chapter 5 effect of modification on different properties of cassava starch
TRANSCRIPT
5. Modified cassava starches …..
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Chapter 5
Effect of modification on different Properties of Cassava starch
Modification of native starches using various chemical reagents makes significant variation in
the structural, physicochemical, thermal and rheological properties of starch. Current session
discuss the various changes occurring in the native starch through the modification process
and the effect on tablet properties.
5.1.1 Structural properties of modified arrowroota starches
FTIR spectroscopy
The FT-IR spectra of the modified cassava starch are shown in Fig 5.1. The characteristic
regions in these spectra can be summarized as follows.
The broad band between 3700 and 3000 cm-1 is assigned to O–H stretching and it is due to
hydrogen bonding involving the hydroxyl groups on the starch molecules. The band at 2922
cm-1 is assigned to CH2 symmetrical stretching vibrations. The band at 1597 cm-1 is attributed
to the scissoring of two O–H bonds of water molecules, while the bands at 860 and 767 cm-1
are due to skeletal stretching vibrations of starch
Comparisons between the spectra of the native and heat-moisture treated starches were carried
out in the spectral region of 1645 to 800 cm-1 (Fig 5.1a). The spectra changed in that the band
intensity at 1000 cm-1 was clearly reduced, accompanied by the disappearance of the peak at
1042 cm-1. The intensity of the band situated at approximately 1547 cm-1 was shown to
increase with an increasing degree of crystallinity. The disappearance of the peak at 1092 cm-1
could hence be interpreted as a sign that amylopectin has become amorphous during heat
treatment.
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Fig 5.1.1a FTIR spectra of modified cassava starches
The FTIR spectrum of acetylated starch showed the presence of carbonyl group at 1654cm-1
indicating the incorporation of ester group in the sample. A comparison of the spectrum of
native cassava starch with acetylated starch clearly indicates the introduction of acetyl moiety
through a band at 1831 cm-1 (Fig 5.1.1a).
The carboxymethyl starch derivative shows new bands at 1590, and 1475 cm-1(Fig 5.1.1b).
Those new bands confirm that carboxymethylation has taken place. Similar observations were
reported for carboxymethylated potato starch, corn starch, and maize starch (Bhattacharya et
al., 1995, Zeljko et al., 2000).
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Fig 5.1.1b: FTIR spectra of modified cassava starches
Enzymatic degradation of cassava starch with ∞-amylase resulted in the decreased or
disappearance of the intensity of bands at 1150 and 1080 cm-1, and the intensity of the band at
1220 cm-1was increased. These results suggest that the ordered structure of native starch was
disrupted as a result of enzymatic degradation and the structure of the modified starch is more
amorphous in nature. It is also interesting to note that the native starch has prominent band at
929 cm-1. This band is sensitive to water and characteristic index of hydrophilicity of starches
(Alexander, 1992). Upon enzymatic modification, the intensity of this band is decreased. The
results of the FTIR spectra (Fig 5.1.1a) suggest the formation of amorphous structure in starch
and decrease in the ordered structure of starch (Sevenou, et al., 2002).
The spectra of native and octenyl succinate modified starch showed several discernable
absorbencies at 1580, 1155, and 900 cm-1, which were attributed to C-O bond stretching (Fang
et al., 2004). Compared to native starch, the spectrum of octenyl succinate modified starch
shows a new peak at 1724 cm-1, the band at 2928 cm-1 is characteristic of the C-H stretching
vibration. Another characteristic peak occurred for octenyl succinte starch prepared using
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water as the solvent 1690 cm-1, which presumably originates from tightly bound water present
in the starch granule.
5.1.2 Physicochemical properties
For native starches, the physicochemical properties like swelling power, solubility, amylose
content, percentage light transmittance and water binding capacity are significantly related to
the average granule size of the starches separated from various sources (Singh and Singh,
2001).But for the starch derivatives, all these properties depend also on the type of reagent
used for modification, the amount of reagent used for particular modification and ultimately to
degree of substitution.
Swelling volume and solubility
The strong swelling power of starch granules makes it easy for them to reach their maximum
viscosity and they are likely to breakdown easily because of their weak intermolecular forces,
thus becoming more sensitive to shear force as the temperature increases. The swelling
volumes of the native and selected modified starches are shown in Table 5.1.1. The swelling
power and solubility values of the native starch were higher than those of heat moisture treated
starch. Heat moisture treatment causes reduction in the values from 40.8 to 24.3 mL/g and
24.7 to17.0% for swelling volume and solubility respectively. This reduction can be the result
of uncoiling double helices that may have been present in a crystalline array in the native
granule.
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Table 5.1.1: Physicochemical properties of modified cassava starches
Starch type Swelling
Volume
(mL/g)
Solubility
Total
amylose
WBC In vitro enzym
digestibility
Degree of
Substitution
(DS)
%
Native starch 24.8±0.36 24.7±0.45 24.73±0.4 79.3±0.19 32.7± 0.54 -
HM 20.3±0.83 17±0.24 25.3±0.06 87.42±0.11 41.5± 1.12 -
HP 12.25±0.1 7.1±0.579 7.6±0.95 55.7±0.05 27.3±.02 0.08±0.01
HPCL 7.5±0.10 4.008±0.19 3.03±0.20 45.4±0.19 20.2± 0.56 -
St. Acet. 27.3±0.32 29.37±0.37 25.7±0.11 92.8±0.84 24.5± 1.09 0.05±0.01
CMS 0.2 ±0.22 90.12±0.17 11.46±0.46 95.3±0.24 29.9± 0.87 0.17±0.02
Enz. modi. 9.4±0.13 79.5±0.23 28.4±0.23 97±0.14 67.2± 1.02 -
O.S DMF 37.5±0.12 48.8±0.19 9.75±0.28 85.5±0.18 32.1±.0.99 0.017±0.01
O.S WATER 15.41±0.72 23.9±0.36
12.80±0.26 75.6±0.12 31.1± 1.12 0.008±0.01
The decrease in solubility suggests that additional interactions may have occurred between
amylose-amylose and amylopectin-amylopectin chains during heat moisture treatment. This
may be also another reason for the reduction in swelling ability. Hoover and Maunal (1996)
and Hoover and Vasanthan (1994), have reported similar results using corn and potato
starches. Several researchers have reported that structural changes within the starch granules,
after HMT, might be responsible for the reduction in swelling capacity and starch solubility
(Leach et al,. 1959). Gunaratne and Hoover, (2002) and Hoover et al., (1994) also reported
structural changes within the amorphous region and crystalline regions of the starch granules
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such as starch chain interaction within the amorphous region and disruption and reorientation
of the starch crystallites caused by heat moisture treatment.
The aqueous solubility of the hydroxypropyl starch was significantly lower than that of the
native cassava starch. The values were decreased considerably by the hydroxypropylation to
12.25 ml/g for swelling volume and 7.1% for solubility. The decreased swelling power and
solubility of the hydroxypropylated starches have been attributed to the increase in
interactions between starch chains due to the introduction of hydroxypropyl groups. In
addition, inter and intra-molecular hydrogen bonds in the starch chains have become more
tightened, thereby the granular structure of the starch is strengthened and the motional
freedom of starch chains in amorphous regions decreases. It is reasonable that though increase
in starch hydrophilicity after hydroxypropylation facilitates water percolation into the starch
granules, the close architecture after hydroxypropylation denies the entry of water molecule,
thereby decreasing the swelling volume and solubility. Previous reports showed that
hydroxypropylation of native starch causes increase in the swelling power and solubility.
(Kaur et al., 2004, Kavitha et al., 1998). But opposite results were found in this study
probably due to the difference in the starch sources.
Hydroxypropylation and crosslinking (dual modification) lowered the swelling power and
solubility to 7.5 ml/g and solubility to 4.008 %. Cross linking of starch reduces the swelling
and increases the rigidity, whereas hydroxypropylation increases hydrophilicity. The ordered
structure of starch is disrupted when native starch is modified by cross linking and
hydroxypropylation. Complex interactions between these factors restrict the swelling power
and solubility of hydroxypropyl cross linked starch.
The acetylation increased the swelling power of cassava starch to 27.3 mL/g. This might be
due to the presence of hydrophilic substituting groups that retain water (Betancur et al., 1997).
Acetylation also increased the solubility to 29.37%. Following introduction of acetyl groups
on starch molecules, structural reorganization occurs as a result of steric hindrance and this
result in repulsion, thus facilitating an increase in water penetration within the granules with
subsequent increase in swelling capacity. Structural disintegration probably weakens the
starch granules after modifications, and this enhanced leachates from the starch increased
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starch solubility. Similar observations have been reported earlier for starches of rice (Gonzalez
and Perez, 2002, Liu and Corke, 1999), wheat (Wootton and Bamunuarachchi, 1979) and
great Northern Bean (Sathe et al., 1981).
Carboxymethyl starch exhibited increased solubility than the native starch forming viscous
solutions. Swelling power could not be determined in carboxymethyl starch sample because of
high solubility. Introducing the carboxymethyl group in its sodium form (–CH2COONa) to the
starch molecule increases its solubility. It is reported that the higher the DS, the higher the
solubility of the CMS (Kittipongpatana et al., 2006) In the present study, CMS with DS of
about 0.17 is completely soluble in cold water. This might be an indication of the superior
even distribution of the etherifying agents along the starch molecule in the initial stages of the
carboxymethylation. The enzyme modified starch followed the same pattern as the
carboxymethyl starch. Enzyme treatment leads to the reduction in the swelling volume from
24.5 to 9.4 mL/g and solubility increased from 24.8 to 79.5%.
Octenyl succinylation caused increase in swelling power (37.5 mL/g) and solubility (48.8%)
which may be due to introduction of bulky OSA groups as indicated by Perez et al,. (1993).
Similar behaviour was reported for succinylation of conavalia (Bentacur-Ancona, et al,. 2002)
and amaranth starches (Bhandari and Singhal, 2002). This is one of the benefits of octenyl
succinylation which allows to utilization of these starches in processes in which a thickening
agent must form gel at lower temperatures, or simply to reduce energy consumption during
cooking. But the OS starch sample prepared using water as the solvent showed reduction in
swelling volume and solubility values. This can be attributed to the retardation of the reaction
efficiency in the presence of water, and this result is also justified by the reduction in the
degree of substitution (0.008) for the OS starch prepared with water as the medium than the
OS prepared by DMF as the solvent medium (0.018). Thus the results confirm that most of the
modifications bring about reduction in swelling volume and solubility. CMS however showed
very low swelling volume and high solubility.
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Amylose contents
The data on amylose contents of the modified cassava starches are exhibited in Table 5.1.1.
During starch modification, the amylose molecule is modified more extensively than the
amylopectin moiety. Modification of amylopectin occurs close to the branch point,
presumably because amorphous region is more accessible to the modifying reagents. Analysis
of the data showed that there is significant variation in amylose content with respect to the
modification process.
Fig 5.1.2: Amylose content of modified cassava starches
As evident from the Fig-5.1.2, heat moisture treatment caused a slight increase in the amylose
content. For hydroxypropylated starch, there was a reduction in the values which were further
reduced by the hydroxyl propylated cross linked starch (3.03%). This may be due to the effect
of the dual modification. For the enzyme modified starch, the amylose content was increased
from 24.73 to 28.4%. The amount of native starch hydrolysis by amylases is reported to be
inversely related to the amylose content (Cone and Wolters, 1990, Vasanthan and Bhatty,
1996, Rendleman, 2000, Carre 2004, Evans and Thompson, 2004, Riley et al., 2004).
Acetylated cassava starches had slightly higher amylose content and increased from 24.73%
to 25.7%. Similar effects of acetylation on the amylose content of the starches have been
observed.
0
5
10
15
20
25
30
Swel
ling
volu
me
%
sample name
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earlier (Betancur et al,. 1997). The presence of acetyl groups has been reported to interfere
with the functioning of amylose and amylopectin fractions of starch and it affects the
absorption of iodine during amylose estimation (Whistler and Daniel 1995, Betancur et al,.
1997). The acetyl groups introduced in rice starch chains impeded the formation of the helical
structure of amylose in some areas, by sterical hindrance, and in consequence, formation of
amylose–iodine complex, which resulted in underestimation (Gonzalez and Perez, 2002).
Octenyl succinate substitution reduced the amylose content markedly (9.75%) for the sample
prepared in DMF but the OS starch prepared using water had higher value (12.80%). This
increase in value may be due to that presence of water retards the substitution of octenyl
succinate group into the glucose moiety. This reduces the reaction efficiency and increase in
the amylose content with respect to the moderately substituted sample prepared in DMF.
Water binding capacity (WBC)
A significant change was noticed in the WBC of cassava starch on modification. There was an
increase in WBC from 79.3% for the native starch to 87.42 % for the HMT starches (Table-
5.1.1). This implies that the hydrophilicity of cassava starch was increased with HMT. These
results are consistent with earlier reports on the water absorption properties of the heat-
moisture treated starches. Adebowale et al., (2005) reported that HMT linearly increased the
WBC of red sorghum starch, which implies that hydrophilic tendency increased with moisture
treatment. For the hydroxypropylated and hydroxypropylated-cross linked starches, the WBC
values were reduced after the modification that is 55.7, and 45.4% respectively. The WBC of
acetylated starch was increased from 79.3 to 92.8% for native starch. Introduction of bulky
hydrophilic groups in the starch molecules caused to imbibe more water leading to an
enhanced WBC. The carboxymethylation caused increase in the WBC (95.3%) values. The
enzymatic modification also increased the water binding capacity. The erosion of amylose
moiety by the enzyme causes more imbibing of water into the molecule ultimately ends in the
increases water binding capacity. During octenyl succinylation, the introduction bulky group
causes increase in the hydrophilicity of the starch molecule. This increased hydrophilicity is
reflected through the higher WBC. The OS starch prepared using the DMF as the medium
showed higher values (85.5%) then the counterpart OS starch prepared using water as the
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medium (75.6%).This may be due to the poor substitution in the presence of water as the
solvent.
In vitro enzyme digestibility
The in vitro ∞ amylase digestibility studies showed that the enzyme susceptibility of cassava
starch was increased by HMT (Table 5.1.1). It is also possible that during heat moisture
treatments, the slight swelling of the granule caused expansion of the naturally present
pinholes and internal cavities in starch granules, allowing the enzyme to penetrate easily into
the granules. According to Juszczak, et al.,(2003), pores present on starch surfaces could
become centers of enzymatic attack. The large increase in hydrolysis of heat-treated starch
could also be attributed to the effect of heat on the weaker areas on the starch granule,
allowing the enzyme to degrade the starch granules more extensively. In addition, it has been
found that heat–moisture treatment is effective in enhancing the adsorption of ∞ -amylase
(Kurakake et al., 1996).
Hydroxypropylation caused decrease in the enzyme digestibility of cassava starch (27.3%) and
for the dual modification where the hydroxypropylated starch was further subjected to cross
linking, the % digestibility values were again reduced. This is because the available OH
groups are substituted during the hydroxypropylation and cross linking, so the number of sites
for the action of ∞ amylase is reduced, finally resulting in the reduced % digestibility.
The acetylated derivatives possessed lower enzyme digestibility when compared to the native
starch. The % digestibility after 30 min of incubation of the starch with amylase was 32.7 %
for the native starch, whereas it was only 24.5% for the acetylated starch. Results proved that
acetylation decreased the percentage of released reducing sugar resulting from ∞ amylase
hydrolysis. During carboxymethylation, the % digestibility was reduced to 29.9, but on the
other hand the enzyme digestibility of enzyme modified starch (67.2%) was higher than the
other starch derivatives. During the production of enzyme modified starch, the amylase
enzyme causes degradation of starch molecule to small molecules like maltodextrins, and
these small molecules of sugars can contribute the higher % digestibility. The % digestibility
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values of OS modified starches were not significantly different from that of the native starch
(32.1 & 31.1%). This could be due to the low substitution level in the derivatives.
5.1.3 Retrogradation studies
Percentage light transmittance
The light transmittance of the gelatinized starch pastes of different modified cassava starch
differed considerably (Table 5.1.2).The light transmittance of all the starch pastes decreased
progressively during refrigerated storage. However, this decrease was more pronounced
during the initial 48h. HMT brought about a reduction in % transmittance from 26.6 to 15.6%.
The compression of the starch granules as a result of HMT leads to lower light transmittance
of the starch paste and consequently there is a decrease in paste clarity (Moorthy et al., 1996).
Similar time-dependent reduction was reported for HMT banana starch.
When starch pastes of hydroxypropylated derivatives were stored for 5 days, the results
obtained showed that the starch turbidity increased as indicated by the reduction in percentage
transmittance (15.01 to 10.25%). For the dual modified starch also, the paste clarity was
decreased on storage and the values were higher than those of the native starch.
Acetylation and enzyme modification produced the most marked decrease in % transmittance.
The light transmittance of acetylated starches decreased during storage from 21.2% to 5.12%.
This may have occurred due to lower levels of retrogradation that prevented the aggregation of
amylose and amylopectin in the starch pastes (Singh et al., 2002).
The % transmittance values of enzyme modified starch showed the maximum values. The
higher degradation of starch caused the higher transmittance of 97.3%, and on storage, the
values were decreased predominantly to 39.9%. In this sense, limited turbidity development in
modified starches lends credence to the fact that higher crystallinity of starch granules and
these developments lead to restriction of starch particles aggregation. This is because higher
levels of crystallinity restricted granule swelling and the amount of leached amylose and
amylopectin.
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Table 5.1.2 % Transmittance of modified Cassava starches
Carboxymethylation also showed the same trend of reduction that is from 9.41 to 4.63%.
Increase in turbidity during storage is due to the interactions between leached amylose and
amylopectin chains that lead to the development of junction zones, which reflect or scatter a
significant amount of light.
Compared to other modified cassava starch derivatives, except enzyme modified starch,
octenyl succinate starch which is prepared in dimethyl sulphoxide showed higher
transmittance of 24.22%. The changes to the granular and molecular structure induced by
octenyl succinylation facilitated better penetration and absorption of water within starch
granules which ultimately lead to more swelling of starch and resulted in more transmittance
of light (Pal et al,. 2000). Bhandari and Singhal (2002) have reported increase in paste clarity
for succinylated derivative of amaranth and maize starches. Increase in paste clarity with
decrease in DS of OSA modified indica rice starch was reported by Song et al., (2006).
Improved paste clarity is a useful property in the manufacture of some foods like jellies,
sausages and fruit pasted which require transparency (Jyothi et al., 2005). The paste clarity of
sample
Transmittance (%)
Number of days
Native starch 35.9 31.35 28.9 25.2 20.1
HM 26.6 20.4 16.1 15.8 15.6
HP 15.0 5.15 16.5 15.5 10.2
HPCL 45.1 40.2 39.2 39.1 39.0
St.Acet. 21.2 15.3 12.1 9.0 5.12
CMS 9.41 8.50 8.28 7.96 4.63
Enz.modi. 97.3 90.8 70.9 47.8 39.9
O.S DMF 24.22 6.63 6.38 6.38 5.92
O.S WATER 5.06 4.93 4.91 4.7 4.51
5. Modified cassava starches …..
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O.S DMF has the minimum value of 5.92% whereas the O.S WATER showed very lower values
compared to all other modified starch derivatives i.e., 5.06 to 4.51%. This is because starch
granule dissociates and ability of the granules to reflect light diminishes (Craig et al., 1989).
Factors such as granule swelling, granule remnants, leached amylose and amylopectin,
amylose and amylopectin chain lengths, inter or intra-molecular bonding, presence of lipids,
cross linking and substitution have been reported to be responsible for turbidity development
in starches during storage. The turbidity values of all the starch pastes from corn fractions
increased progressively during storage and this has been attributed to the interaction between
leached amylose and amylopectin chains that led to the development of functional zones,
which reflect or scatter a significant amount of light (Pereara and Hoover 1999). Amylose
aggregation and crystallization have been reported to be complete within the first few hours of
storage, while amylopectin aggregation and crystallization occurs during later stages. (Miles et
al.,1985).
Least concentration gelation (LCG)
A starch gel is composed of swollen granules, because the amorphous region hydrates and
swells to a gel phase, during the process of heating. The least gelation concentration is used as
the index of gelation. Fig 5.1.2 shows the effect of concentration on the gelation capacity of
native and modified starches. Native starch did not form a gel until it reached 8%
concentration. Gel formation in starches involves swelling and hydration of starch granules,
which occurs predominantly in the amorphous region of starches, and gel strength depends on
strength of intra-granular binding forces within swollen starch in granules. It is reasonable that
following heat moisture treatment, structural reordering and realignment of portions of
amylose and amylopectin enhanced intra-granular binding forces, which improved gel
strength. This explains reductions in LGC following the modifications.
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Fig 5.1.3: Least Concentration of Gellification (LCG) of modified cassava starches
Hydroxypropylation caused a slight variation in the LCG values. The values were reduced to
6.5% for hydroxypropylation whereas for the dual modified starch, it was 7.5%. Acetylation
caused a reduction in LCG values from 8 to 6%. The introduction of acetyl groups during
modification causes inter-molecular repulsion in the starch gel, which accounts for weaker
gels. Acetyl group substitution on starch molecules hampers these intra-granular forces of
interactions by replacing the OH groups on the glucose units, thus limiting formation of strong
gels as compared with those of native starch. It is also reasonable that intra and inter-
molecular electrostatic repulsion after introduction of acetyl groups reduced gel cohesion, thus
resulting in weaker gels.
The carboxymethylation of cassava starch led to an increase in the LCG values from 8 to 11%,
whereas the bio modified starch showed maximum LCG value (15%). During octenyl
succinylation using the DMF the LCG values showed slight reduction to 6.5% but for the OS
starch prepared using water did not show significant variation (8%) and this may be due to the
reduction in the reaction efficiency of the system in the presence of water as the solvent.
0 5 10 15
Native starch
HM
HP
HPCL
St.Acet.
CMS
O.S DMF
O.S WATER
enz.modi.
% LCG
sam
ple
nam
e
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5.1.4 Thermal properties
When dry native starch granules are suspended in a sufficient amount of water and heated,
they absorb water and swell to several times. Continued heating results in a loss of X-ray
crystallinity order as judged by loss of birefringence and crystallinity and as a consequence,
amylose leaches out of the granules (Ellis and Ring 1985). The enthalpy of gelatinization
reflects the loss of molecular order and gelatinization temperature is considered a parameter of
crystalline perfection. Because amylopectin plays a major role in starch granule crystallinity,
the presence of amylose lowers the melting temperature of crystalline regions and the energy
to start gelatinization
Table-5.1.3: Gelatinisation parameters of modified cassava starches
Gelatinization parameers and the enthalpies associated with gelatinization of modified starches
are given in Table 5.1.3. Gelatinization temperatures, gelatinization band and enthalpy of
gelatinization of heat moisture treated starches increased. The onset (To) decrease after HMT
from 63.13 to 60.12°C. The Peak temperature (Tp) often referred to as the gelatinization
temperature of the native cassava starch was 73.92oC and after HMT, the peak temperature
values increased to 76.21°C.The endotherms were shifted to a higher temperature with a
Sample
Gelatinisation Temperatures (°C)
∆H (J/g)
To Tp Te
cassava 63.13±0.04 73.92±0.11 79.51±0.12 13.69
HM 60.12±0.06 76.21±0.19 80.02±0.15 11.43
HP 66.02±0.03 72.08±0.01 79.44±0.12 10.10
HPCL 69.55±0.01 70.75±0.20 83.69±0.11 10.08
St.Acet. 61.42 ±0.05 70.13 ±0.32 79.26 ±0.63 10. 22
CMS 68.53±0.27 72.32±0.12 79.52±0.11 12.79
Enz.modi. 62.12±0.33 74.5 ±0.17 80.12±0.31 11.64
O.S DMF 65.86±0.01 69.5±0.03 74.23±0.01 9.36
O.S WATER 64.69±0.03 68.73±0.02 74.03±0.01 10.71
5. Modified cassava starches …..
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broader shape and the peak areas were unchanged or slightly decreased. The broadened peaks
of the HMT starches indicated greater inhomogeneity in structural organization of amylose
and amylopectin within the granules. As compared to the starch gel morphology, this
inhomogeneous nature may be reflected by the non-uniform staining of gelatinized starch
granules. The differences in the degree of heterogeneity among the starches have been
ascribed to the interplay of many factors: eg. molecular structure of amylopectin, amylose to
amylopectin ratio, crystalline to amorphous ratio and phosphorus content (Gunaratne and
Hoover, 2002). The gelatinization enthalpy decreased during HMT from 13.69 to 11.43J/g.
similar results were observed for HMT sweet potato starch by Collado and Corke (1999) and
Collado et al., (2001). The endotherm was interpreted as revealing the intrinsic stability and
heterogeneity in size and perfection of crystalline region in granular starches (Zobel, 1992).
Starch chain associates within the amorphous region and degree of crystalline order were
altered during HMT. The magnitude of these changes depends on the starch source (Hoover
and Vasanthan 1994).
After hydroxypropylation, decreases were recorded for gelatinization temperatures and
gelatinization enthalpy. There was a slight increase in the To values (63.13 to 66.02°C), while
the Tp value showed a decrease to 72.08°C (Table-5.1.3). The lower Δ H of
hydroxypropylated cassava starch (10.1 J/g) suggested a lower percentage of organized
arrangements or a lower stability of crystals. As indicated by Perera et al., (1997)
hydroxypropyl groups disrupt double helices (due to rotation of the flexible hydroxypropyl
groups) within the amorphous regions of the granules. Introduction of the bulky
hydroxypropyl groups on the polymer backbone facilitates structural flexibility resulting in
reduction of the gelatinization temperature. The enhanced structural flexibility also accounts
for reduced enthalpy of gelatinization of the starch after hydroxypropylation. The increase in
the gelatinization range could be due to the increased inhomogeneity within the starch
granules (Seow and Thevamalar, 1993). Jenkins and Donald (1998) and Liu et al., (1999) have
reported the same results earlier in wheat starch.
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Fig 5.1.4: DSC thermogram of Hydroxypropylated Cassava starch
Hydroxypropylation cum cross-linking caused a reduction in Tp and ΔH values but the To and
Tc values were increased (Fig-5.1.4). The To values were increased to 69.55°C whereas the Tc
values increased to 83.69°C. Rutenburg and Solarek (1984) reported that the formation of
cross linkages retains the integrity of starch granules, thus, more heat was needed for
gelatinization. The Tp values (70.75°C) and enthalpy of gelatinization (10.08 J/g) were
reduced after dual modification process Decrease in thermal parameters is consistent with
fewer crystals being present after modification with a cooperative melting process (Jenkins
and Donald 1998, Liu et al,. 1999, Nurul Islam and Mohd Azemi, 1994, Rutenburg and
Solarek, 1984, Seow and Thevamalar,1993).
Acetylation influenced the onset temperature (To), peak temperature (Tp), concluding
temperature (Tc), and the enthalpy of gelatinization (∆H). On acetylation, the T₀ values were
lowered to 61.42°C and peak temperature (Tp) to 70.13°C respectively, the ∆H values also
showed lower values (10.105 J/g). This may be attributed to the presence of hydrophilic
substitution groups and increase in hydrogen bonding in starch molecules, which favored
gelatinization at low temperature. These results are in agreement with those reported by
5. Modified cassava starches …..
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Wootton and Bamunuarachchi, 1979 (1979). They suggested that introduction of acetyl groups
to the polymer chains resulted in destabilization of granular structure, thus causing increase in
swelling and decrease in gelatinization temperature. Eliasson, et al., (1988) also reported that
acetylation of high amylose corn starch caused the gelatinization temperature of starch to
decrease from 74.6 to 72.1°C.
DSC thermograms of native and carboxymethylated starches (Table 5.1.3) showed that was a
slight decrease in the Tp value which reduced to 72.32°C. The ∆H values also showed the
same pattern, reduced to 12.79 J/g. This change can be explained by the inter-molecular
hydrogen bonds, which stiffen the macromolecular chain, which decreased with the partial
replacement of hydroxyl groups by carboxymethyl group. The increase in the free volume
within the molecules due to the introduction of bulk groups that allows more molecular
mobility also contributes to the reduction in ∆H of starch with carboxymethylation. However,
the processes involved in carboxymethylation tremendously affected the starch crystallinity,
thus making the granules largely amorphous. Similar results were reported in Valetudie, et al.,
(1995) Collado et al., (1999), Jane, et al., (1992).
Fig 5.1.5 : DSC thermogram of enzyme modified cassava starch
5. Modified cassava starches …..
131
During the enzyme modification process, the change in the onset, peak and end set
temperature was not that much significant (Fig-5.1.5). The DSC thermogram was shifted
towards right with small reduction in the ∆H values (11.64 J/g) (Table-5.1.3). This indicates
that a clear relationship exists between gelatinization temperature and susceptibilities to
amylase attack. Crystalline arrangement of the starch granule plays an important role in its
susceptibility to ∞-amylase attack.
The effect of octenyl succinylation on native cassava starch was to shift the endotherm (Fig
5.1.6) to a lower temperature and reduce ΔH from 13.6 to 9.36J/g for O.S DMF and 10.71 for
O.S WATER samples.
Fig 5.1.6: DSC thermogram of Octenyl succinate starch
Bao, et al., (2003) reported that internal bonds of starch granules are weakened by
incorporation of hydrophobic alkenyl group, allowing the starch to swell at a lower
temperature. The Tp and Tc values of O.S DMF were 69.5 and 74.23 °C, whereas for O.S WATER
samples, these were 68.73 and 74.03°C respectively. T₀, Tp, Tc values of modified starches
were found to be comparable to the earlier reports (Baker and Rayas-Durate, 1998; Wotton
and Manatsathit, 1984; Wotton and Bamunuarachchi, 1979a, 1979b). The lower gelatinization
5. Modified cassava starches …..
132
temperature and enthalpy were due to the weakening of hydrogen bonding by the hydrophilic
alkenyl groups, helping starch to swell at lower temperature and hence gradually decreasing
the enthalpy of OS starches ( Bao et al., 2003; Rutenberg and Solarek, 1984). Introduction of
bulky OSA groups in to the backbone of the bio-polymer enhances structural flexibility and
contributes to the reduction of gelatinization temperature of modified starches (Lawal 2004).
Additionally, the OSA effect on the gelatinization temperature of starch is dependent on the
starch base and degree of substitution (Bao et al.,2003 Miller et al., 1991). At molecular
level, this may be expected to involve the cleavage of existing hydrogen bonds between starch
molecules and formation of new bond involving water to give less order structure with
increased entropy (Paton, 1987). The effect can be explained as the weakening of hydrogen
bonding by the hydrophobic octenyl succinyl groups, helping starch at relatively lower
temperature. Similar trend in thermal properties of OSA starches has been reported earlier
(Bao et al., 2003, Shih and Daigle, 2003).
5.1.5 Pasting properties
When a starch granule is heated in excess water, it leads to further granule swelling, additional
leaching of soluble components and total disruption of granules. This process results in the
formation of a viscous starch paste. The viscosity of starch, as a food component is a vital
factor for consideration in its applicability to food systems. Pasting parameters of native and
modified starches are presented in Table 5.1.4. and Fig.5.1.7. The result indicates that pasting
temperature shifted to higher values following heat moisture treatments.
After heat moisture treatment (HMT), there was a very slight breakdown (182cP.) and an
increase in FV (1409 cP), more like a Type C pasting profile. The low breakdown in the
viscosity showed that the granules were quite strong and resisted the breakdown under shear
and heat. The high viscosity with a very low breakdown is a desirable property of the starch
because its paste has a non-cohesive texture suitable for many food and industrial applications.
A similar trend was observed in Taiwanese sweet potato starches (Collado and Corke, 1999)
and potato starches (Stute 1992).
5. Modified cassava starches …..
133
Table 5.1.4: Viscosity parameters of modified cassava starches
The HMT may make the granules resistant to deformation by strengthening the inter-granular
binding force and it was speculated that in the annealed starch swollen gelatinized granules
were more rigid, contributing significantly to high final viscosity (Stute, 1992). The setback
values were significantly increased after HMT (1160 cP). The strengthening of intra-granules-
bonded forces allow the starches to require most heat before structural disintegration and paste
formation occurs (Eliasson 1980). Setback value is a measure of retrogradation tendency,
which appear to be related to the structure of amylose and amylopectin, since small amylose
molecules tend to be retrograded rapidly. Pasting temperatures of the starch samples increased
with increasing moisture content as well. The native starch had pasting temperature of 69.3oC
whereas that of HMT had value of 79.4oC. Increase in pasting temperature after HMT was
consistent with most of the other starches as reported for cocoyam starch by Lawal (2005), as
well as lentil, potato and yam starches by Hoover and Vasanthan (1994). They claimed that
structural rearrangement contributed to these changes. The extent of starch chain associations
within the amorphous regions and the degree of crystalline order are altered during HMT. The
magnitude of these changes is dependent upon the moisture content of the starch sample.
Sample Viscosity parameters (cP) Pasting
Temperature
(°C) PV BD FV
SB
Native starch 2489.3 ±0.11 1487.0 ±0.45 1409.3 ±0.50 406.0±0.35 69.3±0.50
HM 2508±0.16 182±0.26 3481±0.51 1160±0.31 79.4±0.60
HP 108.5±0.7 97.0±0.1 269±0.071 178±1.1 -
HPCL 16.5±0.12 24.0±0.01 5±0.02 8.5±0.35 -
St.Acet. 2567±0.1 1819±0.2 2557±0.1 738±0.0 76±0.5
CMS 89±0.18 22±0.15 22±0.15 34.5±0.35 50.45
Enz.modi. 1314±0.1 318±0. 5 1646±0.21 198±0.11 63.4±0.30
O.S DMF 139.5±0.35 25±0.4 180.5±0.20 66±0.12 Err
O.S WATER 1300±0.21 337±0.12 1708±0.01 745±0.2 75.90
5. Modified cassava starches …..
134
The change in the starch properties due to hydroxypropylation has been observed to be
associated with the amylose content and granule morphology of native starch. The larger the
granule size, the higher is the extent of hydroxypropylation in starches. The study shows that
pasting parameters reduced remarkably after hydroxypropylation. The peak viscosity was
reduced to 108.5 cP, and the final viscosity to 269.cP The setback value which was a measure
of retrogradation was reduced (178 cP ) after hydroxypropylation as a result of the prevention
of structural realignment of starch molecules after gelatinization.
Fig 5.1.7: RVA pasting profile of modified cassava
The change in the starch properties due to hydroxypropylation has been observed to be
associated with the amylose content and granule morphology of native starch. The larger the
granule size, the higher is the extent of hydroxypropylation in starches. The study shows that
pasting parameters reduced remarkably after hydroxypropylation. The peak viscosity was
reduced to 108.5 cP, and the final viscosity to 269.cP The setback value which was a measure
of retrogradation was reduced (178 cP ) after hydroxypropylation as a result of the prevention
of structural realignment of starch molecules after gelatinization.
Cross-linking of the hydroxypropylated starch further reduced the pasting properties. From the
Fig- 5.1.7, it was evident that among all the modified cassava starches, the, HP-CL starch
showed the least values for all the pasting parameters, ie the peak viscosity was reduced to
16.5cP, the breakdown was reduced to 24.0cP and the final viscosity was predominantly
0
500
1000
1500
2000
2500
3000
3500
400050
.15
60.1
571
.75
83.0
594
.45
95.0
595
.05
92.3
80.9
70.2
59.3
50.6
549
.8
VIS
COSI
TY [c
P]
TEMPERATURE [OC]
CASSAVA
HP
HPCL
CMS
St.Acet.
En.Modi.
HM
5. Modified cassava starches …..
135
shifted to 5cP. The hydroxypopylation cum cross linking made the starch highly heat stabile in
such a way that it resists swelling even at 100oC.
Acetylation caused increase in the peak, and final viscosity to 2567cP and 2557 cP from 2489
cP and 1409.cP respectively. (Table 5.1.4). Acetylation influences the interaction between
starch chains by steric hindrance, altering hydrophilicity and hydrogen bonding and resulting
greater swelling of granules, and increased peak viscosity (Liu, et al,. 1997, Sandhu et al,.
2004). Increases in viscosity values after succinylation (Bhandari and Singhal, 2002) and
acetylation (Gonzalez and Perez, 2002; Liu and Corke, 1999; Liu and Ramsden, 1997) have
been reported. The Breakdown Value which is a measure of fragility of the starch was
increased to 1819cP. Following acetylation, the modified starches become partially degraded
and this partially degraded network is not resistant to shear and cannot maintain the integrity
of the starch granules. This accounts for the higher breakdown value observed in starch
acetate. The pasting temperature and setback values were increased for the starch acetate
compared to native starch (76 oC and 738 cP respectively).
Carboxymethylation also caused decrease in the pasting parameters. The peak viscosity was
reduced to 89cP and final viscosity to 22cP. The pasting temperature was reduced to 50.45cP
compared to 69.3cP for the native starch. The breakdown and setback were reduced to 22 and
34.5 cP respectively. The decrease in the viscosity of the carboxymethylated starch can be due
to introduction of the solubilizing carboxymethyl group in its sodium form. It is also possible
that oxidative degradation might have taken place due to the presence of starch has been
decreased.
During enzymatic conversion , the peak viscosity of the starch was decreased (1314cP), but
the final viscosity increased to 1646 cP. The enzyme hydrolysis of starch molecule in solution
reduces the viscosity during the cooking process, but the reorientation of smaller chains during
the cooling process is faster than the unmodified starch (Fig-5.1.7). This may be reason for the
decrease in the peak viscosity and increase in the final viscosity. The pasting temperature was
also reduced to 63.4⁰ C. This indicates a marked decrease in solid properties due to enzyme
action, which leads to decrease in the overall resistance of the sample to flow. The breakdown
and setback was also reduced.
5. Modified cassava starches …..
136
During octenyl succinylation, the incorporation of bulky groups such as OSA alters the overall
pasting capacity of the starches and the modified starches tended to paste more extensively.
Comparing the values of the OS starch prepared by the two solvent systems, the pasting
properties showed higher values for the O.S WATER samples than the O.S DMF. The peak
viscosity and final viscosity showed significant differences in the two samples viz., 1300 and
139.5cP and 1708 and 180.5 cP respectively. The pasting temperature was increased to 75.90
⁰C for the O.S WATER sample which is much higher than the native starch.
5.1.6 Rheological properties
Flow curve
To describe the flow behaviour of modified starch dispersions at 10% concentration, the
viscosity (v) as a function of the shear rate (g ) was plotted . The flow curves (viscosity /shear
rate and shear stress /shear rate) of modified cassava starches at 25oC are presented in Fig.
5.1.8 and 5.1.9 and the viscosity values at different shear rates are presented in table-5.1.5. As
can be seen from Fig 5.1.8, modified starch dispersions exhibit a non- Newtonian
pseudoplastic-type behaviour (n<1) which is characterized by a decrease in viscosity by
increasing in shear rate.
Fig 5.1.8: Flow curve of modified cassava starches (Shear rate Vs viscosity)
-50
0
50
100
150
200
250
300
2
78.8
156
232
309
386
463
539
616
693
770
846
923
1,00
0
Visc
osity
[Pa-
s]
Shear rate [1/S]
Cassava
HP
HPCL
StAcetate
CMS
HM
En.modi.
5. Modified cassava starches …..
137
The plots denote a non- Newtonian behaviour because although flow was not uniform, the
relationship between shear stress and shear rate is not constant (Rao and Steffe, 1992).
Table-5.1.5: Viscosity (cP) of different modified starches at various shear rates.
Sample Shear rate (1/S)
2 258 524 744 1000
Native starch 71.4 2.98 1.94 1.55 1.29
HM 114 3.95 1.91 1.51 1.27
HP 225 2.99 1.52 1.68 1.5
HPCL -1.77 -0.0424 -0.0433 -0.037 -0.0281
St.Acet. 275 8.05 5.64 4.18 3.25
CMS 167 0.893 0.673 0.0517 0.00583
Enz.modi. 3.88 0.517 0.382 0.321 0.284
O.S DMF 73.9 0.346 0.403 0.54 0.509
O.S WATER 0.0947 0.079 0.0515 0.0426 0.0214
This type of behaviour has been reported for dispersions of starch from other botanical sources
also (Tecante and Doublier, 1999,Thebaudin et al., 1998). It was observed that starch
dispersions were less sensitive to the shear stress, since bigger values were required to make it
flow (Table-5.1.6). The yield stress values were high for the carboxymethyl starch (840.46
Pa) whereas it was very low for the enzyme modified starch (3.0875 Pa). But the infinite shear
viscosity was high for the starch acetate (5.694 Pa.S) and low for O.S WATER sample. Yield
stress is another important rheological parameter for predicting the product’s processing
and/or end use performance.
5. Modified cassava starches …..
138
Table 5.1.6: Casson yield stress and infinite shear viscosity of values of modified cassava
starches
Sample
Casson
Yield stress (Pa) Infinite shear
viscosity (Pa.s)
Native starch 199.99 0.8705
HM 261.38 1.3841
HP 377.42 3.472
HPCL - -
St.Acet. 495.53 5.694
CMS 840.46 -
Enz.modi. 3.0875 0.5273
O.S DMF 138.72 -
O.S WATER - 0.325
The stress level required to initiate flow is usually referred to as yield stress and is related to
the level of internal structure in the material, which must be destroyed before flow can occur.
As shown in Table 5.1.6, it is evident that different modified starches showed different yield
stress and infinite shear viscosities. . This may be attributed to the increased nonspecific
interaction between the particles and the crowding due to elimination of solvent at high
concentration (Rha, 1978). Amylose outside the granules forms a three-dimensional network
whose structure is determined by the starch concentration, the structure of the swollen
granules, the ratio amylose/amylopectin, the proportion of solubilised amylose, as well as by
the method of pastes preparation (Cheng, et al., 1996); taking into account these factors and
considering the swelling and solubility patterns of both the results showed characteristic shear-
thinning behaviour (Steffe, 1996). Doublier et al. (1987) described that the pastes of starch are
mixtures of three fractions: (a) soluble macromolecules (amylose), (b) finely dispersed
particles (amylopectin) obtained after the breakage of the swollen granules during the
liberation of amylose when shaking is enough, and (c) solid particles corresponding to
fragments of swollen granules. All starch derivatives showed low magnitudes of Casson yield
5. Modified cassava starches …..
139
stress in the range of 3–850 Pa. Therefore, modified cassava starch dispersions at 10-%
concentration were highly shear-thinning fluids with moderate magnitudes of yield stresses,
Thixotropy
The existence of thixotropy means that the flow history is important in the prediction of
viscosity in processes such as mixing, flow through pipes, centrifugation etc. where viscosity
continues to change for a long time. For the dispersions with 10% solids, certain thixotropic
behaviour is observed, since shear stress is not only related to shear rate, but also to time; this
is confirmed by the formation of a curl when descending on the shear rate values,
phenomenon named as hysteresis (process of deformation in which the load phase and its
discharge phase do not coincide), which is particular for each starch source (Rao, and Steffe
1992, Tecante and Doublier, 1999).
Table 5.1.7: Thixotropy of modified cassava starches.
Sample Structure Recovery
Ratio(%) after 60 s
Native starch 59.672
HM 61.741
HP 62.074
HPCL -
St.Acet. 50.817
CMS 39.769
Enz.modi. -
O.S DMF 31.232
O.S WATER 55.141
On the basis of this thixotropic behaviour, the % recovery ratio were calculated. Among all
the modified cassava starches, HP starch showed the maximum, and the order of percentage
recovery was as following HP> HM> St.Acet> O.S WATER> CMS> O.S DMF
5. Modified cassava starches …..
140
Dynamic rheology
Because gels are viscoelastic materials, dynamic rheological tests to evaluate properties of gel
systems are well suited for studying the characteristics of gels as well as gelation and melting .
From dynamic rheological tests in the linear viscoelastic ranges, the storage modulus, G’, and
the loss modulus G”, and tanδ= (G”/G’), the loss factor can be obtained. G’ values are a
measure of the deformation energy stored in the sample during the shear process, representing
the elastic behaviour of a sample. In contrary, G” value is a measure of the deformation energy
used up in the sample during the shear and lost to the sample after wards representing the
viscous behaviour of a sample. If G’ is much greater than G”, the material will behave more
like a solid. i.e. the deformation will be essentially elastic to recoverable. However, if G” is
much greater than G’, the energy used to deform the material is dissipated and the materials
behaviour is liquid like (Rao, 1999.) On the other hand, the loss factor or damping factor
reveals the ratio of the viscous to the elastic portion of the deformation behaviour. A phase
angle δ=0° or tanδ=0 corresponds to an elastic response and δ=90° or tanδ=∞ is a viscous
response. If the phase angle is within the limits of 0<δ<90° the material is called viscoelastic
(Schramm, 1994, Steffe 1996, Mezger 2002.) Moreover the complex viscosity η*=G*/ω is
another useful parameters where ω is the frequency of oscillation (Rad sec-1) and G*
=�����2 � ��"�2
Frequency sweep analysis
Dynamic frequency sweep tests were done in the limit of linear viscoelastic region to
determine the frequency dependency of elastic and viscous moduli. The viscoelastic
behaviour was obtained over a frequency of 0.1–10 Hz. Fig10 shows changes in storage
modulus (G’), loss modulus (G”), and complex viscosity as a function of frequency
The magnitudes of G’ and G” increased with increase in frequency with the high frequency
dependency. Among the different modified starches in the whole range of frequencies, G’ was
greater than G” for HP, starch acetate and CMS suggesting a solid elastic-like behaviour.
5. Modified cassava starches …..
141
Fig 5.1.9: Frequency sweep of Cassava starches (storage modulus/frequency)
5.1.10: Frequency sweep of Cassava starches (loss modulus/frequency)
G’ was also strictly frequency-dependent and increased with increasing frequency, while for
G” this effect was evident only at higher frequencies. For heat moisture treated starches, the
G’ values are lower than that of G” and with increase in the frequency, the G’ values reduced
but with increase in the frequency the G” increased. The tanδ values tended to move toward
90⁰ which means that the gel is more toward liquid state.
0
1000
2000
3000
4000
5000
6000
0.1
0.14
7
0.21
5
0.31
6
0.46
4
0.68
1 1
1.47
2.15
3.16
4.64
6.81 10
Stor
age
mou
lus
[Pa]
Frequency [Hz]
Cassava
HP
HPCL
StAcetateCMS
0
500
1000
1500
2000
2500
0.1
0.14
7
0.21
5
0.31
6
0.46
4
0.68
1 1
1.47
2.15
3.16
4.64
6.81 10
Loss
mod
ulus
[Pa]
Frequency Hz]
cassava
HP
HPCL
StAcetate
5. Modified cassava starches …..
142
Fig 5.1.11: Frequency sweep of Cassava starches (phase angle /frequency)
In hydroxypropylated starches, the G’, G” and tanδ varied considerably. Rheological
behaviour of the native starch pastes was observed to be strongly influenced by the granule
size distribution, granule shape and amylose content (Kaur et al, 2002; Morikawa and
Nishinari, 2002; Singh et al, 2003; Singh and Singh, 2001, 2003). The hydroxypropylation
resulted in increased peak G’, G” and decreased tanδ of the starch pastes (Fig 5.1.9, 5.1.10,
5.1.11 and Table 5.1.8). The increase in peak in G’ and G” occurs due to the decrease in
associative forces within the starch granules. The reduction in the tanδ of hydroxypropylated
starch also suggests that G’ increases more strongly than G”. Morikawa and Nishinari (2000)
also indicated that the G’ value of the hydroxypropylated potato starch pastes was primarily
governed by the volume fraction of the granules induced by heating. However, the G’ slope
(Table 5.1.9) of the hydroxypropylated starch pastes is much higher than that of native starch,
showing that the elastic properties of starch pastes can be decreased by hydroxypropyaltion.
Differences in the value of slopes of G’, G”, complex viscosity, phase angle versus frequency
are summarized in Table 5.1.9. Acetylation of starch also brings about significant variation
in rheological parameters. The G’ was greater than G” and the tanδ value was reduced to
31.7Pa from the 90 Pa of native starch. Betancur-Ancona et al., (1997) studied the rheological
properties of acetylated Canavalia ensiformis (jack bean) starch and reported a substantial
increase in the apparent viscosity upon acetylation. A similar enhancement viscosity has been
reported by Sathe and Salunkhe (1981) for acetylated Phaseolus vulgaris (haricot bean) starch.
0
20
40
60
80
100
0.1
0.14
7
0.21
5
0.31
6
0.46
4
0.68
1 1
1.47
2.15
3.16
4.64
6.81 10
PHA
SE A
NG
LE [
o ]
FREQUENCY [Hz]
cassava
HP
HPCL
StAcetate
CMS
HM
En.modi.
5. Modified cassava starches …..
143
From the results, it is evident that the starch acetate is strictly having a solid elastic behaviour
compared to native starch. Carboxymethylation also has same effect on the cassava starch,
here also G’ was greater than G” and the tanδ value was again reduced to 13.10 leaving the
same result as acetylated starch i.e. the behaviour of CMS is more toward the solid state.
Table 5.1.8: Dynamic moduli of modified cassava starches at different frequencies
Sample
Frequency 10 (Hz)
Storage
modulus (Pa)
Loss
modulus (Pa)
Complex
viscosity (Pa)
Phase angle
( 0)
Native starch 0.00525 105 1.67 90
HM 0.00682 137 2.17 90
HP 5290 1920 89.5 20
HPCL 0.0134 268 4.26 90
St.Acet. 891 551 16.7 31.7
CMS 3890 909 63.6 13.1
Enz.modi. 0.0142 285 4.53 90
O.S DMF 144 136 3.15 43.5
O.S WATER 0.0147 295 4.69 90
Octenyl succinylation of cassava starch using DMF as the solvent medium makes cassava
starch more solid-like but the interesting fact is that the sample prepared using water as the
medium makes the starch more liquid-like. The G’ values of O.S DMF sample was increased
with increasing frequency and it is greater than the G”, but O.S WATER sample behaves in the
opposite manner. The tanδ also showed the same pattern O.S DMF has the value of 43.50 at
10Hz frequency, whereas the O.S WATER has the value of 90 at 10Hz frequency. The slope
values were not significant.
5. Modified cassava starches …..
144
Table 5.1.9: Slope values of modified cassava starches
Temperature sweep measurements.
The dynamic storage modulus (G’) is a measure of the energy stored in the sample. The G’ of
starch progressively increases to a maximum (peak G’) at a certain temperature and then drops
with continued heating in a dynamic rheometer. The initial increase in G’ can be attributed to
granule swelling. Granules may swell to fill the entire available volume of the system
(Eliasson, 1986), and inter- granular contact might then result in the formation of a three-
dimensional network of swollen granules (Evans and Haisman, 1979; Wong and Lelievre,
1981). With further increases in temperature, G’ decreased, indicating that the gel structure is
destroyed (Lii et al., 1996 a). This destruction is due to the melting of crystalline regions
remaining in the swollen granules, allowing the granules to deform (Eliasson, 1986). The
rheological properties of modified starches exhibit significant differences from those of native
starches when subjected to temperature sweep testing.
On HMT of cassava starch, the storage modulus (G’) increased with temperature as starch
granules kept swelling at temperature above To (Fig 5.1.12 ). Thereafter, starch granules
sample SM slope [Pa / 1/s] LM slope [Pa / 1/s] CV slope[Pa·s / 1/s]
Cassava -1.83771 0.41537 -0.72263
HM 0.17184 0.21167 -0.81568
HP 0.15845 0.19193 -0.83812
HPCL -0.40397 1.8214 0.7026
St.Acet. 0.2495 0.27499 -0.74403
cms 0.12806 0.08052 -0.87498
En.modi -0.81184 0.99239 -0.01198 [
O.S DMF -0.14609 0.2321 -1.07924
O.S WATER -0.37325 1.48703 0.4798
5. Modified cassava starches …..
145
started to deform, which resulted in a decrease in G’ and a value of maximum G’ (G’max) as
postulated by Lii, et l.,(1996a).
The loss modulus (G”) exhibited similar pattern to that for G’ during heating. At the earlier
stages of heating i.e. slightly before 70oC, the G’ was decreased and the starch suspension was
transformed into a “sol” and the amylose molecules were dissolved from the swollen starch
particles. At the temperature above 70.8oC (> To), G’ of all the heated starch suspensions
increased rapidly (as the starch granules kept swelling) to a maximum G’. The initial increase
in G’ was attributed to the interplay of the following factors: the progressive swelling of starch
granules that finally become a close packed network; the solubilized amylose that was
released during the heating process, and the influence in gel volume. The temperature TG max
for native and heat moisture treated starches varied from 68.2 to 90 o C.
Hydroxypropylation of cassava starch leads to significant increase in the G’ and G” with
increase in the temperature. During heating, decrease in associative forces within the starch
granules caused by the introduction of hydroxypropyl groups; results in greater water
penetration and swelling and a consequent increase in G’
Dual-modification (HPCL) resulted in starch pastes with higher peak viscosity and greater
stability than those of native starch pastes (Wu and Seib, 1990). Table 5.1.9 summarizes the
G’and G” values of modified starches at different temperatures. The result revealed that
hydroxypropylation followed by cross linking made the native starch more shear and heat
stable. This may be due to the structural change in the granules after modification, caused
during hydroxypropylation followed by cross linking. Reports revealed that
hydroxypropylation increases the degree of subsequent cross-linking. The phase angle values
were also decreased (Fig 5.1.16).
Hydroxypropylation of cassava starch leads to significant increase in the G’ and G” with
increase in the temperature. During heating, decrease in associative forces within the starch
granules caused by the introduction of hydroxypropyl groups; results in greater water
penetration and swelling and a consequent increase in G’
5. Modified cassava starches …..
146
Dual-modification (HPCL) resulted in starch pastes with higher peak viscosity and greater
stability than those of native starch pastes (Wu and Seib, 1990). Table 5.1.9 summarizes the
G’and G” values of modified starches at different temperatures. The result revealed that
hydroxypropylation followed by cross linking made the native starch more shear and heat
stable. This may be due to the structural change in the granules after modification, caused
during hydroxypropylation followed by cross linking. Reports revealed that
hydroxypropylation increases the degree of subsequent cross-linking. The phase angle values
were also decreased (Fig 5.1.16).
Fig 5.1.12: Temperature sweep of modified cassava starch (Storage modulus Vs temperature)
Fig 5.1.13: Temperature sweep of modified cassava starch (Loss modulus Vs temperature)
0
5000
10000
15000
2000034
.3
40.8
47.1
53.4
59.7
65.8 72
78.1
84.4 90
Stor
age
mod
ulus
[Pa]
Temperature [oC]
Native
HP
HPCL
StAcetate
CMS
HM
En.modi.
0
1000
2000
3000
4000
5000
6000
7000
30
35.7
42.1
48.4
54.6
60.9 67
73.3
79.2
85.5
Loss
mod
ulus
[Pa
]
Temperature [OC]
Native
HP
HPCL
StAcetate
CMS
HM
En.modi.
5. Modified cassava starches …..
147
Fig 5.1.14: Temperature sweep of modified cassava starch (complex viscosity Vs
temperature)
Fig 5.1.15: Temperature sweep of modified cassava starch (phase angle Vs temperature)
Hydroxypropylation of cassava starch leads to significant increase in the G’ and G” with
increase in the temperature. During heating, decrease in associative forces within the starch
granules caused by the introduction of hydroxypropyl groups; results in greater water
penetration and swelling and a consequent increase in G’
Dual-modification (HPCL) resulted in starch pastes with higher peak viscosity and greater
stability than those of native starch pastes (Wu and Seib, 1990). Table 5.1.9 summarizes the
0
1000
2000
3000
4000
5000
6000
7000
3034
.339
.544
.549
.654
.659
.764
.669
.574
.579
.284
.489
.2Com
plex
vis
cosi
ty [P
a -S
]
Temperature [oC]
Native
HP
HPCL
StAcetate
CMS
HM
En.modi.
0102030405060708090
100
3034
.339
.544
.549
.654
.659
.764
.669
.574
.579
.284
.489
.2
Phas
e an
gle
[o ]
Temperature [oC]
Native
HP
HPCL
StAcetate
CMS
HM
En.modi.
5. Modified cassava starches …..
148
G’and G” values of modified starches at different temperatures. The result revealed that
hydroxypropylation followed by cross linking made the native starch more shear and heat
stable. This may be due to the structural change in the granules after modification, caused
during hydroxypropylation followed by cross linking. Reports revealed that
hydroxypropylation increases the degree of subsequent cross-linking. The phase angle values
were also decreased (Fig 5.1.16).
Acetylation results in G’and G” maxima and a decreased tan maximum. From the table 5.1.9
it is clear that the G’ values were high for starch acetate compared to all other modified
starches during the heating cycle, and the TG’ max for starch acetate was 79.2 o C. The complex
viscosity values were also greater for the starch acetate (Fig 5.1.13). These changes occur
because acetylation causes an increase in peak pasting viscosity. During carboxymethylation,
the G’ and G” values were too small compared to all other modified starches with increase in
the temperature. Complex viscosity also followed the same pattern.
Acetylation results in G’and G” maxima and a decreased tan maximum. From the table 5.1.10
it is clear that the G’ values were high for starch acetate compared to all other modified
starches during the heating cycle, and the TG’ max for starch acetate was 79.2 o C. The complex
viscosity values were also greater for the starch acetate (Fig 5.1.13). These changes occur
because acetylation causes an increase in peak pasting viscosity. During carboxymethylation,
the G’ and G” values were too small compared to all other modified starches with increase in
the temperature. Complex viscosity also followed the same pattern.
5. Modified cassava starches …..
149
Table 5.1.10: Storage and loss moduli of modified cassava starches at different
temperatures
Starch Temperature ⁰c
30 50 70 90
Storage modulus (pa)
Cassava 199 116 347 129
HM 6580 1090 580 4110
HP 87.4 77.4 759 1520
HPCL 849 495 330 630
St.Acet. 17500 7820 4970 3880
cms 308 304 293 284
En.modi 620 583 587 617
O.S DMF 308 99.5 5.84 383
O.S WATER 117 101 14.5 96.1
Loss modulus (pa)
Cassava 225 188 308 203
HM 1310 324 405 753
HP 87.4 77.4 759 1520
HPCL 849 495 330 630
St.Acet. 5810 2560 1680 809
cms 182 158 147 146
En.modi 620 583 587 617
O.S DMF 308 99.5 5.84 383
O.S WATER 117 101 14.5 96.1
Octenyl succinylation of the cassava starch caused progressive increase in the G’and G”
values. The G’ value increased to a maximum that is the TG’ max (81.90 C) and then dropped
with continued heating.. The initial increase in G’ may be attributed to the granular swelling of
starch with bulky octenyl group. From Fig 5.1.16, 5.1.17 and 5.1.18 it was found that OS
5. Modified cassava starches …..
150
starch prepared in the water medium showed higher values for dynamic moduli except phase
angle which were approximately same for both samples. (Fig.5.1.18). The rheological
properties of Octenyl succinate modified starch mainly depend on some critical factors like,
reaction conditions and starch source.
Fig 5.1.16: Temperature sweep of Octenyl succinate modified starch (storage modulus
Vs temperature)
Fig 5.1.17: Temperature sweep of Octenyl succinate modified starch (Loss modulus Vs
temperature
0
1000
2000
3000
4000
5000
6000
7000
8000St
orag
e m
odul
us [P
a]
Temperature [0C]
Native
OS.Water
OS.dmf
0
200
400
600
800
1000
1200
1400
Loss
mod
ulus
[Pa]
Temperature [0C]
Native
OS.Water
OS.dmf
5. Modified cassava starches …..
151
Fig 5.1.18: Temperature sweep of Octenyl succinate modified starch (Phase angle Vs
temperature)
5.2 Use of Modified cassava starch in tablets
It was found that native cassava starch cannot meet all the requirements for tabletting as
excipient in tablet formulation. Hence, the present study was undertaken to evaluate the
suitability of different modified starches prepared by physical (heat-moist treatment),
chemical (acetylation, hydroxypropylation and carboxymethylation) and enzymatic (using ∞-
amylase) methods as binding and disintegrating agent in tablet formulations by analyzing the
physical, flow behaviour, pasting and rheological properties of the starches. Tabletting was
also carried out with the selected modified starches as binder and disintegrant and the
properties of the tablets were monitored as per the Indian Pharmacopoeia tests for hardness,
friability and disintegration.
0
10
20
30
40
50
60
70
Phas
e an
gle
[0]
Temperature [0C]
Native
OS.Water
OS.dmf
5. Modified cassava starches …..
152
5.2.1 Identification of proper modified starches for binder and disintegrant
Density, flow and compression studies of modified cassava starches
Binders are agents employed to impart cohesiveness to the granules. This ensures the tablet
remains intact after compression as well as improving the flow qualities by the formulation of
granules of desired hardness and size. The choice of a suitable binder for a tablet formulation
requires extensive knowledge of the relative importance of binder properties for enhancing the
strength of the tablet and also of the interactions between the various materials constituting a
tablet (Mattsson, 2000).
Starch, being a multifunctional excipient in pharmaceutical tablet preparations can be used as
binder and disintegrant. Starch paste made by heating starch with excess water is used as
binder whereas dry starch itself is used as the disintegrant. Hence the selection of suitable
starch material as excipient for binder and disintegrant depends on their physical, flow and
functional properties. In solid state pharmaceuticals, particle size enlargement is an important
unit operation and is performed to impart degree of functionality to particles. These functions
include improved flowability, compressibility and compactability. Binder is an essential
component in this process to impart these properties.
To find out the suitability of various modified starches as a binder and/or disintegrant, the
pasting, rheological and powder flow properties were analyzed. Table 5.2.1 summarizes the
various micromeritic properties of modified cassava starches. The bulk and tapped density
values give a clear idea about the flowability of the powder and granules.
5. Modified cassava starches …..
153
Table 5.2.1: Powder properties of modified cassava starch
Starch
type
Bulk
density,
g.cm-3
True
density,
g.cm-3
Tapped
density,
g.cm-3
Relative
density
cm-3
Hausner
ratio
Angle of
repose,(°)
Compressibilit
%
Hydroxy
propyl
0.42±0.1 1.31±0.10 0.62±.61 0.32±.32
1.25±.44
48.1±.22 10.25±.43
HMT 0.62±0.02 1.37±0.21 0.73±.11 0.44 ±.14 1.18±.32 34.6±.21 14.7±.44
Enz.
modi
0.45±0.16 1.39±0.04 0.61±.21 0.32±.44 1.33± .21 45.6±.42 7.58 ±.21
CMS 0.69±0.43 1.31±0.12 0.78±.13 0.52±.21 1.12±.10 37.5±.21 14.52±.23
Starch
Ace.
0.67±0.32 1.31±0.14 0.78±.08 0.51±.14 1.16±.21 38.6±.09 18.01±.05
Native
Cassava
0.65±0.11 1.47±0.06 0.87±.43 0.44±.09 1.23±.21 48.6±.11 13.35±.55
One of the most important factors affecting the bulk density of a powder and its flow
properties is the inter-particulate interaction. Among the various derivatives the least bulk
density was observed to hydroxypropyl starch (0.421 g.cm-3) and highest value (0.694 cm-3)
was observed for the carboxymethyl starch. The tapped density values also showed the same
pattern. The relative density values (0.320 cm-3) was low for the hydroxypropyl starch
followed by the enzyme modified starch (0.326 cm-3) and highest (0.526 cm-3) value was
obtained for carboxymethyl starch.
The flow rate of granules which is a measure of flowability is considered to be necessary for
successful tabletting (Newmann, 1967). The flow rates were observed to be comparable
although there was a decrease in flow rate with increasing binder concentration, this could be
as a result of increased bonding and cohesiveness between particles leading to reduction in the
flow of granules (Abdulsamad et al, 2008). The angle of repose provides an insight into the
5. Modified cassava starches …..
154
magnitude of cohesiveness of the powder and hence its flowability (Paronen et al,. 1983). On
comparison to the other modified starches, hydroxypropylated and enzyme modified starches
had angle of repose values of 48.150 and 45.690 respectively, which shows the cohesive nature
of the particles which hinders the free flow of these starches, whereas heat moisture treated
and enzyme modified starch has the lowest values of 34.6 and 37.respectively. Favorable
particle properties and the optimal presence of water diminish the cohesiveness of the powder,
resulting in an increased bulk density and enhanced flowability.
The Hausner ratio (i.e. the ratio of tapped density to bulk density) previews the degree of
densification which could occur during tabletting. The higher the ratio, the greater the
propensity of the powder to densify. This phenomenon may cause tablets which lack
uniformity of weight and content to be produced. The Hausner ratio was high for the enzyme
modified starch (1.335) whereas it is low for the carboxymethyl (1.128) starch. The %
compressibility values indicated that the enzyme modified starch has the least value of 7.85
and starch acetate had the highest value of 18.01% (Table 5.2.1)
The pasting studies were carried out at 4 concentrations to examine the suitability as a binder,
and the results showed that as concentration increased, the peak and final viscosity of the
starch paste increased as given in Table 5.2.2. The maximum peak viscosity (2565 cP) was
observed for starch acetate and minimum for hydroxypropylated starch (107 cP) at a
concentration of 10%, whereas the final viscosity was maximum for heat moist treated starch
(3487 cP) and minimum for the enzyme modified (44 cP) and hydroxypropylated starch (273
cP).
5. Modified cassava starches …..
155
Table 5.2.2: Pasting characteristics modified cassava starches as influenced by concentration
Starch type Peak viscosity, cP Final viscosity, cP
2.5% 5% 7.5% 10% 2.5% 5% 7.5% 10%
Enzyme modified 46 168 443 836 11.8 18 30 44
Starch acetate 39 251 982 2565 41 153 371 2557
Hydroxy propyl 19 27 56 107 10 13 40 273
Heat-moist 26 244 1007 2509 21 306 1318 3487
Carboxymethyl 100 850 1250 2357 221 456 755 1340
Native cassava 81 419 1285 2389 83 478 1295 1470
Maize 34 269 760 1734 18 200 625 1493
The distribution of binders is important to impart mechanical strength to compacts. This
distribution is inhibited by high viscosity binder. Also, there was no correlation observed
between physical properties of the binder films and their granule and compact properties.
Hence, rather than film characteristics, the binders should be evaluated for their ability to
improve handling properties, compressibility and compactibility of the granules. Having
registered the minimum peak and final viscosity, the enzyme modified and hydroxypropylated
starches can be considered as most ideal for use as binders.
Rheological properties of various modified starches at the same concentrations are
summarized in Table 5.2.3. The results on the modified cassava starches showed that with the
increase in the concentration of starches from 2.5 to 10%, all the dynamic rheological
parameters showed corresponding increase. But for the enzyme modified starches, with the
increase in the concentration, the storage modulus values were decreased and the loss modulus
values (G”) were higher than that of the with storage modulus (G’) indicating the
predominance of the liquid nature of the gels. Phase angle will give more accurate idea about
the solid-liquid characters of the starch paste. If the value is more towards 900, liquid nature is
predominant and if more towards zero, it indicates solid nature.
5. Modified cassava starches …..
156
Table 5.2.3: Dynamic moduli of modified cassava starches influenced by concentration
Starch type Storage modulus
(G’)(Pa)
loss modulus (G”)(Pa) phase angle (δ=0°)
H MT
2.5% 1080 2260 64.5
5.0% 1310 2150 58.5
7.5% 1500 2200 55.7
10% 2000 1980 44.7
Hydroxypropyl
2.5% 2190 1590 36
5.0% 1565 1880 43.3
7.5% 1760 2020 42.7
10% 1990 1890 43.5
Enzyme modified starch
2.5% 1530 2320 46.5
5.0% 1360 1310 44.1
7.5% 1190 1110 43.0
10% 826 2040 41.9
Carboxymethyl starch
2.5% 417 2020 78.3
5.0% 1400 1900 53.7
7.5% 1650 2060 51.4
10% 1580 1750 47.9
Starch acetate
2.5% 266 1860 81.9
5.0% 308 2100 81.7
7.5% 353 1890 79.7
10% 1250 2040 78.5
5. Modified cassava starches …..
157
Comparing all these starches, lowest values of phase angle (<500) were observed for enzyme
modified and hydroxypropyl starches and hence have equal viscoelastic-solid-liquid nature.
This also justifies the selection of these derivatives as binders.
The other starch derivatives viz., starch acetate, carboxymethyl, heat moist treated starches
may show better functionality as disintegrants since they possess good flow and
compressibility properties as evidenced from Table 5.2.1.
Based on the results obtained from above evaluations, tablets were prepared using
Hydroxypropyl, and enzyme modified starch as binders with suitable commercially available
disintegrant (Starch 1500), and Carboxymethyl starch, starch acetate, HMT starch, and
enzyme modified starch at different concentrations were used as disintegrant with suitable
commercial binders (maize starch) and evaluated for tablet properties by comparing with
respective standard excipients.
5.2.2 Tablet properties (as binder)
The tablets prepared using modified starches as binder were evaluated. Binders have been said
to promote plastic deformation of particles and thereby increasing the area of contact for inter-
particulate bonding (Uhumwangho et al., 2006) subsequently leading to the formation of more
solid bonds in the tablet. The hydroxypropylated and enzyme modified starches at a
concentration of 2.5, 5.0, 7.5, and 10% were examined as binder. The mechanical properties of
the tablet formulations were assessed by the crushing strength and friability of the tablets.
While crushing strength indicates the strength of the tablet, friability values provide a measure
of tablet weakness. The hardness, friability and disintegration time of the tablets were
measured and depicted in Fig. 5.2.1
5. Modified cassava starches …..
158
02468
10121416
2.5 5 7.5 10
Har
dnes
s, K
gStarch Concentration, %
020406080
100120140160180
2.5 5 7.5 10
Dis
inte
grat
ion
tim
e, s
Starch concentraion,%
0
0.5
1
1.5
2
2.5
2.5 5 7.5 10
Fria
bilit
y,%
Starch concetration,%
Faile
d
5. Modified cassava starches …..
159
Fig 5.2.1: Tablet properties of modified cassava starches used as binder at various
concentrations
For all the starches, as the concentration of the binder increased, hardness and disintegration
time increased whereas friability decreased. It has been established that the presence of high
concentration of plasto-elastic binding agent leads to an increase in plastic deformation of the
formulation and consequently to the formation of more solid bonds with increase in tablet
strength and resistance to fracture and abrasion. Even at low concentration of hydroxypropyl
and native cassava starch (upto 5%), tablets failed in the friability test, i.e., complete breakage
and disintegration of the tablets occurred. Enzyme modified starch, when used as a binder
using starch paste of 7.5% resulted in the tablets of hardness 11.5 kg, friability-0.76% and
disintegration time of 40 s which is comparable to that of the tablets made by using maize
starch as binder, i.e., hardness-12.9 kg, friability-0.52% and disintegration time-33.2 s.
The crushing strength-friability ratio (H/FD ratio) also provides a parameter for measuring
tablet strength. Generally, the higher the H/FD value, the stronger the tablet. The H/FD values
for starch used as binder in formulations are presented in Fig 5.2.1. An increase in H/FD
values was observed for the tablets with increase in binder concentration. However, there is
no limiting value for the hardness or crushing strength of the tablets as the use of the tablets
varies widely and also disintegration time varies depending upon the type of application; but
friability should be less than 1%. Hence to quantify the efficiency of tablets, H/FD ratio is
introduced which is the ratio of the tablet hardness (H in Newton) divided by the product of
the friability (F in %) and disintegration time (D in min). It measures the tablet strength
0
100
200
300
400
500
600
2.5 5 7.5 10H
/FD
rat
ioStarch Concentration, %
Cassava Hydroxy propyl Enzyme modifed Maize
5. Modified cassava starches …..
160
(hardness) and weakness (friability) and it subsequently evaluate all the negative effect of
theses parameters on disintegration time (Adebayo and Itiola, 2003). In general, high values
of H/FD ratio indicate a better balance between binding and disintegrating properties.
Comparing the properties of tablets made by using native cassava and maize starch as binder
at 7.5 % concentration, the value of H/FD ratio for cassava starch was 21 whereas for maize
starch, it was about 448. This significantly low value of the native cassava starch showed its
inferior quality as a binder. For the hydroxypropyl starch, this value was very low (Fig 5.2.1).
But when enzyme modified starch was used as a binder at 7.5% concentration, it gave a
maximum value of 503 showing that tablet properties are on par with that of the generally
used maize starch and hence ideal as binding material.
5.2.3 Tablet properties (Disintegrant.)
A disintegrant is normally added to facilitate the rupture of bonds and subsequent
disintegration of the tablets. This increases the surface area of the drug exposed to the
gastrointestinal fluid; incomplete disintegration can result in incomplete absorption or a delay
in the onset of action of the drug.
Effects of disitegrant concentration in tablet properties are depicted in Fig-5.2.2. As the
concentration of the starch increased, disintegration time decreased whereas there was not
much variation or clear cut trend in the hardness and friability values.
0
1
2
3
4
5
6
0 2 4 6 8 10 12
Har
dnes
s, k
g
Starch concentration, %
5. Modified cassava starches …..
161
Fig 5.2.2: Tablet properties of different modified starches at various concentrations as
disintegrant
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10 12
Fria
bilit
y, %
Starch concentration, %
050
100150200250300350400
2 3 4 5 6 7 8 9 10
Dis
inte
grat
ion
tim
e, s
Starch concentration, %
050
100150200250300350400450500
0 2 4 6 8 10 12
H/F
D r
atio
Starch Concentration, %
Enzyme modifed Starch acetate Carboxymethyl Heat-moist
5. Modified cassava starches …..
162
Increase in concentration of starch disintegrant led to a decrease in the disintegration time.
This could be due to enhanced in swelling, which is associated with the increase in starch
concentration (Iwuagwu, and, Onyekweli, 2002; Bi et al., 1999). The higher relative density
led to an increase in the disintegration time; similar observations have been already reported
(Esezobo et al,, 1989, Itiola and Pilpel, 1991). With increase in relative density, there was a
decrease in porosity (Washburn, 1921), consequently water penetration into the tablets can
slow down, swelling would be reduced and development of the active mechanism of
disintegration is reduced. When porosity decreased, more solid bridges are formed, which
make the annihilation of inter-particular force more difficult (Luangtanan-Anan and Fell,
1990). The disintegration time, friability and hardness of the tablets by incorporating heat
moisture treated starch at 10% concentration were 6.5 s, 0.82% and 4 kg, respectively,
whereas for carboxymethyl starch at the same concentration, these values were 11.5s, 0.59%
and 5.08 kg, respectively.
The commercially available carboxymethyl starch gave a value of 7s for disintegration time,
0.54% for friability and 5.33 kg for hardness which is almost comparable with the above
values. The H/FD ratio has been suggested as a better index for measuring tablet quality than
the crushing strength friability ratio (CSFR) because in addition to measuring the tablet
strength (crushing) and weakness (friability), it simultaneously evaluates all negative effects
of these parameters on disintegration time (Upadrashta, et al,. 1992). In general, higher values
of H/FD ratio indicate a better balance between binding and disintegration properties. There
was a general increase in the H/FD ratio for paracetamol tablets with increasing disintegration
concentration. The H/FD ratio was found to be increased as concentration of the disintegrant
increased. Maximum values were observed for carboxymethyl starch (449) and heat moisture
treated starch (451) incorporated tablets. This clearly shows that heat-moist treated starch and
carboxymethyl starch can be used as a disintegrant up to 10%. Considering the H/FD ratio,
the order of the effectiveness of modified starches as disintegrants is : Heat moisture treated
starch> carboxymethyl starch> starch acetate.
5. Modified cassava starches …..
163
5.3 Use of modified cassava starch in film production
The hydrophilic nature of native starches is a major problem that limits its use in the edible
film preparations and their properties depend on the ambient humidity. This can be avoided by
improving the barrier properties of starch based films by addition of various chemicals in
minor amounts. Plasticizers, such as glycerol, are often used to modify the mechanical
properties of the film (Gaudin et al., 1999, Myllarinen, et al., 2002). Plasticizers decrease
intermolecular attractions between adjacent polymeric chains increasing film flexibility, but
also they may cause significant changes in the barrier properties of the film (Garcı´a, et al.,
2000) Chemical derivatization provides a way to solve this problem and to produce films
having good quality. The objective of the present study was to prepare films using modified
cassava starch and characterize the appearance, physicochemical and mechanical properties
with regard to the capsule production.
Depending on the previous results of various physicochemical, thermal and rheological
properties of modified cassava starch, four derivatives (Heat moisture treated starch, Starch
acetate, Hydroxypropyl starch, and enzyme modified starch) were selected for the film
preparation.
5.3.1 Moisture content and film solubility
The moisture content and solubility of the films prepared using the four cassava starch
derivatives are presented in the Table 5.3.1. The solubility of the films prepared with
plasticizer showed higher values than films prepared without plasticizer. It could be concluded
that hydrophilic plasticizers enhanced solubility of film in water. Among the four derivatives,
films prepared using the heat moisture treated and starch acetate showed higher value of 19.1
and 16.1%, whereas the hydroxypropyl and enzyme modified starched showed the lower
value.
5. Modified cassava starches …..
164
Table 5.3.1: Properties of films prepared from modified cassava starches
Starch type Moisture content
(%)
Thickness of films.
(mm)
Water solubility
(%)
HM 19.1±0.03 0.11±0.03 36.33±0.11
HP 14.2±0.01 0.12±0.02 26.2±0.21
Enz. modi. 16.0±0.06 0.10±0.05 39.2±0.10
St. Acet. 13.2±0.03 0.13±0.06 32.3±0.08
This increase in the moisture content in the prepared films may be due to the expansion of granules
during the heat moisture treatment and the extensive breakage of amylose molecule during the enzyme
treatment for the modification process. Increase moisture content is not good for a film which can be
used for the capsule.
Solubility of film in water is an important property of starch based films, thereby the capsules
prepared from it. The maximum solubility was observed for the enzyme modified starch
(39.2%) and lowest for the hydroxypropyl starch (26.2%). Studies revealed that as heating
temperature of film solutions increases, a progressive decrease in film solubility of starches
and this may be due to the increased polymer interaction (Bonacucina et al., 2006) Type and
concentration of plasticizer were also affected the solubility of starch films. Irrespective of the
type, an increase in plasticizer content leads to an increase in films solubility.
5.3.2 Thickness and Colour of films
Thickness of the films prepared from the modified cassava starch is listed in table 5.3.1.The films
prepared using the enzyme modified starch has the least thickness value of 0.10 followed by the heat
moisture treated starch. The order of thickness of films prepared are as follows; enzyme modified >
heat moisture treated starch> hydroxypropyl starch> starch acetate. For the sample thinner than 1mm
the strength reduces as thickness decreased. When thin film are made, the water evaporates fast, and at
room temperature the rate of molecular movement is limited and the molecules in the film do not have
time to respond o the shrinking of the film, the water evaporates slowly and the molecules have enough
time for relaxation. Since thicker films are exposed to a higher degree of water for a long time the
crystallanity in these films are expected to be higher than the thin film. The film thickness was in
5. Modified cassava starches …..
165
proportion to the film’s nature and composition. This has also been demonstrated by Abugoch
et al., (2010) and Sebti, et al., (2007), in which a relationship has been observed between film
thickness and, the content and nature of the film forming polymer.
Table 5.3.1 reveals the color values of the films prepared using the modified cassava starch. Film
color is a critical property that influences the appearance of the capsule to be prepared.
Transparent films are characterized by low values of the area below the absorption curve. The
results obtained showed that all the films prepared from the starches are almost transparent
compared to films prepared without adding glycerol. The studies revealed that with increase in
the concentration of glycerol the opacity of films increase. Comparing the four different films
prepared from the starches, the brightness (L value) is high for enzyme modified starch
(85.21) followed by the starch acetate (80.4) respectively.
Table 5.3.2: Color values of films prepared using modified cassava starch.
Starch source L a b Total color
difference
HM 77.13 -0.20 2.20 3. 22
HP 64.81 0.05 2.73 4.21
Enz. modi. 85.21 0.41 3.69 3. 01
St. Acet. 80.4 0.55 2.43 3.12
The ‘a’ and ‘b’ values ranged in between -.20 to 0.55 and 2.73 to 3.69. The DE values revealed that for
all the films the values were smaller, that is all films were almost transparent, and the more transparent
film was obtained for the enzyme modified starch. The order of opacity of the films was as follows: -
Hydroxypropyl> heat moisture treated starch > Starch acetate > enzyme modified starch.
5.3.4 Mechanical properties of films
Mechanical properties of films shows the typical pattern of polymer materials, since they
exhibited low values of tensile strength at break, and moderate elongation values (Fig. 5.11).
The plasticizer molecules interfered with starch packing, decreasing intermolecular attraction
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and increasing polymer mobility. This involved an increase in elongation and a decrease in
tensile strength as glycerol content rose in film formulations. ( Mali et al., 2002, 2005).
The data also confirms that the mechanical behavior of plasticized modified cassava starch
films depends on glycerol (plasticizer) concentration. The maximum elongation was obtained
for the enzyme modified starch, probably due to the presents of more networks formed during
the enzyme hydrolysis process that cross links more during the casting process of the
filmogenic solution.
Fig. 5.3.1: % elongation values of films prepared using modified cassava starch
The elongation value was less for the hydroxypropyl starch, because it swells more and resists
the spreading during the preparation of the films. Increasing the amount of glycerol in the
preparation has been reported to lead to elongation as rupture decreased (Lourdin et al., 1997
and Gaudin et al., 1999).
Conclusion.
Evaluation of various modified cassava starch revealed that there is significant variation in
different properties of native starch upon modification. There is a predominant variation in the
physical, flow, pasting and rheological properties of the modified cassava starches prepared by
0
20
40
60
HMHP
En.Modist.Acet.
5. Modified cassava starches …..
167
physical, chemical and enzymatic methods. The low viscosity, low bulk density, high
Hausner ratio, low relative density, high angle of repose and low compressibility of the
hydroxypropylated and enzyme modified starches make them suitable as a binder in tablet
formulations. Among these two starches, enzyme modified starch as a binder at 7.5%
concentration produced tablets with the properties comparable to that of the generally used
maize starch. Starch acetate, carboxymethyl, heat moist treated and enzyme modified
carboxymethyl starches possess good flowability, compressibility and high viscosity and
hence they can be as disintegrants. Among these carboxymethyl and heat moist treated starch
can be used as disintegrant, up to 10% concentration in tablets with desirable balanced
hardness, friability and disintegration properties.
Among the different modified cassava starches tested for the film preparation, all four starches
suspensions were able to form films. Among them, starch acetate showed the best
characteristics to form films since they were more transparent and easily removed from the
cast plate. The film forming capacity of starches is related to their amylose concentration ans
well as the architecture of the macromolecular components, mainly the size of the amylose
and amylopectin chains The enzyme modified starch also showed good characteristic of
desirable film but the viscosity of the sample is a major problem that gives very thin film
which is unusable for the capsule production. Even though the starch acetate provided average
quality of film, its hygroscopic nature is the main hindrance in capsule production. So further
study is needed to get a reliable film.