Add preservatives to the packaging not the food

No one wants food that has gone mouldy - least of all when they have only just purchased the product. But consumers are not exactly wild about food preservatives either. Packaging researchers are now introducing coated films to fight the battle of the bacteria.

At first glance, food packaging and an operating theatre don't have much in common. But when you look at the elaborate procedures that are used in sterilising packaging materials, the operating theatre analogy is not so far-fetched after all. Bacteria settle themselves at the exact spot where the foodstuff touches the packaging, and multiply rapidly from there. To put paid to the unwanted settlers, film-packaged foodstuffs often contain added food preservatives such as benzoic or sorbic acid. However, discerning consumers prefer to have as few additives as possible in their food.

This is why scientists at the Fraunhofer Institute for Process Engineering and Packaging IVV, working in the Alliance for Polymer Surfaces POLO, have opted for a different way of scaring off the moulds and microbes. Instead of adding preservatives to the food, they coat the packaging film with them. "This places the substances directly at the surface of the foodstuff, which is where they need to act," explains group leader Dieter Sandmeier. "In that way we can cut food preservatives to a minimum."

The coating layer is applied using special techniques and materials based on substances such as Ormocers. These plastics contain elements of inorganic glass and organic polymers. "We have managed to develop films that can protect solid products from attack by all kinds of bacteria," Sandmeier is pleased to report.

Films like this are not good enough when it comes to protecting liquid foods like milk, however. Because in this case the food preservatives introduced do not remain on the surface as they would on cheese or sausage. They spread through the entire product and are heavily diluted.

Packaging materials for liquids are therefore sterilised with hydrogen peroxide, for example, before being brought into contact with foodstuffs. But this complex procedure is performed at temperatures in excess of 70°C, which is too high for certain plastics such as PET.

The IVV researchers have now taken a leaf out of the medical technicians' book. They sterilise medical instruments with plasma, an ionised gas. There is just one drawback: the treatment takes at least half an hour, or even up to one and a half hours - far too long for an industrial bottling process. The scientists have now optimised the process so that it only takes one to five seconds. In this way they have no problem complying with environmental protection regulations, and energy consumption can be reduced by a factor of up to 1000.