© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Starch/Stärke 55 (2003) 337–344 DOI 10.1002/star.200300193 337 Research Paper 1 Introduction Starch is composed of the two glucose polymers: amy- lose and amylopectin, organised into a radially anisotrop- ic, semi-crystalline structure in the granule [1]. Among ce- real starches, rice has the smallest granules and, in con- trast to wheat, shows a single granule size distribution [2]. In addition, the amylose content of rice starches varies broadly between different varieties. It is well recognised that on storage rice changes in eating quality. Several dif- ferent mechanisms have been implicated in this change [3, 4]. When aqueous starch suspensions are heated, the gran- ules absorb water and swell considerably resulting in a viscosity increase. Concentration, temperature, volume fraction of the swollen granules, the deformability of the granules, as well as the degree of molecular entangle- ment have all been reported to affect the viscosity of starch pastes [5, 6]. The viscosity of starch pastes has of- ten been interpreted in terms of the parameter cQ where c is a concentration and Q is the mass of water adsorbed per mass of dry starch. This starch granule mass fraction (cQ), which has been referred to as the “notional volume fraction” [7], plays an important role in determining the rheological properties of starch dispersions [8]. At low cQ, the granules are sufficiently far apart and do not affect vis- cosity substantially. However, as cQ increases the inter- granular interactions become greater and this is accom- panied by a large increase in dispersion viscosity [9]. A number of studies on the viscosity of rice flour pastes have been published [10-13]. It has been shown that the viscosity of rice flour pastes was inversely proportional to the amylose content of rice at low paste concentration but directly proportional at high (>7%) paste concentrations [10, 11]. The rice flour pastes exhibited shear-thinning be- haviour without a yield stress at concentrations of 2-6% and temperatures of 25-80 °C and their flow properties were represented by a power law model [12]. Latha et al. [13] studied dispersions of steamed rice flour at higher concentrations (10-50%) and measured yield stresses by stress relaxation at the higher concentrations within this range. The data was modelled using Herschel-Bulkley, Casson and Bingham relationships. Little work has been reported on the viscosity of rice starch pastes as opposed to rice flours. In this paper we report the viscosity of a series of starches of different amylose content extracted from rice varieties grown in Thailand, which is one of the major rice exporting countries. The main objective of the work was to relate the viscosity to the amylose content and other aspects of the Piyarat Noosuk a , Sandra E. Hill b , Pasawadee Pradipasena c , John R. Mitchell b a Department of Food Technology, Faculty of Agro-Industry, Prince of Songkla University, Songkhla, Thailand b Division of Food Sciences, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, UK c Department of Food Technology, Faculty of Sciences, Chulalongkorn University, Bangkok, Thailand Structure-Viscosity Relationships for Thai Rice Starches The viscosities of five Thai rice starches with different amylose contents were mea- sured by rotational rheometry and using a Rapid Visco Analyser. The viscosity re- sponse was interpreted in terms of the swelling volume and the structural properties of the granule determined by calorimetry, infrared spectroscopy and X-ray diffraction. It was found that the waxy rice starches had a higher swelling volume resulting in a higher viscosity than the other rice classes. Critical concentrations for close packing, identified from the concentration where the flow behaviour departs from Newtonian, were 2.5%, 1.5% and 1.10% for the high amylose, medium amylose and waxy rice starch, respectively. An exception to this was a commercial high amylose starch that had a critical concentration similar to the medium amylose category. This was ex- plained in terms of a relatively higher loss of short-range molecular order on gelatini- sation as shown by FTIR for this starch. Medium amylose Jasmine rice starch, ex- tracted from rice that had been aged for 5 months, showed differences in granule size, crystallinity, pasting profile and enthalpy of gelatinisation when compared with the re- sults for starch extracted from freshly harvested rice. This was provisionally interpret- ed in terms of granule damage during the extraction process, though more work would be required to confirm this interpretation. Keywords: Rice starch; Paste viscosity; Granule structure Correspondence: John R. Mitchell, Division of Food Sciences, School of Biosciences, University of Nottingham, Sutton Boning- ton Campus, Loughborough, LE12 5RD, UK. Phone: +44-115 9516146, Fax: +44-1159516142, e-mail john.mitchell@notting- ham.ac.uk.