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 (the hydroxyl hydrogen and oxygen) 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 the tightly bound amorphous regions of double helical structures to swell amylopectin, thus causing crystalline structures to melt and break free. At ambient temperatures these crystalline regions do not allow water to enter. Heat causes such regions to become diffuse, the amylopectine chains begin 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 this stress/pressure caused by this swelling phenomenon eventually rupture the structure organization of the granule and allows for leaching of amylose molecules to surrounding water.
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. Geltemperature 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 disgestable or to thicken/bind water in roux sauce, a soup.
Cooked, unmodified starch, when cooled for a long enough period, will thicken (or gel) 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 is starch cooked and then dried in the starch factory on a drum dryer or in an extruder making the starch cold water soluble. A spray dryer is used to obtain high quality pregelatinized starch powder.
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.
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