of starch molecules in the presence of water and heat, allowing the hydrogen

bonding sites to engage more water. This irreversibly dissolves the starch

granule in water. Water acts as a plasticizer.

Three main processes happen to the starch granule: granule swelling,

crystal or double helical melting, and amylose leaching.

During heating, water is first absorbed in the amorphous space of starch, which

leads to a swelling phenomenon. Water then enters via amorphous regions

the tightly bound areas of double helical structures of amylopectin. At

ambient temperatures these crystalline regions do not allow water to enter.

Heat causes such regions to become diffuse, the amylose chains begin to

dissolve, to separate into an amorphous form and the number and size of

crystalline regions decreases. Under the microscope in polarized light starch

loses its birefringence and its extinction cross.

Penetration of water thus increases the randomness in the starch granule

structure, and causes swelling, eventually soluble amylose molecules

leach into the surrounding water and the granule structure disintegrates.

The gelatinization temperature of starch depends upon plant type and the amount

of water present, pH, types and concentration of salt, sugar, fat and

protein in the recipe, as well as starch derivatisation technology used. Some

types of unmodified native starches start swelling at 55 °C, other types at

85 °C. The gelatinization temperature of modified starch depends on for example

on the degree of cross-linking of the amylopectin, the degree of acid

treatment, acetylation. Gel temperature can also be modified by genetic

manipulation of starch synthase genes. Gelatinization temperature also depends

on the amount of damaged starch granules. These will swell faster.

Damaged starch can be produced, for example, during the wheat milling

process, or when drying the starch cake in the starch plant. There is an inverse

correlation between gelatinization temperature and glycemic index.

Gelatinization improves the availability of starch for amylase hydrolysis. So

gelatinisation of starch is used constantly in cooking to make the starch

digestable or to thicken/bind water in roux sauce, a soup.

Retrogradation Cooked, unmodified starch, when cooled

for a long enough period, will thicken and rearrange itself again to a more

crystalline structure; this process is called retrogradation. During cooling,

starch molecules gradually aggregate to form a gel. Molecular associations

occur: Amylose-Amylose ; Amylose-Amylopectin;

Amylopectin-Amylopectin. A mild association amongst chains come together

with water still embedded in the molecule. Due to the tightly packed

organization of small granule starches, retrogradation occurs much more slowly

compared to larger starch granules. High amylose starches require more energy to

break up bonds to gelatinize into starch molecules, leading to a rigid and stiff

gel. A mild association amongst chains come together with water still embedded

in the molecule. Due to strong associations of hydrogen

bonding, longer amylose molecules will form a stiff gel. Amylopectin molecules

with longer branched structure, increases the tendency to form strong

gels. Granule size do not directly impact starch performance, but it is one

of the main factors affecting starch gelatinzation and retrogradation. High

amylopectin starches will have a stable gel, but will be softer than high

amylose gels. Retrogradation restricts the

availability for amylase hydrolysis to occur.

Pregelatinized starch Pregelatinized starch is starch cooked

and then dried in the starch factory on a drum dryer or in an extruder making

the starch cold-water-soluble. Also spray dryers are used to obtain dry

starch sugars and low viscous pregelatinized starch powder.

Determination A simple technique to study starch

gelation is by using a Brabender Viscoamylograph. It is a common

technique used by food industries to determine the pasting temperature,

swelling capacity, shear/thermal stability and the extent of

retrogradation. Under controlled conditions, starch and distilled water

is heated at a constant heating rate in a rotating bowl and then cooled down.

The viscosity of the mixture deflects a measuring sensor in the bowl. This

deflection is measured as viscosity in torque over time vs. temperature, and

recorded on the computer. The viscoamylograph provides the audience

with the beginning of gelatinization, gelatinization maximum, gelatinization

temperature, viscosity during holding, and viscosity at the end of cooling.

DSC or Differential scanning calorimetry is another methods industries use to

examine properties of gelatinized starch. As water is heated with starch

granules, gelatinization occurs, involving an endothermic reaction.

The initiation of gelatinization is called the T-onset. T-peak is the

position where the endothermic reaction occurs at the maximum. T-conclusion is

when all the starch granules are fully gelatinized, and the curve remains

stable. See also

Dextrin Modified starch

Starch References

External links Food Resource, Starch, Oregon State

University Corn starch gelatinization, filmed with

microscope, Youtube