Fermented hydrocolloid gums

Microbial gums are extracellular polysaccharides secreted by bacteria in fermentation and recovered by alcohol precipitation. Xanthan, from Xanthomonas campestris, and gellan, from Sphingomonas elodea, dominate. Their appeal over plant gums is consistency of structure — a fermentation makes the same polymer every time, where a crop does not — but the reason they are used at all is a set of rheological behaviours that few other polymers offer.

Xanthan: why it thins under shear

Xanthan has a cellulose backbone carrying a charged trisaccharide side chain on every second glucose. Those side chains fold back along the backbone and stabilise it as a rigid, ordered helix, and the helices associate weakly with one another into a tenuous network.

At rest, that network gives the solution a yield stress: it behaves as a very weak solid and does not flow until a threshold force is applied. Under shear the weak associations break and the rigid helices align with the flow, so viscosity collapses — and when shear stops, the associations reform almost immediately.

This explains the everyday behaviour. A dressing holds herb particles and spice in suspension indefinitely because the yield stress supports their weight; it pours readily because pouring exceeds that threshold; and it recovers instantly rather than staying thin. The rigid, side-chain-protected helix is also why xanthan tolerates salt, acid and heat far better than most polysaccharides: the backbone is shielded, so the ordered structure survives conditions that would collapse a flexible random coil.

Gellan: gelation switched by ions and by acyl groups

Gellan is a linear tetrasaccharide repeat. Heated, it exists as random coils; cooled, the chains form double helices, and cations — calcium and magnesium especially — bridge the carboxyl groups to bind helices into a continuous network. Gelation is therefore triggered by ions, not merely by cooling, and gel strength depends on the ion concentration in the food.

The more useful control is chemical. Native gellan carries acetyl and glyceryl substituents; removing them by alkali treatment produces “low-acyl” gellan. The bulky acyl groups obstruct helix packing, so high-acyl gellan gives soft, elastic, cohesive gels while low-acyl gellan gives firm, brittle gels that fracture cleanly. Blending the two gives intermediate textures. A single organism’s polymer thus spans a wide texture range through one downstream processing choice.

Low-acyl gellan also forms what is called a fluid gel — a weak network at very low concentration that suspends particles in a beverage without perceptible thickness, because the yield stress is high relative to the viscosity.

What limits them

Charged polysaccharides interact with proteins, so a gum that behaves well in water may flocculate or synerese in a protein-rich system; xanthan and galactomannans such as locust bean gum also interact synergistically, gelling in combination where neither gels alone. These interactions are the practical difficulty in formulation.

Both gums are essentially indigestible and function as soluble fibre. At the concentrations used for texture, that is nutritionally minor; at higher intakes, fermentation in the colon produces gas, and tolerance is individual. Reports of effects on the gut microbiota from emulsifier and hydrocolloid intake exist and are actively debated, and the evidence at normal dietary exposures is not settled.