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The interaction of xanthan and glucomannan (konjac mannan, KM) has been monitored using differential scanning calorimetry (DSC), electron spin resonance (ESR), and mechanical spectroscopy. The DSC and ESR results indicate that interaction occurs in water immediately following xanthan side chainbackbone association. The process is thermodynamically driven and occurs to reduce xanthan-water contacts.In the presence of electrolyte, both techniques show that the xanthan conformational change shifts to higher temperatures and depends on the nature of the cation. Divalent cations give rise to a greater shift than monovalent cations. The results indicate that xanthan/KM interaction occurs at temperatures much lower than the conformational transition. The rheological properties (i.e., storage (GO and loss (G") moduli and gelation temperature, Tt,i) are greatly influenced by the presence of electrolyte. In all instances, G', G", and gelation temperature are reduced when electrolyte is present, with divalent cations having a greater effect than monovalent cations. The reductiori in G', G", and Tgei follows the lyotropic series with increasing effects in the order K+ ~Cs+ < Na+ ~NHt+ ~Ba2+ « Mg2+ ~Ca2+. These observations are explained by the fact that electrolyte promotes xanthan self-association at the expense of xanthan/KM interaction.

synopsisPoly(viny1 alcohol) (PVA) gels were prepared by freezing and thawing aqueous solutions at temperatures from -20 to 15'C. The temperature was varied periodically by use of a computer.The endothermic DSC peak was observed for the PVA gels at about 6OT for five specimens of different degrees of saponification (DS). Another endothermic peak was also observed in the range 67-WC. and this peak shifted to higher temperature with increasing DS. These endothermic peaks shifted to lower temperature on immersion of the PVA gels in water. The dynamic Young's modulus E' at room temperature was also decreased by immersion of PVA gels in water; E' decreased monotonically with increasing temperature for PVA gels without immersion in water, while it increased up to a certain temperature and then decreased with increasing temperature for PVA gels in water. The X-ray diffraction showed a characteristic crystalline pattern for PVA gels of higher DS, and this peak was intensified by stretching the gel. 1 5 35 55 15 36 &Io I8 Temp ('c)Temperature dependence of E' and tans for 15% (w/w) PVA gels (DP 1700, DS 97.5 f 0.5 mol %) for various numbers of freeze-thaw cycles; (A) and (B) and symbols for numbers of freeze-thaw cycles as in Fig. 4. Fig. 5.

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