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The maximum viscosity in okenia and corn starches was obtained in the heating stage and decreasing of viscosity began before the ‘holdup’ stage. This effect is due to starch concentration that could be explained by the volumetric fraction occupied by the granules, i.e. at higher concentration, a more compact system is formed, increasing the resistance to shear rates in the heating stage; but the combined effect of the agitation and the friction causes the fragmentation of the granules and in consequence, an increase in the BU values in the molecular dissociation and re-association stages (Kayisu, Hood, & Vansoest, 1981). Fig. 7 shows the variation of the shear stress in function of the shear rate for the dispersions of okenia and corn starches with 5 and 10% solids. The plots denote a non-Newtonian behavior because although they exhibit a uniform flow, the relationship between shear stress and shear rate is not constant (Steffe, 1992). This type of behavior has been reported for dispersions of starch from other botanical sources (Kapoor & Bhattacharya, 2001; Rao & Tattiyakul, 1999; Tecante & Doublier, 1999; Thebaudin et al., 1998). For the dispersions with 5% solids, certain thixotropic behavior is observed, since shear stress is not only related to shear rate, but also to time; this is confirmed by the formation of a curl when descending on the shear rate values, phenomenon named as hysteresis (process of deformation in which the load phase and its discharge phase do not coincide), which is particular for each starch source (Steffe, 1992; Tecante & Doublier, 1999). Likewise, it was observed that okenia starch was less sensitive to the shear stress, since bigger values were required to make it flow. Amylose outside the granules forms a three-dimen-sional network whose structure is determined by the starch concentration, the structure of the swollen granules, the ratio amylose/amylopectin, the proportion of solubilised amy-lose, as well as by the method of pastes preparation (Cheng, Tsai, & Tseng, 1996); taking into account these factors and considering the swelling and solubility patterns of both starches, it was noticed that okenia presented higher granules swelling but lower solubility; suggesting that the amylose within these granules in combination with the amylopectin could be strengthening such a structure. When the concentration of solids increased to 10% (Fig. 7), the hysteresis was much smaller (slight thixotropic behavior). There was also a significant increase in the shear stress values when varying the concentration from 5 to 10% solids; this change in the flow behavior agrees with the idea that the viscosity is dependent on the concentration (Crockford & Knight, 1997; Fasihuddin & Peter, 1998). |
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