Lassi, drinking yoghurts and fruit-flavoured milk drinks share one problem. At their pH, milk protein wants to clump together. Without help, the drink throws a sediment, turns sandy in the mouth or separates into a watery layer.
High methoxyl pectin is the classic answer, and it is one of the better-studied stabilisers in dairy. The research explains both why it works and why it sometimes does not.
Why acidified milk drinks separate
Casein, the main milk protein, exists as micelles that stay dispersed in fresh milk. As milk is acidified, the micelles lose that protection and aggregate, which is exactly what happens when yoghurt sets. In a drink, the aggregates are broken up by stirring or homogenisation, but they remain prone to sticking together again and settling.
How pectin holds the drink together
Pectin is negatively charged. A study in Biomacromolecules measured pectin adsorbing onto casein micelles at and below pH 5.0, in more than one layer, and concluded that the attachment is driven by electrostatic attraction. The coated particles then keep each other at a distance.
Exactly how that coating stabilises the drink is still discussed. One study proposed that stability comes partly from a weak network of pectin-coated casein. A later study found no gel network in stable drinks and proposed a mechanism resembling steric stabilisation, where the adsorbed layers physically keep the particles apart, combined with secondary adsorption.
Most of the pectin never touches the protein
The same work on the mechanism found that in acidified milk systems of practical concentration, less than 20% of the pectin added interacts directly with the casein micelles. The rest stays in the serum and plays no part in stabilising the final drink. It is not wasted, though: that excess was found to be crucial for getting enough pectin adsorbed during mixing. Once on the protein, the pectin was found to be adsorbed irreversibly.
The pH window
This is where many problems start. Pectin only protects the protein within a range of pH, and the range depends on the pectin.
| What was tested | What was found | Source |
|---|---|---|
| Pectins of different degree of esterification, pH 4.2 to 3.0 | Pectins with a degree of esterification of about 67 to 71 stabilised the milk at pH above 4.0. Below pH 4.0 the dispersions contained aggregates. Pectins below 50 did not stabilise at all. | Liu and others, 2006 |
| Pectin compared with soybean soluble polysaccharide | Below pH 4.2 the soybean polysaccharide stabilised better, and between pH 4.2 and 3.2 it was unaffected by pH, while pectin-stabilised drinks depended on pH. | Nakamura and others, 2006 |
The practical point is to measure the pH of the finished drink, not the recipe target. A fruit preparation, a longer fermentation or a different culture can move the final pH out of the window without anyone changing the stabiliser.
The process matters as much as the pectin
- When it goes in. When pectin was added to skim milk before acidification, it adsorbed onto the casein from the start of acidification.
- Homogenisation. In the studies above, the acidified milk was homogenised with the stabiliser already present, so that fresh particle surfaces met the pectin.
- Heat after stabilising. Heating a pectin-stabilised drink at 75 °C for 20 minutes changed the particles' surface charge in a way that indicated partial desorption of the pectin.
- Dilution. Diluting stable drinks with water increased pectin adsorption but decreased stability, which matters wherever a drink is made up from a concentrated base.
Before changing the stabiliser, check
- The pH of the finished drink, measured on several batches.
- The type of pectin and its degree of esterification.
- When the pectin is added, and whether homogenisation comes after it.
- Any heat treatment after the stabiliser is in.
- Any dilution step, and the storage temperature.