| heal.abstract |
Xanthan effect on swelling power (SP), solubility index (SOL) and granules status of wheat starch dispersions (2%w/w) was investigated. Control samples and samples with xanthan (0.09%w/w at the final solution) were prepared and heated at temperatures from 60 to 90°C for 5 or 30 min. Additionally, the viscosity of starch-xanthan mixtures was calculated at different shear rates. Regarding these mixtures two preparation techniques were used: separate preparation of starch and xanthan solutions and subsequent mixing (a), or mixture of the powders prior to addition of water (b). Samples were heated at two different temperatures 75, 90°C and times 5, 30 min. According to SP values and granules dimensions at 75°C, xanthan addition enhanced swelling. It also enhanced starch polymers leakage at temperatures <80°C. At higher temperatures lower SOL values were found than those of control samples. Furthermore, in the presence of xanthan the proportion of large granules was greater than this found in aqueous solution. However, xanthan induced granules folding, which was noticed even at relatively low temperatures (75°C). With respect to viscosity, all samples showed pseudoplastic behaviour. Experimental values were fitted well by the Ostwald-de Waele model (σ= k·γ̇n). Consistency values (k) and flow index (n) were mainly influenced by the preparation technique and the heating temperature. Mixtures prepared with technique (a) had increased viscosity, consistency and pseudoplasticity. Mixtures prepared with technique (b) had irregular granules' shape and size due to xanthan addition and bad adhesion between continuous and dispersed phase, which influence viscosity negatively. © 2003 Elsevier Ltd. All rights reserved. |
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