Starch gelatinization is a process of breaking down the intermolecular bonds
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