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The effects of starch compositions on the gelatinization and retrogradation behaviors of mixed cassava and rice starches were investigated by Brabender viscoamylography, differential scanning calorimetery, dynamic rheometry, scanning electron microscopy and X-ray diffractometry. The rice starches used were prepared from indica (TCN1) and japonica (TC189) rice. All the results of mixed cassava and rice starches were also compared with those of mixed cassava starch and rice flours in order to clarify the influence of the interactions between starch molecules on the physical properties of mixed flours when application.
The results of Brabender viscoamylography indicated that, for the mixed cassava-TC189-TCN1 starch systems of 5:1:1-1:1:5, the pasting temperatures are in the range of 64.0-66.5℃ and the peak and hot-paste viscosities are lower than those of pure starch systems. Besides, the Setback viscosity of the 1:1:5 system is the highest among the mixed systems. By the evidence of SEM, aggregates of cassava and rice starches and the entanglement of leaching-out materials from starch granules were observed in the above paste systems. When the total concentration of the starch systems was 25%, the gelatinization enthalpy changes of the mixed systems appear to be lower than those of pure starch systems, and increasing with the increment of cassava starch concentration. On the other hand, the storage modulus (G'') increase during gelatinization is lower for the mixed systems mainly composed with TCN1 rather than the other mixed or pure systems. Generally, the changes in the gelatinization behavior depend on the characteristics of starch compositions, e.g. the granular rigidity and size, the amount, chain length and the degree of branch of solubles.
On retrogradation, the changes in retrogradation enthalpy and G'' value varying with storage time depend on the composition of mixed starches. When the composition of TCN1 or cassava starch increases, the retrogradation rate or extent, respectively, of the mixed starch systems increases. As shown by X-ray diffractograms, an exceptional signal of 2θ=17 or 22 appears in the retrogradated, mixed starch systems, in addition to the typical signals of B-type crystallites. SEM data suggested that the network structures of mixed starch gels seem not to be influnced by the mixed ratio of starches.
The above changes in the physical properties of mixed cassava and rice starches during gelatinization and retrogradation are similar to those of the mixed cassava starch and rice flours, indicating the property of the latter systems is governed by the interations between starches.
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