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| Gelatinization of Starch | to Top |
Starch in its processed, commercial form is composed of starch grains or granules with most of the moisture removed. It is insoluble in water. When put in cold water, the grains may absorb a small amount of the liquid. Up to 60 to 70°ree;C the swelling is reversible, the degree of reversibility being dependent upon the particular starch. With higher temperatures in irreversible swelling called gelatinization begins.
Starch begins to gelatinize between 60 and 70C, the exact temperature dependent on the specific starch. For example, different starches exhibit different granular densities, which affect the ease with which these granules can absorb water. Since loss of birefringence occurs at the time of initial rapid gelatinization (swelling of the granule), loss of birefringence is a good indicator of the initial gelatinization temperature of a given starch. The largest granules, which are usually less compact, begin to swell first. Once optimum gelatinization of the grains has occurred, unnecessary agitation may fragment the swollen starch grains and cause thinning of the paste.
The gelatinization range refers to the temperature range over which all the granules are fully swollen. This range is different for different starches. However, one can often observe this gelatinization because it is usually evidence by increased translucency and increased viscosity. This is due to water being absorbed away from the liquid phase into the starch granule.
Gelatinization Changes in Starch
From the changes brought about by this process shown below, a starch paste may occur and/or a starch gel if conditions are correct. These changes are listed and may be seen by clicking on the heading of this paragraph.
 | Raw starch that has not had moisture added does not undergo gelatinization. By definition, gelatinization is a phenomena which takes place in the presence of heat and moisture. The dry raw starch, if heated, would undergo dextrinization. This certainly would affect the starch paste viscosity and starch gel strength. The paste viscosity would be decreased and gel strength decreased. If a "limited amount" of moisture is added to the raw starch you may get partial gelatinization. This condition exists in baked products. |
 | Cornstarch at a 5% level in 95% water would have a slight change occur if heat is initiated. Water might be slightly ADSORBED onto the surface of the granule. Actually, in the research from which these images came, I found that I got a difference in paste viscosity and ultimate optimum gelatinization temperature as measured by viscosity if I allowed cornstarch to sit in water at room temperature. This led me to believe that there is some initiation of adsorption upon the granule at room temperature (27C). |
 | If this 5% dispersion of cornstarch was heated to 40C I would expect more water would be ADSORBED onto the surface of the granule, the hydrogen bonding between the starch polymers within the granule might begin to be loosened slightly. In some types of starches water might even begin to be ABSORBED into the granule. |
 | If this 5% dispersion of cornstarch was heated to 50C I would expect more water would be ADSORBED onto the surface of the granule, the hydrogen bonding between the starch polymers within the granule would begin to be loosened.. This would allow the water to penetrate into the granule becoming ABSORBED by the granule. Additionally, some of the amylose may begin to work itself off the granule surface, thus, opening the structure even more. |
 | If this 5% dispersion of cornstarch was heated to 60-65C I would expect more water would be ADSORBED onto the surface of the granule, the hydrogen bonding between the starch polymers within the granule would loosen. This would allow the water to penetrate into the granule becoming ABSORBED by the granule. Additionally, some of the amylose would work itself off the granule surface, thus, opening the structure even more. This in turn would allow even more of the water to become ABSORBED and more amylose to work itself out into a colloidal dispersion outside of the granule. The long amylose polymer is a colloid in characteristics. |
 | This is intermediate between 60 and 70C. The precise changes are affected by rate of heating, condition of the starch and other factors. |
 | If this 5% dispersion of cornstarch was heated to 70-90C I would expect more water would be ADSORBED onto the surface of the granule, the hydrogen bonding between the starch polymers within the granule would loosen. This would allow the water to penetrate into the granule becoming ABSORBED by the granule. Additionally, the amylose would work itself off the granule surface, thus, opening the structure even more. This in turn would allow even more of the water to become ABSORBED and more amylose to work itself out into a colloidal dispersion outside of the granule. The long amylose polymer is a colloid in characteristics. At some point between 60-95C we would likely have gelatinization occur. This might be measured by loss of birefringence, increased viscosity, translucency, increased susceptibility to enzyme action, x-ray diffraction or some other chemical or physical means. At this point, the starch granule is swollen as much as possible. It is a starch sol until you remove it from the heat and begin to allow the amylose and some amylopectin to recrystallize, i.e. realign. |
 | In some instances, when heated to 90C the starch granule could reach optimum gelatinization and be a nice swollen granule sack. In other cases, this may allow the sack to "implode" and loose their contents as there is not enough structure and hydrogen bonding to hold the polymers together. It is interesting that overcooking, as with overstirring, will decrease the starch paste colloidal sol viscosity. |
Updated: Wednesday, December 5, 2007.
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