Solution Properties

A. Solubility

To date, most of the studies on gellan gum have focused on the low-acyl materials. These are produced as mixed salts, predominantly in the potassium form but also containing divalent ions such as calcium. Typical levels of the major cations in Gelrite are: Ca²⁺, 0.75%; Mg²⁺,0.25%; Na⁺,0.70%; and K⁺,2.0%. Low-acyl gellan gum is only partially soluble in cold water. Solubility is increased by reducing the ionic content of the water and by conversion of the gum to the pure monovalent salt forms, but complete solubility of Gelrite is only achieved in deionized water using the pure monovalent salt forms. Low-acyl gellan gum is dissolved by heating aqueous dispersions to at least above 70℃. Progressively higher temperatures are required as the ionic strength of the aqueous phase is increased. Except in the case of Gelrite at low concentrations in the absence of ions, subsequent cooling of the hot solutions always results in gel formation. Gels can be formed with Gelrite in concentrations as low as 0.05%. Suppression of solubility by the inclusion of ions is a useful tool for the practical utilization of low-acyl gellan gum. In this way, the gum can be easily pre-dispersed in water without encountering hydration problems, and can be activated simply by heating. Use of gellan gum in this manner is analogous to the use of native starches, which, being cold-water-insoluble granules, can be conveniently slurried in water prior to cooking. Solutions of gellan gum will react in the cold with mono and divalent ions to form gels and, depending on the types and levels of ions, the resulting gels may not melt on heating. To circumvent this usually undesirable situation, it is recommended that, in applications where partial or complete pre-solution of gellan gum is unavoidable, the gellan gum be incorporated above 70℃. Bearing in mind the above considerations, there are a number of alternative ways of incorporating low-acyl gellan gum into a given system. It may be added alone or in combination with other dry or liquid ingredients to a cold mix that is then heated and cooled to induce gelation. Alternatively, it may be added to a mix that has been pre-heated above 70℃. The preferred method of addition is best determined by consideration of the ingredients in the formulation and processing conditions. The ions present in the system have a major impact on the quality of the final gel and for best results ions additional to those inherently present in the system may be required. These can also be added in the cols or after heating.

B. Rhcology of Solutions

Native gellan gum on heating and cooling in the presence of cations forms cohesive, elastic gels similar to those obtained by heating and cooling mixtures of xanthan gum and locust-bean gum. Since this texture dose not appeal to most consumers, native gellan gum alone is not expected to see widespread utility as a gelling agent. However, when dispersed in cold water, it provides extremely high viscosities. A possible limitation to its use as a thickener is high sensitivity to salt. This effect is shown in Fig.3, which compares the viscosities of 0.3% solutions of xanthan gum and native gellan gum at different concentrations of salt. The viscosities recorded are K values derived from the ‘power-law’ equation, η=Kγ ^n-1, and are approximations of the viscosities at one reciprocal second. The well-known stability of xanthan gum viscosity to changes in salt concentration is apparent. In contrast, the viscosity of the native gellan gum displays a strong dependence on salt concentration. The native gellan gum solutions are highly thixotropic and the apparent high viscosities appear to be the result of the formation of a gel-like network. Similar thixotropic behaviour is observed when low concentrations of xanthan gum /locust-bean gum are dispersed in cold water.