STRUCTURAL AND TEXTURAL PROPERTIES OF CALCIUM-INDUCED, HOT-MADE ALGINATE GELS

The accepted mechanism of alginate gelation at ambient temperature is by formation of egg-box junctions between pairs of polyguluronate chai n sequences and an included array of site-bound calcium ions. We now p resent evidence (circular dichroism and low-amplitude mechanical spect roscopy data) of similar regular interactions at high temperatures whe re all the active ingredients (alginate, salt and sequestrant) are hyd rated in a hot medium. During controlled cooling, a second transition is obtained at low temperatures, which may be attributed to the latera l interaction of dimeric chains and thus the development of an extende d, three-dimensional network. A subsequent temperature increase melts the second wave of structure formation, thus producing substantial the rmal hysteresis, but preserves the 'weak gel' properties of the algina te-calcium reaction observed at elevated temperatures. At levels of sa lt higher than 40% calcium conversion aggregation can occur even at 90 degrees C, leading to precipitation rather than a homogenous network. Prolonged refrigeration (72 h at 4-5 degrees C) sustains a gradual mo lecular rearrangement to a self-supporting (visual evidence), firmer ( frequency sweeps) network. Surprisingly, the strongest high guluronate alginate is not only more elastic than its low molecular weight count erpart (strain sweeps in dynamic oscillation) but it also exhibits hig her yield strain on compression testing than the high molecular weight , high mannuronate sample. The distinct differences in the long-range properties of high guluronate and high mannuronate alginate samples wi th comparable molecular weights might serve as a guide for the develop ment of reformed products with the desired characteristics.