EFFECTS OF PH AND DMSO CONTENT ON THE THERMAL AND RHEOLOGICAL PROPERTIES OF HIGH METHOXYL PECTIN-WATER GELS

It was shown that high methoxyl pectin forms a gel with a high elastic modulus in the presence of dimethyl sulfoxide (DMSO). Differential sc anning calorimetry (DSC) and dynamic viscoelastic measurements were ca rried 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 (p H = 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). Endo thermic peaks accompanying the gel-to-sol transition observed in heati ng DSC curves were sharpest for the acidic pectin gels with 0.168 mf D MSO 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 amoun ts of DMSO lower the gelling ability. The mean end-to-end distance r(m ) of chains which connect junction zones decreases, and the bonding en ergy epsilon increases with increasing DMSO content up to 0.277 mf, th en r(m) increases and epsilon decreases beyond this DMSO content. Judg ing from the heat absorbed on forming 1 mole of junction zones, juncti on 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 carboxy lic groups, hence the elastic modulus, the endothermic enthalpy accomp anying gel-to-sol transition, and gel-to-sol transition temperature in crease with decreasing pH.