Calcium binding and calcium-induced gelation of sodium alginate modified by low molecular-weight polyuronate

• Ca-binding behavior of normal alginate was modified by guluronate-rich fraction.
• Competitive Ca-binding occurred between normal alginate and the G-rich fraction.
• Rheological properties of normal alginate was modified by the G-rich fraction.
• The G-rich fraction decreased elasticity and increased deformability of normal alginate.
• Functions of the G-rich fraction were different from those of pectin fraction.

The functions of low molecular-weight Mw polyuronate on the calcium binding and calcium-induced gelation of normal sodium alginate (ALG) have been investigated. Mannuronate- and guluronate-rich fractions were prepared from ALG at two different Mw for each. In the mixtures of ALG and each alginate fraction, changes in the relative viscosity of dilute solutions and rheological properties of the gels were examined after calcium addition. In dilute solutions, the mannuronate-rich fractions did not substantially alter the calcium binding behavior of ALG regardless of Mw. On the contrary, the guluronate-rich fractions changed the calcium binding behavior of ALG, and more calcium was required for increase in the relative viscosity relating to the formation of egg-box dimers and subsequent aggregations. These results were more evident when Mw of guluronate-rich fractions was lower. Gel rheology was also different between the mannuronate- and the guluronate-rich fractions. In the gels, both fractions decreased the storage modulus in the linear viscoelastic regime with increased yield strain, but these effects of the guluronate-rich fractions were greater than the mannuronate-rich fractions when compared at equivalent Mw. These functions of the guluronate-rich fractions were quite different from those of low-methoxyl pectin fraction. By using well-characterized polyuronate samples, calcium-induced gelation for the mixture of ALG and each low Mw polyuronate was compared on the molecular level.

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