Crosslinking of starch

Cross linked starch is a highly polymerized glucose C6H1OO5 composed of numerous dehydrated glucose units with three hydroxyl groups located on the C2, C3, and C6 carbon atoms. These free hydroxyl groups form crystalline grains through hydrogen bonding. Starch granules are insoluble in water and suspended in water to form starch emulsion. When heated, the hydrogen bond strength weakens and ruptures. The original crystalline structure of the granules is destroyed, forming a sticky paste. This phenomenon is called gelatinization, and the temperature required to rise is called the melting temperature. This is an important property of starch, and different varieties of starch such as corn, potatoes, native and others have differences in gelatinization properties and paste formation properties, which affect their applications.

The application of reagents with 2 or 3 functional groups, such as formaldehyde, epichlorohydrin, sodium triphosphate, etc., can react with starch to form etherification or esterification bonds between two different starch molecules, which are stronger than hydrogen bonds and enhance the stability of starch granules. They are difficult to gelatinize under heat and have a high gelatinization temperature. Starch molecules are linked together by strong bonds, known as cross-linking, and the product is called cross-linked starch, which is an important type of modified starch. The cross-linking effect is significant, and a very low degree of cross-linking can greatly enhance the swelling and gelatinization properties of modified starch granules. The dosage of the reagent should be below 0.1% of the starch weight. High degree cross-linking can increase the stability of starch granules, and they will not gelatinize even when heated in boiling water. Control the process conditions to produce cross-linked starch of different degrees, suitable for the requirements of different applications. Crosslinking has a more significant impact on starch such as sticky corn, potato, cassava, etc. The viscosity and adhesiveness of this type of starch paste are highly stable, but after cross-linking, it can change the properties of the paste to be similar to ordinary corn starch.

The particle morphology of cross-linked starch is the same as that of the original starch, with little change. Highly cross-linked starch granules have a tight structure, do not gelatinize when heated, and have high fluidity. They are suitable as anti sticking agents and lubricants for rubber products. As a lubricant used for surgical rubber gloves, it will not gelatinize or become sticky during high-temperature disinfection. It can be used as the carrier of insecticides or disinsectants, and spray is convenient to use. Zinc amide has high stability and is suitable for application in dry batteries. Cross linked starch particles will expand when heated, which has a good effect on papermaking application and can be absorbed and raised by wet paper. Used for slurry sand, it is easy to be adsorbed onto the fiber surface, increasing wear resistance. Used as adhesive for casting sand cores, coal bricks, and ceramics.

Crosslinking and other reactions such as oxidation, acidification, esterification are commonly used in industry to prepare composite modified starch, which has better properties. For example, composite modified starch with high thermal and cold viscosity can be prepared by crosslinking with epichlorohydrin and oxidizing with sodium hypochlorite. After being stored at room temperature for a week, the viscosity remains basically unchanged, and it still becomes paste after further heating, with its original thermal viscosity. The application of this composite modified starch in paper coating can be stopped on weekends. The unused coating can be left at room temperature without changing its properties. When starting next week, it will be heated and the coating will still have the same properties as the newly formulated coating. Oxidized starch has good adhesive properties, good film-forming ability, and high transparency, but its thermal viscosity stability is poor and it is easily reduced when heated. Crosslinking can change this disadvantage.