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Starch gelatinization


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.

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 are 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, or 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.


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