In high pressure processing, food and beverage products in their final packaging are subjected to extreme levels of hydrostatic pressure in order to kill potentially harmful microorganisms and make the product shelf stable. The level of pressure used to achieve this is greater than what you would experience at the deepest part of the ocean. Given this extreme treatment, you might expect that the packaging required for these products would be highly specialized, unique, and very expensive.
In reality, though there are specific qualifications that must be met, the packaging used for HPP products is actually quite normal.
Since pressure is applied to the package using water, the material obviously needs to be water resistant. During the course of the treatment, the package can be expected to compress by about 15% of its total volume, so the material also needs to be able to flex somewhat and subsequently return to its original shape. These qualifications make plastic an obvious choice for packaging HPP treated products, and luckily there are plenty of common polymers that have the appropriate characteristics.
A common area of confusion for people new to the concept of HPP is how we manage to keep the packaging from imploding or being crushed under the tremendous pressure to which it is subjected. The answer is Pascal’s Law, which states that pressure applied to a confined, incompressible fluid is transmitted throughout the fluid such that the same pressure is felt everywhere. In other words, since HPP uses water, and water is an incompressible fluid, the packaging receives pressure equally from all sides instantaneously. Because there is no pressure differential over any part of the surface, the product is never in danger of being crushed. This also means that HPP can be applied to packaging of any shape.
Certain demands are also naturally placed on the package’s seal during the process. Again, because pressure is being applied evenly over the entire surface of the product, the seal does not experience any extraordinary stress. Yet being as the package does flex to some degree during the process, the seal has to be able maintain its integrity throughout this moderate shape change. Just like with the main packaging material, this is not a case of having to reinvent the wheel, but certain options stand up to the rigors of HPP better than others.
We mentioned earlier that any shape package can be subjected to HPP. While this is true, it does not indicate that shape is irrelevant. Because HPP uses batch processing, as opposed to continuous processing, the amount of product that can be loaded into the holding cylinder determines how much can be processed at one time, affecting the efficiency (and therefore the cost) of the procedure. Therefore, in some cases it is worth considering how the size and shape of the packaging will affect its ability to fit into the cylinder.
There is one more packaging concern that has nothing to do with the process itself: barrier permeability. One of the advantages of HPP is that it significantly increases shelf life, so in most applications it makes sense to package the food using a high barrier material – that is, one that allows a lower level of oxygen transfer. This ensures that your product’s shelf life will not be compromised by inferior packaging materials.
So as you would expect, there are certain packaging requirements when it comes to HPP. However, they are not as extraordinary as many people imagine. Common materials and designs are perfectly acceptable, as long as they are intelligently chosen.