Structures of Four Waxy Rice Starches in Relation to Thermal, Pasting, and Textural Properties

Cereal Chem. 79(2):252–256 Waxy rice starches from three japonica cultivars (Taichung Waxy 1 [TCW1], Taichung Waxy 70 [TCW70], Tachimemochi) and one indica cultivar (Tainung Sen Waxy 2 [TNSW2]) were characterized for chemical and physicochemical properties. The amylopectin structures were different for the four waxy rice starches in terms of degree of polymerization (DP), average chain length (CL), exterior chain lengths (ECL), and distribution of chains, indicating the existence of varietal differences. The order of swelling power was TCW1 > TCW70 > TNSW2 > Tachimemochi; the order of water solubility index was TCW70 > TNSW2 > Tachimemochi > TCW1. The low water solubility index of TCW1 might be ascribed to a high DP. All starches shared similar gelatinization temperatures and enthalpies but showed distinct retrogradation patterns. TNSW2 showed the highest retrogradation rate, followed by TCW2, Tachimemochi, and TCW70. TCW70 exhibited the highest overall pasting viscosity, followed by TNSW2, TCW1, and Tachimemochi. The hardness of waxy rice starch pastes from a Brabender amyloviscograph increased rapidly after storage at 5°C for one day and remained the same or slightly increased after seven days of storage, whereas the opposite trend was observed for adhesiveness. The lower degree of retrogradation of TCW70 was probably a result of a larger amount of A chain and a shorter ECL. The changes in hardness correlated with the amount of A and B1 chains. The texture attributes of waxy rice starch pastes were significantly affected by amylopectin retrogradation during storage. Waxy rice (Oryza sativa L.), also called glutinous or sweet rice, is characterized by its opaque appearance and very low amylose content. Waxy rice can be classified into two subspecies, indica and japonica, and there are many cultivars in each subspecies. In Asia, indica and japonica waxy rices are used for different rice products. Indica waxy rice is used in rice tamale and rice pudding for its tenderness, while japonica waxy rice is used for sweetened rice cake due to its stickiness (Villareal et al 1993). Earlier studies have shown that waxy rices with high gelatinization temperature (GT) had a harder texture than did the low-GT rices (Antonio and Juliano 1974). The intermediate-GT waxy rice starch had a higher gel viscosity than did the low-GT starch (Perdon and Juliano 1975). Because the amylose content in waxy rice starch is usually <2%, the amylopectin structure plays a determinant role in the physicochemical properties of this starch, including the average degree of polymerization (DP), average chain length (CL), degree of branching, and chain-length distribution profile. Shi and Seib (1992) reported that the retrogradation of waxy starches was directly proportional to the mole fraction of unit chains with DP 14–25 and inversely proportional to the mole fraction of unit chains with DP 6–9. The DP of amylopectin negatively correlated with the hardness of stale cooked rice for low-GT waxy rices (Villareal et al 1993). The amylopectin content affected the swelling behavior of cereal starches (Tester and Morrison 1990; Morrison et al 1993) and the integrity of swollen starch granules determined the rheological properties of a starch paste or gel (Tsai et al 1997). Although varietal differences in amylopectin structure of waxy rice starches and the relation to physicochemical properties have been reported (Perdon and Juliano 1975; Juliano and Villareal 1987; Villareal et al 1993; Lu et al 1997; Tsai et al 1997; Villareal et al 1997), there is limited information on the contribution of starch structure to the different pasting and textural properties of waxy rice starches. Because many cultivars of waxy rice with different functionalities are commercially available and waxy rice starch consists of essentially 100% amylopectin, they serve as a good model to study the structure-functionality relationship of amylopectin. This study was initiated to provide additional information to better understand the relationship of amylopectin structure to thermal, pasting, and textural properties of four waxy rice starches. MATERIALS AND METHODS Starches Samples of four cultivars of milled waxy rice were obtained from the Taiwan Agricultural Research Institute, Taiwan, in 2000. There were three japonica cultivars (Taichung Waxy 1 [TCW1], Taichung Waxy 70 [TCW70], Tachimemochi) and one indica cultivar (Tainung Sen Waxy 2 [TNSW2]). Starch was isolated from milled rice by following a modified alkaline steeping method (Yang et al 1984). Dried starch was defatted with water-saturated-1-butanol (WSB) by shaking the suspension (5 g of starch in 25 mL of WSB) on a rotary shaker for 24 hr at room temperature and then centrifugation (15 min at 3,000 rpm). Defatted starch was dried in a convection oven at 40°C overnight. The apparent amylose content was determined by measuring the iodine affinity of defatted starch according to Schoch (1964). Structural Characterization The carbohydrate distributions of native and debranched maltodextrins were evaluated by using high performance size-exclusion chromatography (HPSEC) and high-performance anion-exchange chromatography with pulsed amperometric detection (HPAECPAD). A Waters HPSEC system equipped with 515 HPLC pump with an injector of 100 μL sample loop, an in-line degasser, and a 2410 refractive index detector maintained at 40°C was used in this study. The separation of both native and debranched starches was accomplished by a series of Shodex OHpak columns, including a guard column, KB-802 and KB-804 columns, and columns were maintained at 55°C with a column heater. The mobile phase was 0.1M NaNO3 and 0.2% NaN3 at a rate of 0.7 mL/min. Dextran standards, ranging from 5,200 to 872,300 of weight average molecular-weight (Mw) (PSS Polymer Standards Service-USA) and α 1-4 linked sugars with degree of polymerization (DP) 1 to 7 (Sigma Chemical Co.) were used to construct the regression line for molecular weight (MW) determination. The debranched maltodextrins were prepared by following the method of Kasemsuwan et al (1995), except that 6 mg of starch was dissolved in 3.2 mL of distilled water. The branch chainlength distribution of debranched starch was further elucidated with a Dionex DX-500 HPLC system (Sunnyvale, CA) by following the method of Kasemsuwan et al (1995). Sugars with DP 1–7 were used to identify the chromatographic peaks. The assignment for the chromatographic peaks with DP > 7 was based on the assumption that each successive peak represented a saccharide that was 1 DP longer than that of the previous peak. A mixed bed exchange resin 1 Department of Food Science, University of Arkansas, Fayetteville, AR 72704. 2 Corresponding author. Phone: 501-575-3871. Fax: 501-575-6936. E-mail: yjwang@ uark.edu Publication no. C-2002-0205-01R. © 2002 American Association of Cereal Chemists, Inc.