chapter 2 review of literature - information and...
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Chapter 2
REVIEW OF LITERATURE
2.1. PECTIN
2.1.1. Introduction
Pectin plays an important role in food processing as food additives and as a source
of dietary fiber. Pectin gels are very important in creating or modifying the texture of
jams, jellies, confectionary and in low fat dairy products. They are also used as
ingredients in the pharmaceuticals industry and lower the glucose response. In order to
understand their type and content, pectins are separated based on their solubility by
sequential extraction in water or buffer solutions, solutions of chelating agents, dilute
acids, or dilute sodium hydroxide or sodium carbonate. It is also considered a safe
additive with no limits on acceptable daily intake (FAO, 1969; Gnanasambandam &
Proctor, 1999). Factors affecting the functionality of pectins include composition, degree
of methylation, solubility, pH, temperature and presence of soluble solids. Good quality
of pectin based on the high degree of esterification and intrinsic viscosity with low acetyl
content. Degree of methylation is related to the rate of gel formation. High methoxyl
pectins gel in the presence of sugar gel but low methoxyl pectin gel in the presence of
calcium. Gel strength depends on the length of molecule. At very low molecular weight,
pectin is unable to form gels under conditions (Pagan et al., 1999). Although pectin
occurs commonly in most of the plant tissues, the number of sources that may be used for
the commercial manufacture of pectin is limited. Various sources of pectin include citrus
peels, dried apple pomace, sugar beets, sunflower heads, residues of mango, guava,
papaya, coffee and cocoa processing. Currently half of the commercial pectins used in the
food industry are extracted from citrus peels (Voragen et al., 1995). Citrus pectins are
light cream to light tan in color whereas apple pectins are often darker. Gnanasambandam
& Proctor (1999) isolated pectins from soy hull, a co-product of soyabean processing
using nitric acid as well as enzymes. Enzymatic pretreatment of soyhulls increased
content of alkali soluble pectins as well as X-ray diffraction intensities. Studies showed
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that enzyme pretreatment improved the galacturonic acid content of pectins prepared
using conventional acid extraction process. Pectins are natural ionic polysaccharides with
many applications in food and pharmaceutical industry because of their ability to form
gels in the presence of divalent cations such as calcium (Braccini & Perez, 2001). Pectin
isolated from sugar beet pectin differs from other pectins in that it has higher branched
region in the form of acetyl content, which make sugar beet pectin unable to form gel.
Pectins also contain a small amount of protein around 2%, which make used as a
emulsion stabilizer also (Siew & Williams, 2008). Pectin is present not only in the
primary cell walls but also in the middle lamella between plant cells where it helps to
bind the cells together. The amount, structure and chemical composition of the pectin
differs between plants, within a plant over time and in different parts of a single plant.
During ripening, pectin is broken down by the enzymes pectinase and pectin esterase,
resulting in the process where the fruit becomes softer. This is because the middle lamella
that primarily consists of pectin breaks down and cells become separated from each other.
Pectin is thus also a natural part of human diet but does not contribute significantly to
nutrition. In human digestive system, pectin escapes the digestion in small intestine but is
acted upon by microbial growth of large intestines. Therefore, it also acts as soluble
dietary fiber. Pectin also reduces cholesterol absorption by increasing the viscosity in the
intestinal tract (Srivastva & Malviya, 2011). Pectin is a linear polysaccharide consisting
of a few hundred to one thousand saccharide units. The average molecular weight of
pectin varies from 50,000 to 150,000 (Whistler & Bemiller, 1997). It consists of D-
galacturonic acid units with very small amount of neutral sugars. The poly galacturonic
acid is partly esterified with methyl groups and the free acid groups may be partly or fully
neutralized with sodium, potassium or ammonium ions. (Monsoor et al., 2001).
Depending on the pectin source and the extraction mode, carboxyl groups are partially
esterified by methanol and in some cases, hydroxyl groups are partially acetylated.
Neutral sugars such as galactose, glucose, rhamnose, arabinose and xylose may also be
present bounded to the galacturonic acid as side chains and inserted into the main chain.
(Rolin & De Vries, 1990). Depending upon the degree of esterification, pectins are
divided in two categories: high-ester pectin with DE higher than 50% and low-ester
pectin, with DE lower than 50% (Thakur et al., 1997). In HMP, the gel is formed by
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building a junction zone resulting from the cross-linking of homogalacturan through
hydrogen bond and the hydrophobic interaction between methoxyl groups. In low-ester
pectin, junction zones are formed by calcium cross-linking between free carboxyl groups
(Willats et al., 2006). Pectin is also used in fillings, sweets, as a stabilizer in fruit juices
and as a source of dietary fiber. In nature, around 80% of carboxyl groups of galacturonic
acids are esterified with methanol. This proportion is although reported to decrease more
or less during pectin extraction. The ratio of esterified to non-esterified galacturonic acid
determines the behavior of pectin in food applications (Srivasatva & Malviya, 2011).
2.1.2. Pectin Isolation
The extraction of pectin involves the hydrolysis of insoluble protopectin into
soluble pectins and then leaching them out of the fruit tissues. Several methods have been
reviewed for the hydrolysis. Extraction with hot water is the simplest and oldest method
for recovering pectic substances from plant tissues (Hermann (1919)). A wide range of
reagents could be used for the extraction of pectin. The most commonly used are mineral
acids such as sulphurous acid, sulphuric acid, hydrochloric acid, phosphoric acid and
nitric acid. HCl is most widely used because it is cheap and high yield of pectin obtained
(Sudhakar, 1991). Francis & Bell (1975) reviewed the commercial pectins and postulated
that its anhydrouronic acid content, methoxyl content, degree of esterification, jelly grade
and jelly units, mainly judges the quality of pectin. Anhydrouronic acid content indicates
the percentage of other organic material present while ash content represents the
inorganic impurities. In order to understand their type and content, pectins are separated
based on their solubility by sequential extraction in water or buffer solutions, solutions of
chelating agents, dilute acids or dilute alkalis. Enzymes are also used in pectin extraction
i.e. endo-polygalacturronase or combination of pectin esterase/endo-polygalacturonase
and endo-arabinase and endo-galactanase. Less degrative non-pectolyic enzymes might
be useful in preparation of pectin ingredients in their native state without altering the
properties (Gnanasambandam & Proctor, 1999). Sakai & Okushima (1980) also prepared
pectin from citrus peels using. In this method, a strain of Trichosporon penicilatum (a
protopectin degrading enzyme producer) was used. Microorganism was added in the
citrus peels and fermentation was allowed for a period of 15-20 hrs at 30 °C. 20-25 g
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pectin / kg peels were obtained having high amount of neutral sugars. Prickly pear fruit
skin was used by Habibi et al. (2005) to isolate pectin by acid extraction followed by
fractionation. Fractionation was done to characterize the pectin structure using Anion
exchange chromatography as well as size exculsion chromatography. Results revealed
that alcohol soluble pectin 1 fraction was neutral having linear β- galactan. One of the
acid fractions alcohol soluble pectin 2 with galacturonic content consisted of repeating
disaccharide units. Alcohol soluble pectin 3 showed high galacturonic acid content
having alternate homogalacturonan blocks and rhamnogalacturonana blocks with same
amount of galactopyranosyluronic acid residues in each block. Researchers have also
extracted pectin from sunflower heads by alkali and optimize the extraction process using
response surface methodology. Most important factors found out to be temperature and
washing time of sunflower heads, pH, temperature, washing time and solvent: solid ratio
and their interactions significantly affected pigment removal. It has been observed that
slightly alkaline solution and lower solvent: solid ratio can be used to speed up the
pigment removal without affecting the pectin quality and yield (Shi et al., 1996).
Microwaves are also used by some researchers to extract pectin from orange peels
(Kratchanova et al., 2004). Orange peels were exposed to microwaves for different
duration of 5-15 minutes with different power of 0.45 to 0.9 kW. Pectin was then
extracted using HCl. Microwave pretreatment of peels led to destructive changes in the
plant tissues. The changes resulted in an increase in the capillary porous characteristics
and the water absorption capacity of the plant material. Theses changes led to increased
pectin yield and improve its characteristics by improving the water absorption capacity of
peels. Mango peels were studied to evaluate the impact of different extraction conditions
on the yield and some biochemical characteristics of mango peels pectins in order to
access the feasibility of using mango peel as a source of pectin (Koubala et al., 2008).
Isolated pectin was then compared with lime pectin. Extraction conditions shown to have
a deep impact on the extraction yields and on the biochemical and macromolecular
characteristics of the extracted pectins. Ammonium oxalate led to high extraction yields
and proved to be an outstandingly interesting extractant. HCl also led to high extraction
yields but the extracted pectin was partly degraded. Water came out as a poor extractant
with respect to extraction yields. Whatever the extraction method used, the pectins
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recovered were highly methylated. High methylated pectins from gels with high amounts
of sugar and acid. Many factors influence the conditions of gel formation and the gel
strength achieved but under equal conditions, gel strength increases with the molar mass
of pectin used (Voragen et al., 1995). Ammonium oxalate extracted pectins are thereby
most likely to exhibit good gelling properties i.e. high average molar mass, intrinsic
viscosity and a high degree of methylation. Extraction process was developed by
Turquois et al., (1999) to extract pectin from sugar beet pulp and potato pulp with high
gelling properties rather than yield. Rheological measurements were done to study the
effect of concentration of the extracted product, calcium content, sequestrant effect and
hydration temperature. Study included acidic and alkaline procedures for pectin
extraction. Extraction process developed maintains the structural integrity of the pectins
as much as possible. Theses procedures yielded products, which possessed both high
pectic substance content with low degree of esterification and a high gelling ability in the
presence of calcium. Sugar beet and potato pulps could be used as new sources of
pectins. Low quality apples were also used by Rascon-Chu et al., (2009) to extract pectin
and determine their gelling ability using acid extraction. Extracted pectin was then
characterized for composition and functional properties. Studies revealed that low quality
apple resulted in pectin with high galacturonic acid content, high intrinsic viscosity
allowing the formation of firm physical gels. Concentration of the pectins in a system
also affected the textural properties of gels. Hardness and adhesiveness found to be
increased with increase in pectin concentration. Therefore, the pectin recovered could be
used as a food additive to texturize or stabilize different food products. A new method
was developed by Cho et al., (2003) in which pectin was concentrated efficiently
decreasing the amount of ethanol need for the precipitation of pectin. Cross-flow
filtration was used to concentrate and purify isolated pectin. Cross microfilteration,
effectively concentrated pectin extracts which saved 75% of ethanol consumption
required for pectin precipitation. Undesirable impurities were also effectively removed.
2.1.3. Properties of Pectin
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Alkali salts of pectinic and pectic acids are usually water soluble in water. Dry
powdered pectin has tendency to hydrate very rapidly and form clumps on addition of
water. These clumps consist of semi dry packets of pectin contained in an envelope of
highly hydrated outer coating. Further solubilisation of such clumps is very slow. Dilute
pectin solutions are Newtonian but at moderate concentration, they exhibit non-
Newtonian, pseudo plastic behavior characteristics. Factors that increases gel strength
will increase the tendency to gel, decrease solubility and increase viscosity and vice-
versa. These properties of pectins are a function of their structure. Solutions of
monovalent salts of pectins exhibit stable viscosity because each polymer chain is
hydrated, extended and independent. Reduction in the pH reduce the ionization, the
polysaccharide chains no longer repel each other over their entire length and can
associate and form a gel. Pectins are mainly used as gelling agents but can also act as
thickener, water binder and stabilizer. Low methoxy pectins form thermo reversible gels
in the presence of calcium ions at low pH whereas high methoxyl pectin rapidly form
thermally irreversible gels in the presence of sufficient sugars such as sucrose and at low
pH. The lower the methoxyl content, slower is the gel set (Kohn, 1982). Effect of
chemical and physical modification on the thermal behavior of pectins was studied by
Einhorn-Stoll et al., (2009) to understand the physical state and state transitions resulting
from structural changes during preparation and processing of pectin materials. It has been
observed that degradation parameters varied with the degree of modification. All
chemically modified pectins were more sensitive to thermal degradation as compared to
their native materials. Transition temperatures as well as weight loss found to be
decreased with increased degree of modification. Very different behavior has been
observed in case of mechanically degraded pectins that showed starting of thermal
degradation earlier and ended later with decreased molecular weight. Pectin origin also
influenced the thermal degradation. Chemical changes in the cell wall as well as turgor
changes has been directed towards characterization of changes in pectic substances
during processing and storage operations, Pectins can be demethylated and
depolymerised by both enzymatic and non-enzymatic reactions. A simple model has been
proposed by Chang et al., (1993) to study the basic interaction between pectin molecules
and other cell-wall constituents effecting chemical and textural changes of vegetable
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tissues during different cooking treatments. Krall & McFeeters (1998) studied the effect
of temperature, degree of methylation, pH and calcium on the hydrolysis of pectin. It has
been concluded that rate of pectin hydrolysis decreased at pH 3. Dynamic light scattering
study had been carried out by Kjoniksen et al., (2004) to study the temperature-induced
association and gelation of aqueous solutions of low methoxy pectin at different
temperatures and polymer concentrations. Results revealed that as the temperature
decreased formation of association complexes promoted and strengthened with increased
polymer concentration. Slow relaxation time for the highest polymer concentration rises
strongly with decreasing temperature suggested enhanced polymer chain association. At
low temperature of 10 °C, gelation occurs in the semi dilute regime and a transparent gel
is formed. Low temperature stabilized the association complexes and the gel net work
through intermolecular hydrogen bonds, which are broken-up at higher temperatures.
Panchev et al., 1989 proposed a model to describe the kinetics of apple pectin extraction
including the dissolution of pectin from protopectin and the degradation of dissolved
pectin. Minkov et al., (1996) also proposed a mathematical model to study the hydrolysis
of solid-phase protopectin to solid-phase pectin and the transformation of solid-phase
pectin into liquid-phase pectin. Cho & Hwang (2000) proposed mathematical model to
evaluate yield and intrinsic viscosity of pectin in acidic solubilisation of apple pomace.
Using experimental data, yield and intrinsic viscosity models were determined as forms
of an Arrhenius-type equation and an exponential function of temperature. Comparative
study on functional properties of beet and citrus pectin was studied by Mesbahi et al.,
(2005). The highest yield of pectin was obtained at pH 1, temperature 90° C with
extraction periods of 4 hrs. Results indicated that the extracted beet pulp pectin could be
used in certain foods such as ketchup tomato sauce as a thickener or as an agent
increasing the viscosity. Beet pectin did not have good gelling properties in the food
products and could not produce a strong network to trap free water as much as citrus
pectin does. However, beet pectin showed higher hydrodynamic volume when dissolved
in water therefore can be used as a thickening agent. Degradation of pectins in alkaline
was studied by Renaurd & Thibault (1996) at mild alkaline pH and temperatures between
15-45 °C and kinetic constants of pectin saponification was reported by the liberation of
methanol. At higher pH a marked deviation was observed from the expected first order
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kinetics with respect to methyl-esterified carboxyl groups. Deviation may be attributed to
the changes in pH that occurred during saponification. Physicals gels i.e. low methoxyl
pectins were evaluated for their rheological properties by Gigli et al., (2009). Influence of
ionic strength and temperature on the gelation of low methoxyl pectin was studied.
Studies revealed that gel transition is very sensitive to the ionic strength of the medium
while the viscoelastic properties of the gel structure were retained up to 60 °C. The
mechanical spectrum of low methoxyl pectin gel approaches the beginning of the
terminal region where the rheological behavior is mainly dominated by viscous flow, thus
indicating a liquid like character of a material and thus accounting for the large
proportion of solvent contained in the network. Effect of ultrasound waves on the acid
desertification of low methoxyl pectin was studied by Panchev et al., (1994). Treated
samples were then evaluated to study the effect of temperature, time and nitric acid on the
yield and quality of low esterified pectins. Ultrasonication of the treated material resulted
in pectin with decreased degree of esterification and higher yield. Optimum ultrasound
treatment proved to be within 24-30 minutes.
2.1.4. Pectin Modification
When a substantial portion of the methyl ester groups is removed by hydrolysis,
the modified pectin attains the ability to form uniform strong gels in the presence of
bivalent cations over a wide pH range. This property makes low-methoxyl pectin useful
for numerous applications in which ordinary pectin cannot be used (Graham & Shepherd,
1953). Modification of the pectins is done to increase their reactivity by partial hydrolysis
of the ester groups. Pectins can modified by saponification catalyzed by mineral acids,
bases, slats of weak acids, enzymes, and concentrated ammonium systems. The acid and
base hydrolysis is the simplest efficient procedures for modifying pectins. Modification
decreased the methoxyl content and increased the free carboxyl groups content of the
pectin after saponification. Chemical modification also increased the sorption capacity of
the pectins for heavy metals simulating the electrolyte composition of human body
(Kupchik et al., 2006). Low methoxyl pectin found its application in the making of jellied
fruit cocktails, low-solids gels, milk puddings, candy centers and coatings for various
food materials. Low methoxyl pectins are versatile materials, which make possible the
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preparation of many new food products and the preparation of old food products in new
or easier ways (Graham & Shepherd, 1953). Pectins with higher degree of esterification
will gel at higher pH because they have fewer carboxylate anions at any given pH.
Dissolved pectins are decomposed spontaneously by de-esterification as well as by
depolymerisation. Rate of degradation depends on pH, water activity and temperature.
Maximum stability is found at pH 4. At low pH values and elevated temperatures,
degradation due to hydrolysis of glycosidic linkages is observed. Deesterification is also
favored by low pH. De-esterification of high methoxyl pectin slows the setting and
gradually adapts the characteristics of low methoxyl pectin. High temperature along with
high temperature favors elimination process that results in loss of viscosity and gelling
properties. At alkaline pH pectin is rapidly de-esterified and degraded even at room
temperature. Powdered high methoxyl pectin slowly lose their ability to form gels if
stored under humid or warm conditions while low methoxy pectin are more stable and
loss should not be significant after one year storage at room temperature (Srivastva &
Malviya, 2011). Khondkar et al., (2007) have studied rheological behaviors of starch and
pectin gels. Study was conducted to observe the influence of pectin on starch properties.
Starches were mixed wit hydrocolloids to improve their rheological properties (Descamp
et al., 1986). Cross linked of pectin and starch affected the rheological properties by
increasing the elasticity of starch gels. Cross linked starch-pectin mixtures (2:3 and 3:2)
showed quite high storage and loss moduli indicating that these gels have greater degree
of elasticity and very well structured.
2.1.5. Applications
Starch films and coatings have been used for various food and pharmaceutical
applications. Native and modified starches can be used for making edible films and
coatings. Films prepared from starches are isotropic, odorless, tasteless, non-toxic and
biodegradable. The starch films have low oxygen permeability, starch coatings are
nutritious, safe and economic and have been used in the storage and marketing of foods.
It has been observed by various researchers that starch films are similar to plastic films in
various properties like physical characteristics, chemical resistance and mechanical
properties. Maize starch was used to demonstrate a novel method, which enables the
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preparation of starch films and coatings with good thickness control. 5% starch was
gelatinized in de-ionized water at 120 °C for 30 min with small addition of dispersant and
ethanol followed by ultrasonication to stabilize starch solution (Pareta & Edirisinghe,
2006). Pectin-starch blends are also used for making films. Theses films exhibit
extremely good modulus and tensile properties. Such films are biodegradable, recyclable
and may help satisfy increasing consumer and regulatory demands for materials with
these properties. To take the advantage of its polymeric properties, starch has been
suggested that starch must be gelatinized to disintegrate granules and overcome the
strong crystalline intermolecular forces before mixing with pectin. The disruption of the
starch granules and the resulting degree of solubilisation are highly dependent on the time
and temperature conditions to which the starch is exposed (Coffin et al., 1995). A
potential industrial use for the pectin/starch blends includes edible bags for soups and
noodle ingredients (Fishmann et al., 2000). To protect sensitive functional compounds
and deliver them safely to the intestine and colon, various edible coatings find attractive
approach for the food industry.
Pectin coating was also utilized to enhance spray-dry stability of pea protein-
stabilized oil-in-water emulsions by Gharsallaoui et al., (2009). Effect of drying on the
physical stability of oil-in-water emulsions containing pea protein-coated and pea
protein/pectin-coated droplets has been studied. It has been observed that pea protein-
pectin coating developed provide superior stability to oil droplets in terms of ageing and
pH changes, which may be due to increased steric repulsion by pectin that formed a less
charged protective layer around the protein interfacial film surrounding the oil droplet.
Starch has been found to be potential alternative to commonly used coatings that escapes
digestion in small intestines. The combination of pectin with starch is a potential formula
for food grade coatings. Dimantov et al., (2004) mixed pectin with high amylose
cornstarch to prepare coatings. Coatings were then evaluated for their surface
characterization and dissolution properties. It has been observed as the amount of
cornstarch increased in the coating system the roughness of the film increased whereas
dissolution of the coatings decreased at stomach and intestine pH with increased amount
of cornstarch in the coating system. Pectin/starch blends were also plasticized with
glycerol using extrusion to make edible films by Fishman et al., (2000). Studies revealed
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that plasticized pectin and pectin starch films have a large glass transition at about -50°C
indicating that these films are reasonably flexible at room temperature. Higher storage
modulus of these films makes them ideal for many thin film applications like edible bags
for soup, medical delivery systems etc.
Zsivanovits et al., (2004) have also studied mechanical properties of different types of
pectins. The material properties of pectin networks should show as strong dependence on
the concentrations, particularly hydration of the network. The hydration of the network is
influenced b the balance between the osmotic stress and cross-linking of the network,
which tends to restrict swelling and the affinity of the network for water, which drives
swelling. Mechanical behavior of pectins was examined at high concentrations relevant to
the behavior of pectin in the plant cell wall and as a film-forming agent. Mechanical
properties were examined as a function of counter ion type, concentration and extent of
hydration. Result revealed that swelling and stiffness of the films are strongly dependent
on pectin source and ionic environment. At a fixed osmotic stress, both Ca+
or Mg+
ions
reduce swelling and increase the stiffness of the films. Study has concluded that swelling
of high methoxyl pectin films at a constant osmotic stress is dependent on the source of
the pectin. Swelling is also dependent on counter ion type and concentration. The swollen
films behave as viscoelastic solids with a simple proportionality between polymer
concentration and tensile modulus.
2.2. KIDNEY BEAN
2.2.1. Grain characteristics
Legumes are processed and consumed in a variety of forms all over the world.
These methods are being widely used to improve nutritive value of legumes, primarily by
reducing the level of heat labile, non-nutritive compounds and by increasing the
bioavailability of nutritional components. Beans are an important legume crop consumed
throughout the world. Polyphenolic components present in beans act as antioxidants.
Lower glycemic index and the presence of alpha amylase inhibitors are proving
beneficial for diabetic patients as well as reduction of cholesterol level (Singh et al.,
2010). Legumes are commonly used as a source of protein and carbohydrate in human
diet in many countries of the world. Legumes are edible fruits and seeds of pod bearing
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plants belonging to the family Leguminosae, containing about 750 genera and 16000-
19000 species (Allen & Allen, 1981). The grain legumes are ranked fifth in terms of
annual world grain production (171 million tons) (Ratnayake et al., 2001). India is the
largest producer and consumer of pulses in the world, accounting for 33% of the world
area and 22% of the world production of pulses (Singh et al., 2004). Legumes are sources
of complex carbohydrate, proteins and dietary fiber, having significant amounts of
vitamins and minerals and high energetic value (Tharanathan & Mahadevamma, 2003).
Protein content ranged between 17 to 40% in contrast to 7-13% of cereals, and being
equal to the protein content of meats (18-25%) (de Almeida Costa et al., 2006). Legumes,
including annual oilseeds, are high in protein, micronutrients, vitamins, minerals and
plant fibers. In addition, legumes are able to fix nitrogen from the air (through their
symbiotic association with the rhizobium bacteria), and they are adaptable to a variety of
cropping systems. Legumes are the major source of protein and constitute an important
supplement to the predominantly cereal-based diet of Asians (Singh, 1988). Cereals are
deficient in amino acid lysine, which is compensated for by the surplus in legumes, while
legumes are deficient in sulfur containing amino acids, which is compensated for by a
relative surplus in cereals (Thirumaran & Seralathan, 1988). Whole legume seeds were
recommended for consumption as the endosperm is main source of starch and protein and
seed coat is a good source of dietary fiber. Legume seed proteins mostly consist of salt-
soluble globulins, which are synthesized during seed development and hydrolyzed during
germination to provide nitrogen and carbon for developing seedlings (Chau et al., 1998).
The grains of food legumes are similar in structure but differ significantly from each
other in size, shape, color and thickness of the seed coat. Legume seeds have two major
parts; seed coat and the kernel (embryo and cotyledons). On an average, pulses (including
soybean) contain 11% seed coat, 2% embryo and 87% cotyledons. Legume proteins are
of two types – storage and structural – more versatile and useful in the Indian diets.
Storage proteins (70-80 percent) occur within the cells in discrete protein bodies. About
20-30 percent is the structural proteins responsible for cellular activities including
synthesis of structural and storage proteins. The cotyledons, account for 93 percent of
methionine and tryptophan of the whole seed, while the seed coat is the poorest in these
amino acids. The embryo is rich in methionine and tryptophan but it contributes only
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about 2 percent of their total quantity in the seed (Kapoor & Gupta, 1977). Legume
proteins are deficient in methionine and trytophan. Dietary fibers are necessary to prevent
various diverticular and degenerative diseases. Recommended daily intake levels range
between 25-50 gm of fiber. Legumes are excellent sources of dietary fibers. It ranges
from as low as 6 percent in peanuts to as high as 25 percent in kidney beans and green
gram (Paul & Southgate, 1978; Kamath & Belavady, 1980). Low dietary fiber intake is
linked with increased incidence of cancer of the colon and rectum, diverticular disease,
coronary heart disease, diabetes and gallstone in affluent societies of the West (Burkitt &
Trowell, 1975). The hypocholeslterolemic effect is attributed to the dietary fiber fraction
of legumes (Cummings, 1978; Hellendoorn, 1979) because of its high content of pectins,
gums and galactants. Dietary fiber also absorbs bile salt. It is aided by saponins. Kidney
bean (Phaseolus vulgaris L.) is the most widely produced and consumed food legume in
Africa, India, Latin America and Mexico (FAO, 2002). This bean usually contains 20–
30% protein on a dry basis, and the protein has a good amino acid composition but is low
in sulphur-containing amino acids (notably methionine) and tryptophan (Gueguen &
Cerletti, 1994; Sathe, 2002). Dry beans have recovered prestige in the diets of developed
countries. This is due, in part, to health problems related to meat consumption, as well as
the discovery of the benefits of legumes in the diet and the protection they afford against
colon disease (Champ, 2001; Hangen & Bennink, 2003; Lee et al.,1992; Mathres, 2002).
Kidney beans have numerous health benefits, e.g., they reduce heart and renal disease
risks, lower glycemic index for persons with diabetes, increase satiation, and prevent
cancer. Furthermore, kidney beans are regarded as an important source of protein and
minerals for livestock feed production, as well as potential raw materials for processing
into human food (Shimelis & Rakshit, 2007). Cookability has been defined as 'the
conditions by which seeds achieve a degree of tenderness during cooking, acceptable to
the consumer'. In most countries, legumes are commonly prepared for traditional
consumption by soaking for varying periods and then boiling. A characteristic property of
nearly all dried legumes is the long cooking time of 3-4 h required to attain the required
degree of softness and palatability ('doneness'). In high altitude regions, such as the
highland plateau of Africa, cooking time is even further increased. Traditional processing
methods and pretreatments designed to reduce cooking time include soaking in water for
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periods of up to 24 h. Water absorption is an important determinant of the rate of
hydration and of cooking properties. Water absorption is to some extent determined by
heredity, but it is also influenced by environmental factors, such as agronomic and
storage conditions. Starch and protein are the major components involved in the
hydration process. Agbo (1982) demonstrated significant differences in processed food
quality between two dry bean strains of the same genotype that differed only in a single
gene for seed-coat color. Initial moisture content, seed-coat thickness, texture and
permeability, and storage temperature have been shown to affect water uptake in cowpea
(Sefa-Dedeh & Stanley 1979, Moscoso et al., 1984) and dry kidney bean. Phytic acid
content and Cookability found to be correlated. Hard texture of grains is due to calcium-
pectic complexes formed. Phytic acid chelates the calcium and reduce the formation of
complex thus results in texture softening. (Kohn 1968, Kumar et al.,1978, Mattson et al.,
1950).
Long storage periods under tropical conditions result in hard to cook (HTC). This
phenomenon has been reported in several species of legumes including cowpea and red
kidney bean (Jackson & Varriano-Marsten 1981, Sefa-Dedeh et al., 1979). HTC results
from deterioration during storage and reduced water absorption (hard shell) and
cookability of cotyledons (Sclerema), accompanied by deleterious changes in texture and
flavor. Mejia (1979) reported a significant correlation between an increase in tannin
content and hardness, attributable to temperature- and humidity-dependent changes in
condensed tannins, and continued development of tannin from low-molecular mass non
tannin material. The loss of cookability of dry kidney bean in storage has been related to
the reduction in phytic acid phosphorus, and changes in the ratio of monovalent to
divalent cations in soaked bean. The reduction in phytic acid and monovalent cations
results in lower solubilisation of pectic substances through chelation and ion exchange
during cooking (Moscoso et al., 1984). HTC is overcome by the use of salt solution for
soaking the legumes. Salt alters the configuration and conformation of native proteins,
thus increasing their solubility, reducing steric hindrance, and exposing more peptide
bonds to hydrolysis. Salts also break the hydrogen bonds between protein and condensed
tannins. Salt reduces the calcium- and magnesium-mediated interactions between phytic
acid and protein and between minerals and pectin, altering the microstructure of black
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bean, making them more porous and permitting easier penetration of heat and water
(Sievwright & Shipe 1986). Soaking dry bean in food-grade salt reduced HTC reported
by Rockland & Metzler (1967).
Digestibility of legumes is limited by few antinutritional factors like (ANFs) like
trypsin inhibitor and others. These are chemical substances, which, although non-toxic
generate adverse physiological responses and interfere with the utilization of nutrients.
ANFs are protease inhibitors, lectins, goitrogens, antivitamins and phytates, saponins,
oestrogens, flatulence factors, allergens and lysinoalanine (Liener, 1981). Some other
ANFs are cyanogens, favism factors, lathyrism factors, amylase inhibitors, tannins,
aflatoxins and amines. Although only a few legumes may contain all these ANFs, many
contain a few of them. Most of the ANFs are heat-labile and since humans only consume
legumes after cooking, it would not constitute any major health hazard. Heat stable
compounds such as polyphenols and phytates are, however, not easily removed by simple
soaking and heating. These could be reduced by germination and/or fermentation.
Legumes are rich source of polyphenolic compounds. Till recently, some of these (e.g.
tannins), were considered as anti-nutrients due to their adverse effects on protein
digestibility. However, nowadays, there is considerable interest in the antioxidant activity
of these compounds and in their potential health benefits, especially in the prevention of
cancer and cardiovascular disease (Menon, 2000). Dark colored legumes like red kidney
beans, black beans, black gram and soybean have higher amount of these polyphenolic
compounds. Cooking improved the antinutrients, protein and starch digestibility of food
legumes (Rehman & Shah, 2005). Cooking reduce the tannin and phytic acid content.
Maximum improvement in protein and starch digestibility was observed by cooking
legumes at 121 °C for 10 minutes.
2.2.2. Flour
Physicochemical and functional properties of flours prepared from common beans
and green mung beans were studied by Dzudie & Hardy (1996). Common beans flours
showed significantly better water and oil absorption capacity than green mung bean flour.
Mung bean flour showed higher bulk density, emulsion capacity whereas flour from
common bean showed more stability. Studies revealed the samples are rich in protein,
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potassium, phosphorus and calcium with good functional properties. Therefore, flours
form these legumes can be used as a protein supplement in human diets. Rheology of the
Bengal gram flour was also studied by Bhattacharya et al., (1992). Effect of
concentration, particle size, consistency index and apparent viscosity were investigated.
Bengal flour suspension was pseudo plastic and exhibited yield stress. Particle size and
concentration of flour strongly influenced the rheological behavior of the suspension.
Chemical composition of raw or soaked beans played an important role in indicating the
texture of beans after cooking (Pujola et al., 2007). Amount of protein and amylose
present in raw beans provide a good indication of these substances in cooked beans.
Magnesium content in the raw seeds showed a strong correlation with that found in
cooked seed coat. Varietal differences also found to play role in having greater tendency
to lose starch during processing. Functionality has been defined as any property of a food
ingredient having great impact on its utilization, except its nutritional quality, functional
properties affect the processing applications, food quality directly and indirectly and
ultimately their acceptance in food and food formulations (Mahajan & Dua., 2002).
Functional properties of 10 legume flours have been investigated by Sosulski et al.,
(1976). Functional properties of the flours are due to protein content, complex
carbohydrates, pectins and mucilage. Protein composition as well as non-protein
components may contribute substantially to the emulsification properties of protein-
containing products like legume flour McWatters, (1983). Flours from different black
gram cultivars were investigated for functional, thermal and pasting properties and were
correlated with each other (Kaur & Singh, 2007). Flours shown to have low breakdown
viscosity indicating their resistance to break during cooking or thermostability in other
words.
2.3. Starch
Starch is the most abundant storage reserve carbohydrate in plants. Starch is a
versatile and useful polymer not only because of the ease with which its physicochemical
properties can be altered through chemical or enzymatic modification and/or physical
treatment (James et al., 2003). Most starches are composed of a mixture of two molecular
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entities (polysaccharides), a linear fraction, amylose, and a highly branched fraction,
amylopectin. Amylose contributed to 15 and 25% for most starches. The ratio of amylose
and amylopectin in starch varies from one starch to another. The two polysaccharides are
homoglucans with only two types of chain linkages, an α -(1 → 4) of the main chain and
an α -(1 → 6) of the branch chains. Hydrophilic properties of the polymer are imparted
by the abundance of hydroxyl groups along the amylose molecules, giving it an affinity
for moisture. Because of their linear nature, mobility, and the presence of many hydroxyl
groups along the polymer chains, amylose molecules have a tendency to orient
themselves in a parallel fashion and approach each other closely enough to permit
hydrogen bonding between adjacent chains. As a result, the affinity of the polymer for
water is reduced and the solution becomes opaque. Amylopectin is a highly branched
polysaccharide. The structure consists of α – D -glucopyranose residues linked mainly by
(1 → 4)-linkages (as in amylose) but with a greater proportion of nonrandom α - (1 → 6)-
linkages, which gives a highly branched structure. Amylopectin is one of the largest
biological molecules and its molecular weight ranges from 106 to 10
9 g × mol
–1,
depending on botanical origin of the starch, fractionation of starch, and method used to
determine the molecular weight. Amylopectin has limited mobility in solution because of
the branched nature and eliminate the possibility of significant levels of inter chain
hydrogen bonding. On average, amylopectin has one branch point every 20 to 25
residues. The branch points are not randomly located. Linear branched chains with DP
~15 in amylopectin are the crystalline regions present in the granules. Another unique
feature of amylopectin is the presence of covalently linked phosphate monoesters. They
occur largely in starch from tuberous species, especially potato starch. Phosphate
monoesters increased the electrostatic repulsion between which results in the change of
gelatinization and pasting properties of starch (Qiang, 2005). Functional properties of
starch depend on the ratio of amylose: amylopectin which further depend on the botanical
origin from which the starch extracted (Swinkles., 1985). Starch is used as ingredient and
provides texture to several foods. Thus, it can be used as thickeners, stabilizers, binders,
adhesives, tonics, coagulants, gelling and forming agents, emulsion and foam stabilizers
and water retention agents (Freidman, 1995). The reduced bioavailability of pulse
starches has been attributed to the presence of intact tissue/cell structure, high levels of
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amylose, high content of viscous soluble dietary fiber component, the presence of a large
number of antinutrients and strong interactions between amylose chains, which could
influence the rate and extent of pulse starch digestibility related to slow and moderate
postprandial glucose and insulin responses (Hoover & Sosulski, 1985; Edward, 1993:
Hoover & Zhou, 2003). Swelling factor, amylose leaching, gelatinization temperature,
gelatinization enthalpy, relative crystallinity and chain length distribution of amylopectin
affected the digestibility of the pulse starches (Chung et al., 2008).
2.3.1. Physicochemical properties
Physicochemical properties like swelling power, solubility, and transmittance
were reported to be significantly correlated with the average granule size of the starches
from various sources. (Zhou et al., 1998). Native starch granules are insoluble in cold
water but swell in warm water. When starch granules are heated in the presence of water,
an order-to disorder phase transition occurs. Swelling of starch granules exert a pressure
on neighboring crystallites and tends to distort them. Further heating leads to uncoiling or
dissociation of double helical regions and break-up of amylopectin crystallite structure.
Starch molecules have tendency to contract to obtain a random coil conformation
providing a constraint in direction of the chain against swelling. Further hydration
resulted in increased mobility permitting a redistribution of molecules and the smaller
linear amylose molecules diffuse out. Heating and hydration both weakened the granule
to the point where it can no longer hold the pressure developed inside the starch granule
and eventually a sol results. Collapse (disruption) of molecular orderliness within the
starch granule resulted in irreversible change in properties such as granular swelling,
crystallite melting, loss of birefringence, viscosity development, and solubilisation
(Flory, 1953). Leach et al., (1959) concluded that strong bonds resists the swelling of
granules whereas weak bonds undergo very rapid and unrestricted swelling and at
relatively low temperature. The property of starch depends on the physical and chemical
characteristics such as granule size distribution, amylose/amylopectin ratio and mineral
content. Swelling power has been reported to be influenced by strongly bonded micellar
network (Gujska, 1994). Amylose content known to be affected by climatic conditions,
botanical sources and soil type during growth (Julaino et al., 1964; Morrison et al., 1984).
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Amylose plays a very important role in restricting the initial swelling because swelling
proceeds more rapidly after amylose has been exuded. Granules become more susceptible
to shear disintegration as they swell and release soluble material as they disintegrate.
Legumes have been characterized by a high amylose content of 25-65% (Hoover &
Sosulski, 1991). El-faki et al., 1983 and Lineback & Ke (1975) reported amylose content
of chickpea to be ranged between 30-32% and 28-33%, respectively. Hoover & Sosulski
(1991) reported that swelling power and solubility also get affected by temperature,
which might be due to melting of the crystallites. The swelling power of starch is
associated more with granule structure and chemical composition, particularly amylose
and lipid content. Presence of lipid results in the formation of amylose-lipid complex,
which are believed to restrict swelling and amylose leaching. Once the amylose-lipid
complexes dissolve, the rate of amylose leaching out of the granules increases
substantially.
Pulse starch contains varying amount of phosphate monoester derivatives, which
result in increased paste clarity and viscosity (Jane et al., 1996). The starch granules of
pulse have greater stability against mechanical shear than those of the fragile swollen
wheat starches because of the hot paste viscosity that does not show any breakdown point
in legumes (Iyer & Singh, 1997). Clarity of starch is one of the important attributes in
starch applications. Swelling and brittleness of the starch molecules affect the clarity of
starch pastes. Solutes like sucrose and glucose increased the starch paste clarity whereas
lipids increased the opacity. Salt reduce the transmittance as well as visual clarity of
potato starch paste. Starch paste clarity affected by the phosphorus. Phosphorus is present
as phosphate monoester and phospholipids in various starches. Phosphate monoesters are
covalently bound to the amylopectin fraction of starch and known to increase starch paste
clarity and viscosity, while the presence of phospholipids results in opaque and lower
viscosity pastes. When a beam of light is reflected back and the starch appears white and
opaque due to the surface of the granule being larger than the wavelength of light.
Seperation of starch chains during gelatinization decreases the reflecting ability of starch
granules and thus increases the percentage transmittance of a starch paste (Hoover et al.,
1996).
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2.3.2. Thermal properties
Whenever a material undergoes a change in physical state (e.g. melting) or
transforms from one crystalline form to another or whenever it reacts chemically, heat is
either absorbed (endothermic) or liberated (exothermic). Various techniques are used to
understand the changes occur during gelatinization like Differential Scanning
Calorimetry (DSC), X-ray scattering (Jenkins & Donald., 1998). DSC has been widely
used to study thermal behavior of starches as it helped to understand phase transition in
starch upon heating in presence of water (Ghaisi et al., 1982). DSC is a technique
whereby the difference in energy input into a substance and a reference material is
measured as a function of temperature while both materials are subjected to programmed
heating or cooling. When thermal transitions occurs, the energy absorbed bin the
transition, a recording of this balancing energy yields a direct calorimetric measurement
of the energy transitions which is then recorded as a peak. The area under peak is directly
proportional to the enthalpic change (Karim et al., 2000). Thermal properties predict the
qualities suitable for industrial use. Temperature and water content leads to a change in
organization of the granule during gelatinization. Gelatinization caused a collapse of
crystalline order within the starch granules, which resulted in irreversible changes in
properties like swelling, solubility, loss birefringence. The point of initial gelatinization
and the range over which it occurs are governed by starch concentration, method of
observation, granule type, and heterogeneity within the granule population under
observation (Atwell, 1988). Gelatinization occurs initially in the amorphous regions as
opposed to crystalline regions of the granules due to weak hydrogen bonds in amorphous
regions. Amylopectin played a major role in starch granule crystallinity; the presence of
amylose lowers the melting point of crystalline regions and the energy to start
gelatinization (Flipse et al., 1996). Recrystallization of amylopectin branch chains has
been reported to occur in less ordered manner in stored starch gels as it is present in
native starch gels. This explains the behavior of amylopectin retrogradation endotherms
at a temperature below that for gelatinization (Ward et al., 1994). Transition temperatures
are influenced by the molecular architecture of the crystalline region corresponding to the
distribution of amylopectin short chains (DP 6-11) and not by the proportions of
crystalline regions corresponding to the amylose/amylopectin ratio (Noda et al., 1998).
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Transition temperatures were positively related to long chains amylopectin. Tp and Tc
showed significant positive correlation with peak viscosity, breakdown, setback and final
viscosity. Transition temperature of starches was dependent on the proportion of granules
and amylopectin and more on the former. Starches with higher proportion of the longer
side chain amylopectin fraction showed higher transition temperature, which is consistent
with their greater crystallinity as indicated by higher gelatinization enthalpy (Singh et al.,
2011). Cooke & Gidley (1992) reported that ΔHgel reflects the loss of double helical order
rather than the loss of crystallinity whereas according to Tester & Morrison (1990a.
1990b) ΔHgel reflects the overall crystallinity of amylopectin. Legumes have higher
transitions temperatures, which make them suitable for application where high processing
temperatures are used to assure its thickening effects since use of starches with low
gelatinization temperatures in canned products resulted in formation of
pyrodextrinization and reactions with other components of the system to certain extent
(Betancur et al., 2001).
2.3.3. Retrogradation properties
Retrogradation is a term used for the behavior of gelatinized starch on cooling and
storage. It is of great importance as it affects quality, acceptability and shelf-life of starch
containing foods (Biliaderis, 1991). Starch retrogradation has been used to describe
changes in physical behavior following gelatinization. Retrogradation is the reassociation
of starch molecules forming an ordered structure such as double helices during storage. In
an initial step, two chains may associate. Ultimately, under favorable conditions, a
crystalline order appears and physical phase separation occurs (Atwell, 1988).
Retrogradation is important in industrial use of starch, as it can be a desired end-point in
certain applications but it also causes instability in starch pastes. Starch retrogradation is
influenced by the botanical source (e.g., cereal starch vs. tuber starch) and the fine
structure of amylopectin (e.g., chain length and distribution), molecular size and size
distribution of starch affect the rate of retrogradation. Amylose is responsible for short
term (Goodfellow & Wilson, 1990) whereas amylopectin for long-term rheological and
structural changes of starch gels (Gudmundsson, 1994). Moisture content in the starch gel
and storage temperature can affect the rate and extent of starch retrogradation. Certain
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polar lipids, surfactants, and sugars can retard or reduce the extent of retrogradation. At
lower water contents, water acts as a plasticizer, which will affect the Tg (glass transition
temperature) of a partially crystalline polymer (Bizot, 1997). Therefore, the amount of
water will affect the glass transition of starch-based foods, hence the properties,
processing, and stability of many starch-based food products (Slade & Levine., 1987). On
ageing starch molecules in pastes, gels and baked foods begin to associate resulting in
precipitation, gelation and changes in consistency and opacity followed by gradual
increase in rigidity and phase separation between polymer and solvent (D’Appolonia &
Morad., 1981; Kulp & Ponte., 1981). Retrogradation is also desirable in some cases
where hardening and reduced stickiness is required (Collona et al., 1992). Amylopectin is
responsible for retrogradation of starch (Eliasson, 1985). Stability of these crystallites is
very less than amylose due to limited dimensions of the chain (Miler et al., 1985).
Amylose content found to be influenced the retrogradation (Baik et al., 1997; Fan &
Marks., 1998). Amylopectin and other intermediate materials also play an important role
in starch retrogradation during refrigerated storage. The intermediate materials with
longer chains than amylopectin may also form longer double helices during
retrogradation (Yamin et al., 1999). Starch retrogradation is influenced by the botanical
source (e.g., cereal starch vs. tuber starch) and the fine structure of amylopectin (e.g.,
chain length and distribution).
2.3.4. Morphology
Morphological features depends upon the size, shape and size distribution of
granules of starch from different botanical sources. Granule size reported to be influence
the pasting properties of starch (Ao & Jane, 2007; Shinde et al., 2003). Geera et al., 2006
reported that granule size is related to the molecular architecture of amylopectin and its
molecular arrangement with the starch granule. Morphology may be depended on the
biochemistry of the chloroplast or amyloplast as well as the physiology of the plant
(Badahuizen, 1969). In nature, starch exists in the form of granules, which can differ in
size and shape. The origin of starch granules can be inferred from their size, shape, and,
the hilum position (the original growing point of granule). X-ray diffraction has been
used to study the crystallinity change and to characterize the transition of crystal structure
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during starch gelatinization. Starch granules are categorized based on packing of parallel
stranded double helices in the granule, into three types- A, B and C type. A-type granules
are found in cereals like maize, rice, B-type granules are found in tubers like potato and C
type granules which is a mixture of A & B are found in legumes. A having close packing
and B having loose packing with more amount of inter-helical water (Cooke & Gidley.,
1992). Legume seed starch granules are bean-like with a central elongated or starred
hilum. The size of starch granules vary from 2 to 100 μm in diameter. The tightly packed
A type structure would be expected to be more stable. A type starch has a higher melting
temperature and hence is more stable than the B type (Gidley, 1987). C-type starches
showed unique characteristics differ from A- and B- type granules starches since legume
starches have higher amylose content which lowers the melting point of crystallites and
the energy for starting gelatinization (Flipse et al., 1996).
2.3.5. Pasting properties
When starch is cooked, the flow behavior of a granule slurry changes markedly as
the suspension becomes a dispersion of swollen granules, partially disintegrated granules,
then molecularly dispersed granules. The cooked product is called a starch paste. In
general, a starch paste can be described as a two-phase system composed of a dispersed
phase of swollen granules and a continuous phase of leached amylose. It can be regarded
as a polymer composite in which swollen granules are embedded in and reinforce a
continuous matrix of entangled amylose molecules (Ring, 1985). If the amylose phase is
continuous, aggregation with linear segments of amylopectin on cooling will result in the
formation of a strong gel. Consistometer was used by Ceaser (1932) and Ceaser & Moore
(1935) to study the pasting characteristics and starch containing products. Brabender
viscoamylograph and viscoamylograph was used to study pasting properties. Sandstedh
& Abbott (1961) reported that starch concentration affected the pasting properties while
Mazurs et al., (1957) develop graphical presentation of amylograph data to compare
properties, which are independent of starch concentration. Rapid Visco Analyzer (RVA)
was then introduced as an alternative to Brabender viscoamylograph to measure pasting
characteristics. RVA has the advantages of using a small sample size, short testing time,
and the ability to modify testing conditions. Starches have been classified into four types
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based on their gelatinized paste viscosity profiles. Type I is high swelling starches, which
are characterized by a high peak viscosity followed by rapid thinning during cooking
(potato, tapioca, waxy cereal). Type II is moderately swelling starch, which shows a
lower peak viscosity, and much less thinning during cooking (normal cereal starches).
Type III consists of restricted swelling starches, which show a relatively less pronounced
peak viscosity and exhibit high viscosity that remains constant or increases during
cooking (legume starches). Type IV is highly restricted starch, which does not swell
sufficiently to give a viscous solution (high amylose maize starch) Schoch & Maywald
(1968). Starches that are capable of swelling to a high degree are also less resistant to
breakdown on cooking and hence exhibit viscosity decrease significantly after reaching
the maximum value. Increase in viscosity during cooling indicated the tendency of
various constituents present in hot paste to reassociate or retrograde as the temperature of
the paste decrease. Viscosity of the gelatinized starch suspension may be attributed to the
frictional dissipation of energy in the movement of the swollen starch granules relative to
one another (Miller et al., 1973). Cooked starch behaved as non-Newtonian fluids due to
secondary bonds between the hydrodynamic units, either directly or through intermediate
water molecules. (Schutz., 1971).
2.3.6. Rheological properties
Starch rheology is to study the stress-deformation relationships of starch in
aqueous systems. The rheological properties of starch are important to both food and
industrial processing applications. During processing, starch dispersions will be subjected
to combined high heating and shearing that affect their rheological change as well as the
final characteristics of the product. Starch gelatinization, especially granular swelling,
changes the rheological properties of starch. The subsequent retrogradation will further
modify the rheological properties of starch. Rheology is directly related to the
microstructure of starch. The rheological properties of starch are influenced by many
factors such as the amylose/amylopectin ratio, minor components, the chain length of
amylose and amylopectin molecules, the concentration of starch, shear and strain, and
temperature (Qiang, 2005). Dynamic rheometery allows the continuous assessment of
dynamic moduli at various temperatures (Ferry, 1980). There is a great opportunity to
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utilize various strains or deformation forces to obtain a more complete view of a
material’s physical properties. Very low strain allow measurements without disturbing or
destroying the inherent gel structure, which is of great value in describing the time and
temperature dependent changes in starch gels during aging ( Karim et al., 2000). The
storage modulus (Gɂ) is a measure of the energy stored in the material and recovered for it
per cycle while (Gʺ) measure the energy dissipated or lose per cycle of sinusoidal
deformation (Ferry,1980). Initial increase in Gɂ could be attributed to the degree of
granular swelling to fill the entire available volume of the system (Eliasson, 1986) and
intergranule contact might for a three- dimensional network of the swollen granules
(Evans & Haisman, 1979; Wong & Lelievre, 1981), further increase in temperature
resulted in decreased Gɂ indicated the disruption of gel structure during prolonged heating
(Tsai et al.,1997). Destruction of gel structure may be attributed to the melting of
crystalline region remaining in the swollen granules, which deforms and loosens the
particles (Eliasson, 1986). Another parameter, which maybe useful in indicating the
physical behaviour of a system is the loss tangent (tan δ). It is the ration of the energy lost
to the energy stored for each cycle of the deformation, i.e. tan δ= Gʺ/ Gɂ. It is a useful
indicator of the relative contributions of the viscous (Gʺ) and elastic (Gɂ) components to
the viscoelastic properties of a material.
2.4. Modification of starch
Modification of starch was carried out to overcome the shortcomings of native
starches such as insolubility in cold water, loss of viscosity, and thickening power after
cooking. In addition, retrogradation occurs after loss of ordered structure on starch
gelatinization, which results in syneresis or water separation in starchy food systems.
However, these shortcomings of native starch could be overcome, for example, by
introducing small amounts of ionic or hydrophobic groups onto the molecules. The
modifications alter the properties of starch, including solution viscosity, association
behavior, and shelf life stability in final products. Another purpose of starch modification
is to stabilize starch granules during processing and make starch suitable for many food
and industrial applications. Starch can be physically modified to improve water solubility
and to change particle size. The physical modification methods involve the treatment of
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native starch granules under different temperature/moisture combinations, pressure,
shear, and irradiation. It also includes mechanical attrition to alter the physical size of
starch granules. Starch is widely modified by chemical methods. The most common
chemical modification processes are acid treatment, cross-linking, oxidation, and
substitution, including esterification and etherification. Chemical modification can be
carried out on three starch states:
• In suspension, where the starch is dispersed in water, the chemical reaction is carried
out in water medium until desired properties are achieved. The suspension is then
filtered, washed, and air-dried.
• In a paste, where the starch is gelatinized with chemicals in a small amount of water, the
paste is stirred, and when the reaction is completed, the starch is air-dried.
• In the solid state, where dry starch is moisturized with chemicals in a water solution, air
dried, and finally reacted at a high temperature (i.e., ≥ 100˚C).
The most common chemical modification includes oxidation, esterification, and
etherification. The chemical modification of starch results in enhanced molecular stability
against mechanical shearing, acidic, and high temperature hydrolysis; obtaining desired
viscosity; increasing interaction with ion, electronegative, or electropositive substances;
and reducing the retrogradation rate of unmodified starch.
Another type of starch modification that helps improve the application of starch is
crosslinking. Researchers have used different type of crosslinking agents so far like
phosphorus oxychloride, sodium tripolyphosphate, epichlorohydrin (Hoover & Sosulski,
1985, Rutenberg & Solarke, 1984; Woo & Seib, 1997; Wuzurberg, 1986) to improve the
mechanical properties as well as water stability of starch products (Kunaik &
Marchessault, 1972; Seker & Hanna, 2006). Starch with a low level of cross-linking
shows a higher peak viscosity than that of native starch and reduced viscosity breakdown.
The chemically bonded cross-links may maintain granule integrity to keep the swollen
granules intact, hence, prevents loss of viscosity and provides resistance to mechanical
shear. Increasing the level of cross-linking eventually will reduce granule swelling and
decrease viscosity. At high cross-linking levels, the cross-links completely prevent the
granule from swelling and the starch cannot be gelatinized in boiling water even under
autoclave conditions. Cross-linking of legume starches has been shown to decrease
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amylose-leaching, water binding capacity, α-amylase digestibility, granular swelling but
also increased thermal stability and setback viscosity (Hoover et al., 2010).
Cross-linked starches are used in salad dressings to provide thickening with stable
viscosity at low pH and high shear during the homogenization process. Cross-linked
starches with a slow gelatinization rate are used in canned foods where retort sterilization
is applied; such starches provide low initial viscosity, high heat transfer, and rapid
temperature increase, which are particularly suitable for quick sterilization (Rutenberg &
Solarke, 1984). Cross-linked starches have been applied in soups, gravies, sauces, baby
foods, fruit filling, pudding, and deep fried foods (Wuzurberg, 1986).
2.5. Pectin-Starch Blends
Starch films are similar to plastic films in various properties like physical
characteristics, chemical resistance and mechanical properties. Maize starch was used to
demonstrate a novel method, which enables the preparation of starch films and coatings
with good thickness control. 5% starch was gelatinized in de-ionized water at 120 °C for
30 min with small addition of dispersant and ethanol followed by ultrasonication to
stabilize starch solution (Pareta & Edirisinghe, 2006). Pectin-starch blends are also used
for making films. Theses films exhibit extremely good modulus and tensile properties.
Such films are biodegradable, recyclable and may help satisfy increasing consumer and
regulatory demands for materials with these properties. To take the advantage of its
polymeric properties, starch has been suggested that starch must be gelatinized to
disintegrate granules and overcome the strong crystalline intermolecular forces before
mixing with pectin. The disruption of the starch granules and the resulting degree of
solubilization are highly dependent on the time and temperature conditions to which the
starch is exposed (Coffin et al., 1995). A potential industrial use for the pectin/starch
blends includes edible bags for soups and noodle ingredients (Fishmann et al., 2000). To
protect sensitive functional compounds and deliver them safely to the intestine and colon,
various edible coatings find attractive approach for the food industry.
Pectin coating was also utilized to enhance spray-dry stability of pea protein-stabilized
oil-in-water emulsions by Gharsallaoui et al., (2010). Effect of drying on the physical
stability of oil-in-water emulsions containing pea protein-coated and pea protein/pectin-
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coated droplets has been studied. It has been observed that pea protein-pectin coating
developed provide superior stability to oil droplets in terms of ageing and pH changes,
which may be due to increased steric repulsion by pectin that formed a less charged
protective layer around the protein interfacial film surrounding the oil droplet. Starch has
been found to be potential alternative to commonly used coatings that escapes digestion
in small intestines. The combination of pectin with starch is a potential formula for food
grade coatings. Pectin was mixed with high amylose cornstarch to prepare coatings by
Dimantov et al., (2004). Coatings were then evaluated for their surface characterization
and dissolution properties. It has been observed as the amount of cornstarch increased in
the coating system the roughness of the film increased whereas dissolution of the
coatings decreased at stomach and intestine pH with increased amount of corn starch in
the coating system. Pectin/starch blends were also plasticized with glycerol using
extrusion to make edible films by Fishman et al., (2000). Studies revealed that plasticized
pectin and pectin starch films have a large glass transition at about -50°C indicating that
these films are reasonably flexible at room temperature. Higher storage modulus of these
films makes them ideal for many thin film applications like edible bags for soup, medical
delivery systems etc. Mechanical properties of different types of pectins have also been
studied by Zsivanovits et al., (2004). The material properties of pectin networks should
show as strong dependence on the concentrations, particularly hydration of the network.
The hydration of the network is influenced b the balance between the osmotic stress and
cross linking of the network, which tends to restrict swelling and the affinity of the
network for water, which drives swelling. Mechanical behavior of pectins was examined
at high concentrations relevant to the behavior of pectin in the plant cell wall and as a
film-forming agent. Mechanical properties were examined as a function of counter ion
type, concentration and extent of hydration. Result revealed that swelling and stiffness of
the films are strongly dependent on pectin source and ionic environment. At a fixed
osmotic stress, both Ca+
and Mg+ ions reduce swelling and increase the stiffness of the
films. Study has concluded that swelling of high methoxyl pectin films at a constant
osmotic stress is dependent on the source of the pectin. Swelling is also dependent on
counter ion type and concentration. The swollen films behave as viscoelastic solids with a
simple proportionality between polymer concentration and tensile modulus.