Collapse, shrinkage and structural changes in dried alginate gels containing fillers

Fillers are widely used in the food industry, among other reasons to strengthen mechanical properties. Shrinkage, collapse, mechanical properties and pore-size distribution were studied in cellular solids composed of alginate and different fillers (bentonite, cornstarch and rice-starch) immersed in a 60°Bx sucrose solution then vacuum-oven- or freeze-dried. Alginate gels filled with bentonite (5 and 10%) shrank by as much as 76% of their original volume, and were characterized by a dense, strong matrix, as revealed by SEM micrographs and mechanical property analysis. The calculated mean pore area was 0.8radical dot10−12 m2. A 30% reduction in shrinkage was achieved in samples containing 10 and 30% cornstarch that were immersed for 1.5 and 5.5 h, respectively and vacuum-oven-dried. Alginate filled with 30% cornstarch and immersed for 5.5 h in the 60°Bx sucrose solution maintained its cylindrical shape, with no obvious collapse during the drying process. Mean pore area was 1.2radical dot10−12, 1.0radical dot10−12 and 0.67radical dot10−12 m2 for samples containing 10, 20 and 30% cornstarch filler, respectively. SEM studies revealed that the alginate matrix covers the densely packed starch granules. Increasing the filler concentration strengthened the matrix, but made it more brittle. Freeze-dried samples showed less shrinkage. Mechanical tests showed that freeze-dried specimens are weaker than those dried in a vacuum-oven. Both starch and bentonite incorporation created the expected changes in the dry matrices, with respect to their role as fillers (additives to polymers, differing in chemical composition and morphology), and to their ability to induce changes in terms of a particular property, e.g. influencing and increasing its brittleness (i.e. producing a crunchier product).