Within the packaging technology, use is often made of a hydrogen peroxide-containing sterilization agent in order to sterilize a packaging material for a food so as to make for the storage of the food during extended shelf-lives or storage times without harmful effects on the quality properties of the food, such as flavour, aroma, colour and consistency. Depending upon the type which is to be packed, the requisite extermination or deactivation of microorganisms may vary from one type of food to another. For food types of the type which have a pH value of 4.6 or lower, e.g. juice and wine, the extermination need not be as extensive as for foods with higher pH values, e.g. milk, since a lower pH value per se counteracts the growth of surviving microorganisms during the storage time of the food. On the other hand, higher pH values have a weaker, or non-existent, inherent counteraction to such growth and, as a result, require a more extensive extermination of microorganisms in order to attain the desired extended shelf-life of the food.
The expression “sterilize” and equivalent variations of this expression are taken to signify sterilization levels ranging from practically total extermination of microorganisms to a more moderate sterilization level which may be sufficient to impart to foods with lower pH values (so-called high-acid foods) extended shelf-life.
According to a prior art sterilization method, packaging containers for a food with extended shelf-life are produced from a web of plastic-coated paper in that the web is, for purposes of sterilization, led through an aqueous bath of hydrogen peroxide (60-75° C.; approx. 35 weight %) during a sufficient stay-time in the bath in order to exterminate or deactivate microorganisms on the surface of the web.
After the passage through the hydrogen peroxide bath, remaining residual hydrogen peroxide is removed from the surface of the web by a combined mechanical and physical treatment of the web in connection with, or immediately after the exit of the web from the hydrogen peroxide bath.
The sterilized and treated web is reformed into a tube by both longitudinal edges of the web being united with one another in an overlap joint or seal, and the tube is filled with the pertinent food which has been sterilized separately by means of a suitable sterilization treatment prior to the filling operation. The tube is divided into continuous packaging units by repeated transverse sealing of the tube transversely of the longitudinal direction of the tube and below the filling level of the tube, at the same time as the packaging units are separated from one another by incisions in the transverse sealing zones. The separated packaging units are thereafter given the desired geometric configuration by a fold forming- and sealing operation. Known commercial examples of such aseptic packaging containers are TETRA BRIK®, TETRA CLASSIC® and TETRA PRISMA®
According to another prior art method, similar packaging containers are produced for a food with extended shelf-life from flat-folded tubular blanks of plastic-coated paper in that the blanks are first raised into open packaging cartons. The tubular packaging cartons are provided with a bottom closure by means of a fold forming- and sealing operation, whereafter the packaging cartons provided with a bottom are sterilized by means of gaseous or liquid hydrogen peroxide which is brought into contact with the inner walls of the packaging cartons in order to exterminate or deactivate any microorganisms present. After the contact with hydrogen peroxide, remaining residues of hydrogen peroxide are driven off physically, usually by means of air.
The sterilized packaging cartons are thereafter filled with a separately sterilized food and are finally provided with, a top closure, Known commercial examples of such packaging containers for foods with extended shelf-life are TETRA REX® and TETRA APTIVA®.
According to yet a further known method which is reminiscent of the immediately previously described method, plastic bottles are produced for foods with extended shelf-life, e.g. injection moulded PET bottles, in that the bottles, prior to filling, are sterilized by means of gaseous hydrogen peroxide which is injected into the bottles through the open bottle mouth for contact with the inner walls of the bottle. After contact with the hydrogen peroxide, the interior of the bottle is ventilated by means of air so as to drive off remaining residual quantities of hydrogen peroxide from the inner surfaces of the bottle.
The bottles are thereafter filled with the pertinent food and closed by means of, for example, a screw cap for further transport and handling.
A sterilization by means of hydrogen peroxide, as described above, most generally entails that the sterilized surfaces, in this case the inner walls of the sterilized packaging container and the bottle, respectively, display remaining residual quantities of hydrogen peroxide. Even if these residual quantities are normally extremely slight, most often 03 ppm or less, and have no harmful effect on the food, they nevertheless constitute an unnecessary consumption of hydrogen peroxide and thereby an unnecessary economic loss in the sterilization operation.
In order to reduce remaining residual quantities of hydrogen peroxide, it has previously been proposed in the art to combine such a chemical sterilization using hydrogen peroxide with a physical sterilization employing UV radiation, as described in, for example, WO-A-80/01457. As a result of this combination of chemical and physical sterilization, the concentration (and thereby the consumption) of hydrogen peroxide may be reduced to 10 weight % or even less, if the physical sterilization is carried out using a UV radiation within a wavelength range of below 325 nm.
Even if a combined chemical and physical sterilization using hydrogen peroxide and UV radiation thus implies that the consumed quantity of hydrogen peroxide may be reduced, at the same time as a synergistically improved sterilization effect may be achieved, it nevertheless does not fully resolve the problem involving remaining residual quantities of hydrogen peroxide. This is because, int. al., certain plastics in contact with aqueous hydrogen peroxide display a tendency to absorb the hydrogen peroxide which may therefore readily penetrate into and remain on and/or in such an absorbing plastic surface, for example the outer plastic coating of a packaging laminate. The depth of penetration of the hydrogen peroxide may in certain cases even be so great that the absorbed hydrogen peroxide is not accessible to, and therefore cannot be ventilated off using air. The penetration ability of the hydrogen peroxide into plastic may vary from one plastic to another, but is particularly serious in plastics of a hydrophilic nature, such as, for example, PET, in which it is readily absorbed and remains in position at relatively large penetration depths.
There is thus still a need to be able to reduce the consumed quantity of hydrogen peroxide in sterilization of packaging material by means of a sterilization agent containing hydrogen peroxide.