Understanding nanostructural differences in hydrogels from commercial carrageenans: Combined small angle X-ray scattering and rheological studies

Abstract

Hydrogels from commercial carrageenans (κ-C, ι-C, λ↑-C (high viscosity) and λ↓-C (low viscosity)) were prepared with and without the addition of salts (KCl and CaCl 2 ). FT-IR and 1 H NMR characterization evidenced that while the κ-C and ι-C grades were relatively pure carrageenans, the two λ-C grades were λ-, κ-, θand μcarrageenan hybrids. The effect of carrageenan and salt concentration on the hydrogel strength were evaluated through a response surface design and a detailed structural characterization was carried out by small angle X-ray scattering (SAXS) and rheology. The low amount of sulphate substitution in κ-C enabled intramolecular association, giving rise to strong hydrogels, even in the absence of salts. On the other hand, ι-C, λ↑-C and λ↓-C produced much weaker hydrogels and required the addition of salts to induce intramolecular association by ionic cross-linking. SAXS results suggested the formation of similar structures of double helices in κ-C and ι-C with the addition of salts; however, distinct network structures were attained. In the case of κ-C, a Gauss-Lorentz gel model was suitable to describe the hydrogel structure and the addition of K + promoted the formation of more ordered and densely packed structures. On the other hand, larger but weaker aggregates, with marked periodicity, were observed in ι-C, with Ca 2+ inducing the formation of more densely packed networks. The complex composition of the λ-C grades gave rise to more heterogeneous branched network structures, properly described by a correlation length model, where the gelation mechanism was mostly governed by the κ-carrageenan component.

Rheological and structural characterization of carrageenan emulsion gels

“…1, illustrate that the highest gel strength values were obtained within the middle range of all the studied variables. This is in contrast with what has been previously reported for κ-C hydrogels, where higher strength values were obtained when increasing the κ-C and KCl concentrations [28]. Whereas the creation of stronger carrageenan networks is favourable for the development of hydrogels, these strong interactions between the polysaccharide chains may not be completely desirable in the emulsion gels since it could impede a proper dispersion of the oil droplets within the polysaccharide matrix.…”

“…(6), corresponded to a formulation of 2.0% ι-C + 0.5% KCl and 0.5 OWR. The same behaviour has been reported for ι-C hydrogels, where the maximum ι-C and KCl concentrations produced the strongest hydrogel [28]. The reduced strength of ι-C emulsion gels as compared with κ-C can be attributed to the electrostatic repulsive forces exerted by the greater amount of sulphate groups in the ι-carrageenan chains (the main component in the ι-C grade), hence producing weaker interactions between the polysaccharide chains.…”

“…In general, the results showed that the κ-C emulsion gels were the strongest compared with those obtained from ι-C and λ-C. This was already anticipated since the lower amount of sulphate substitution in κ-carrageenan (the main component in the κ-C grade) is known to facilitate the formation of stronger hydrogenbonded gel networks [28]. For the κ-C emulsion gels, the statistical analysis showed that the linear term of OWR, all the quadratic terms and the interaction between κ-C and OWR presented significant effect on the gel strength (P < 0.05).…”

“…Different commercial carrageenan grades, in the form of powders were kindly donated by CEAMSA (Pontevedra, Spain): κ-carrageenan (extracted from Euchema cottonii), composed by 92% κ-carrageenan and 8% ι-carrageenan; ι-carrageenan (extracted from Euchema spinosum), composed by 82% ι-carrageenan and 18% floridean starch; and λ-carrageenan (high viscosity grade) (extracted from Gigartina pistillata), composed by 29% λ-carrageenan, 27% κ-carrageenan, 2% μ-carrageenan, 20% θ-carrageenan and 19% starch [28]. KCl was obtained from Sigma-Aldrich (Spain) and sunflower oil was purchased in a local market (Valencia, Spain).…”

“…However, a detailed structural characterization of carrageenan emulsion gels is not available to date and the underlying gelation mechanism is not completely understood. A recent work [28] demonstrated that the gelation mechanism and type of gel networks originated by different commercial carrageenan grades are significantly different and can be further altered by the inclusion of cations. Specially, the addition of K + was seen to induce the formation of stronger κ-C and λ-C hydrogels.…”