Starch cross-linking is used to strengthen starch granules and prevent them from breaking down due to factors like heat, acidity, or mechanical shear. The extent of cross-linking and the source of starch affect the properties of modified starch. A small amount of cross-linking can stabilize the granule structure and increase granule swelling during heating, leading to higher swelling powers and paste peak viscosities. Studies have shown this to be true for different types of modified starch.
When starch granules undergo cross-linking reactions, it strengthens their structure and makes them more resistant to viscosity breakdown. However, the effects of cross-linking can vary depending on the extent of the reaction and the source of starch. Very low levels of cross-linking increase swelling powers and paste peak viscosities, while higher levels of cross-linking generally reduce granule swelling, solubility, and paste clarity. Cross-linked starches also exhibit increased pasting temperatures, stability to shear, and tolerance to acid pH conditions. Gelatinization temperatures and enthalpies decrease with very low levels of cross-linking, but increase with more substantial degrees of cross-linking.
Cross-linked starch rheology is influenced by the concentration and type of swollen granules present in a paste. Increasing cross-linking levels make the granules firmer. However, cross-linking alone cannot provide retrogradation stability and freeze-thaw stability, which require additional stabilization to create dual-modified starches. The susceptibility to enzyme-catalyzed hydrolysis reduces to some extent with increasing levels of cross-linking.
Cross-linking reactions use reagents that can form connections between adjacent starch chains. Common reagents used include phosphoryl chloride, sodium trimetaphosphate, sodium tripolyphosphate, epichlorohydrin, and adipic-acetic mixed anhydride. The optimal conditions for the reaction depend on the reagent used. For example, STMP and STPP reactions are generally conducted by heating starch granules impregnated with the reagent and a catalyzing base. Cross-linking is favored at pH values above 8 and 10 for STPP and STMP, respectively. In contrast, POCl3, EPI, and adipic-acetic mixed anhydride reactions are conducted in aqueous granular suspensions under basic conditions. Addition of a stabilizing salt is reported to enhance reaction with POCl3 and STMP. Other reagents have also been explored. Reactive extrusion and extended exposure to UV or polarized light have been used to generate cross-linked starches.
Cross-linked starches look similar to their native starch counterparts and the reaction primarily occurs in the amorphous regions of the granules. The type of reagent used affects the uniformity of the reaction throughout the granule. The molecular cross-links occur mainly between AM and AP, and the degree of cross-linking affects starch properties. The distarch form predominates in cross-linking reactions, with phosphodiester:monoester ratios ranging from 3:1 to more than 4:1.