Effect of sucrose on the mechanical and acoustic properties of freeze-dried agar, κ-carrageenan and gellan gels

Agar, κ-carrageenan and gellan gels (2%) with and without infused sucrose (5, 10 and 12.5%) were freeze dried and the mechanical and acoustic signatures of the resulting solid sponges recorded. For comparison, agar gels with 5 and 10% starch were also prepared and likewise tested. The presence of the sugar, or starch, in the dried gels increased their density in a manner that could not be predicted from the corresponding stoichiometric relations between the gum and additive indicating that the sugar, or starch, were not inert fillers. In general, the presence of the sucrose in the dried gel solid matrix increased the brittleness of the sponges. The effect could be quantified in terms of the increase in the mechanical signature's apparent fractal dimension (Richardson's and Kolmogorov's). The more brittle dried gels also had a "richer" acoustic signature whose apparent fractal dimension was determined with the "blanket" algorithm. Despite the irregular and jagged appearance of all the dry gels stress-strain relationships, they could be described by the same kind of a three parameter empirical model originally derived for soft baked products and polymeric foams. Although the added sugar, or starch, stiffened the dehydrated gels the effect could not be quantified unambiguously because of the differences in the density and in the overall shape of the stress-strain curve.