Electrical technique for dehydrating milk, without heat

Powdered milk is widely used by the food industry as it allows the product to be stored and transported in a stable condition for long periods of time without the need for refrigeration. However, dehydrating milk into powder requires an energy-intensive process with high temperatures, which is effective but has high energy costs and can also alter heat-sensitive components, such as whey proteins, enzymes or vitamins.

Now, a team from the Institute of Microelectronics of Barcelona (IMB-CNM-CSIC) and the Center for Innovation, Research, and Technology Transfer in Food at the Autonomous University of Barcelona (CIRTTA-UAB) has explored a non-thermal alternative to this process.

The electrospray technique uses an electric field, rather than heat, to remove water from liquid foods, such as milk at room temperature (20°C). The researchers used this method, which is currently being patented, to concentrate the milk by removing a significant amount of water without altering the structure of its proteins.

Their study, which has been published in the journal Innovative Food Science and Emerging Technologies, has shown promising results with the water content of the milk being reduced by nearly half; although the process will need to be optimised further in order to produce completely dehydrated milk.

An electric field instead of heat

In milk, as in any liquid, droplets tend to stick together due to surface tension. To turn it into powder, industrial processes use heat to evaporate the water it contains and concentrate its components. Electrospray takes a different approach: by applying a strong electric field, it overcomes surface tension and breaks the liquid into millions of microdroplets. It’s as if the electricity were shattering the milk droplets, causing a single drop to break down into increasingly smaller droplets, where the water can evaporate more effectively at room temperature.

Image credit: IMB-CNM-CSIC

The electrospray technique is already used in fields such as tissue engineering, nutraceuticals (nutrition and pharmacology) and sensor manufacturing, but the researchers said its application in food processing had barely been explored and had never been studied for dehydrating milk.

Researcher Bernat Perez, who developed this line of research in his doctoral thesis at CIRTTA-UAB and IMB-CNM and is the study’s first author, explains that, since this is a novel technique in the food sector, “tests with skim milk have made it possible to determine the influence of critical process variables, such as injection flow rate, the distance between the tip and the collector, and the milk’s fat content”.

How the technique was tested

The study was conducted in the Micro and Nanofabrication Clean Room at the IMB-CNM-CSIC, using an electrospray system (Fluidnatek LE-10) adapted to inject reconstituted skim milk at various concentrations and flow rates, applying a voltage of 29 kV and maintaining a distance of 10 cm between the tip and the collector.

The injection flow rate proved to be a determining factor in water removal. At a high flow rate (0.2 mL/min), the milk’s solids content barely changed after the process. In contrast, at a low flow rate (0.01 mL/min), the milk became significantly more concentrated: in the case of reconstituted milk with 12% total solids, its solids content increased from 12.3 to 36.8%. According to the researchers, this is because, at low flow rates, each drop receives a greater electrical charge relative to its volume, which promotes its fragmentation and greater water evaporation.

The researchers also analysed the size of the deposited droplets, the amount of water removed, and, using gel electrophoresis, whether the process altered the structure of the main milk proteins, such as caseins and whey proteins. Manuel Castillo, a professor in the Department of Animal and Food Science at the UAB and a researcher at CIRTTA-UAB, explains that the results “suggest that electrospray drying, unlike conventional thermal methods, does not alter these key components of milk”.

A first step towards heat-free food drying

The authors emphasise that this is a first step towards validating the technology: under the tested conditions, a completely dry milk powder was not obtained, and aspects such as processing capacity, energy efficiency and the scalability of the technique still need to be demonstrated. In addition, the method for dehydrating liquid products is in the process of being patented.

Llibertat Abad, a researcher at IMB-CNM-CSIC, notes that “the next steps in this work will focus on optimising parameters such as liquid flow rate, applied voltage, and the distance the droplets travel during the process; as well as exploring configurations with multiple emission points that allow for scaling up production, evaluating energy consumption, and assessing the microbiological stability of the final product”.

All in all, the study opens up a new line of research for the dehydration of high-value liquid foods, such as milk, juices or other heat-sensitive products. Although it is still far from industrial application, this non-thermal technology points towards a new way to transform liquids into stable, easy-to-store products while better preserving their original characteristics.

Top image credit: iStock.com/Luis Echeverri Urrea