Calcium binding and calcium-induced gelation of normal low-methoxyl pectin modified by low molecular-weight polyuronate fraction

Functions of low molecular-weight polyuronate fractions in the calcium binding and calcium-induced gelation of normal low-methoxyl pectin (LMP) were investigated. The fractions from pectin with two different degrees of esterification (DE) and also from alginate with two different mannuronate/guluronate (M/G) ratios were prepared within the weight average molecular-weight range from ca. 40–65 kg/mol. In the mixtures of LMP and each polyuronate fraction, changes in the relative viscosity (ηr) of dilute solutions and in rheological properties of gels were examined in the presence of calcium. Different results were indicated between the pectin and the alginate fractions. The addition of the pectin fractions, regardless of its DE, increased ηr of dilute solutions and increased dynamic storage modulus (G′) of gels with showing greater effects at the lower DE. On the contrary, the addition of the alginate fractions, regardless of its M/G ratio, increased the critical threshold concentration of calcium required for the increase in ηr of dilute solutions and decreased G′ of gels with showing greater effects at the lower M/G ratio (i.e. guluronate rich). Based on the results, molecular associations of the mixtures were presented schematically to compare the functions between the pectin and the alginate fractions