Phase behavior of inulin and waxy maize starch systems as a model of carbohydrate-carbohydrate interactions.

摘要

At low moisture contents (20% w/w, w.b.), inulin's glass transition temperature (Tg) was measured using differential scanning calorimetry (DSC), and its relative crystallinity was determined through X-ray diffraction analysis. Inulin was found to be a semi-crystalline material, which presented both a Tg and a crystalline X-ray diffraction spectrum. At room temperature, its glass transition occurred at 18% moisture content (d.b.), corresponding to water activity (aw) values close to 0.75. When pre-solubilized and stored below its Tg, inulin remained in a highly amorphous state (∼13% relative crystallinity), while at conditions above Tg, pre-solubilized inulin recrystallized and reached native inulin's relative crystallinity (40%).; At moisture contents ≤20%, gelatinized waxy maize starch WMS remained in the glassy, amorphous state, since its glass transition at 25°C occurred at moisture contents of above 28% (w.b.). Concentrated mixed samples of inulin and WMS showed the existence of two Tg's, which corresponded to those of the individual components. Thus, phase separation existed due to a lack of interaction and plasticization effects between the two biopolymers.; Mixed inulin and WMS systems were also analyzed at high moisture contents (>60% w/w, w.b.). Steady shear rheological data were fitted to the Carreau model and to the Herschel-Bulkley model. A sudden change in steady and dynamic rheological properties was observed in samples with total polymer concentrations ≥30% (w/w, w.b.), conditions above inulin's coil-coil overlap concentration, resulting in closely packed inulin-filled, phase-separated WMS gels. An extended Cox-Merz rule was successfully applied to the analysis of concentrated inulin gels.; Fluorescence and light microscopy were used to characterize the phase-separated systems, in which two phases with completely different morphologies co-existed. The mechanism of phase separation was determined to be that of nucleation and growth of inulin crystallites. Polarized light microscopy and DSC were used to verify the crystalline nature of inulin aggregates in the samples. A ternary phase diagram that summarizes the interactions between inulin and WMS at high moisture contents was generated.