Plant-based milk alternatives are a fast-growing area of beverage development. A review in the Journal of Food Science and Technology links the demand to cow milk allergy, lactose intolerance, calorie concerns and a growing preference for vegan diets. The same review is frank about the difficulty: many of these drinks face technological problems in processing or preservation, and for legume-based drinks, sensory acceptability is the major limiting factor.
What a plant-based milk actually is
The review describes these drinks as fluids made by breaking down plant material such as cereals, legumes, oilseeds or nuts, extracting it in water and homogenising the result, giving particles in the range of 5 to 20 µm that imitate cow's milk in appearance and consistency.
A second review, in Comprehensive Reviews in Food Science and Food Safety, describes two main production routes: breaking down plant material to form a suspension of its natural oil bodies in water, or building an oil-in-water emulsion by homogenising oil, water and emulsifiers. Either way, the drink is full of particles that can move.
Why it goes wrong
Plant-based drinks contain relatively large dispersed particles: fat globules, solid particles from the raw material, proteins and starch granules. According to the Journal of Food Science and Technology review, these make a stable product hard to achieve because the solid particles settle. The same large particles give a sandy, gritty or chalky mouthfeel, and the low fat content leaves the drink lacking creaminess.
The physics is the same as in dairy drinks: how fast a particle settles or rises depends on its size, its density difference from the liquid and the liquid's viscosity. Our article on why dairy beverages separate covers that model in more detail.
Three factors that govern stability
The review identifies three factors that govern the stability of plant-based milks:
- Particle size. Smaller particles settle more slowly and are less noticeable in the mouth.
- Emulsion formation. Whether the fat is properly emulsified or free to separate.
- Protein solubility. Protein that is not in solution behaves as one more particle that can settle.
It reports that stability improves with heating at high temperature, which hydrates and softens plant tissue so that it breaks down better, with homogenisation, and with stabilisers and emulsifiers.
The Comprehensive Reviews paper lists the functional ingredients used to build these drinks: oils, emulsifiers, thickeners, antioxidants and minerals. Each is a formulation decision with a bearing on stability, not only the ones added for it.
Fortification belongs in the stability trial
Many plant bases lack nutrients found in milk. The review gives oat milk as an example: it lacks calcium and needs fortification to serve as a milk alternative. Mineral fortificants add further particles to a system that is already prone to settling, so they belong in the formulation from the first stability trial, not added once the base seems settled.
A diagnostic starting point
| What you see | What it points to |
|---|---|
| Chalky or gritty in the mouth | Particle size too large |
| Sediment at the bottom of the pack | Particles settling: raw material solids, undissolved protein, starch or mineral fortificant |
| A cream or oil layer at the top | An unstable emulsion, or fat not fully homogenised |
| Thin, watery body | Low solids, or too little structure in the liquid phase |