Modified Starch

2. Review of Literature

8

Chapter 2

REVIEW OF LITERATURE

2.1 Starch-chemistry.

Starch is the predominant carbohydrate reserve in many plants; Starch is semicrystalline in

nature with varying levels of crystallinity. The packaging of amylose and amylopectin within

the granules has been reported to vary among the starches from different species. The activity

of the enzymes involved in starch biosynthesis may be responsible for the variation in

amylose content among the various starches (Krossmann and Lloyd, 2000). Starch granule

differences amongst various plant species are accounted for, not only by the ratio of

constituent molecules, but also by their location and interaction and it is probably the most

commonly used hydrocolloid. Starch is a morphological complex polymer substance, (Fig.

2.1). The crystalline composition consists of around 15-45% of the starch granules. The

crystallinity is exclusively associated with the amylopectin component, while the amorphous

regions mainly represent amylose (Zobel, 1988a, 1988b).

Fig 2.1: Backbone of starch molecule

Amylose: Amylose is defined as a linear molecule of D-glucopyranosyl units joined by (1

4) linkage, but it is today well established that some molecules are slightly branched by (1

6) linkages (Fig 2.2). Amylose solutions can be easily characterized by size-exclusion

chromatography coupled on-line to multi-angle laser light scattering (SECMALLS). It is the


9

smaller of the two polysaccharides making up starch molecule. The amylose is essentially

linear but not purely and its solution properties are generally regarded as typical for those of

a linear polymer (Biliaderis, 1991). The inside of the helix is lipophilic where there are only

hydrogen atoms. On the outside, there are hydrophilic hydroxyl groups. The exact position of

amylose in the granules is uncertain, but it is generally believed that it acts as an amorphous

space filler in the granules, whereas the amylopectin is highly branched with shorter chains

arranged as double helices in clusters of a partially crystalline character (French, 1984,

Zobel, 1988). Amylose is located in the granule as bundles between amylopectin clusters and

or randomly dispersed. They could be located therefore among the amorphous and crystalline

regions of the amylopectin clusters (Robin et al., 1974). In starch granules, the amylose chain

displays a natural twist in a helical conformation with six anhydroglucose units per turn

(Zobel, 1988a). Amylose is probably the first biopolymer for which a helical structure was

proposed. The ability of amylose to form complexes with butanol provides a method for

separating amylose from amylopectin by selective precipitation (Schoch, 1968).

Amylopectin- Amylopectin is the highly branched component of starch and it is formed

through chains of -D glucopyranosyl residues linked together mainly by (1.4) linkages

but with 56% of (1,6) bonds at the branch points (Fig 2.2).


10

Fig 2.2: Structure of amylose and amylopectin molecules

The multiplicity in branching is a common feature of both amylopectin and glycogen. The

basic organization of the chains is described in terms of the A, B and C chains as defined by

Peat et al (1952). Thus, the outer chains (A) are glycosidically linked at their potential

reducing group through C6 of a glucose residue to an inner chain (B); such chains are in turn

defined as chains bearing other chains as branches. The single C chain per molecule likewise

carries other chains as branches but contains the sole reducing terminal residue. The ratio of

A-chains to B-chains is an important parameter which is also referred to as the degree of

multiple branching.

Minor components- Minor components associated with starches correspond to three

categories of materials: (i) particulate material, composed mainly of cell-wall fragments; (ii)

surface components, removable by extraction procedures; and (iii) internal components.

Lipids represent the most important fraction associated with the starch granules. Starch

quality is also influenced by the presence of lipids, proteins and phosphorous. Lipid levels

are lower in tuber than in cereal starches. In tuber starches, lipids are only found on the


11

granule surface, while starches from cereal endosperm have surface and integral lipids

(Morrison, 1988) Starch also contains several different minerals in small amounts, and the

most important mineral is phosphorus (Bulen et al., 1998).

2.2 Structural properties of starch

The extent of crystallinity of native starch granules ranges from about 15% for high-amylose

starches to about 4550% for waxy starches. The granules have a hierarchical structure that

can be observed readily by light and electron microscopy. The morphology of starch granules

depends on the biochemistry of the chloroplast or amyloplast, as well as physiology of the

plant (Badenhuizen, 1969). The granule is a partially crystalline material, i.e., within it, there

are amorphous and crystalline regions and the degree of crystallinity is reported to be in the

range 15-35% (French, 1984). The long range molecular order in the starch granules can be

studied by the X-ray powder diffraction technique. Depending on the plant origin, native

starch exhibits three different X-ray diffraction patterns. A- pattern, characteristic of cereal

starches like wheat, barley, rye, oat, maize, rice etc with characteristic d-spacing at 5.8, 5.2

and 3.8 , B- pattern characteristic of certain tuber and stem starches like potato which have

characteristic d-spacing at 15.8, 5.9, 5.2, 4 and 3.7 A0 and retrograded starch (Ring et al.,

1987), and C-pattern, intermediate between A- and B-types which is found in legume

starches and some tuber and seed starches (French, 1984). It has the characteristic d-spacing

found in the A-pattern and the 15.8 A0 d-spacing of the B-pattern (Zobel, 1988a). The sharp

diffraction patterns in the XRD are usually associated with crystalline material and the non-

sharp areas with amorphous regions.

The crystallinity of starch has been assigned to the well-ordered structure of the

amylopectin molecules inside the granules. The absolute crystallinity of starch from four

varieties of cassava was found to lie between 8-14%. (Moorthy et al., 2002). Cassava starch

possesses A, C or a mixed pattern with three major peaks at 2=15.3, 17.1 and

23.5(Rickard et al 1991/14) ; Sweet potato starch also has A (Takeda et al., 1986 ,Gallant

et al., 1982) C (Zobel, 1988, Chiang and Chen, 1988) or intermediate pattern (Tian et al.,

1991). Takeda et al., observed A pattern for two varieties of sweet potato while it was CA

for another variety with absolute crystallinity of 38%. Colocasia, Xanthosoma, Pachyrrhizus,


12

Arrowroot, Amorphophallus and D dumetorum starches possess A pattern (Moorthy,2001,

Gallant et al., 1982) and Dioscorea alata, Dioscorea esculenta, Dioscorea rotundata,

Dioscorea abysinica and Dioscorea cayensis starch possess B patterns It was found that the

XRD pattern of extracted starch is the same throughout the growth period of Diosocrea

rotundata. Canna edulis and Curcuma sp. starches exhibit B XRD pattern. The absolute

crystallinity of Canna starch was 26% (Zoebel, 1988a).

Tuber crops

Starchy tubers and root crops are important subsidiary or subsistence food in tropical and sub

tropical countries. Although a wide range of tuber crops are grown worldwide, only five

species account for almost 99% of the total world production. These are potato (Solanum

tuberosum, 46%), cassava (Manihot esculenta, 28%), sweet potato (Ipomea batatas, 18%),

yams (Dioscorea spp., 6%) and taro (Colocassia, Cytosperma, Xanthosoma spp., 1%). Root

and tuber crops are grown worldwide and usually have low commercial value for direct

consumption. The starch of such crops would be a good source for different food industries

(Alves et al., 1999, Amani et al., 2004, Brunnschweiler et al., 2005, Moorthy et al., 1993).

The tropical root starches have widely varying physicochemical and functional properties

unlike the cereal starches which possess almost similar characteristics (Moorthy, 1994). The

large variability in the starch properties can be attributed to the differences in the

morphological and structural features of the starches

2.3 Functional properties of starch

Applications of starch in food and industry depend on various functional properties like

viscosity, swelling, retrogradation etc., which in turn depend on the source of starch,

presence of various ingredients and processing conditions. These properties are discussed

below.

Swelling volume

The swelling power (SP) is the ratio of the wet weight of the sedimented gel to its dry weight

of starch (Crosbie, 1991). Swelling factor (SF) is the ratio of the volume of sedimented gel to


13

the volume of dry starch granules with a density of 1.4 g/ml (Tester and Morrison, 1990).

Swelling apparently is a property of amylopectin. High proportions of long chain (degree of

polymerization >35) molecules in amylopectin contributed to the increase in swelling (Sasaki

and Matsuki, 1998). When swollen granules are the dominant structural feature in aqueous

starch systems, starch concentration is an important factor. In the dilute regime, the viscosity

is governed by the volume fraction of swollen granules (Steeneken, 1989). In the

concentrated regime, the viscosity is governed by particle rigidity. The swelling power and

solubility provide evidence of the magnitude of interaction between starch chains within the

amorphous and crystalline domains. The extent of this interaction is influenced by the

amylose to amylopectin ratio, and by the characteristics of amylose and amylopectin in terms

of molecular weight/distribution, degree and length of branching and conformation (Hoover,

2001).

Gelatinization properties

Gelatinization is a major step which exhibits featured characteristics of starch. The granules

absorb water and swell, and the crystalline organization is irreversibly disrupted (Fig 2.3).

The gelatinization temperature of most starches is between 60 and 800C. In general, there is a

negative relationship between the amylose content of starch and the gelatinization

temperature. Collapse of crystalline order within the starch granules manifests itself as

irreversible changes in properties, such as granule swelling, pasting, loss of optical

birefringence, loss of crystalline order, uncoiling and dissociation of the double helices, and

starch solubility (Atwell et al., 1988, Hoover, 2001, Stevens and Elton, 1981). The order-

disorder transitions that occur on heating an aqueous suspension of starch granules have been

extensively investigated using DSC (Donovan, 1979, Jenkins, 1994). Starch transition

temperatures and gelatinization enthalpies by DSC may be related to characteristics of the

starch granule such as degree of crystallinity. Gelatinization occurs initially in the amorphous

regions, as opposed to the crystalline regions, of the granule, because hydrogen bonding is

weaker in these areas. The differences in transition temperatures between the different

starches may be attributed to the differences in the degree of crystallinity. The gelatinization

and swelling properties are controlled in part by the molecular structure of amylopectin (unit

chain length, extent of branching, molecular weight, and polydispersity). Differential


14

Scanning Calorimetry (DSC) is the most common technique used to study the thermal

properties of starches. It measures first-order (melting) and second-order (glass transition)

transition temperatures and heat flow changes in polymeric materials and gives information

on order-disorder phenomena of starch granules (Biliaderis et al., 1986a). Gelatinization is an

endothermic process.

Fig 2.3 Swelling of starch granule during heating process in the presence of water

(Mechanism for Starch Gelatinisation. (Harper, 1981b)

Retrogradarion

The molecular interactions (hydrogen bonding between starch chains) after cooling of the

gelatinized starch paste have been called retrogradation (Hoover, 2001). During

retrogradation, amylose forms double-helical associations of 40-70 glucose units (Jane and

Robyt, 1984) whereas amylopectin crystallization occurs by association of the outermost

short branches. In the case of retrograded starch, the value of enthalpy provides a quantitative

measure of the energy. Starch retrogradation enthalpies are usually 60-80% lower than

gelatinization enthalpies and transition temperatures are 10-26 C lower than those for


15

gelatinization of starch granules. The crystalline forms are different in nature from those

present in the native starch granules (Karim et al., 2000). Fig 2.4 summarizes the changes

during heating and cooling of starch suspensions.

Fig 2.4: Terms to describe changes induced by heating and cooling (b) Physico-

chemical changes that take place during heating and cooling. (Svegmark 1993)

Retrograded starches show lower gelatinization temperatures and enthalpy than native

starches because they have weaker starch crystallinity (Sasaki et al., 2000). The crystalline

forms are different in nature from those present in the native starch granules (Karim et al.,

2000). Both amylose and amylopectin fractions are important in the retrogradation process.

Amylose undergoes rapid crystallization as soon as cooling begins and retrogradation

depends on the amylose content in the sample, the amount that is free and uncomplexed with

lipids, and its molecular weight distribution. Amylopectin, on the other hand, recrystallizes

slowly and the degree of retrogradation depends on the chain length distribution of


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amylopectin. Recrystallization and retrogradation of amylopectin is dominant at a higher

concentration of solids and the polymer formed is more loosely bound than retrograded

amylose and hence, highly susceptible to amylolysis (Ring et al., 1988).

Pasting properties

Continued heating of starch in excess water with stirring causes the granules to further swell,

the amylose to leach more, and the granules to disintegrate, forming a viscous material called

paste (BeMiller, 2007). Pasting occurs after or simultaneously with gelatinization. Pasting

properties of starch are important indicators of how the starch will behave during processing

and are commonly measured using various viscometers like Brabender Viscometer and Rapid

Visco Analyzer (RVA). Initially heating starch suspension results in swelling of starch

granules. As heating continues, an increase in viscosity can be observed, which reflects the

process of pasting. The temperature at the onset of viscosity increase is termed pasting

temperature. Viscosity increases with continued heating, until the rate of granule swelling

equals the rate of granule collapse, which is referred to as the peak viscosity (PV). PV

reflects the swelling extent or water-binding capacity of starch and often correlates with final

product quality since the swollen and collapsed granules relate to texture of cooked starch.

Once PV is achieved, a drop in viscosity, or breakdown, is observed as a result of

disintegration of granules. Break-down is a measure of the ease of disrupting swollen starch

granules and suggests the degree of stability during cooking (Adebowale and Lawal, 2003).

Minimum viscosity, also called hot paste viscosity, holding strength, or trough, marks the end

of the holding stage at the maximum temperature of the RVA. Cooling stage begins and

viscosity again rises (setback) which is caused by retrogradation of starch, particularly

amylose. Setback is an indicator of final product texture and is linked to syneresis or weeping

during freeze-thaw cycles.


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Fig 2.5: Starch granules during cooking. (Fapet Oy, Helsinki Finland from Pigment Coating

and Surface)

Viscosity normally stabilizes at a final viscosity or cold paste viscosity, which is related to

the capacity of starch to form viscous paste or gel after cooking and cooling (Batey, 2007,

Newport Scientific, 1998). Other components naturally present in the starchy material or

additives interact with starch and influence pasting behavior.

Rheology

Rheology is the study of the flow and deformation of materials (Barnes et al., 1989) and

associate the physical flow behavior with the materials internal structure and a distinction is

made between liquid, solid and viscoelastic materials. Which property dominates, and what

the values of the parameters are depend on the stress and the duration of stress application.

Thus, a given material can behave like a solid or like a liquid, depending on the time scale of

the deformation process. If the experiment is performed relatively slowly, the sample appears

to be viscous rather than elastic; if the experiment is performed relatively fast, it appears to

be elastic rather than viscous (Barnes et al., 1989). Shear stress, shear rate and viscosity are

the building blocks of understanding rheology. Fig 2.6 is a depiction of the velocity gradient

created in a liquid between two parallel plates of area A. One plate is positioned at y = 0, and

the other is positioned at y = d. The plate at y = d is moved at a relative velocity U while the


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plate at position y = 0 remains stationery. The force F exerted on the liquid creates a velocity

gradient where the small arrows are proportional to the local velocity. This motion creates

internal friction. The resistance to this force is the shear viscosity. Shear strain is described

as "the movement of a layer of material relative to parallel adjacent layers, and is generally

referred to as shear. The change in shear strain per unit time, known as the shear rate y,

creates the velocity gradient. The force parallel to the plate at y = d is known as the shear

stress. Steady shear measurements are traditionally regarded as the most important material

properties that result in the knowledge of the material response.

Fig 2.6: Shear deformation of a material

There are generally two types of materials, Newtonian and non-Newtonian. Newtonian

materials are characterized by constant viscosity over a large range of stress. Examples of

Newtonian materials include water, alcohol, and most oils. Non-Newtonian materials are

defined by remarkable changes in the viscosity of suspensions with changes in stress. Non-

Newtonian suspensions exhibit elastic properties as well as viscous properties. Some exhibit

properties of a viscous fluid and an elastic solid. These particular characteristics are known as

viscoelasticity. Elasticity describes the ability to store mechanical energy reversibly during

deformation. Viscoelastic properties are attributed to the breakdown and reformation of the

network structure of applied shear stress. Linear viscoelasticity is based on the superposition

principle that implies that the strain at any time is directly proportional to the stress.


19

The general aims of rheological measurements are:

to obtain a quantitative description of the materials mechanical properties

to obtain information related to the molecular structure and composition of the to

material

characterize and simulate the materials performance during processing and for quality

control

Several methods are available for measuring the rheological properties of a solution, but the

geometry of the measurement device is of great importance. Several different measurement

geometries exist, like spindle type, concentric cylinders, cone and plate etc. (Fig.2.7).

The most common types of fundamental rheological tests used in cereal testing are: (i) flow

viscometry (ii) small and large deformation shear creep and stress relaxation; (iii) large

deformation extensional measurements; and (iv) small deformation dynamic shear

oscillation. Here we discussing about the two major rheological analyses, the flow curve and

the dynamic shear oscillation tests.

Fig 2.7: Different measuring systems used for rheological analysis.


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Steady shear flow curves

When shear is slowly imparted to such a system, it becomes progressively easier as

successive increments of force are applied. The nature of the interlocking or interwoven

structure dictates whether initial flow occurs with difficulty until sufficient structure is lost or

whether a sufficient initial force is required to initiate motion, that is, whether a yield value

has to be exceeded. In any case, continued shearing breaks further linkages, so that the

apparent viscosity drops with increasing shear. A flow curve, viscosity (h) versus shear rate

(g), across a wide range of shear rates can provide important information about storage

stability; optimal conditions for mixing, pumping, and transferring; and end-user

applications. It also provides important information regarding the ways in which the structure

changes to comply with the applied shear in different conditions, such as storage, processing,

and application. If the shear rate changes during an application, the internal structure of the

sample will change and the change in stress or viscosity can then be seen.

Yield stress phenomena

The yield stress measurement is crucial for modified starch. Yield stress (y) is defined as the minimum shear stress required for initiating flow. Yield stress can be measured using a stress

ramp experiment. Yield stress can also be defined as the stress below which a material will

not exhibit a fluid like behavior. This means that subjecting a material to stresses less than

the yield stress will lead to a non permanent deformation or a slow creeping motion over the

time scale of the experiment.

Thixotropy

Time-dependent flow measures the increase or decrease in viscosity with time, while a

constant shear is applied. The flow is called thixotropic if viscosity decreases with time or

rheopetic if it increases. Thixotropic behavior describes a degradation of the structure during

the loaded phase; thus, a reduction in viscosity with time occurs when shear is applied.

During the relieved phase, the original structure is recoverable. The extent of structural

recovery is dependent on the time allowed for the recovery. Therefore, a thixotropic material

will have a shear thinning behavior when a gradually increasing shear is applied. This is


21

because the orientation of the structures molecules or particles will change to align with the

flow direction. However, its original orientation can be restored over a period of time after

the external force is removed. There is a delay in time for the structure to recover completely.

Viscoelastic test methods (dynamic shear oscillation)

Two different types of methods are available to determine the linear viscoelastic behaviour of

a material: dynamic and static methods (Barnes et al., 1989). The dynamic methods involve

the application of harmonically varying stress or strain. The static methods involve the

imposition of a step change in the stress or strain and the observation of the subsequent

development of the strain or stress as a function of time. Analyses of the viscoelastic

materials are designed not to destroy the structures, so that the measurements can provide

information about the intermolecular and inter-particle forces of the materials (Martin, 1993).

Oscillatory measurements provide information about the structure and elasticity of a material.

They can, for example, be used to determine the storage stability. The oscillation strain

sweep measurement is used to find a range of strain at which the rheological properties of the

samples are independent of the applied strain. These strain values should not be exceeded in

further measurements. A strain amplitude sweep is utilized to determine the linear

viscoelastic region of material response, which is used to establish the correct parameters for

subsequent dynamic testing. The maximum strain at which G remains constant is called the

critical strain and defines the limit of the linear viscoelastic region (LVE). In general, the

material can respond to this type of deformation through two mechanisms: elastic energy

storage and viscous energy dissipation. Quantitatively, these responses can be represented as

storage modulus (G), energy stored per unit volume, and loss modulus (G), energy

dissipated per unit deformation rate per unit volume. Storage modulus (G) is proportional to

the extent of the elastic component (contributed by crosslinking, entanglement, and/or

aggregation) of the system, and loss modulus (G ) is proportional to the extent of the viscous

component (contributed by the liquid like portion) of the system (Larson 1999).

Typically, the strength of interaction or internal structure in an emulsion is measured by the

magnitude ofthe ratio G/G = tan , which is called damping factor ( is phase angle). The


22

smaller the tan (or the greater G), the stronger the interaction (Radebaugh and Simonelli,

1983).

Phase angle tan d is associated with the degree of viscoelsticity of the sample. A low value in

tan d or d indicates a higher degree of viscoelasticity (more solid like). The phase angle d can

be used to describe the properties of a sample.

d = 90 G*= G and G= 0 viscous sample

d = 0 G*= G and G= 0 elastic sample

0 < d < 90 viscoelastic sample

d > 45 G> G semi liquid sample

d < 45 G> G semi solid sample

Complex viscosity - h*

Complex viscosity describes the flow resistance of the sample in the structured state,

originating as viscous or elastic flow resistance to the oscillating movement.

Static methods

Static methods are either creep tests at constant stress or relaxation tests at constant strain.

The creep test is used far more often than the relaxation test. In the creep test, a constant

stress is applied and the strain of a sample is determined as a function of time. In the

relaxation test the sample is subjected to a predetermined strain, and the stress required

maintaining this strain is measured as a function of time (Marriott, 1988). The creep/recovery

test is therefore an alternative for obtaining the relaxation time and viscoelastic properties of

a material.

2.4 Starch modification.

The industrial applications of starch are often limited because it is used mainly in its

unmodified form. Very often the viscosity of cooked native starch is so high that it precludes


23

its use in specific applications. For example, tuber starches on cooking give high peak

viscosity, which upon continued cooking and cooling drops, contrary to cereal starches,

which show moderate host paste viscosity, but result in substantial increase in setback

viscosity upon cooling; some starch dispersions are gummy and not palatable; amylose-rich

starches form rigid, opaque gels on cooling (due to retrogradation), which on storage lose

water (syneresis), whereas amylopectin-rich starches (waxy-type) form soft gels.

To meet the demanding technological needs of today, the properties of starch are modified by

a variety of modification methods. Starch modification is aimed at correcting one or some of

the short comings, which will enhance its versatility and satisfy consumer demand. Thus, the

various chemically or otherwise modified starch derivatives offer significant value addition

and give scope to develop a variety of fabricated food products having varied texture and

mouth feel. These modifications are aimed at introducing desirable alterations in the starch

structure so that its behavior is predictable and controllable (Fig 2.8).

Fig2.8: Modification of starch


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Therefore, the modified starch derivatives are the products of either glucosidic bond cleavage

(acid modification to dextrins) or forming new functional groups (carbonyl group formation

during oxidation), or substitution of free available hydroxyl groups (by etherification or

etherification) or bridging of molecular chains by cross-linking reactions. The various

modifications employed are physical modification, chemical and enzymatic modifications.

Physical Modification

The common physical modification methods of starch include pregelatinisation, heat-

moisture treatment (HMT), annealing, steam treatment, extrusion and gamma irradiation

(Eliasson and Gudmundsson,1996, Sair, 1967, Raja, 2000, Bao and Corke, 2003). Physical

modification involves the simultaneous action of several conditions such as temperature,

pressure, moisture and shear. Temperature and moisture contents during processing of starch

alter its functional properties.

Pre-gelatinization: Pre-gelatinization is the simplest of all starch modifications. It is

effected by the cooking of aqueous starch slurry and subsequent drying. These starches are

very useful in the preparation of ready-to-eat convenience foods; they give a palatable texture

and help to hold other components in a uniform suspension. The market for such starches is

steadily expanding. They are also useful as wall paper adhesives. The process for the

production of pregelatinised starch involves drying of 30-40 % (dry solid) starch slurry on a

roller drum drier heated to 160-1700C by direct steam. The product exhibits high

transparency, high viscosity and good color carrier properties Drum drying is the most

common method of producing prelatinised starch. In general there are two types of drum

dryers, the single and double drum dryer and are used in large scale manufacture of

pregelatinised starch.

Heat moisture treated starch (HMT): Heat moisture treatment of starches is defined as the

physical modification that involves the treatment of starch granules at low moisture levels for

a certain period at a temperature above glass transition points. The studies on HMT of tuber

and root starches indicated that the extent of starch chain association within the amorphous

region and the degree of crystalline order of the starch granules is altered during HMT

(Gunaratne and Hoover, 2001). Structural changes within the amorphous region and


25

crystalline region of starch granules such as starch chain interaction within the amorphous

region and disruption and reorientation of starch crystallites are caused by heat-moisture

treatment. (Hoover and Manuel, 1996b). It seems that disruption of starch crystallites and

reorientation of amylopectin double helices by heat-moisture treatment allow more reaction

reagents to access the crystalline region increasing derivatization in that region. Heat

moisture treatment makes significant variations in the XRD pattern of starches.

Transformations from B to A or B+A type by HMT have been reported for potato starch by,

Gunaratne and Hoover (2002), Hoover and Vasanthan (1994). As stated by Genkina, et al.,

(2004), HMT often results in transformation of the less thermodynamically stable B-

polymorphic structure (with hexagonal packing of double helices and about 36 water

molecules inside every cell) to a more stable monoclinic structure of A-type polymorphs

(with about six water molecules inside the helices)

Extrusion: Extrusion cooking, because of its low cost and continuous processing capability

is a popular means of modifying the functional characteristics of starches. Numerous studies

have reported on the complexities of extrusion process and modeling of the process.

Extrusion cooking can be described as a process whereby the moistened materials are cooked

and worked into viscous, plastic like dough. Extrusion cooking represents a more modern and

versatile process, but depending on the specific mechanical energy input and product

temperature, solubility is the pronounced functional characteristic. The improved functional

characteristics favour application in various fields (Colonna et al., 1989).

Annealing: Annealing refers to treatment of starch in excess water (


26

for normal and high amylose starches. Amylopectin regions in the starch are perfected by

annealing process (Knutson, 1990). The swelling power and solubility of the starches are

reported to be lowered by annealing (Lorenz and Kulp, 1978, Lorenz and Kulp, 1984)

The pregelatinised and heat-moisture treated starches are the major physically modified

products from cassava starch (Sriroth, 2002). The process for the production of pregelatinised

starch involves drying of 30-40 % (dry solid) starch slurry on a roller drum drier heated to

160-1700C by direct steam. The product exhibits high transparency, high viscosity and good

colour carrier properties.

Pyrodextrins: Pyrodextrins are starch derivatives obtained by either dry heating or heating

of the aqueous starch slurry with or without pH change (Fig 2.9)

Fig 2.9: Pyrodextrinisation of starch

The three major reactions taking place during dextrinization are glycosidic bond cleavage (by

hydrolysis), glycosidic bond formation (transglycosidation), and repolymerization. They are

commercially very useful products for various applications

Photooxidation: Photooxidation of starch in the presence of atmospheric oxygen gives rise

to gluconic and glucuronic acids. The former further degrades to yield D-arabinose. The


27

reaction is of free radical type involving glycosidic bond cleavage, as well as the cleavage of

C-5 and C-1 bond of glucose residues (Fiedorowicz et al., 2001)

Micronization: The micronization (a physical damage induced by McCrone micronizing

mill) of barley starch showed amylopectin of low molecular weight being preferentially

solubilized in cold water. Depending upon the conditions, more amylose is extracted into

cold water (Tomasik and Zaranyika 1995).

Polarized light treatment: Some studies have been made on the degradation of starch by

polarized light. Moonlight is assumed to be a source of polarized light (Hoover, 1998).

Preliminary exposure to polarized light did not affect the crystalline structure of starch,

although some changes in melting temperature and transition enthalpy were seen. Prolonging

the exposure led to some degree of cross-linking, as shown by increased molecular weight.

This has been attributed to activation of enzymes adhering to starch granule surface.

Sensitivity of amylopectin to illumination exceeds that of amylose (Whistler 1998).

UV light induced starch degradation: In the case of UV light induced starch degradation,

the radiation is first absorbed by acetal chromophore at C-1 of glucose unit followed by

further photoreaction. Formation of peroxide ion at C-1 leads to gradual chain scission and

reduced molecular weight, paste clarity/viscosity, and melting enthalpy. Prolonged

irradiation leads to cross-linking with increases in molecular weight (Tharanathan, 1995).

The surface derivatization of starch: The surface derivatization of starch granules is

another approach for bringing in desirable changes. The complexing of amylose by lipid

molecules influences both thermal and rheological properties of wheat starch, whereby the

leaching of amylose molecules from the granules to the water is restricted (Setser and

Racette, 1992, Radhika et al., 2008)

Chemical modifications

Chemical modification of starch generally occurs via the introduction of functional groups

that change starch properties. Thus the aim of chemical modification is to modify cooking

characteristics, decrease retrogradation, decrease the gelling tendencies of pastes, increase


28

freeze-thaw stability, decrease gel syneresis, improve film formation, improve adhesion and

improve emulsion stability.

Acid thinned starch: This is one of the earliest methods of starch modification, and the

derived degradation products have a vast application potential. In its simplest methodology,

the native granular starches are subjected to treatment with acids, either at room temperature

(for a period of several days) or at elevated temperature (for several hours). The extent of

degradation is measured by the release of reducing sugar (called dextrose equivalent). Acid

modification is widely used in the starch industry to prepare thin boiling starches for use in

food, paper, textile and other industries (Rohwer and Klem, 1984). The amorphous regions of

starch are more rapidly hydrolyzed than are the crystalline regions during acid hydrolysis at

temperatures below the gelatinization temperature (BeMiller, 1965). Kerr (1952)

demonstrated that in early stages of acid modification, the amount of amylose or linear

fraction in starch increased and amylopectin was preferentially hydrolyzed, inferring amylose

was protected by forming a resistant complex with particles of amylopectin. The

retrogradation rate of acid-thinned starch gels increased as hydrolysis proceeded (Kang etal

1997). Acid modification also increased solubility and gel strength and decreased viscosity of

starches (Kim and Ahn, 1996, Osunsam et al., 1989). The viscoelastic properties of starches

are also affected by acid hydrolysis. Virtanen, et al, (1993) reported that the gel of acid

modified oat starch, although less rigid, was more elastic than the corresponding native starch

gel. Dynamic rheological tests showed that the dispersions of acid modifiedwaxy corn starch

behaved as Newtonian liquid-like solution, while the unmodified counterpart behaved like

weak gels (Chamberlain and Rao, 1999).

Oxidation-Oxidation is an important modification method for bringing about changes in

physicochemical properties of starch. Oxidative agents modify starch by forming new

functional groups in the molecule. Oxidation with hypochlorite or more rarely with

potassium permanganate is an old method but still used. It involves conversion of primary

hydroxyl group to carboxyl group. Oxidized starches (Wurzburg, 1986) are mainly prepared

by treating starch with sodium hypochlorite, the rate of the reaction being influenced by the

acidity of the reaction medium. Whistler and Schweiger (1965) demonstrated that

hypochlorite oxidation of corn amylopectin was most rapid at neutral pH while the reaction


29

rate decreased with increasing acidity and alkalinity. Similar results were observed on wheat

and corn starches. The type and amount of functional groups formed in the starch molecule

depend on the reaction pH as well. The formation of carbonyl group was found to be higher

under acidic conditions while the amount of carboxyl group increased with increasing pH

(Schmorak et al., 1963).

Substitution: Substituted starches are prepared by treating starches with various

chemicals under controlled conditions. Studies have shown that substitution decreases the

extent of syneresis (exudation of water during frozen storage), gelatinization transition

temperatures, and pasting temperatures Esterification is an important modification method of

starch. The commonly used reagents for esterification are acetic anhydride, acetic acid, vinyl

acetate, succinic anhydride, alkenyl succinic anhydrides, citric acid and formic acid.

Fig: 2.10: Starch modification reactions ( Rudrapatnam N. Tharanathan 2002)


30

Starch acetate: A common starch modification is acetylation, which is the esterification of

starch polymers with acetyl groups to form starch acetates (Jarowenko, 1986). Acetylation

has been reported to increase the water absorption and to lower the pasting temperature and

set back of rice starch (Gonzalez and Perez, 2002). Gelatinisation temperature was

significantly reduced by the introduction of acetyl groups. Factors such as amylose to

amylopectin ratio, intragranular packing and the presence of lipids mainly govern the degree

of substitution during acetylation of starches from different sources (Singh, Kaur et al.,

2004a and Singh, Chawla et al., 2004). Starches with low amylose content have been

observed to exhibit a higher degree of substitution after acetylation. Rutenberg and Solarek

(1984) reported that the introduction of acetyl groups upon acetylation reduces the bond

strength between starch molecules and thereby increases the swelling power and solubility of

the starch granule, decreases the coagulation of the starch, and provides improved freeze-

thaw stability. The extent of physico-chemical property changes in the acetylated starch

compared to the native starch is proportional to the degree of acetylation or degree of C = O

substitution incorporated into the starch molecules. The C = O bond of the acetyl group

experiences a different molecular environment depending on whether it is a substituent on

amylose or on amylopectin. (Phillips et al., 1999). Equation depicting the reaction of starch

with acetic anhydride is given below (Fig 2.11).

Fig 2.11: Formation and structure starch acetate.

Starch succinates : Starch succinates are prepared by a base catalyzed reaction of succinic

anhydride in aqueous medium. Succinylation increases hydrophilicity of the starches. The


31

starch side chain carboxylic groups in succinates provide useful properties such as metal

chelation (Jeon et al., 1999). Low DS starch succinates could be obtained by refluxing starch

with succinic anhydride in pyridine at 115C for varying reaction times without prior

gelatinization (Lohmer and Rist, 1950, Rutenberg and Solarek, 1984, Bilmers and Tessler,

1995). Bhandari and Singhal (2002) have optimized the reaction conditions for the

preparation of succinate derivatives from corn and amaranth starches in non-aqueous medium

and a starch: pyridine ratio of 2:1 was found to be vital for the reaction to take place.

Bhandari et al., (2002) have studied the rheological properties of succinylated corn and

amaranth starches. The effect of various reaction conditions (pH, time, temperature and

reagent concentration) on the succinylation of Canavalia ensiformis starch was studied by

Betancur et al., (2002). Typical equation depicting the reaction of starch with succinic

anhydride is given below (Fig 2.12).

Fig 2.12: Formation and structure of starch succinate

Starch Octenyl succinate : Starch octenyl succinates can be prepared by the esterification of

starch with a substituted dicarboxylic acid anhydride, 1-octenylsuccinic anhydride (OSA) at

pH 7-9 (Thomas and Atwell, 1997). They are used as emulsifiers and emulsion stabilisers in

salad dressings, in beverages etc and as clouding agents used to stabilise the oil-water

interface of an emulsion. One key application of OSA treated starch is the replacement of

gum arabic in systems that require emulsion stabilisation or encapsulation.

Octenylsuccinylation of starch lowers the gelatinisation temperature, improves paste clarity,

provides stability to retrogradation and modifies the texture of the starch (Trubiano, 1986).

Modification with octenyl succinic anhydride has been reported to increase the paste

viscosity and swelling volume and reduce the gelatinisation temperature of rice starch (Bao et

al., 2003, Shih and Daigle, 2003). High DS OSA derivatives produced firmer starch gels.


32

Park et al., (2004) have studied the rheological properties of corn starch octenyl succinates.

The OSA starch pastes exhibited high shear-thinning behavior. Equation depicting the

reaction of starch with octenyl succinic anhydride is given below (Fig 2.13).

Fig 2.13: Formation and structure of starch octenyl succinates

Starch citrates -Citric acid is considered harmless in food applications compared to other

substances used for starch derivatisation (Klaushofer et al., 1978a). Starch citrates are used in

various food products to increase the dietary fiber contents in the form of resistant starch

(RS4). Weppner et al., (1999) have reported the synthesis of citric acid esters of corn, pea,

potato and wheat starches and they have obtained derivatives with resistant starch content up

to 57.5 %. They have also observed that the resistant starch content increased with increase in

the DS. The effect of various reaction conditions on resistant content in the corn starch citrate

was investigated by Xie and Liu (2004). When the reaction was carried out at 140C for 7 h,

the highest RS content of 87.5 % was obtained in the waxy corn starch citrate having a DS of

0.16. When heated, citric acid is dehydrated to yield the citric anhydride which can react with

starch present in the reaction medium to form the starch citrate. On further heating, additional

dehydration occurs which results in the formation of cross-links between starch chains

(Wing, 1996). Equation depicting the reaction of starch with citric acid anhydride is given

below (Fig 2.14).


33

Fig 2.14: Formation and structure of starch cirates

Cross-linked starch-Cross-linked starches constitute a major class of modified starches.

Cross-linking reinforces the H-bonds in the granules with chemical bonds. Therefore, cross-

linked starches are more resistant to acid, heat and shearing than the native starch. The most

widely used cross-linking agents include STMP, STPP, phosphorus oxychloride,

epichlorohydrin (EPI) and adipic acetic mixed anhydride (Wu and Seib, 1990, Yeh and Yeh,

1993, Yook et al., 1993, Wurzburg, 1986c and Bergthaller, 2004). Cross-linking using

multifunctional reagents introduces intermolecular bridges which result in restricted swelling

of the granules during gelatinisation and minimize granule rupture (Eliasson and

Gudmundsson, 1996, Woo and Seib, 2002, Reddy and Seib, 1999). Reagents such as

epichlorohydrin, phosphorous oxychloride, metaphosphate, citric, or adipic acids, react with

starch forming intermolecular cross-linking of molecules. Langan (1986) has reported that

pastes from cross-linked starches are more viscous, heavily bodied and are more stable to

heat, shear and low pH. Cross-linking reduces loss of viscosity and formation of stringy paste

during cooking (Woo and Seib, 2002). According to Liu et al., (1999) the effect of cross-

linking is different for the waxy and non-waxy starches. Cross-linking minimizes granule

rupture, loss of viscosity and the formation stringy paste during cooking (Woo and Seib,

1997), yielding starch that is suitable for canned foods and other food applications (Hirsch

and Kokini, 2002, Rutenberg and Solarek, 1984). Jane, Radosavljevic, and Seib (1992) found

that cross-linking of starch chains occurred mainly in amylopectin region of the starch

Another factor that may influence the extent of cross-linking is the size distribution of starch

granule population (Hung and Morita, 2005). During cross-linking, small size granules have

been reported to be derivatized to a greater extent than the large size granules (Bertolini et


34

al., 2003). Equation depicting the reaction of starch with cross linking agent is given below

(Fig 2.15).

Fig 2.15: Formation of crosslinked starch

Hydroxypropyl Starch (HP): Modification by hydroxypropylation is especially interesting

because of its toxicological safety and accommodative properties (Wurzburg, 1986c).

Hydroxypropylation is a form of starch etherification. Hydroxypropyl starch (HPS)

derivatives are prepared by reacting starch with propylene oxide as etherifying reagent,

leading to introduction of hydroxypropyl groups onto the polymeric chain of starch.

Propylene oxide is reactive as a result of its highly strained three-membered epoxide ring.. In

forming HPS, part of the hydroxyl groups of the anhydroglucose unit (AGU) would be

converted into O-(-2-hydroxypropyl) groups (Bergthaller, 2004). Though all the three

hydroxyl groups (at O-2, O-3 and O-6) are amenable for substitution, the derivation mainly

takes place at O-2. The substitution of HP groups takes place preferentially in the amorphous

domains of amylopectin, with most of it at the C-2 hydroxyl group (67-78%), whereas at O-3

and O-6 it is 15-29% and 2-17%, respectively. Bulky HP groups prevent the alignment of

starch chains (due to steric effects) and reduce starch retrogradation characteristics. The

substituent disturbs the association of the polysaccharide chains preventing retrogradation

due to the hydrogen bonds. Improved functional properties of hydroxypropylated starches

such as extended shelf life of cold storage products (freeze-thaw stability), higher peak

viscosity and paste clarity, and decreased gelatinization temperatures are well documented

(Hoover etal 1988 , Kim and Eliasson, 1993, Liu et al, 1999, Pal et al., 2002, Perera et al.,

1997). The disruption of H-bonds due to the introduction of hydroxypropyl groups weakens

the granular structure of starch and this effect alters its pasting properties (Seow and

Thevamalar, 1993, Wootton and Mantsathi, 1983, Choi and Kerr, 2003, Yeh and Yeh, 1993,


35

Kim et al., 1992, Liu et al., 1999b). The degree of syneresis has been observed to decrease

with increasing MS of hydroxypropyl potato starch (Eliasson and Kim, 1992). HP starches

have excellent film forming properties that are of use in biodegradable films. In general,

starch ethers are more stable to cleavage by acids. Equation depicting the reaction of starch

with propylene oxide is given below (Fig 2.16).

Fig 2.16: Formation and structure of hydroxypropyl starch

Hydroxypropyl (HP) starches are designed to withstand vagaries of cooking, such as high

temperatures, shear forces, and extremes of pH. HP starches offer pastes of increased clarity

and freeze-thaw and cold storage stabilities. The substituent disturbs the association of the

polysaccharide chains preventing retrogradation due to the hydrogen bonds. Improved

functional properties of hydroxypropylated starches such as extended shelf life of cold

storage products (freeze-thaw stability), higher peak viscosity and paste clarity, and

decreased gelatinization temperatures are well documented (Hoover et al., 1988, Pal et al.,

2002, Perera et al., 1997). Kim, et al., (1992) have used light microscopy for

hydroxypropylated potato starches and suggested that hydroxypropylation mainly takes place

at the central region of the granules. It has been reported that granule swelling is essential

for the substitution reaction to take place in granular starch (Hauber et al., 1992).


36

Starch phosphorylation: Starch phosphorylation is the earliest method of starch

modification. The reaction gives rise to either monostarch phosphate or distarch phosphate.

Phosphorylated starch is produced through esterification of the starch with phosphorylating

agents such as sodium tripolyphosphate (STPP), sodium trimetaphosphate (STMP), sodium

orthophosphate, phosphorus oxychloride or sodium hexametaphosphate (Kerr and Cleveland,

1962). Monosodium hydrogen phosphate (NaH2PO4) and disodium hydrogen phosphate

(Na2HPO4) can produce derivatives with DS up to 0.2 (Bergthaller, 2004). Phosphorylation is

usually done by a dry heat reaction in the temperature range 140-1600C. The phosphate

diester starches have the phosphate esterified with two hydroxyl groups, very often from two

neighboring starch molecules. This leads to the formation of a covalent bridge or cross-

linking. Phosphate cross-linked starches show resistance to high temperature, low pH, high

shear and leads to increased stability of the swollen starch granule. They improve viscosity

and textural properties of the starch. As a thickener and stabilizer, starch phosphate diesters

are superior to unmodified starches. They also provide resistance to gelling and

retrogradation, and do not synerise on storage. Starch phosphate can substitute gum arabic (at

~ 0.5% level) in sugar syrups, ice cream mixes, salad dressing, and pudding. Waly et al.,

(1994) have studied the effect of various reaction conditions on the phosphorylation of starch

with urea, phosphoric acid and tetra sodium phosphate decahydrate. The effect of pH on the

phosphorylation of sago starch with STMP and STP was studied by Muhammad et al.,

(2000). Efforts have been made to replace the conventional processes for the production of

starch phosphates with an extrusion cooking process Phosphorylation of rice starch by

extrusion cooking showed that increased barrel temperature (120 180 0C) resulted in greater

phosphorus incorporation into the starch. Phosphorylation at low levels of substitution

resulted in greater solubility, swelling power, paste viscosity and clarity for rice starch.

Starch granule size has impact on the effect of phosphorylation on the properties of rice

starch. Phosphorylated rice starch has been reported to exhibit a reduced degree of

hydrolysis by acid or amylase. Equation showing the reaction of starch with phosphorylating

agents is given below (Fig 2.17).


37

Fig 2.17: Formation and structure of starch phosphate.

Carboxymethyl starch- Here the hydroxyl groups of starch are partly substituted with

sodium monochloroacetate (SMCA) to give carboxymethyl starch (CMS). The

carboxymethyl substitution of starch hydroxyl groups gives rise to derivatives that are

coldwater-soluble. This modification procedure has a positive effect on the applicability in

the areas like in the textile, papermaking and pharmaceutical industries. To prevent starch

gelatinization, the reaction has to be carried out in an organic medium. Carboxymethyl

starch, under the name sodium starch glycolate, is used in the pharmaceutical industry as a

disintegrant and as sizing and printing agent in the textile industry. Highly substituted

derivatives are possible. Ohtani et al., (1977) have studied the carboxymethylation of

hydrolyzed hemicellulose and starch, which are useful as builders for detergents. Stojanovic

et al., (2000) have reported the synthesis of carboxymethyl starches in an ethanol/water

medium under different experimental conditions. The starch type was found to influence the

DS values of the products in ethanol medium. CMS is reported to have increased water

solubility and with increase in DS, the solubility also increased. The effects of various

reaction conditions on the carboxymethylation of arrowroot starch in isopropanol water

media were investigated by Kooijman et al., (2003). Ragheb et al., (1997) have synthesized

carboxymethylated derivatives of native and oxidized starches of different molecular sizes

and studied their application in textile printing.


38

Resistant Starch

The greater awareness on the part of consumers of the relationship between a nutritious diet

and health and well-being has been one of the reasons for the increase in popularity of novel

foods with good nutritional properties (Prez-Alvarez, 2008b, Sanz, et al., 2008a). Resistant

starch refers to the portion of starch and starch products that resist digestion as they pass

through the gastrointestinal tract. RS is an extremely broad and diverse range of materials

and a number of different types exist (RS1 4). At present, these are mostly defined

according to physical and chemical characteristics (Nugent, 2005). They provide the mouth-

feel of high fat emulsions in low fat or fat free products, lend a glossy, fat-like appearance,

allow less fat pickup in some fried products, and are of use in the formulation of dietetic

foods. They are either partially or totally undigested, thus contributing zero calories to the

food on consumption. The four distinct classes (Fig 2.18) of RS in foods are: (1) RS1

physically inaccessible starch, which is entrapped within whole or partlymilled grains or

seeds; (2) RS2 some types of raw starch granules (such as banana and potato) and high-

amylose (high-amylose corn) starches; (3) RS3 retrograded starch (either processed from

unmodified starch or resulting from food processing applications); (4) RS4 starches that are

chemically modified to obtain resistance to enzymatic digestion (such as some starch ethers,

starch esters, and cross-linked starches) (Ratnayake and Jackson, 2008, Sanz et al., 2009)

Fig 2.18: Different type of resistant starch


39

Dual modified (substituted and cross-linked) normal maize, waxy maize, tapioca, potato and

normal wheat starches are available commercially with varying degrees of

hydroxypropylation and cross-linking. The temperatures and enthalpies of gelatinization of

the dual modified (hydroxypropylated/cross-linked or acetylated/cross-linked) waxy wheat

and maize starches are generally lower than those of the unmodified starches. Gelatinization

temperatures (To; Tp; Tc) of cross-linked waxy wheat starch and its hydroxypropylated/cross-

linked and acetylated/cross-linked forms are 5-7 C below than those of unmodified forms of

waxy maize starch when measured in excess water (Reddy and Seib, 2000). Dual-

modification, hydroxypropylation and crosslinking, is commercially carried out (Lopez,

1987, Tessler, 1975, Tuschhoff, 1986, Wurzburg, 1986, Yeh and Yeh, 1993). Starches with

high amylose content can be stabilized by initially reacting them with propylene oxide and

this reaction is inhibited by adding cross linking agents to yield modified starches having

outstanding high temperature and short time retort properties (Tessler, 1975). Phosphorus

oxychloride, sodium trimetaphosphate, and epichlorohydrin are generally used as cross

linking reagents by several authors (Luallen, 1985, Smolka and Alexander, 1985, Takahashi

et al., 1989, Tessler, 1975, Valle, et al., 1978, Wu and Seib, 1990, Yeh and Yeh, 1993,

Yook et al., 1993).

Enzyme modified starch: Enzymatic modification of native starch may be considered

as one of the techniques to modify native starch by decreasing the molecular weight .The

Enzyme conversion of starch (Fig 2.19) is used to produce derivatives with varying

viscosity, gel strength, thermo reversibility and sweetness. In enzymatic modification

techniques, the gelatinised starch is subjected to degradation by enzymes resulting in various

products (Alexander, 1992).


40

Fig 2.19: Enzyme hydrolysis of starch

Selective enzyme hydrolysis of starch produces a range of products like glucose, maltose,

oligosaccharides and polysaccharides with varying chain length and dextrose equivalent (DE)

(Taggart, 2004). The major enzyme modified products are linear dextrins of varying DE,

high fructose syrups, glucose syrups, dextrose, maltodextrins and cyclodextrins (Raja, 1995,

Blanchard and Katz, 1995). The commonly used enzymes are and amylases, iso amylase

and pullulanase. The amylase selectively attacks the (1, 4)-linkages of the starch and

produces maltodextrins and low DE dextrins. However, amylase hydrolyzes every other 1,

4- linkages to give lower molecular fragments and higher DE syrups like maltose.

Isoamylases and pullulanase give high DE syrups through hydrolytic attack at specific sites

such as 1, 6-linkages in the starch. Cyclodextrins are produced by the enzyme hydrolysis of

starch using Cyclodextrin glycosyltransferases (Kumar, 1995). Cyclodextrins find application

in food processing, pharmaceutical and agrochemical industries for preparation, separation,

purification and protection of pharmaceuticals, fragrance, flavors and steroids.


41

2.5 Applications of modified starches

Starch is a staple in the diet of much of the worlds population and is also widely used in the

Western world in the food and beverage industries as a thickener and a sweetener, as well as

having some manufacturing applications in the paper and textile industries. The more

prevalent use of starch for industrial purposes will become economically viable when its use

as a raw material rivals those derived from petroleum-based products. Starch based products

have traditionally been used by the water treatment industry as a coagulant or flocculant aid.

Potato starch is associated with better performance than other types because of its high

potassium content. Starch based products have been displaced to a large extent by synthetic

poly electrolytes because of the superior performance and lower dosage rates required of the

latter. The textiles industry is an important market for starch. There are three main

applications of starch: sizing printing, and finishing. Adhesives are a traditional application

for starch. Starch based adhesives are primarily used for paper bonds with the most important

sector corrugated board production. The binding and bonding properties of starches with high

levels of amylopectin make a good addition to adhesives, especially on bottle labels, which

are often subject to water and high humidity. These all are the traditional and declining

markets of starch (Garth Entwistle, 1997), Some of the new and developing markets for

starch are,

Mineral oil drilling - Starch can be used in the oil industry as a drilling aid included in

water-based fluids. Drilling fluids, in the form of circulating aqueous clay suspensions are

used to stabilize bore hole walls and enable drilled solids to be transported to the surface.

Starch products are incorporated into water-based drilling fluids to control fluid loss. These

products are degradable and reduce environmental damage when they replace oil-based

fluids.

Agrochemicals-Starch is of interest in the agrochemical industry as an encapsulation agent

for pesticides, and for the production of aqueous base pesticide formulations. Starch

encapsulation leads to the safer handling of pesticides and can improve the efficiency of the

active ingredient with an improved delivery to the target pest and a reduction in losses due to

evaporation, leaching and light decomposition.


42

Biodegradable plastic-films-Growing interest has been shown in the incorporation of starch

into plastics to make them biodegradable. The use of starch as a thermoplastic material is of

recent origin. Compared to starch films, low dextrose equivalent dextrins and corn syrup

were more resistant to water vapor transport. Films of starch hydrolysates have shown some

resistance to oxygen transmission.

Cosmetics and toiletries-Starch and starch derivatives, could potentially be used for the

production of a wide range of cosmetics and toiletry products. The use of sorbitol in

toothpaste and cosmetic creams is an example of a well established use of a starch derivative

in this sector. Starch grafted co-polymers after saponification, find important applications as

absorbents in disposable soft goods designed to absorb body fluidsnappies, incontinence

pads, female sanitary products. Enzyme catalyzed esterification of n-butylglucosidea

process patented by Cerestar, gives a product with a number of equally interesting features

that are useful within cosmetic formulations.

Pharmaceuticals : Starch considered as the most used EXCIPIENT in pharmaceutical

formulations. It has many pharmaceutical applications and it is used mainly in tablets as a

filler, binder or disintegrant

Excipients are increasingly being recognized for the critical role they play in pharmaceutical

products. Providing specific special functionalities in formulations, pharmaceutical

excipients contribute enormously to the efficacy of a product. They bind tablets together

under the stress of direct compression, control the release of active ingredients, help tablets

disintegrate and dissolve efficiently and influence absorption. In a tablet formulation, a range

of excipient materials is normally required along with the active ingredient in order to give

the tablet the desired properties

Filler: Fillers are used to make tablets of sufficient size for easy handling by the patient and

to facilitate production. Tablets containing a very potent active substance would be very

small without additional excipients. Good filler will have good compactability and flow

properties, acceptable taste, will be non-hygroscopic and preferably chemically inert. It may

also be advantageous to have a filler that fragments easily, since this counteracts the negative

effects of lubricant additions to the formula (de Boer et al., 1978.).


43

Binder: A material with a high bonding ability can be used as a binder to increase the

mechanical strength of the tablet. A binder is usually a ductile material prone to undergo

plastic (irreversible) deformation. Typically, binders are polymeric materials, often with

disordered solid state structures. Of special importance is the deformability of the peripheral

parts (asperities and protrusions) of the binder particles (Nystrm et al., 1993.) The effect of

the binder depends on both its own properties and those of the other compounds within the

tablet. A binder is often added to the granulation liquid during wet granulation to improve the

cohesiveness and compactability of the powder particles, which assists formation of

agglomerates or granules.

Disintegrating agent: 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; There are several types of disintegrants, acting with different

mechanisms: (a) promotion of the uptake of aqueous liquids by capillary forces, (b) swelling

in contact with water, (c) release of gases when in contact with water and (d) destruction of

the binder by enzymatic action (Rudnic and Kottke, 1999). Starch is a traditional

disintegrant; the concentration of starch in a conventional tablet formulation is normally up to

10% w/w. The starch particles swell moderately in contact with water, and the tablet disrupts.

So-called superdisintegrants are now commonly used;

Glidant, antiadherent and lubricant: Glidants are added to increase the flowability of the

powder mass, reduce interparticular friction and improve powder flow in the hopper shoe and

die of the tabletting machine. An anti adherent can be added to decrease sticking of the

powder to the faces of the punches and the die walls during compaction, and a lubricant is

added to decrease friction between powder and die, facilitating ejection of the tablet from the

die. However, addition of lubricants (here used as a collective term, also including glidants

and antiadherents) can have negative effects on tablet strength, since the lubricant often

reduces the creation of inter-particular bonds (de Boer et al., 1978)

Flavours, sweetener and colourant: Flavour and sweeteners are primarily used to improve

or mask the taste of the drug, with subsequent substantial improvement in patient


44

compliance. Colouring tablets also has aesthetic value, and can improve tablet identification,

especially when patients are taking a number of different tablets (Susanne bredenberg, 2003)

Tablets account for more than 80% of all pharmaceutical dosage forms administered to

people Pharmaceutical formulation is the process by which active ingredients of drugs are

converted into preparations which are safe, effective and convenient to use.Of the two oral

solid dosage forms commonly employed in this country, the tablet and the capsule, the tablet

has a number of advantages. One of the major advantages of tablets over capsules, which has

recently proved significant, is that the tablet is an essentially tamperproof dosage form.

In consideration of the following may be cited as primary potential advantages of tablets.

1. They are a unit dose form, and they offer the greatest capabilities of all oral dosage

forms for the greatest dose precision and the least content variability.

2. Their cost is lowest of all oral dosage forms.

3. They are the lightest and most compact of all oral dosage forms.

4. They are in general the easier and cheapest to package and ship of all oral dosage

forms.

5. Product identification is potentially the simplest and cheapest, requiring no additional

processing steps when employing an embossed or monogrammed punch face.

6. They may provide the greatest ease of swallowing with the least tendency for hang-

up above the stomach, especially when coated, provided that tablet disintegration is

not excessively rapid. They lend themselves to certain special release profile

products, such as enteric or delayed-release products

7. They are better suited to large-scale production than other unit oral forms. They have

the best combined properties of chemical, mechanical and microbiologic stability of all

the oral forms.

2.6 Tablet Formulation

A tablet formulation typically consists of an active pharmaceutical ingredient (API) together

with nonactive ingredients, or excipients, such as fillers or diluents, binders or adhesives,


45

disintegrants, lubricants and glidants, colours, flavours and sweeteners (Fig 2.20). It might

also be necessary to add miscellaneous components such as buffers, depending on the

application. Common goals in pharmaceutical development and research work are to develop

formulations of required stability, with specific release profiles and to ensure that operating

conditions are robust during production (Peck et al., 1989).

Fig 2.20: Tablet and capsule manufacturing process

A need for new excipients

With the increasing number of new drug moieties with varying physicochemical and stability

properties, there is growing pressure on formulators to search for new excipients to achieve

the desired set of functionalities. The growing performance expectations of excipients to

address issues such as disintegration, dissolution, and bioavailability. The continued

popularity of solid dosage forms, a narrow pipeline of new chemical excipients, and an

increasing preference for the direct-compression process creates a significant opportunity for

the development of high-functionality excipients. . Fig 2.20 illustrates the different factors to

be taken in consideration when selecting a new excipient for the pharmaceutical tablet

production. The characterization of pharmaceutical excipients using a material science

approach has helped to design drug formulations to obtain a desired set of performance

properties. For tablets, a better understanding of the compression properties of the material

alone and in combination with other potential components helps in developing desirable

formulations as well as acceptable products (Ashok et al., 2006). When formulating tablets,


46

the choice of excipients is extremely critical (Fig 2.21). It must fulfill certain requirements

such as compressibility, good binding functionality, powder crystallinity, flowability and

acceptable moisture content.

Fig 2.21: Factors considered when excipient is selected for oral solid dosage forms

2.7 Sources of new excipients

Pharmaceutical excipients are inorganic or organic compounds, which are necessary

formulation of medicinal preparations suitable for direct application to a patient, although,

they are not holders of any pharmacological activity. As with drug substances, excipients are

derived from natural sources or are synthesized either chemically or by other means. They

range from simple, usually highly characterized, organic, or inorganic molecules to highly

complex materials that are difficult to fully characterize. In earlier days, excipients were

considered inactive ingredients. Excipients are now known to have defined functional roles in

pharmaceutical dosage forms. These include (i) modulating solubility and bioavailability of

the active ingredient(s); (ii) enhancing stability of the active ingredient( s) in finished dosage

forms; (iii) helping active ingredients maintain a preferred polymorphic form or

conformation; (iv) maintaining pH and osmolarity of liquid formulations; (v) acting as


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antioxidants, emulsifying agents, aerosol propellants, tablet binders, and tablet disintegrants;

(vi) preventing aggregation or dissociation; and (vii) modulating the immunogenic response

of active ingredients (e.g., adjuvants) and many others. It is becoming increasingly apparent

that there is an important relationship between the properties of the excipients and the dosage

forms containing them ( Katdare and Mahesh, 2006)

The development of innovative pharmaceuticals is critical to improving the health care and

standard of living for billions of people around the globe. Commonly used excipients such as

corn starch, lactose, talc, and sucrose did not present significant questions of safety, and were

largely ignored by the regulatory community. Advancements in pharmaceutical technology

have rendered this view of excipients as simple inert pharmaceutical fillers obsolete.

Starch is usually called classical loosening agent. This effect is related not as much to the

swelling of starch grains (the degree of starch swelling in water at 37C does not exceed 5

10%) as to the ability of rendering tablets porous, which favors permeation of liquids

(Andreev, 2004). This behavior is consistent with the fact that starch as the loosening agent,

while neither significantly influencing water-soluble material nor improving the

disintegration characteristics of sugar-based tablets, is highly effective in lactose-based

tablets.

Both native starch and its modifications attract the attention of pharmacists developing drugs

with new compositions and pharmacological properties. The reasons for this interest are

predominantly as follows:

(i) The tendency to increased production of ready-to-use medicinal;

(ii) Variety of the structure and properties of starch produced from different raw materials;

(iii) Ability of native and modified starch to produce densification and stabilization of

various drug compositions;

(iv) The possibility of increasing the gel-forming and emulsifying properties of starch by

means of directed modification;


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(v) The possibility of increasing the stability of drugs under conditions of freezing thawing

cycles, high temperatures (sterilization), and strongly acid media ( Kul'fuis and Arende-

Scholte 2000, Bolhuis et al., 1981)

Starches from different botanical sources may not have identical properties with respect to

their intended use. Starch obtained from Ensete ventricosum, has been evaluated as tablet

binder using chloroquine phosphate, dipyrone and paracetamol as model drugs. Dioscorea

starch (composition on dry weight basis: 0.1% ash, 0.5% protein, 1% fat and 98.4% starch)

obtained from Dioscorea abysinica, was also evaluated as tablet binder (Odeku and Picker-

Freyer 2007). It showed better binding ability to that of maize starch and exhibiting

somewhat lower crushing strength and higher porosity. The binding performance of starches

obtained from taro (Colocassia esculenta) and sweet potato (Ipomoea batatas) was found to

be similar to that of commercial corn starch. Starches obtained from the seeds of Sorghum

bicolor, performed as well as maize starch and better than acacia in binding properties .The

uses of other alternative starches from rice, barley and wheat starches plantain starch from

Musa paradisiacia and tapioca, dried fibrous remnant material obtained from cassava,

Manihot esculenta have also been extensively reported in various tablet formulations

(Sanghvi et al., 1993).

Native starch upon such processing usually acquires a new superstructure. Precooked starch

described in the US Pharmacopoeia is good swelling and partly soluble. Swelling starch is

used in the production of tablets as a filling and binding component for wet granulation

process. The soluble part of a molecule acts as a binding agent, while the insoluble part

performs the role of filler possessing good disintegrating (loosening) properties. In

comparison with the tablets involving microcrystalline cellulose (MCC) and polyvinyl

pyrrolidone (PVP) as binding components, the compositions involving partly soluble

precooked starch possess comparable strength at a significantly shorter disintegration time.

The swelling starch in a powdered form can be introduced into the initial mixture prior to

granulation. During the granulation process, it is necessary to add water. This starch can be

additionally modified so as to improve its free-running ability (The USSR State

Pharmacopoeia 1989). The pregelatinization degree of starch paste influences the properties

of the resulting tablets (Itiola and Pilpel, 1986). A properly made paste is translucent rather


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than clear (which would indicate virtually complete conversion to glucose) and produces

cohesive tablets that are readily disintegrated when properly formulated (Marshall et al.,

1993)

Different chemically substituted starches are also used in the tablet production, starch acetate

and succinates are of special importance for pharmaceutical industry. Carboxymethyl starch

can be used in various tablet compositions, it is capable of preventing the detrimental

influence of hydrophobic lubricants (such as magnesium stearate) on the disintegration

characteristics (Eur. Patent No. 015963/1982 , Kul'fuis and Arende-Scholte 2000)

The European Pharmacopoeia (2002) defines tablets as solid preparations each

containing a single dose of one or more active substances and usually obtained by

compressing uniform volumes of particles. Tablets are intended for oral administration.

Some are swallowed whole, some after being chewed, some are dissolved or dispersed in

water before being administered and some are retained in the mouth where the active

substance is liberated. Despite the long and continuing history of the development of new

technologies for administration of drugs, the tablet form remains the most commonly used

dosage form

2.8 Tablet manufacturing process

The manufacture of conventional tablets is a cost-effective process and involves different

processing steps including operations where particles are engineered with the intention of

optimizing functional properties such as the technical performance during tabletting and drug

release properties. The manufacturing of tablets requires certain qualities of the powder, low

segregation tendency, good flowability and compactability being examples. It is also

important that the materials constituting a tablet, i.e. drug and excipients, are chemically and

physically stable during processing and storage (Jonas Berddren, 2003). The different step in

the tablet manufacturing process is shown in Fig.2.22. The manufacturing starts with the

granulation process and is summarized below.


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Fig 2.22: Different steps in tablet manufacture process.

Granulation

For the powder mixture to flow evenly and freely from the hopper into the dies, it is usually

necessary to convert the powder mixture to free flowing granules. The most important

reasons for a granulation step prior to tableting are to improve the flow properties of the mix

and hence the uniformity of the dose, to prevent segregation of the ingredients in the hopper

of tablet machines, to improve the compression characteristics of the tablet mixture and to

reduce dust during handling. A granule is an aggregation of component particles that is held

together by the presence of bonds of finite strength. Granulation usually refers to processes

whereby agglomerates with sizes ranging from 0.1 to 2mm are produced. There are various

techniques of producing granules such as dry and wet granulation, extrusion (Johansson et

al., 1998, 2001) or spray drying.

Dry granulation

Dry granulation is a valuable technique in situations where the effective dose of a drug is too

high for direct compaction and the drug is sensitive to heat, moisture or both, which

precludes wet granulation. The blend of powders is forced into dies of a large heavy-duty

tableting press and compacted to slugs. The slugs or roller compacts are then milled and


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screened in order to produce a granular form of tableting material which flows more

uniformly than the original powder mix (Davies and Newton 1996).

Wet granulation

The most frequent procedure for the preparation of aggregates or granules in this context is

wet granulation,. This is carried out by adding a liquid binder or an adhesive to the powder

mixture, passing the wetted mass through a screen of the desired mesh size, drying the

granulation and then passing through a second screen of smaller mesh to reduce further the

size of the granules. Granulation is mainly performed in planetary mixers with low speed and

low shear forces. In wet granulation, liquid bridges are developed between particles, and the

resulting tensile strength of these bonds increases as the amount of liquid increases. During

drying, inter-particulate bonds result from fusion or recrystallisation and curing of the

binding agent. The formation of crystal bridges has been shown to be a major influence on

the physical characteristics of tablets especially if the solid is more soluble in the granulating

fluid (Fell and Newton, 1971b, Sebhatu et al., 1997).

Variables that affect granulation properties

Effect of binders on granule properties

None of the pharmaceutical ingredients is more fundamental than the binding agents used in

the formulation of granules. Most binding agents used for wet granulations, such as starch

paste, acacia mucilage, gelatin solution, simple syrup, methylcellulose solution and corn

syrup are hydrophilic in nature. These binders increase the bulk density and reduce the

porosity of the powder, thereby diminishing the effective surface area for evaporation. The

most significant changes in the physical properties affected by binder dilution were found in

granule friability and bulk density. Specifically, the more dilute binder solution resulted in

less friable granules. Also considerable influence is observed on interparticulate porosity and

thus on flow rate while insignificant effects were observed on average particle size and

granule density (Sofia mattsson, 2000)


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Effect of processing variables on granule properties

Modified Starch

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