1 Introduction In the food industry the use of native starches is limited by their lack of stability under the conditions of temperature, shear, pH and refrigeration commonly applied to processed foods. However, by introduction of functional groups into starches using a range of chemical modification techniques, starches can be produced with improved or specific proper- ties for extended usefulness in food or industrial applica- tions. The chemically modified starches have markedly al- tered physicochemical properties, compared to their parent starches [1]. Acetylation and cross-linking are two widely used methods for making modified starches. A mechanism for the effect of acetylation was proposed: introduction of the acetyl group reduces the bond strength between starch molecules and thus makes the starch granule swell and solu- bilize more, giving improved freeze-thaw stability [1]. Cross-linked starch is more resistant to acid, heat, and shear- ing than native starch [2], and is therefore suitable for appli- cations such as in canned food. Much research has been reported on acetylated and cross- linked starch. Acetylation increases solubility, swelling pow- er and viscosity but decreases gelatinization temperature of rice starch [3]. Hardness, adhesiveness and cohesiveness of gels made from the starches are increased by acetylation. In a study of Xanthosoma violaceum starch it was found that acetylation increases solubility and reduces gelatinization temperature, viscosity and swelling power of the starch [4]. Acetylated cassava starch is more soluble than native starch, while the clarity and sol stability of the derivatives are im- proved, and viscosity is reduced [5, 6]. We previously showed that acetylation of maize starch of a range of amy- lose contents reduces gelatinization temperature, increases peak viscosity and enzymatic digestibility, and decreases starch gel hardness [7]. Thickening power, hot paste stability and clarity of opti- mally cross-linked waxy barley starch match that of cross- linked waxy maize starch [8]. By adding cross-linked waxy barley starches with varying degrees of cross-linking to white pan bread in place of wheat starch it was found that the swelling power of the cross-linked waxy barley starch de- clines as the level of cross-linking increases [9]. More amy- lopectin than amylose molecules seem to be cross-linked, however, it is very difficult to provide direct evidence of where the modification takes place [10]. The characteristics of cross-linked rice starch show that cross-linking increases heat of gelatinization and shear stability, and it reduces solu- bility in dimethyl sulfoxide and freeze-thaw stability [11]. Cross-linked starch exhibits a three-dimensional network structure under scanning electron microscopy. Cross-linking greatly increases the stiffness of potato starch gels [12]. The objectives of this study were to evaluate the proper- ties of normal and waxy rice starches after cross-linking and acetylation. Properties studied include gelatinization temper- ature, pasting profiles, shear stability, swelling power and solubility, gel texture, and rate of retrogradation. 2 Materials and Methods 2.1 Starch samples Normal rice starch (NR), waxy rice starch (WR), cross- linked normal rice starch (NRX), cross-linked waxy rice starch (WRX), acetylated normal rice starch (NRAc), and acetylated waxy rice starch (WRAc) were all obtained from the Bangkok Starch Industrial Co., Thailand. Acetylation followed a commercial process adjusting pH to 9–10, adding vinyl acetate, and stopping the reaction by neutralizing to pH 7. The degree of substitution (DS) of WRAc was 0.0157, and for NRAc was 0.0183. The commercial process for cross- linking used sodium trimetaphosphate in the reaction and was equivalent for both starches. The degree of cross-linking was assessed by use of a Rapid Visco Analyzer (see Section 2.3) such that final viscosity of NRX was double that of NR. 2.2 Differential scanning calorimetry (DSC) Thermal analysis was performed with a Mettler DSC20 instrument (Mettler, Naenikon-Uster, Switzerland) equipped with a Mettler TC11 data analysis station. Starch (2.5 mg, d.b.) was weighed directly into a 40 μL pan and then 7.5 mg of deionized water was directly added into the pan by a mi- crosyringe. After sealing, the pan was left for 1 h to allow the sample to mix and equilibrate. Then the sample was heated from 30 °C to 110 °C at a heating rate of 10 °C/min. An emp- ty pan was used as a reference. Onset temperature (T o ), inter- mediate temperature (T p ), and completion temperature (T c ) of gelatinization and endothermic enthalpy of gelatinization (∆H) were recorded. 2.3 Pasting properties ARapid Visco Analyser model 3-D (RVA) (Newport Sci- entific Pty. Ltd., Warriewood, Australia) was employed to determine the pasting properties of the starch samples. Starch/Stärke 51 (1999) Nr. 7, S. 249–252 © WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999 0038-9056/99/0707-0249$17.50+.50/0 249 Physical Properties of Cross-linked and Acetylated Normal and Waxy Rice Starch Huijun Liu, Lawrence Ramsden, and Harold Corke, Hong Kong (China) Waxy and normal starches present wide biological diversity in their structure. The objective of this study was to investigate the effect of chemical modification on the physical properties of cross-linked and acetylated normal (NR) and waxy (WR) rice starch. Cross-linking in- creased shear stability and decreased swelling power and solubility of NR and WR, but increased viscosity, pasting temperature, and heat of gelatinization of WR, and decreased pasting temperature and heat of gelatinization of NR. Acetylation increased viscosity and solubility of NR and WR, while it increased the swelling power of NR, and de- creased the swelling power of WR. Cross-linking increased hardness and adhesiveness of NR and WR gels, while acetylation increased hard- ness but decreased adhesiveness of the gels. Freeze-thaw stability re- sults showed that both acetylation and cross-linking decreased retrogra- dation of NR, but increased it in WR.