Two gellan gum samples can carry identical specifications and still behave completely differently in your process. If you only ever look at one number to explain that, make it this one.
What molecular weight actually means
Gellan gum is a polymer — a long chain built from repeating tetrasaccharide units. Molecular weight describes how long those chains are.
Three numbers matter:
- Mw (weight-average molecular weight) — the value normally quoted.
- Mn (number-average molecular weight) — always lower than Mw.
- PDI (polydispersity index) — Mw divided by Mn. It tells you how broad the chain-length distribution is.
PDI is the number most buyers ignore and most formulators should not. Two polymers with the same Mw can behave differently if one is uniform and the other is a mixture of very long and very short chains.
Why it changes performance
Molecular weight sits underneath almost every property you care about:
| Property | How molecular weight influences it |
|---|---|
| Viscosity of the solution | Higher molecular weight means higher viscosity at the same concentration |
| Gel strength | Longer chains form a denser, more effective network |
| Suspension efficiency | Higher molecular weight gives more yield stress per unit of gum |
| Clarity | Very high molecular weight can increase turbidity in some systems |
| Mouthfeel | Chain length affects how thick and how "slimy" the product feels |
This is why a viscosity figure on a certificate of analysis is not a gel strength guarantee. Viscosity and gel strength respond to molecular weight differently, and both are also affected by ions, pH and processing.
Why low acyl grades are lighter
Native, high acyl gellan gum has a higher molecular weight than deacylated material. The alkaline deacylation step that removes the acyl groups also cleaves chains, so LA grades typically sit on the lower end of the range.
Typical commercial material falls somewhere in the broad range of 10⁵ to 10⁶ daltons, with deacylated grades toward the bottom and high acyl grades toward the top. Treat any single figure your supplier quotes as specific to their process, not as a universal constant.
The important point for formulators is that HA and LA differ in two ways at once — acyl content and molecular weight — and both contribute to the dramatic difference in gel texture between them.
How molecular weight is measured
Three approaches are common:
- Size exclusion chromatography (SEC / GPC) with refractive index, viscometry or light scattering detection. This gives Mw, Mn and PDI. Multi-detector systems are the most informative.
- Intrinsic viscosity. Much cheaper, and a reasonable proxy for molecular weight once the relationship between the two has been established for that polymer.
- Rheology. The simplest proxy of all — apparent viscosity of a standard solution under defined shear.
Most suppliers rely on the third. That is not a criticism; for routine quality control a standardised viscosity test is practical and sensitive to change. It simply answers a narrower question than Mw does.
The trap when comparing suppliers
Because viscosity is easy to measure and molecular weight is not, supplier comparisons usually happen on viscosity. This creates three common errors:
- Different test conditions. Viscosity values are meaningless without concentration, temperature, shear rate or spindle speed, and hydration protocol. Always compare like with like.
- Confusing causality. A more viscous powder is not automatically a better gelling agent. For suspension applications, viscosity may be exactly what you want. For firm gel applications, it may be irrelevant.
- Ignoring the distribution. A supplier whose material is highly polydisperse may pass a viscosity test while giving inconsistent gels.
What to ask for
A practical supplier questionnaire on this topic:
- Do you measure molecular weight or only viscosity? Under what conditions?
- What is your typical Mw range and batch-to-batch tolerance?
- What is your internal viscosity specification, including concentration, temperature and method?
- How does your process control chain length — fermentation time, agitation, deacylation conditions?
If a supplier cannot answer, that does not automatically disqualify them. It does mean you should qualify their material with your own performance test rather than trusting the specification sheet alone.
A note on storage effects
Molecular weight is not fixed after manufacture. Residual enzymatic activity and, under harsh conditions, prolonged heat exposure can shorten chains during storage. That is one mechanism behind gel strength drifting downward over a product's shelf life — worth keeping in mind when you set a specification window rather than a single target.
For data sheets and technical support on gellan gum (E418), see www.cinogel.com.
Part of the E418.org gellan gum knowledge base.