Cryogelation of xanthan

Xanthan solutions have gel-like properties, including the ability to hold particles in suspension, but the network structure breaks down readily, allowing the solutions to flow freely. These ‘weak gel’ properties are attributed to tenuous association of ordered chains, and are enhanced by cations, particularly Ca2+. In the present work it has been found that much stronger and more cohesive networks are formed when solutions of xanthan are frozen and thawed. Solutions (0.2–2.0 wt%) were prepared in deionised water at 80 °C, frozen rapidly by transfer to a freezer at −20 °C, held at −20 °C for 24 h, and allowed to thaw in a refrigerator at 5 °C. Their rheological properties were then characterised by small deformation oscillatory measurements of storage and loss moduli (G′ and G″) at 5 °C. The concentration-dependence of gel moduli for the xanthan cryogels was unusual, with G′ reaching a maximum value of ∼100 Pa at 0.6 wt% (in comparison with ∼4 Pa for unfrozen xanthan at the same concentration), but then decreasing slightly at higher concentrations. Gel strength was enhanced by subjecting the samples to a second freeze–thaw cycle, or by lowering the freezing temperature to −80 °C. Melting of cryogel structure occurred over the approximate temperature range 20–40 °C, with the moduli at higher temperature (70 °C) becoming the same as those for unfrozen samples. Addition of sugars (sucrose, glucose, fructose, maltose) at concentrations up to ∼10 wt% had no effect on the strength of the cryogels, but higher concentrations caused a progressive reduction in moduli, with no cryogelation at sugar concentrations above ∼20 wt%. Incorporation of low levels of Ca2+ (4 mM) also abolished cryogelation. These results are interpreted as follows. During freezing, xanthan chains are forced to align and associate by conversion of solvent (water) to ice crystals. The forced associations survive on thawing, to give the cryogel network. Addition of sugars restricts conversion of liquid water to ice, thus reducing or eliminating network formation. Strengthening of the normal weak gel structure of xanthan by high concentrations of polymer or incorporation of Ca2+ restricts alignment and further association during freezing, with consequent loss of cryogel structure.