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 guluronaterich
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, calciuminduced
gelation for the mixture of ALG and each low Mw polyuronate was compared on the molecular
level.