PURIFICATION OF COMMERCIAL GELLAN TO MONOVALENT CATION SALTS RESULTS IN ACUTE MODIFICATION OF SOLUTION AND GEL-FORMING PROPERTIES

Lithium, sodium, potassium, and ammonium salts of the industrial polysaccharide gellan were prepared. The salts were freely soluble in water at room temperature (25 degrees C). The opinion had been generally held that heating to 100 degrees C was necessary for gellan to achieve complete solubility in the presence of mono- or multivalent cations. Then, upon cooling, the solutions would form gels. These conclusions were based on the properties imposed upon gellan samples by the presence of contaminating divalent cations. Commercial gellan samples contain calcium and magnesium at levels exceeding 0.9%, sufficient for counterion formation with over one-third of gellan's carboxyl groups. Purification was rapid and included sequential treatments with a cation-exchange (H+) resin, LiOH, NaOH, KOH, or NH4OH, and an anion-exchange (Cl-) resin. About 95% of the divalent cations and nearly 90% of the phosphate that contaminated commercial gellan were removed. The purified monovalent salts of gellan set in the presence of divalent cations and provide well-defined agents for gelling media used for propagation of microbes and plants. In a manner analogous to sodium alginate, solutions of lithium, sodium, potassium, or ammonium gellanate form beads when dropped into solutions of divalent cations. This property was exploited for entrapment of enzymes and cells in beads.