The number of plant-based beverages on supermarket shelves is constantly increasing. The physicochemical properties of these beverages, such as their colour or foamability, vary according to the plant they are made from, and determine their use as an alternative to milk.
Because beverages with foamed milk have become increasingly popular in recent years, the foaming properies of plant-based beverages (PBBs) used as milk substitutes are of great interest.
Foamability is important for cappuccino and latte macchiato
Different processing designs and compositions influence the product functionalities of PBBs, such as their foaming properties. Generally speaking, there are two main mechanisms for producing milk foam: one works by gas injection and the other by mechanical agitation. The foaming process directly affects bubble-size distribution. For speciality coffees such as cappuccino or latte macchiato, air or steam is injected through a nozzle. As soon as gas bubbles are introduced into the system, surface-active molecules begin to populate the air-fluid interface. In milk foam, milk proteins normally stabilise the interface.
How are plant-based beverages produced?
The grinding of the raw material is usually the first major processing step in the manufacture of PBBs, possibly after a preliminary treatment such as soaking. Comminution in water is an important step, since it largely determines particle size distribution in a PBB. The separation of undissolved substances from the watery slurry using various separation methods before or after homogenisation yields a soluble extract. A subsequent heating step, usually a form of UHT treatment, leads to a product with a long shelf life of up to six months at room temperature. The homogenisation of PBBs prevents creaming and/or sedimentation during storage.
What factors influence the foaming behaviour of milk and PBBs?
The main objective of this study was to analyse the selected physicochemical properties of PBBs and milk in a comparative study, with the focus being on their foaming properties. A total of 27 UHT plant-based beverages purchased at major Swiss retailers (80% market volume) were analysed and compared with two UHT whole-milk samples. The phytic-acid* content of the PBBs was also analysed to study the influence of this antinutritional substance on foaming behaviour in greater detail.
Some PBBs are as foamable as milk, if not more so
In milk, the main proteins (caseins) are colloidally soluble, with a diameter of around 50-500 nm. The particle-size distribution of homogenised milk is usually dominated by fat globules <1.7 µm in diameter. It was discovered that after homogenisation, the particle-size distributions of PBBs were between 5 and 20 µm, which is significantly higher than for milk.
Viscosity and pH values were similar to or higher than those of cow’s milk, and PBBs were less white in colour than cow’s milk. Interestingly, some PBBs had good foaming properties, either comparable to or even better than those of milk.
PBBs differ greatly in terms of their product properties
Since their protein composition and functionality differ from that of milk and their raw materials are naturally found in a dry state and must therefore be dispersed in water, PBBs can differ greatly in terms of their properties. Although the processing principles are similar for all materials, certain steps in the production process can have a major effect on a range of product properties.
Added substances and phytic acid in PBBs
Additives can change flow behaviour and foamability in PBBs. Starches are meant to contribute to emulsion stability; calcium ions can bind to proteins and alter their properties. Salt can affect the solubility of proteins, and therefore the foamability of PBBs. Phytic acid* binds to proteins and reduces their solubility, enzymatic activity and digestibility.
A higher phytic-acid content in PBBs produced a greater volume of foam at the temperature of 60°C, considered the optimum temperature for the consumption of hot beverages. On the other hand, phytic acid should be reduced for nutritional reasons, which could reduce foamability.
*Phytic acid*: Phytic acid is one of the bioactive substances. In plants such as legumes, cereals and oilseeds, it acts as a store for phosphates and cations, which the seedling needs for its growth. Because of their complex-forming properties, they can bind insolubly in the human stomach and gut with minerals such as the calcium, magnesium and iron consumed with food, rendering them non-bioavailable.
- The foamability of some PBBs was comparable to or even better than that of milk.
- The foam heights of the PBBs differed widely, ranging from 41.5 mm to 173 mm at room temperature (milk foam height: 134.8 mm) and 50.9 mm to 203.6 mm at 60° C (milk foam height: 179.3 mm).
- The viscosities and pH values of PBBs were similar to or higher than those of cow’s milk. PBBs were less white, and their average particle sizes were usually markedly larger than those of cow’s milk.
- The levels of antinutritional phytic acid in the PBBs might influence foam height at 60° C, the temperature considered optimum for the consumption of hot beverages.