9
Retrogradation in starches is commonly measured using a DSC, texture analyzer
or a rheometer. While the DSC analysis is sensitive to chain ordering of the amylopectin fraction, the mechanical tests are sensitive to chain entanglements with/without the formation of crystallites (Shevkani et al. 2017). Rheological techniques are
suited to monitor gel firmness (rigidity) on ageing and are usually carried by measuring the storage modulus (G`). These techniques do not evaluate the role of one
component or process only, but the combined effect of all the components of the gel.
Karim et al. (2000) have reviewed the methods to study retrogradation of starches.
DSC measures retrogradation enthalpy (ΔHret) and transition temperatures for the
melting of recrystallized starch. ΔHret is the amount of energy required for dissociation of the re-associated amylopectin. Hence, it provides a quantitative measure
of retrogradation, higher ΔHret means higher retrogradation tendencies.
Pasting Properties
Heating of starch continuously in excess of water causes starch granules to swell
and burst. Then the amylose leaches out and the granules disintegrate resulting in
the formation of 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, starch is generally regarded as the most important constituent of rice in terms of cooking quality
and functionality. The pasting properties of starch are used in determining the suitability of starch in different foods and other allied products. The most important
pasting characteristic of granular starch dispersion is its viscosity. High paste viscosity suggests suitability as a thickening agent in foods and as a finishing agent in
the textile and paper industries. The pasting characteristics of starch are determined
either using a Brabender Visco Amylograph or a Rapid Visco Analyzer (RVA)
(Wickramasinghe and Noda 2008; Tukomane and Varavinit 2008; Lin et al. 2009).
Rheometers (Park et al. 2007; Li et al. 2008a, b) or viscometers, which record viscosity profiles continuously with changes in temperature are also used to measure
the pasting properties. RVA is used to determine pasting properties such as peak,
final, setback, and breakdown viscosity, pasting temperature, and peak time. In this
test, starch is mixed with water and allowed to hydrate rapidly on heating, then held
constant for a specific time, and finally cooled to measure the pasting properties. As
heating continues, an increase in viscosity can be observed by swelling of starch
granules, 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 waterbinding capacity of starch and often correlates with final product quality since the
swollen and collapsed granules relate to the texture of cooked starch. Once PV is
achieved, a drop in viscosity, or breakdown, is observed as a result of disintegration
of granules. Breakdown is a measure of the ease of disrupting swollen starch
Starch: An Overview
Retrogradation in starches is commonly measured using a DSC, texture analyzer
or a rheometer. While the DSC analysis is sensitive to chain ordering of the amylopectin fraction, the mechanical tests are sensitive to chain entanglements with/without the formation of crystallites (Shevkani et al. 2017). Rheological techniques are
suited to monitor gel firmness (rigidity) on ageing and are usually carried by measuring the storage modulus (G`). These techniques do not evaluate the role of one
component or process only, but the combined effect of all the components of the gel.
Karim et al. (2000) have reviewed the methods to study retrogradation of starches.
DSC measures retrogradation enthalpy (ΔHret) and transition temperatures for the
melting of recrystallized starch. ΔHret is the amount of energy required for dissociation of the re-associated amylopectin. Hence, it provides a quantitative measure
of retrogradation, higher ΔHret means higher retrogradation tendencies.
Pasting Properties
Heating of starch continuously in excess of water causes starch granules to swell
and burst. Then the amylose leaches out and the granules disintegrate resulting in
the formation of 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, starch is generally regarded as the most important constituent of rice in terms of cooking quality
and functionality. The pasting properties of starch are used in determining the suitability of starch in different foods and other allied products. The most important
pasting characteristic of granular starch dispersion is its viscosity. High paste viscosity suggests suitability as a thickening agent in foods and as a finishing agent in
the textile and paper industries. The pasting characteristics of starch are determined
either using a Brabender Visco Amylograph or a Rapid Visco Analyzer (RVA)
(Wickramasinghe and Noda 2008; Tukomane and Varavinit 2008; Lin et al. 2009).
Rheometers (Park et al. 2007; Li et al. 2008a, b) or viscometers, which record viscosity profiles continuously with changes in temperature are also used to measure
the pasting properties. RVA is used to determine pasting properties such as peak,
final, setback, and breakdown viscosity, pasting temperature, and peak time. In this
test, starch is mixed with water and allowed to hydrate rapidly on heating, then held
constant for a specific time, and finally cooled to measure the pasting properties. As
heating continues, an increase in viscosity can be observed by swelling of starch
granules, 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 waterbinding capacity of starch and often correlates with final product quality since the
swollen and collapsed granules relate to the texture of cooked starch. Once PV is
achieved, a drop in viscosity, or breakdown, is observed as a result of disintegration
of granules. Breakdown is a measure of the ease of disrupting swollen starch
Starch: An Overview