The foil permits heating in a conventional microwave oven of a foodstuff in a package made from the material, while providing enough insulation for the product in its frozen or chilled state said Qinetiq, a UK Ministry of Defence spin-off company..
The invention could provide manufacturers in the convenience market with an alternative packaging for their frozen food products. Conventional microwavable food packaging uses polymeric or paper-based materials transparent to microwave radiation. The use of electroconductive materials, such as metal foils, within microwave ovens can cause arcing and damage.
However metal foil is attractive for its highly reflective properties, making it useful for packaging chilled and frozen microwavable foodstuffs. This would usefully prolong the time for which the foodstuff can remain cool or frozen, such as the time between being purchased and refrigerated at home. Similarly, foil packaging would tend to keep the foodstuff hotter for longer after heating in the package.
Qinetiq filed the patent for a thin polyester wrapping covered with tiny squares of a metal such as aluminium. By spacing the aluminium squares or patches based on the wavelength of microwave rays the packaging cabn pass the thermal radiation through to the food instead of reflecting it.
The patches could be separated with tracks of clear polymer made of polyester, polypropylene, polyethylene or nylon, the patent states.
The patent also lists other advantages to the foil. The metal may act as a barrier to chemical migration and the permeation of oxygen into the food, leading to enhanced shelf life. The patches may also have significant reflectivity in the visible and ultraviolet (UV) radiation helping foods resist discolouration and oxidation.
The patent sets out several scenarios for how the material may be manufactured in bulk. Under one plan vacuum-coated aluminium-on-polymer films already in use may be adapted to the technique. Such films are used in non-microwavable food packaging, such as for potato crisps and similar snack foods.
To adapt the film packagers could gravure print an etch-resistant ink onto the metal surface of the existing film in a pattern corresponding to the patch configuration. The material is then chemically etched with a standard solution such as sodium hydroxide, hydrochloric acid or ammonium peroxodisulphate to remove the exposed metal. The ink can then be removed from the resultant patches by a solvent if required
Alternatively the patch elements can be deposited onto the polymer substrate in the desired configuration from the outset by vacuum coating the metal through a mask which leaves portions of the substrate uncoated around each patch.
A third process would be to make use of a metal foil with a heat-sensitive adhesive backing. Such materials are already used as the basis of "glittery" gift wraps and similar products.
The metal-coated films can be configured to provide different levels of microwave transparency or infrared reflectivity so as to optimise the heating conditions for different foodstuffs in the different compartments when exposed to the same microwave energy.
The package could also include microwave susceptors, discrete metal elements that heat up when exposed to microwaves to produce browning effects. The films may also be formed into flexible bags for packaging of micowavable foodstuffs, or pouches for the heating of home-prepared foods, the patent application states.
See the patent application here.