Demand for UHT milk and milk-protein drinks is growing, a 2019 review in Comprehensive Reviews in Food Science and Food Safety notes. These drinks are microbiologically stable, but they keep changing physically and chemically in the pack, and some of those changes limit shelf life.
The review describes three severe defects. They look different, have different causes and need different fixes, so it pays to be precise about which one is in front of you.
| What you see | What it is | Where to look first |
|---|---|---|
| The whole drink has thickened or set | Age gelation: an irreversible protein network through the pack | Raw milk quality and enzymes, heat load, protein level |
| A compact layer at the bottom | Sedimentation: protein-rich material settling quickly | Heat load, protein level, and any cocoa or coffee |
| Fat collected at the top | Creaming | Fat globule size and homogenisation |
Age gelation has two routes
The review describes at least two mechanisms. In one, heat-stable enzymes, either native to milk or produced by bacteria that grew in the raw milk, survive processing and slowly break down the proteins until the milk destabilises and gels. The other is physico-chemical. Factors known to affect it include the milk or protein concentration, the heat load during processing, including direct compared with indirect UHT, and the composition of the milk.
A stabiliser can also cause gelation. A study of carrageenan-stabilised UHT milk drinks found excessive gelation in long storage with severe heating, lower pH or kappa rather than iota carrageenan.
Sediment
Similar factors to those behind age gelation affect sedimentation. Flavoured drinks add their own load: a review of sediment in cocoa and coffee milk drinks found that polysaccharides, proteins and calcium from the cocoa or coffee powder are the main components of the sediment.
Cream
There are relatively few studies on creaming in UHT milk during storage, which the review suggests means it is less common or less damaging than the other two.
Why high-protein drinks are harder
A 2024 review of milk protein stability under UHT treatment explains that the severity of heating changes protein structure, which can cause fouling in the plant and sedimentation or gelation in storage, and that this is more pressing as high-protein drinks grow. It identifies two main strategies used by industry: chelating agents and polysaccharides. A review of casein-based ready-to-drink beverages adds that stability depends on the type of protein, the mineral content, the pH and the flavour ingredients.
A diagnostic order
- Name the defect: gel, sediment or cream.
- Note the age at which it appears, and whether it worsens with warmer storage.
- Check raw milk quality and the heat load of the process.
- Check the protein level, the minerals and the final pH.
- Only then look at the stabiliser type and amount.
The dairy troubleshooter ranks the likely causes for UHT and flavoured milk defects against what you are seeing.