Physicochemical Properties of Rice Starch Modified by Hydrothermal Treatments

Cereal Chem. 84(5):527-531 Rice starches of long grain and waxy cultivars were annealed (ANN) in viscosity peak heights, lower setbacks, and greater swelling consistency. excess water at 50°C for 4 hr. They were also modified under heat-moisThe modified starch showed increased gelatinization temperatures and tore treatment (HMT) conditions at 110°C with various moisture contents narrower gelatinization temperature ranges on ANN or broader ones on (20. 30, and 40%) for 8 hr. The modified products were analyzed by HMT. The effects were more pronounced for HMT than for ANN Also, rapid-viscosity analysis (RVA). differential scanning calorimetr y (l)SC), the typical A-type XRI) pattern for rice starch remained unchanged after and X-ray diffraction (XRD). Generally, these hydrothermal treatments ANN or HMT at low moisture contents, and the amorphous content inaltered the pasting and gelling properties of rice starch, resulting in lower creased after HMT at 40 moisture content. Rice is the staple food of over half the world's population. Its popularity has increased greatly in recent years because rice foods are nutritious, gluten-free, and hypoallergenic, making them highly desirable for the health-conscious populace. Rice starch, the main component in rice, is mostly responsible for the functional properties of rice ingredients. However, starch generally needs modification to develop specific properties such as swelling consistency (Hoover and Vasanthan 1994; Lii et al 1996). Hydrothermal treatments of starch, particularly annealing and heat-moisture treatment, have been studied extensively in recent years. Essentially, annealing (ANN) is the treatment of starch in excess water at temperatures below the gelatinization temperature (for rice 50-60°C), whereas heat-moisture treatment (HMT) is the treatment at relatively lower moisture and higher temperature conditions (for rice <40% and 100-120°C, respectively). In the processing of starch, these hydrothermal conditions are often applied, resulting in physicochernical changes for the isolated starch products. For instance, Krueger et al (1987) demonstrated that commercial maize starch, isolated by wet milling including a lengthy steeping step at >45°C, was already annealed and had property changes due to partial annealing of the starch granules. Similarly, annealing was involved when rice flour was treated by alkaline protease digestion at 55°C for 5-30 hr, and the isolated rice starch showed significant changes in properties including lower starch pasting consistency (Lumdubwong 2000). In laboratory investigations, starches are often characterized by RVA for pasting properties, DSC for gelatinization transitions, and XRD for changes in crystalline status. After ANN or HMT, the starches normally display greater RVA pasting temperatures and stability, and lower pasting viscosities (Knutson 1990; Jacobs et al 1995; Gunaratne and Hoover 2002; Singh et al 2005; Vermeylen et al 2006). They also show higher DSC gelatinization temperatures, lower gelatinization enthalpies, and narrower (for ANN) or broader (for HMT) gelatinization temperature ranges. These RVA and DSC results are often confirmed by XRD, providing more information on the changes involving the crystallites in the starch granule. As expected, the effects of the harsher HMT are different and often more pronounced than those of ANN. For instance, in the treatment of potato starches, gelatinization temperatures increased on both ANN and HMT. However, on HMT only, the gelatinization enthalpy decreased and the XRD pattern changed from USDA-ARS-SRRC, New Orleans, I.A. 2 Louisiana State University Agricultural Center, Baton Rouge, LA. Karachi University, Karachi, Pakistan. dol: 10.1 094/CCHEM-84-5-0527 © 2007 AACC International, Inc. B-type to A-type (Slute 1992: Vermeylen et al 2006). As enthalpy values represent the number of double helices that unravel and melt during gelatinization (Cooke and Gidley 1992), the results indicate that the potato starch lost some of its crystallites, in number or in size, on HMT but not on ANN. The changes for the ANN modified potato starch that could not be detected by the traditional XRD were most likely due to alterations in binding forces from interactions between the crystalline and the amorphous regions (Stute 1992; Vermeylen et al 2006). The effects of ANN and HMT on starch may vary, depending on the botanical origin of the starch, and treatment conditions (particularly moisture content, temperature, and duration of the treatment). Generally, tuber and root starches have the B-type XRD pattern, and cereal and legume starches have the A-type XRD pattern. In crystallites of the A or B starch, double helices are found in pairs. The differences between A-type and B-type starches arise from water content and the manner in which these pairs are packed in the crystals (Imberty et al 1991). The lattice of B-type starch has a large void in which numerous water molecules can be accommodated. This void is not present in A-type starch. Depending on the chain length and water content, the doublehelix pairs associate to give the A and B starches (Imberty et al 1991). In B-type starches, the packing of double helices is less compact than in A-type starches (Gidley 1987). Consequently, during ANN or HMT, the double helices of B-type starches would he more mobile, and more prone to disruption than those of Atype starches. As a result, tuber starches are more susceptible to property changes than legume or cereal starches with these treatments (Hoover and Vasanthan 1994; Gunaratne and Hoover 2002). Recently, the framework approach of the side-chain liquid-crystalline polymers (SCLCP) has been used to study the structure and physical properties of starch (Waigh et al 2000). According to the SCLCP model, starches consist of three components: flexible backbones, flexible spacers (amorphous units), and mesogens (the rigid units of double helices of the amylopectin side chains). Based on data from mainly DSC and XRD analyses, the model can be applied to explain various starch behaviors. For instance, in the case of starch hydration, the addition of water brings about the self-assembly of the amylopectin helices in the lamellae to form a crystalline smectic hexagonal phase, causing the appearance of the smectic periodicity, whereas the amorphous backbone and spacers are in a plasticized liquid phase with the appearance of the B-type inter-helix spacing. Consequently, during gelatinization, at low water contents (<5%), a single DSC endoderm is observed due to the glassy nematic helix-coil transition. Intermediate water contents (>5%, <40%) result in two endotherms. The first is due to the rearrangement of dislocations of constituent amylopectin helices leading to a smectic-nematic transition. The Vol. 84, No. 5, 2007 527 40 60 80 100 120 second is the helix-coil transition as the amylopectin helices unwind in an irreversible transition. In excess water (>40%) the lamellae break up and helix-coil transition occur at the same point, as free unassociated helices are unstable. As physical treatments of food ingredients are considered safer and more desirable than chemical treatments, hydrothermal treatments of starch have been used for the improvement of functional properties of starch (Tester et al 1998; Shin et al 2005; Loisel et al 2006). However, relatively limited studies on hydrothermal treatments of rice starches are available in the literature (Jacobs et al 1995; Anderson et al 2002; Derycke et al 2005). In this study, rice starches of waxy (WXY) and long grain (LG) cultivars were treated under ANN or HMT conditions. The physicochemical properties of the modified products were characterized by RVA, DSC, and XRD. MATERIALS AND METHODS Waxy starch (WXS) and long grain starch (LGS) were obtained from A&B Ingredients (Fairfield, NJ ). According to the producer, the starch samples contained 0.55% protein (N x 6.25), 1% fat, 1% ash, a maximum of 12.5% moisture, and a minimum of 97% starch. The amylose contents are listed as 2% for the WXS and 20% for the LGS. All other chemicals were of reagent-grade. Annealing and heat-moisture treatment. Annealing was conducted by dispersing starch samples (LGS or WXS, 100-200 g each, dry wt) in water (1:4 starch to water). The mixture was then heated while shaking at 50°C for 24 hr. The residue was freeze-dried to obtain ALGS and AWXS. Heat-moisture treatment was conducted by adding water to starch samples (100 g each, dry wt) to achieve various moisture contents (20, 30, and 40%) and equilibrating them at 4°C overnight. The mixture was then covered and heated at 110°C in a convection oven (LindherglBlue model M01490A, Asheville, NC) for 8 hr, and the products were dried at 50°C overnight before analysis. The native (untreated) starch was used as the control. All analyses were done in triplicate. RVA analysis. Starch pasting properties were analyzed using an RVA-31) Rapid ViscoAnalyzer (Foss North America, Eden Prairie, MN). Samples were prepared by combining starch (3.0 g, 12% moisture basis) and distilled water to produce a total sample weight of 28.0 g. The mixture was stirred manually for 1 min to facilitate dispersion before testing. The initial speed of sample stirring in the analyzer was 16/sec for 10 see, followed by 2.7/sec for the remainder of the test. The heating and cooling cycles were programmed as sample held at 50°C for I mm, heated to 95°C in 3.8 mm, held at 95°C for 2.5 mm, cooled to 50°C in 3.8 mm, and finally held at 50°C for 1.4 mm. The total time of analysis was 12.5 mm; three pasting curves were run per sample. DSC analysis. A Q 100 DSC (TA Instruments, New Castle, DE) was utilized to measure the degree of starch gelatinization properties for the sample. Sample (20 mg) and distilled water (40 mg) were transferred into a DSC pan. The pan was hermetically scaled and inserted in the calorimeter. Thermal curves that included onset temperature (T,,), peak temperature (Tn), the endothermic peak area, and ending temperature (Te) were achieved at a heating rate of5'C/min from 25 to 140°C. Universal Analysis 2000 software was used to determine the onset and ending temperatures (Q1000 DSC manual, TA Instruments, New Castle, DE). The software drew a tangent line at the steepest point of the DSC curve and a baseline connecting the starting and the ending points of the peak. The intersections of the baseline with the DSC curve determined the onset and ending temperatures. Gelatinization energy (enthalpy, AH), the area that is calculated by drawing a straight line between onset temperature and ending temperature, is recorded as J/g on a dry weight basis. X-ray diffraction (XRD) analysis. Starch samples that were freeze-dried and ground into powder were hydrated to 75% relative humidity by incubation in a sealed vessel containing a saturated NaCl solution. The vessel with the hydrated sample was left at room temperature for 24-48 hr. Hydrated sample ( 1 g) was then pressed into a 10 x 25 mm pellet with a hydraulic press before being tested using an automated powder X-ray diffracto-