13 Chapter 5

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 OH 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 OH 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).


115

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.


117

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 24.80.36 24.70.45 24.730.4 79.30.19 32.7 0.54 -

HM 20.30.83 170.24 25.30.06 87.420.11 41.5 1.12 -

HP 12.250.1 7.10.579 7.60.95 55.70.05 27.3.02 0.080.01

HPCL 7.50.10 4.0080.19 3.030.20 45.40.19 20.2 0.56 -

St. Acet. 27.30.32 29.370.37 25.70.11 92.80.84 24.5 1.09 0.050.01

CMS 0.2 0.22 90.120.17 11.460.46 95.30.24 29.9 0.87 0.170.02

Enz. modi. 9.40.13 79.50.23 28.40.23 970.14 67.2 1.02 -

O.S DMF 37.50.12 48.80.19 9.750.28 85.50.18 32.1.0.99 0.0170.01

O.S WATER 15.410.72 23.90.36

12.800.26 75.60.12 31.1 1.12 0.0080.01

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

amyloseiodine 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 heatmoisture 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


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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


127

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 60.12C. 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 76.21C.The endotherms were shifted to a higher temperature with a

Sample

Gelatinisation Temperatures (C)

H (J/g)

To Tp Te

cassava 63.130.04 73.920.11 79.510.12 13.69

HM 60.120.06 76.210.19 80.020.15 11.43

HP 66.020.03 72.080.01 79.440.12 10.10

HPCL 69.550.01 70.750.20 83.690.11 10.08

St.Acet. 61.42 0.05 70.13 0.32 79.26 0.63 10. 22

CMS 68.530.27 72.320.12 79.520.11 12.79

Enz.modi. 62.120.33 74.5 0.17 80.120.31 11.64

O.S DMF 65.860.01 69.50.03 74.230.01 9.36

O.S WATER 64.690.03 68.730.02 74.030.01 10.71


128

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 66.02C), while

the Tp value showed a decrease to 72.08C (Table-5.1.3). The lower 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.


129

Fig 5.1.4: DSC thermogram of Hydroxypropylated Cassava starch

Hydroxypropylation cum cross-linking caused a reduction in Tp and H values but the To and

Tc values were increased (Fig-5.1.4). The To values were increased to 69.55C whereas the Tc

values increased to 83.69C. 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 (70.75C) 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 T values were

lowered to 61.42C and peak temperature (Tp) to 70.13C 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


130

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 72.1C.

DSC thermograms of native and carboxymethylated starches (Table 5.1.3) showed that was a

slight decrease in the Tp value which reduced to 72.32C. 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


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 H 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 74.03C respectively. T, 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


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).


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 0.50 406.00.35 69.30.50

HM 25080.16 1820.26 34810.51 11600.31 79.40.60

HP 108.50.7 97.00.1 2690.071 1781.1 -

HPCL 16.50.12 24.00.01 50.02 8.50.35 -

St.Acet. 25670.1 18190.2 25570.1 7380.0 760.5

CMS 890.18 220.15 220.15 34.50.35 50.45

Enz.modi. 13140.1 3180. 5 16460.21 1980.11 63.40.30

O.S DMF 139.50.35 250.4 180.50.20 660.12 Err

O.S WATER 13000.21 3370.12 17080.01 7450.2 75.90


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


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 63.4 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.


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

C 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


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 products processing

and/or end use performance.


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


139

stress in the range of 3850 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


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 tan= (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 =0 or tan=0 corresponds to an elastic response and =90 or tan= is a viscous

response. If the phase angle is within the limits of 0


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 tan values tended to move toward

90 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


142

Fig 5.1.11: Frequency sweep of Cassava starches (phase angle /frequency)

In hydroxypropylated starches, the G, G and tan 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 tan 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 tan 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 tan 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.


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 tan 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 tan 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.


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[Pas / 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


145

started to deform, which resulted in a decrease in G and a value of maximum G (Gmax) 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

Gand 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).






146









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.


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)







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.


148







Acetylation results in Gand 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 Gand 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.


149

Table 5.1.10: Storage and loss moduli of modified cassava starches at different

temperatures

Starch Temperature c

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 Gand 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


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


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


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.


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

0.420.1 1.310.10 0.62.61 0.32.32

1.25.44

48.1.22 10.25.43

HMT 0.620.02 1.370.21 0.73.11 0.44 .14 1.18.32 34.6.21 14.7.44

Enz.

modi

0.450.16 1.390.04 0.61.21 0.32.44 1.33 .21 45.6.42 7.58 .21

CMS 0.690.43 1.310.12 0.78.13 0.52.21 1.12.10 37.5.21 14.52.23

Starch

Ace.

0.670.32 1.310.14 0.78.08 0.51.14 1.16.21 38.6.09 18.01.05

Native

Cassava

0.650.11 1.470.06 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


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).


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.


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 (=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


157

Comparing all these starches, lowest values of phase angle (


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


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


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, %


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, %


050

100150200250300350400

2 3 4 5 6 7 8 9 10

Dis

inte

grat

ion

tim

e, s


050

100150200250300350400450500

0 2 4 6 8 10 12

H/F

D r

atio

Starch Concentration, %

Enzyme modifed Starch acetate Carboxymethyl Heat-moist


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.


163

5.3 Use of modified cassava starch in fil

Documents

13 Chapter 5