When you think of processes that could benefit from a high-tech makeover, the drying of fruit may not be the first one that comes to mind. It turns out, however, that the use of "ionic wind" for fruit-dehydration both saves energy and preserves nutrients.
For most of us, the application of heat may seem like the obvious choice for the drying of fruit. Unfortunately, though, subjecting fruit to high temperatures reduces its flavor and destroys much of its nutrient content.
Instead, commercial food producers typically utilize a "non-thermal drying" process, in which fans blow unheated air across the fruit to gradually evaporate the moisture out of it. This technique does leave most of the flavor and nutrients intact, but it also takes a long time and uses a lot of electricity.
Seeking an alternative to the power-hungry fans, scientists at Switzerland's Empa research institute looked to a phenomenon known as ionic wind. To understand how it works, you first have to consider that air is composed of gases such as oxygen, carbon dioxide and nitrogen. The atoms that make up each of the gas molecules in turn consist of negatively charged electrons and heavier positively charged protons.
When a suspended wire is positively charged to a high voltage, the electrons are thus drawn towards it, while the protons are repelled. In fact, the electrons actually split off from the nearby gas molecules, leaving the protons behind.
As a result, the gas molecules are now only positively charged, so they move away from the wire, towards a grounded collector located beneath it. While on their way to that collector, they collide with neutrally charged gas molecules, sending them heading in the same direction. All of those moving molecules ultimately create a wind.
In previous studies, scientists only had limited success using ionic wind technology to dry fruit. This time around, however, colleagues at Canada's Dalhousie University took the extra step of placing the fruit on a grounded mesh instead of an impermeable tray. This made a huge difference, as moisture could now evaporate from all sides of the fruit, allowing it to dry twice as fast and in a more uniform fashion.
"Now this isn't exactly rocket science, but so far no one has considered this adaptation for the drying with ionic wind," says the study leader, Prof. Thijs Defraeye.
When compared to conventional non-thermal drying techniques, the mesh-augmented ionic wind method was found to be much quicker, plus it dried the fruit more evenly while preserving more nutrients, and it consumed less than half as much energy. According to the researchers, it should also be fairly easy to scale the system up for industrial use – Empa is now working with a Swiss retailer on commercializing the technology.
Source: Empa
