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Heating of tapioca starch dispersions at low concentration (<10% w/w) forms highly viscous and translucent pastes. In this work, a completely cold soluble tapioca starch powder is produced by spray drying of a previously gelatinized starch paste. Rehydrating the powder yields a viscous paste with a loss in elasticity. Its combination with nongelling food hydrocolloids, such as xanthan gum or ι-carrageenan, restores the mechanical properties and creates alternative starch based thickening agents with a stable structure. Rotation and oscillatory rheological measurements show significant differences in viscosity–temperature, strain, and frequency dependence between the native tapioca starch paste and the spray dried one. The addition of xanthan gum or ι-carrageenan to the spray dried tapioca starch has a much stronger influence on its properties than their addition to the native starch. The different chain flexibilities of the polyelectrolytes cause disparate effects on the mechanical behavior of the mixed systems. By confocal laser scanning microscopy (CLSM) it is shown, that the mixture of starch and ι-carrageenan leads to a well-mixed phase with intermolecular interactions between the hydrocolloid and the starch molecules. In contrast, the addition of xanthan gum induces a separation into local micro domains resulting in a mutual exclusion, which is also confirmed by CLSM. [ABSTRACT FROM AUTHOR]

Abstract: Adding high amounts of sugar influences the physical properties of hydrocolloid systems enormously, due to their mutual interactions with each other as well as with water molecules. The impact of the two disaccharides sucrose and trehalose on the thermo-mechanical properties of agarose based hydrogels is investigated. Agarose, a typical gelling agent, is combined with the nongelling agents sodium-alginate and xanthan, two polyelectrolytes which differ significantly in their flexibility. Material properties, such as viscoelasticity, water holding capacity, gelling temperature and thermal stability can be adapted by the controlled addition of sugar. These properties are investigated by strain and temperature dependent oscillatory rheological measurements as well as by controlled drying experiments. The experiments clearly indicate the competition of the different carbohydrates for water. The differences in water binding of sucrose and trehalose are clearly visible. A phenomenological model is presented and illustrates the different effects of the sugar molecules on the hydrocolloid systems. It is suggested that hydrate shells and the differences in local water binding on atomistic and nano scales have strong effects on macroscopic properties. [Copyright &y& Elsevier]