Gellan is a bacterial polysaccharide, discovered in 1978, that has become one of the most flexible tools a formulator has. It sets clear jellies, holds fruit pieces in place, and keeps particles suspended in drinks. The catch is that "gellan" on a specification can mean two quite different ingredients.
In its native form the gellan chain carries acyl groups. A review of gellan gelation in Food Hydrocolloids notes that these are normally removed by alkaline treatment in commercial production, giving the deacylated, or low acyl, form. The native form is sold as high acyl gellan. The backbone is the same. The behaviour is not.
Two forms, two textures
Low acyl gellan chains pair up into double helices as a solution cools, and a gel forms when those helices aggregate. The gels are firm and brittle, and they melt at a higher temperature than the one at which they set, a gap known as thermal hysteresis.
High acyl gellan forms its helices at a much higher temperature than the deacylated polymer. Yet the gels are weaker and more elastic, with no hysteresis between setting and melting. Work on the acylated polymer explains why: the acetyl groups sit on the outside of the helix and stop the helices aggregating.
| Low acyl | High acyl | |
|---|---|---|
| How the gel forms | Helices form, then aggregate | Helices form, but aggregation is blocked |
| Texture | Firm and brittle | Soft and elastic |
| Setting and melting | Melts at a higher temperature than it sets | No gap between setting and melting |
Ions and pH steer the low acyl form
Because low acyl gellan gels by aggregation, anything that changes the charge on the chains changes the gel. The same review sets out the rules:
- Salt raises the setting point. Cations raise the temperature at which the chains order, and divalent cations such as calcium do so more strongly than monovalent ones such as sodium.
- Counterions count too. The ions that arrive with the gum itself behave like added salt.
- Lower pH promotes gelation. Reducing the negative charge on the chains lets the helices come together more easily.
In practice, the water, the milk minerals, the acid and any calcium in a recipe all shift where and how firmly low acyl gellan sets. A formula that behaves perfectly in a laboratory can behave differently in a plant whose water carries a different mineral load, which is worth checking before blaming the gum.
Fluid gels: a gel that pours
If gellan is sheared while it gels, it does not form one continuous network. A study of gellan fluid gels describes the result as gelled micro-particles suspended in a solution of un-gelled polymer, a system that pours like a liquid yet behaves elastically. The researchers made fluid gels from high acyl gellan, low acyl gellan and a blend of the two.
The same principle is why gellan is used to keep particles in suspension: a structure strong enough at rest to hold a particle in place, and weak enough to pour and drink. It also means the process, and in particular how the product is sheared as it cools, matters as much as the grade chosen.
Emulsions need care
Gellan is also used to structure emulsions. In one study of emulsion-filled gels, low acyl emulsions were unstable under the processing conditions used, so the work continued with high acyl gellan. Raising the gellan level produced stronger gels, and the behaviour of the whole system was controlled by the gelled continuous phase rather than by the oil droplets.
Choosing between them
- A firm, clear gel that holds its shape: start with low acyl, and control the ions and pH it meets.
- A soft, elastic, flexible texture: start with high acyl.
- A pourable structure that holds particles: think in terms of a fluid gel, and design the cooling and shear alongside the grade.
- Somewhere in between: blends of the two forms are used as well.
A broad review of thickeners and gelling agents in the Journal of Food Science and Technology credits gellan with fast setting and clarity. Both are only delivered when the form matches the job.
What to record in a trial
- The form of gellan and how it was hydrated.
- The mineral content of the water and the recipe, and the final pH.
- How the product was cooled and sheared, not only the target temperatures.
- Gel strength or suspension measured at the storage temperature, at a fixed age.