Quiz 21: Retrogradation and gelatinization of starch

 

Answer is Retrogdation of starch

 

Gelatinization of starch

 

Native starch granules are insoluble in cold water. When heated in an aqueous medium, starch granules absorb water and swell. At the initial phase swelling is reversible but in later stage, it becomes irreversible as temperature is increased, and the granule structure is altered significantly. At higher temperature, the starch polymers vibrate vigorously, break intermolecular bonds and allow their hydrogen bonding sites to engage more water molecules. The water penetration leads to an increased separation of starch chains, which has an effect in increasing the randomness and decreasing the number and size of crystalline regions. Due to continued heating complete loss of crystallinity of starch granules are observed. As melting temperature Tm value is exceeded, the viscosity becomes very high.  This point is regarded as the gelatinization point or gelatinization temperature.

 

In bread it is the setting temperature, when the viscous dough changes to a solid sponge. In most baked products the starch crystallites melt at between 60 C and 90 C.

 

Retrogradation of starch

 

Starch retrogradation is a process in which gelatinized starch molecules re-associate to form a double helix crystalline structure. It is the main cause of staling of bread.  Technically, staling is the time-dependent re-crystallization of amylopectin from the completely amorphous state of a freshly heated product to the partially crystalline state of a stale product. The most important requirement of starch re-crystallization is the availability of sufficient moisture, at least locally within the matrix, for mobilizing long polymer chain segments. In fresh bread, the branched chains of amylopectin are unfolded and spread out within the limits of available water. These chains of the amylopectin polymer gradually aggregate, aligning with one another by various types of intra-molecular bonding. This effect results in increasing rigidity of the internal structure of the swollen starch granules causing crumb hardening

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and if v heat a stale bread it becomes more stale due to slight moisture loss

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