Effect of starch, sucrose and their combinations on the mechanical and acoustic properties of freeze-dried alginate gels

Alginate gels (2%) with and without incorporated starch and/or sucrose were freeze-dehydrated to produce cellular sponges. The stress-strain relationships of these dried gels were all irregular and jagged, typical of brittle cellular solids. The stiffness was assessed in terms of the fitted ('smoothed') apparent engineering stress at two pre-selected engineering strain levels (40 and 60%) using a polynomial model. Their brittleness was assessed in terms of the apparent fractal dimension of their stress-strain curves using the compass and box counting algorithms which yield the Richardson's and Kolmogorov's dimensions, respectively. The presence of starch in the dried gels solid matrix invariably increased their stiffness, but not in a manner that could be predicted on the basis of a stochiometric relationship or density increase. Sucrose, in contrast, could have the opposite effect suggesting that its presence in the solid matrix might interfere with its mechanical integrity. All the dried gels had a 'rich' acoustic signature as judged by the magnitude of their apparent fractal dimension, which was determined by the 'blanket' algorithm. But if there were differences between the different dried gels' acoustic signatures, they were too subtle to be detected by the apparent fractal dimension alone.