Effects of urea concentration on thermal and rheological properties of rice starches

Starch is an important ingredient of many processed foods. When starch is heated in excess water, it loses its ordered molecular arrangement and swells irreversibly; this is termed gelatinization. These changes are responsible for the unique characteristics of starch-containing products. Because the purpose of incorporating starch in various applications is often related to the viscosity development upon gelatinization, a thorough understanding of the basis for the increase and maintenance of viscosity is critical. The gelatinization properties and rheological behavior of starch during heating are results of several cooperative factors: the structure of the hydrated starch molecules (Asaoka et al 1984, 1985; Tester and Morrison 1990a,b; Shi and Seib 1992; Sasaki and Matsuki 1998; Jane et a1 1999), interactions between lipids and amylose (Morrison 1995), and noncovalent bondings between starch molecules (Tako and Hizukuri 1997, 1999). Thus, any changes in these structural characteristics would result in subsequent changes in the rheological behavior of starch (Mita and Matsumoto 1981). Hydrogen bonding has been postulated to play a critical role in starch gelatinization. Hydrogen bonds are continuously broken and reformed throughout the course of heating (Caesar 1950). Water and starch molecules form a network through hydrogen bonding, which is responsible for viscosity increase. Dimethyl sulfoxide (DMSO) or DMSO-based solvents are widely used to solubilize starch for structural characterization (Solorza-Feria et al 2002; Han and Lim 2004; Short et al 2005; Zhong et al 2006). DMSO is an effective hydrogen bond acceptor by disrupting inter- and intramolecular starch-starch/water-starch hydrogen bonds and forming DMSO-starch hydrogen bonds (French 1984; Jackson 1991). McGrane et al (2004) used various hydrogen bond-forming and breaking agents to study the rheological properties of amylose gels. They reported that the use of intermolecular hydrogen bond-breaking agents such as urea reduced gel strength significantly, presumably by decreasing the intermolecular network formation between water and amylose. The objective of this study was to examine the role of hydrogen bonding in starch gelatinization by measuring and comparing the thermal and rheological parameters of two rice starches with different amylose contents during heating in aqueous urea solution of various concentrations.