are replaced by the hydrogen
bonds with water, and swelling is regulated by the
crystallinity of the starch (Tester & Karkalas, 1996).
Among various pop corn grain fractions, medium grain
fraction had highest swelling power and solubility. Dent
corn bold grain fraction had higher swelling power as
compared to its counterpart dent corn long grain frac-
tion. Baby corn starch has a low amylose content and
swelling power, so lower amylose content–higher swell-
ing power applies only for starches granules obtained
from same corn type. Sandhya Rani and Bhattacharaya
(1989) indicated that starch granules with low amylose
content being less rigid, swell freely when heated. The
starch granules with higher amylose content, on the
other hand, being better reinforced and thus more rigid,
probably swells less freely. WBC of the starches from
different corn types ranged between 96% and 107%,
lowest for starch from dent corn bold grain fraction and
highest for baby corn and pop corn medium starch was
observed (Table 1). The differences in WBC of starches
from different corn types may be attributed to the
variation in their granule structure. Pop corn medium
grain fraction had higher WBC than its counterpart’s
small and large grain fractions. Dent corn long grain
starch had higher WBC than dent corn bold grain
starch. The engagement of hydroxyl groups to form
hydrogen and covalent bonds between starch chains
lowers WBC (Hoover & Sosulski, 1986). The differences
in degrees of availability of water-binding sites among
the starches may have also contributed to the variation
in WBC (Wotton & Bamunuarachchi, 1978). The tur-
bidity values of gelatinized pastes of the starches sepa-
rated from different corn types is shown in Fig. 1. Pop
corn starch pastes showed highest turbidity values
whereas lowest values were observed for dent corn
starch pastes after 120 h of storage at 4 C (Table 1).
The turbidity values of the starch paste from all corn
fractions increased progressively during storage. The
increase in turbidity during storages has been attributed
to the interaction between leached amylose and amylo-
pectin chains that led to development of function zones,
which reflect or scatter a significant amount of light
(Perera & Hoover, 1999). Turbidity development in
starch pastes during storage have been reported to be
affected by factors such as granule swelling, granule
remnants, leached amylose and amylopectin, amylose
and amylopectin chain lengths (Jacobson, Obanni, &
BeMiller, 1997). The dent corn bold grain starch paste
showed lower turbidity than its counterpart long grain
starch paste which may be due to the presence of fewer
1.8
1.6
1.4
1.2
1
Pop corn (small)
Pop corn (medium)
Pop corn (large)
Dent corn (bold)
Dent corn (long)
Baby corn
0.8
0.6
0
20
40
60
80
100
120
Time (h)
Fig. 1. Effect of storage duration on the turbidity of starch pastes from
different corn types.
K.S. Sandhu et al. / Journal of Food Engineering 64 (2004) 119–127
123
granule remnants in the starch paste, which in turn de-
pends on the granule morphology.
starch granules in ranges from 1 to 7 lm for small and
15 to 20 lm for large granules. Baby corn starch showed
the presence of smallest size granules with mean dia-
meter of 6.33 lm whereas pop corn medium grain
fraction starch had largest granules with mean diameter
of 13.64 lm (Table 1). Pop corn medium grain fraction
starch had higher mean diameter than its counterpart
small and large fractions. Dent corn bold grain starch
had a higher mean diameter than its counterpart long
grain fraction starch. Baby corn starch had small oval
shape granules whereas starches from other corn types
showed the presence of polyhedral shape granules. The
difference in granule morphology may be attributed to
the biological origin, biochemistry of the amyloplast and
physiology of the plant (Badenhuizen, 1969; Svegmark
& Hermansson, 1993). When viewed under scanning
electron microscope, the surface of the granules showed
the presence of surface pores. Fannon and BeMiller
(1992) also observed the presence of pores on the surface
of corn, sorghum and millet starch granules. Leach and
Schoch (1961) suggested that these pores are related to
the botanical source of the starch. Additional indirect
evidence for the presence of pores and cavities on the
surface of starches has been provided by Hellman and
Melvin (1950).
3.2. Retrogradation properties of corn starches
The extent of retrogradation in starches from differ-
ent corn types is shown in Table 2. Starch retrograda-
tion occurs when starch molecules begin to reassociate
in an ordered structure (Atwell, Hood, Lineback, Var-
riano-Marston, & Zohel, 1988). Starch gels are meta-
stable and nonequilibrium systems and therefore
undergo structural changes during storage (Ferrero,
Martin, & Zantzky, 1994). Pop corn large grain fraction
starch gel showed highest (2.7–17.3%) while baby corn
starch gel showed lowest (2.5–6.4%) retrogradation
values observed during the 120 h of storage at 4 C. Pop
corn large grain fraction starch gel showed higher ret-
rogradation than its counterpart’s small and medium
grain starch gels. A higher rate of retrogradation was
observed in dent corn long grain starch gel than its
counterpart bold grain starch gel. The retrogradation in
starch gels from different corn types increased with the
increase in the storage period. The retrogradation
properties of the starch gels are indirectly influenced by
the structural arrangement of starch chains within the
amorphous and crystalline regions of the ungelatinized
granule, which in turn influence the extent of granule
breakdown during gelatinization and the interaction
that occurs between starch chains during gel storage
(Perera & Hoover, 1999).
3.4. Thermal properties of corn starches
The results of various DSC parameters such as
transition temperatures (T
o
;
T
p
;
T
c
), range (R), enthalpies
of gelatinization (DH
gel
), and PHI of starches from dif-
ferent corn types are summarized in Table 3. DH
gel
of
corn starches ranged from 8.9 to 10.9 J/g. The lowest
and highest DH
gel
values among different corn types were
in the starches isolated from baby corn and dent corn
bold grain fraction. DH
gel
reflected the loss of double
helical rather than the crystalline order (Cooke & Gid-
ley, 1992). The gelatinization enthalpy values of starches
has been reported to be affected by factors such as
granule shape, percentage of large and small granules,
and the presence of phosphate esters (Stevens & Elton,
1971; Yuan, Thompson, & Boyer, 1993). The lower
DH
gel
of baby corn starch may be attributed to its small
granule size and lowest amylose content. T
o
, T
p
and T
c
of
starches from different corn types ranged between 66.3–
69.3, 71.5–73.1 and 76.5–78.0 C, respectively. Dent
corn bold grain fraction showed the highest T
o
(69.3 C),
followed by dent corn long grain fraction while it was
lowest for baby corn starch (66.3 C). No significant
differences were observed in T
o
and T
p
values among
different fractions of dent corn and pop corn. Among
various pop corn fractions, T
c
was observed to be
highest for pop corn small grain fraction starch (77.2 C)
and lowest for pop corn medium fraction starch (76.5
C). Perera, Lu, Sell, and Jane (2001) reported value of
T
o
for normal corn starches to be 64.4 C. Highest T
p
3.3. Morphological properties of corn starches
The SEMs of starches separated from different corn
types shows the presence of starch granules ranging
from small to large and oval to polyhedral (Fig. 2).
Singh et al. (2003) reported angular shape for corn
starch granules. A representative curve of granule dia-
meter of corn starches is shown in Fig. 3. The figure
clearly indicates that diameter of majority of starch
granules ranged between 6 and 30 lm with some gra-
nules having diameter in the range of 0.4–4 lm. Singh
et al. (2003) reported average size of individual corn
Table 2
Effect of storage duration on the retrogradation of starch gels from
different corn types and their fractions
Corn type Fraction
Retrogradation (%)
24 h
48 h
120 h
Dent corn Bold
1.9
5.1
14.4
Dent corn Long
2.5
7.3
15.6
Pop corn
Small
2.3
4.5
16.7
Pop corn Medium 2.0
5.1
14.9
Pop corn Large
2.7
7.7
17.3
Baby corn –
2.5
3.8
6.4
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