Microstructure of Aggregated and Nonaggregated κ-Carrageenan Helices Visualized by Atomic Force Microscopy

Gelation of kappa-carrageenan is believed to involve two steps: helix formation on cooling and a further specific cation (salt) induced side-by-side aggregation of helices. Samples that should contain aggregated and also nonaggregated "helices" of kappa-carrageenan were prepared in aqueous solutions, spread onto freshly cleaved mica surfaces, and visualized under butanol using atomic force microscopy. In the presence of an excessive amount of a gel-promoting salt, KCl, kappa-carrageenan appeared to form rigid rodlike structures considered as large aggregates of double helices. Even when the side-by-side interhelical aggregation was suppressed by diluting random coiled solutions prior to cooling, by adding an aggregation-impeding salt, NaI, or by transforming kappa-carrageenan into the tetramethylammonium (TMA) salt, branched rodlike structures were still evident, suggesting that the side-by-side aggregation of helices is not a prerequisite for kappa-carrageenan to form a network structure, at least locally. Even in the absence of factors that promote side-by-side aggregation, kappa-carrageenan helices appeared to be capable of associating and forming gel networks.