Phase diagrams of hybrid carrageenans extracted from Ahnfeltiopsis devoniensis and Chondrus crispus.

NaCl and KCl phase diagrams of two kappa/iota-hybrid carrageenans (KI) are established, and the rheological properties of obtained solutions and gels are reported. KI were extracted from Ahnfeltiopsis devoniensis and Chondrus crispus seaweeds and showed different chemical composition, 48 mol% of kappa carrageenan (K) and 52 mol% of iota carrageenan (I), and 78 mol% of K and 22 mol% of I, respectively. Phase diagrams are systematically compared those of blends of commercial K and I (K+I) showing equivalent chemical compositions. Results confirm that KI clearly differ from mixtures of K and I. K+I form gels at lower polysaccharide concentration and ionic strength, and exhibit gel separation from a liquid phase when large amount (>0.1 mol/L) of KCl is used. In contrast, no syneresis was found in KI gels formed under similar conditions. Both KI and K+I gels are strain hardening, and show a concentration scaling of the elasticity with exponents ranging from 1.1 to 3.2 depending on the type of salt and ionic strength. The strain at break of KI gels does not show salt specificity and is similar to the strain at break of K+I gels in KCl under similar salt and polysaccharide concentrations. K+I gels in NaCl are more fragile than in KCl, thus showing salt specificity.

Postharvest culture in the dark: An eco-friendly alternative to alkali treatment for enhancing the gel quality of kappa/iota-hybrid carrageenan from Chondrus crispus (Gigartinales, Rhodophyta).

After harvesting from natural stocks, female gametophytes of the red seaweed Chondrus crispus were cultured for several days under a light-deprived regime in order to determine if dark treatment can improve the gel quality of carrageenan extracted from the seaweed. An increase in the gel strength of native carrageenan was observed after dark treatment for 10d, but not after 5d, as compared to that of freshly harvested (baseline) seaweed. Corresponding decrease in sulfate and increase in 3,6-anhydrogalactose (3,6-AG) contents of the carrageenan extract were also observed. We posit that during dark treatment, the production of sulfated carrageenan precursor units was prevented while the enzymatic conversion of the prevailing precursor units to the gelation-promoting 3,6-AG was allowed to proceed. The observed increase in carrageenan gel strength with a 10-d dark treatment was comparable to the effect of the widely-used industrial procedure of alkali treatment in improving the extract's gel quality. Hence, postharvest culture in the dark for 10d can be an eco-friendly alternative to alkali treatment.

Thermal and viscoelastic properties of kappa/iota-hybrid carrageenan gels obtained from the Portuguese seaweed Mastocarpus stellatus.

The potential use of the underexploited Portuguese seaweed Mastocarpus stellatus as a source of natural thickening and gelling agents for food applications has been investigated. The alkaline pretreatment duration and extraction parameters (pH, temperature, and extraction duration) of seaweeds have been systematically varied in order to produce kappa/iota-hybrid carrageenans exhibiting a wide range of chemical properties. The mechanical spectra and the setting and melting temperatures of gels obtained by cooling 1.5 wt % extracted biopolymers solutions in 0.05 mol/dm3 KCl were measured by means of small amplitude oscillatory shear experiments. Gels showing elastic storage moduli ranging from 250 to 2750 Pa with no water syneresis could be obtained, and the relationships between the thermal properties and elasticity of the gels were evidenced. Gels mechanical properties are shown to correlate well with polysaccharides chemical parameters such as the degree of sulfate groups, the molecular weight distribution, and the relative content in iota-carrageenan monomers as determined by FTIR spectroscopy.

Effect of extraction parameters on the chemical structure and gel properties of kappa/iota-hybrid carrageenans obtained from Mastocarpus stellatus.

Extraction parameters (temperature, pH, duration) and alkaline pre-treatment duration have been systematically varied in the aim of exploring their impact on both chemical structure and gelling properties of carrageenan biopolymers obtained from Mastocarpus stellatus seaweeds, collected on the Northern coast of Portugal. Increasing the alkaline pre-treatment duration PT leads to kappa/iota-hybrid carrageenans containing less sulphate groups and biological precursor monomers. As a result, gel properties in the presence of KCl are improved as demonstrated by the increase in the Young's modulus with parameter PT. Increasing the extraction duration t ameliorates the biopolymer yield with no significant change in the complex kappa/iota-hybrid carrageenan chemical structure. However, smaller molecular weights are obtained and gel properties are seen to be negatively affected. Extraction temperature and pH have dramatic effects on the biopolymer gel strength, and a set of extraction parameters optimized with respect to extraction yield and gel properties is reported. In addition, kappa/iota-hybrid carrageenans obtained throughout this study exhibit a wide range of gel strengths in KCl, and allow us to present correlations between gel thermal properties and the kappa/iota-hybrid carrageenans chemical structure.