Effects of pH and DMSO content on the thermal and rheological properties of high methoxyl pectin-water gels
1993
Watase, M. | Nishinari, K.
It was shown that high methoxyl pectin forms a gel with a high elastic modulus in the presence of dimethyl sulfoxide (DMSO). Differential scanning calorimetry (DSC) and dynamic viscoelastic measurements were carried out for high methoxyl pectin-DMSO gels prepared at pH 2.4 and 6.86. Elastic moduli of pectin gels as a function of DMSO content showed a maximum around 0.2 mole fraction (mf) DMSO; however, acidic gels (pH = 2.4) needed less DMSO to form a gel and the elastic moduli for the acidic gels were larger than those for neutral gels (pH = 6.86). Endothermic peaks accompanying the gel-to-sol transition observed in heating DSC curves were sharpest for the acidic pectin gels with 0.168 mf DMSO and for neutral pectin gels with 0.277 mf DMSO. A small amount of DMSO, less than 0.3 mf, promotes gel formation, whilst excessive amounts of DMSO lower the gelling ability. The mean end-to-end distance r(m) of chains which connect junction zones decreases, and the bonding energy epsilon increases with increasing DMSO content up to 0.277 mf; then r(m) increases and epsilon decreases beyond this DMSO content. Judging from the heat absorbed on forming 1 mole of junction zones, junction zones in acidic gels are more heat-resistant than those in neutral gels. The electrostatic repulsion between carboxylate ions is reduced at lower pHs because of the suppression of the dissociation of carboxylic groups, hence the elastic modulus, the endothermic enthalpy accompanying gel-to-sol transition, and gel-to-sol transition temperature increase with decreasing pH.
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