Drying fruit with ionic wind

Small but powerful! If the fruit slices are placed on a mesh, they are dried faster and more evenly. Image: Empa

Ionic wind is not generated by the rotating blades of a fan; it is created by connecting, for instance, a metal wire to a high-voltage source with a positive voltage of 10,000 to 30,000 volts. This charges the wire positively and ionizes the surrounding air. Air consists of various gases such as oxygen (O₂), nitrogen (N₂) or carbon dioxide (CO₂). Each of these molecules consists of atoms, which in turn consist of positively charged elementary particles – the protons – and negatively charged particles – the electrons. The electrons are attracted by the positively charged wire, while the much heavier protons are repelled by the wire. These electrostatic forces ultimately cause electrons to "split off" from the (electrically neutral) gas molecules, the remaining molecules are now positively charged – or "ionized". The positive ions collide with other air molecules on their way away from the wire towards the grounded collector located below the wire and set them in motion. This impulse, or rather the particle movement triggered by it, then creates the ionic wind, which is also known as electrohydrodynamic airflow.

The collision between ions and neutral air particles creates a so-called ionic wind, which can be used to dry the fruit slices on the mesh. Image: Empa

Researchers tried to make use of ionic wind with different approaches for the industrial drying of food – but so far without remarkable success, because an upscaling was not possible. Empa researcher Thijs Defraeye from the "Biomimetic Membranes and Textiles" lab and his team pursued the idea further and varied various process parameters. First, the researchers did not place the food to be dried on a tray as it was done previously, but used a mesh instead. "Now this isn't exactly rocket science, but so far no one has considered this adaptation for the drying with ionic wind," says the Empa researcher.

What sounds like a small change makes a huge difference, though: The water can now evaporate from all sides of the fruits or vegetables. As a result, the ionic wind dries the food twice as fast as on an impermeable tray, which was used by researchers over the world so far. But above all, the ionic wind dries fruit and vegetables more uniformly on the mesh. In contrast to the previous approaches of electrohydrodynamic drying, the new design is also easier to scale up – and thus extremely interesting for industry.