Rheology and thermodynamics of starch-based hydrogel-mixtures
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Abstract
The present work presents two different ways to achieve physically modified tapioca starch and the influence on mechanical properties of an aqueous starch paste is investigated. A completely cold soluble tapioca starch powder is produced by spray drying a previously gelatinized starch paste. Rehydrating the received white powder forms a viscous paste with significant loss in elasticity. Heating of a native tapioca starch suspension in contrast yields highly viscous paste with dominating elastic behavior. The combination of the spray dried tapioca starch with nongelling food hydrocolloids, such as xanthan gum, ι-carrageenan, and guar gum restores the mechanical properties and creates new starch-based thickening agents with stable structure. Rheological measurements of a gelatinized native tapioca starch paste compared to a rehydrated paste made from the spray dried starch show significant differences in viscosity and viscoelastic properties, which depend on temperature, amplitude, frequency, or shear rate. Further rheological, optical, and scattering investigations indicate weakening of the amylose network structure generated by the harsh shear and heat conditions during the spray drying process. The addition of water soluble hydrocolloids stabilizes the degraded gel structure by different mixing behaviors. According to the molecular nature of the added hydrocolloids, such as chain flexibility or charge distribution, different phase behavior in the starch-based composite system is induced. Thus, the stepwise replacement of starch by hydrocolloids influences the mechanical properties to various extents. The mixtures of spray dried tapioca starch and hydrocolloids have hydrocolloid dominated functional properties, and mixing the cold soluble tapioca starch with xanthan gum, ι-carrageenan, or guar gum on dry basis, a thickening effect with a weak gel character can be easily produced by hydration without heating. Depending on the desired application, the mechanical properties and texture can be controlled and tuned by the choice of hydrocolloid and by varying the mixture composition. Amylose and ι-carrageenan molecules are thermodynamically compatible, and a stable and well-mixed phase is formed. The mixture with xanthan gum and guar gum results in a phase separation where the hydrocolloid molecules separate into local domains. Phase separation is induced by thermodynamic incompatibility and mutual exclusion effects between amylose and xanthan or guaran molecules. The different phase behavior in the mixed systems is supported by confocal laser scanning microscopy and by covalent labeling of the hydrocolloids with specific fluorescence dyes.