What the acid actually does to the protein
Below pH 4.6, casein micelles dissociate and the protein carries a net positive charge. Instead of repelling each other, the aggregates attract the negative charges on a polysaccharide. Two things follow:
- The protein no longer stabilises itself. Without a hydrocolloid, or with the wrong one, it sediments within days.
- The hydrocolloid has to be anionic and it has to adsorb onto the protein surface. Xanthan and CMC work partly by viscosity; pectin and gellan work by adsorption and network formation.
The difference matters in the mouth. A drink stabilised by viscosity feels thick and coats the tongue. A drink stabilised by a network feels like milk.
Why gellan works in this system
Low acyl gellan sets on cooling in the presence of calcium, and milk supplies calcium at roughly 1.2 g/L. That means the base does the crosslinking for you, exactly as it does in coconut water. The network that forms is fine enough to hold the casein aggregates in place while leaving the drink pourable.
Two properties make it particularly useful here compared with other gelling agents: it holds its structure at pH 3.5 to 4.0, and it survives heat treatment. A pectin-stabilised drink can lose body through a UHT cycle; a gellan network is set after the heat, not before.
Dose and system
| System | Stabiliser | Typical use | Result |
|---|---|---|---|
| Acidified milk drink, pH 3.9 to 4.2 | LA gellan alone | 0.025% – 0.045% | clean, thin, milk-like body |
| Acidified milk drink, long shelf life | LA plus HM pectin | 0.02% gellan with 0.2% to 0.4% pectin | suspension plus protein protection |
| Drinking yoghurt, pH 4.0 to 4.3 | LA | 0.02% – 0.035% | reduced whey separation |
| Sour cream style, pH 4.4 to 4.6 | HA | 0.03% – 0.06% | soft body, no grittiness |
| Acidified plant protein | LA plus pectin | 0.03% – 0.05% | depends heavily on the protein source |
The upper end of each range is where the drink starts to read as a thin gel rather than a drink. That threshold is lower in a product with low solids, and higher when sugar and fat are present.
The three stabilisers, compared
High methoxyl pectin and CMC are the incumbents. Both work; they fail differently.
| Property | LA gellan | HM pectin | CMC |
|---|---|---|---|
| Mechanism | network, ion-set | adsorbs onto casein | viscosity plus some adsorption |
| Dose | 0.02% – 0.05% | 0.2% – 0.5% | 0.3% – 0.6% |
| Mouthfeel | thin, clean | slightly full | can be slippery |
| Heat stability of the network | high | moderate | moderate |
| Protein protection | limited on its own | strong | moderate |
| Effect of over-dosing | visible gel | thick, gluey | very thick |
The practical combination for a long shelf-life acidified milk is pectin for protein protection plus gellan for structure. The pectin keeps the casein dispersed; the gellan keeps the whole thing from separating. Using gellan alone works for shorter shelf life or for products where the protein load is low.
Processing sequence
- Standardise the milk or protein base. Protein at 1.0% to 1.5% is the usual band for a drinking product.
- Hydrate the gellan in hot water at 80 to 85 °C, dry blended with sugar. Do not add it dry to milk; the calcium will cause it to hydrate unevenly.
- Combine with the milk base and homogenise. Single stage at 15 to 20 MPa is common; a second stage improves stability if fat is present.
- Heat treat. UHT at 137 to 142 °C for 2 to 4 seconds, or pasteurise if the shelf life allows.
- Acidify after heat treatment, slowly, with the acid diluted and the batch under shear. This is the step that decides the product. Fast acid addition at high temperature produces a grainy drink that no stabiliser can rescue.
- Cool below the gellan set point without high shear, then fill aseptically.
The acid addition rate deserves more attention than it usually gets. Casein aggregates form in the first seconds of acidification, and their size is set by the local pH drop at the point of addition. Dilute acid, high shear and a controlled flow rate give small aggregates and a smooth drink.
When it goes wrong
| Symptom | Likely cause | What to change |
|---|---|---|
| Sediment within a week | dose too low, or pH below 3.6 | raise dose, check final pH |
| Whey layer at the top | network too weak, or pectin level too low | raise gellan, add pectin |
| Grainy or sandy texture | acid added too fast or too hot | dilute the acid, cool first, add under shear |
| Drink sets into a gel | dose too high for the protein level | reduce, or blend with pectin and lower gellan |
| Chalky taste | excess calcium salt | reduce added calcium, rely on milk calcium |
| Separation only after UHT | network formed before the heat cycle | check where the set point falls relative to the process |
Where gellan is the wrong choice
If the product is a fermented yoghurt rather than an acidified drink, the requirement is different: the gel has to survive the fermentation and the filling shear. That case is covered in gellan gum in yogurt. If the base is a plant protein rather than milk, the charge behaviour changes with the protein source, and how proteins interact with gellan gum is the relevant background. For pH behaviour more generally, see gellan gum in acidic foods.
Does gellan gum precipitate milk protein?
No. It adsorbs onto the positively charged casein aggregates below pH 4.6, which is what holds them in suspension. Precipitation in an acidified milk usually comes from acidifying too fast or too hot, not from the gum.
Can gellan gum replace pectin in an acidified milk drink?
Partly. Gellan gives better structure and heat stability, but pectin is better at protecting the protein against aggregation over a long shelf life. The most common formulation uses both, with less pectin than a pectin-only system would need.
What pH should an acidified milk drink be?
Between 3.8 and 4.2 for most products. Below 3.6 the drink becomes sharply acidic and the protein can be destabilised again; above 4.4 the product sits too close to the isoelectric point at 4.6 and will separate regardless of the stabiliser.