Question

what is principle of strach gelation？

Answer 1

Starch is the most common carbohydrate in human diets and used as gelling (pasting) and thickening agents in foods, paper making, and some other industries. The physicochemical properties of starch obtained from different botanical sources vary due to the difference in their amylose content, grain size and chain length distribution of amylopectin molecules. We discuss the principle of gelation, and the gelatinization and retrogradation processes of amylose, amylopectin and starch molecules.

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.

(1) During heating, water is first absorbed in the amorphous space of starch, which leads to a swelling phenomenon.

(2) 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.

(3) 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 derivatization technology, are used. Some types of unmodified native starches start swelling at 55 °C, other types at 85 °C. The gelatinization temperature of modified starchdepends 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 gelatinization of starch is used constantly in cooking to make the starch digestible or to thicken/bind water in roux, sauce, or soup. The starch molecules (amylose and amylopectin) play a dominant role in the center of the tetrahedral cavities occupied by water molecules, and the arrangement is partially similar to a tetrahedral structure in a gelatinization process. The arrangement should lead to a cooperative effect stabilizing extended regions of ice-like water with hydrogen bonding on the surface of the polymer molecules, where hemiacetal oxygen and hydroxyl groups might participate in hydrogen bonding with water molecules.