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ABSTRACT
Cross-linked carboxymethyl rice starches (CL-CMRS) were prepared from reactions between a native Klong Luang 1 (KL1) rice starch and varied concentrations (2.5-15% w/w) of sodium trimetaphosphate (STMP) in simultaneous carboxymethylation and cross-linking reactions set up using methanol as a solvent. Physicochemical as well as pharmaceutical properties of CL-CMRS were evaluated in relation to the amount of STMP used and the degree of cross-linking (DCx). At a low DCx, the viscosity of CMRS solution was enhanced through the formation of cross-linked polymeric network and chain entanglement. At higher concentrations in the preparation reaction, STMP caused proportional decreases in the water solubility and ≤70-fold of the solution viscosity, but promoted swelling and water uptake of the modified starches. Rheological behavior of the nonsoluble but swellable CL-CMRS was similar to that of commercial superdisintegrants sodium starch glycolate (SSG), and cross-carmellose sodium (CCS). The swelling and water uptake of CL-CMRS were 5-7 and 6-25 times higher, respectively, than that of the native starch. Disintegration test of tablets containing 1 and 3% w/w of native and modified rice starches showed that M-KL1-5 and M-KL1-10 could be developed as tablet disintegrants.
Cereal Chem. 87(3):214-220
Chemical modification is a process commonly employed in the alteration of the physicochemical properties of native starch to improve functionality for uses in the food and pharmaceutical industries. The hydroxyl groups on the starch molecule are targets of substitution by esterification (acetylation, phosphorylation), etheriftcation (hydroxypropylation, carboxymethylation), and crosslinking (distarch formation) (Roberts 1967). Esterification generally lowered the gelation temperature and retarded rétrogradation. Etherification improved water-binding capacity, viscosity, paste clarity, and free-thaw stability; cross-linking reduces elasticity of native starch and increases granular tolerance toward swelling, high shear, acids, and temperature (Singh et al 2007). These improved characteristics allowed the use of modified starches in many types of canned, refrigerated, and frozen foods, as well as application as drug excipients in the pharmaceutical industry.
The physicochemical properties of starch can be further manipulated by a double or dual modification, which is a combination of two reactions done consecutively, sequentially, or simultaneously on starch. The combination is usually a substitution and a cross-linking reaction (Wattanachant et al 2003; Raina et al 2006), although dual-modified starches prepared from two different substitution reactions have also been reported (Koh et al...