Study on thermal and rheological properties of native rice starches and their corresponding mixtures

Starches were isolated from four rice cultivars collected from different Provinces of China: one common waxy rice (Nuo Mi, NM) and non-waxy rice samples, with well-defined physicochemical properties, classified as low (Xian Mi, XM), intermediate (Gong Mi, GM), and high (Jin You Mi, JYM) apparent amylose content (AC) were chosen to cover a broad range in AC and thermal characteristics. The order-disorder transitions that took place on heating aqueous starch suspensions (approximately 70% moisture) containing individual native rice starches and their blends (in various ratios and combinations) have been investigated using differential scanning calorimetry (DSC). Only one endotherm was observed for thermal properties of individual native starches, whereas two endotherms were observed in some cases for mixed starches. Individual rice native starch suspensions and mixed starches (20% and 40%) were also studied using dynamic rheometry by small amplitude oscillatory to characterize viscoelastic changes during the heating-cooling cycle. These results show that T-G'max, G'(5), and tandelta(5) are certainly governed by AC. Waxy starches influenced the rheological properties of mixed starches and the increase in G' was lower in common waxy starch and in some of its blends. Depending on the proportion of the native rice starch in the mixture, the G' peak maxima and G, for rice starch blends were generally between those observed for their individual starches components, i.e., they showed an intermediate change in gel rigidity (G') upon heating and cooling. The hardness for the gel mixtures was also investigated after 3 and 7 days of storage at 4 +/- 0.5 degreesC as an index of starch retrogradation. Waxy blends of 25% were most effective at reducing the rate of firmness as shown by the values of hardness when mixed with low and intermediate AC non-waxy starches. As aging progressed over 3-5 days, significant hardness was observed in the individual components samples compared with their blends. (C) 2004 Elsevier Ltd. All rights reserved.