**3.1. Dynamic rheology and gelatinization**

During this first stage of heating, starch granules swell during the process of gelatinization. Soluble polymer molecules leach from the swollen granules and the rheological properties, such as storage modulus (G') and loss modulus (G'') of the starch increase to a maximum. A sharp increase in G' may occur between 60-80° C (Ahmed et al., 2008) caused by the formation of three-dimensional (3D) gel network developed by leached out amylose and reinforced by strong interactions among swollen starch particles (Fig. 1). Similar changes can occur when viscosity is measured.

The swelling of the granules is important for both viscosity increase and viscoeleasticity of the produced dispersions. Granules' morphology and rigidity, complexes with other components (e.g. lipid-amylose), amylose content, protein content are some factors that determine both peak values of the viscoelastic parameters and their breakdown thereafter.

Concerning their botanical source, among native starches (corn, rice, wheat and potato), potato starches exhibit the highest swelling power and final viscoelastic values. Their shape and size differs with respect to starches of other botanical sources. Starch granules of potato are smooth-surfaced and of different shapes form oval and irregular to cube-shaped. Starch

granules of corn are angular-shaped, while those of rice are pentagonal and angular-shaped. Finally, wheat starch granules are spherical (B-granules) and lenticular-shaped (A-granules). Viscoelastic Properties of Starch and Non-Starch Thickeners in Simple Mixtures or Model Food 221

C)

After gelatinization the process of pasting follows. Under continuing heating, granules are further swollen and finally disrupted (Fig.1). A hot paste is then created consisting of swollen granules, granule fragments, and soluble materials. The botanical source of the starch, water content, temperature and shearing during heating determine the consistency of this paste. The network created consists of dissolved starch polymers (amylose and amylopectin) and a

Rheologically, a peak value of both viscosity and G', G'' is reached resulting mainly from maximum swelling. Furthermore a plateau e.g. constant values may occur (from 80-85°

due to irreversible swelling and solubilisation of amylose (Ahmed et al., 2008) followed by a sudden drop of G' under extensive heating and shear and time. Granules disintegrate. At this point a hot paste is created (Fig. 1). The height of the peak at a given concentration reflects the ability of the granules to swell freely before their physical breakdown. Α sudden drop after the maximum indicates the breakdown on cooking as well as a great ability to

G' decrease indicates the gel structure disruption due to the ''melting'' of the crystalline regions or disentanglements of the amylopectin molecules in the swollen particles that softens the particles (Tsai et al., 1997). The network collapses due to the loss of interactions

The distinction between a paste and a gel is not always evident. A paste usually refers to the hot freshly cooked system and gel is formed after cooling. Both are viscoelastic materials. As the hot pastes, especially of amylose –containing starches, begin to cool they become more elastic and develop solid properties. The transition from a viscous to an elastic gel can be determined by storage and loss moduli thus the setback can be found as a transition point from viscous to solid one (BeMiller, 2011). This setback is known as retrogradation (Atwell et al., 1988). At this critical gel point the system is wall-to-wall connected (percolation threshold) and is characterized by a critical behavior with G'(ω) and G''(ω) obeying the

The first phase of retrogradation begins as the paste cools and a formation of entanglements and/or junction zones is created between amylοse molecules resulting in an elastic gel. This phase may last up to 48 h. The second phase of retrogradation involves amylopectin changes, which is a much slower process that may proceed for several weeks depending on the storage temperature. Both G' and G'' increase upon cooling and during short-storage, G'

Briefly, chemical modification leads to a considerable change in the rheological and pasting properties of starches. Storage (G') and loss modulus (G'') of acetylated, hydroxypropylated

discontinuous phase of swollen granules, empty (ghost) ones and fragments.

**3.2. Pasting and viscoelastic properties** 

swell (Adebowale & Lawal, 2003).

between the particles (Ahmend et al., 2008).

**3.3. Retrogradation and viscoelastic properties** 

same power law: G'(ω)~ G''(ω) ~ ωn (Doublier & Cuvelier, 2006).

and G'' increase indicating that the gels become firmer.

**4. Modified starches** 

Moreover, potato starch granules are the largest (<110μm) in size followed by wheat (<30μm), corn (<25μm) and rice (<20μm) starches. Τhe granule size of potato starch is variable and ranges form 1 to 20μm for small and from 20 to 110 μm for large potato granules, whereas rice starch granules commonly range from 3 to 5μm in size (Hoover, 2001; Singh et al., 2003). Large and cubical or irregularly shaped granules in potato starch exhibit higher storage and loss modulus and lower tanδ than the small and oval granules (Singh & Singh, 2001). Thus, potato starch shows higher G', G'' and lower tanδ than corn, rice and wheat starches during the heating cycle. Furthermore, starch dispersions may exhibit significantly higher G' values (~100 times) as compared to flour dispersions at the first period of heating (40-60 ° C) (Ahmed et al., 2008).

Amylose amount is also quite important for controlling the viscoelastic properties of starch dispersions. Amylose results in higher G' indicating a well-cross-linked nature. Specifically G' can increase exponentially as a function of amylose content (Biliaderis & Juliano, 1993). Concentration effects are also linear for wheat and maize starch in the range of 6-30% (Ring, 1985) and follow a power law in the case of rice starches (8-40%).

Starch is a complicated viscoelastic structure. Under heating it can be described as a composite system, in which gelatinization may be regarded as an example of a phaseseparated composite gel, primarily governed by the volume fraction occupied by the swollen particles, whereas the continuous phase makes an additional contribution due to its own viscoelastic properties (Alloncle & Doublier, 1991; Dickinson,1992).

**Figure 1.** Viscoelastic changes of starch suspensions under heating and further cooling. Curves can shift to both axes accordingly to the factors that are mentioned. Changes in starch granules are also shown. Peak and plateau values can be seen at maximum starch swelling. Thereafter network breakdown and paste formation is shown. (Modification according to data from Ahmed et al., 2008, Food Hydrocolloids 22, pp 278-287 and Singh et al, 2007a, Starch/Stärke, 59, pp. 10-20)

## **3.2. Pasting and viscoelastic properties**

220 Viscoelasticity – From Theory to Biological Applications

period of heating (40-60 °

granules of corn are angular-shaped, while those of rice are pentagonal and angular-shaped. Finally, wheat starch granules are spherical (B-granules) and lenticular-shaped (A-granules). Moreover, potato starch granules are the largest (<110μm) in size followed by wheat (<30μm), corn (<25μm) and rice (<20μm) starches. Τhe granule size of potato starch is variable and ranges form 1 to 20μm for small and from 20 to 110 μm for large potato granules, whereas rice starch granules commonly range from 3 to 5μm in size (Hoover, 2001; Singh et al., 2003). Large and cubical or irregularly shaped granules in potato starch exhibit higher storage and loss modulus and lower tanδ than the small and oval granules (Singh & Singh, 2001). Thus, potato starch shows higher G', G'' and lower tanδ than corn, rice and wheat starches during the heating cycle. Furthermore, starch dispersions may exhibit significantly higher G' values (~100 times) as compared to flour dispersions at the first

Amylose amount is also quite important for controlling the viscoelastic properties of starch dispersions. Amylose results in higher G' indicating a well-cross-linked nature. Specifically G' can increase exponentially as a function of amylose content (Biliaderis & Juliano, 1993). Concentration effects are also linear for wheat and maize starch in the range of 6-30% (Ring,

Starch is a complicated viscoelastic structure. Under heating it can be described as a composite system, in which gelatinization may be regarded as an example of a phaseseparated composite gel, primarily governed by the volume fraction occupied by the swollen particles, whereas the continuous phase makes an additional contribution due to its

**Figure 1.** Viscoelastic changes of starch suspensions under heating and further cooling. Curves can shift to both axes accordingly to the factors that are mentioned. Changes in starch granules are also shown. Peak and plateau values can be seen at maximum starch swelling. Thereafter network breakdown and paste formation is shown. (Modification according to data from Ahmed et al., 2008, Food Hydrocolloids

C) (Ahmed et al., 2008).

1985) and follow a power law in the case of rice starches (8-40%).

22, pp 278-287 and Singh et al, 2007a, Starch/Stärke, 59, pp. 10-20)

own viscoelastic properties (Alloncle & Doublier, 1991; Dickinson,1992).

After gelatinization the process of pasting follows. Under continuing heating, granules are further swollen and finally disrupted (Fig.1). A hot paste is then created consisting of swollen granules, granule fragments, and soluble materials. The botanical source of the starch, water content, temperature and shearing during heating determine the consistency of this paste. The network created consists of dissolved starch polymers (amylose and amylopectin) and a discontinuous phase of swollen granules, empty (ghost) ones and fragments.

Rheologically, a peak value of both viscosity and G', G'' is reached resulting mainly from maximum swelling. Furthermore a plateau e.g. constant values may occur (from 80-85° C) due to irreversible swelling and solubilisation of amylose (Ahmed et al., 2008) followed by a sudden drop of G' under extensive heating and shear and time. Granules disintegrate. At this point a hot paste is created (Fig. 1). The height of the peak at a given concentration reflects the ability of the granules to swell freely before their physical breakdown. Α sudden drop after the maximum indicates the breakdown on cooking as well as a great ability to swell (Adebowale & Lawal, 2003).

G' decrease indicates the gel structure disruption due to the ''melting'' of the crystalline regions or disentanglements of the amylopectin molecules in the swollen particles that softens the particles (Tsai et al., 1997). The network collapses due to the loss of interactions between the particles (Ahmend et al., 2008).

#### **3.3. Retrogradation and viscoelastic properties**

The distinction between a paste and a gel is not always evident. A paste usually refers to the hot freshly cooked system and gel is formed after cooling. Both are viscoelastic materials. As the hot pastes, especially of amylose –containing starches, begin to cool they become more elastic and develop solid properties. The transition from a viscous to an elastic gel can be determined by storage and loss moduli thus the setback can be found as a transition point from viscous to solid one (BeMiller, 2011). This setback is known as retrogradation (Atwell et al., 1988). At this critical gel point the system is wall-to-wall connected (percolation threshold) and is characterized by a critical behavior with G'(ω) and G''(ω) obeying the same power law: G'(ω)~ G''(ω) ~ ωn (Doublier & Cuvelier, 2006).

The first phase of retrogradation begins as the paste cools and a formation of entanglements and/or junction zones is created between amylοse molecules resulting in an elastic gel. This phase may last up to 48 h. The second phase of retrogradation involves amylopectin changes, which is a much slower process that may proceed for several weeks depending on the storage temperature. Both G' and G'' increase upon cooling and during short-storage, G' and G'' increase indicating that the gels become firmer.
