In most cases fully preserved foods are still synonymous with tins, but these days one can also find fully preserved foods in plastic packaging that is fully or semi rigid and in cardboard cartons. In recent years flexible, sterilisable packaging has become increasingly popular, a trend that began in Europe and is becoming increasingly used in Norway.
Both the heat treatment and the sealing of the tin (folding) or flexible plastic packaging (welding) are critical control points in the production of hermetically sealed food containers. To ensure that these critical control points are in accordance with prevailing requirements and internal routines, there must be continuous measurement and documentation as part of the company's quality control system. In addition to this the public authorities have laid down regulations with requirements that the company must regularly test that the containers are sterile with the aid of microbiological analyses.
If there is any suspicion of unsterile products it is important that the reason is found quickly and the unsafe product destroyed. In most cases there is a microbial cause of unsterile containers, but there may also be non-microbial causes. In the case of microbial contamination, the next step is to find out whether the lack of sterility was due to conditions:
- before heat treatment
- during heat treatment (too little heat treatment)
- after heat treatment (reinfection)
The history of the tin or container with all background information, for both microbial and non-microbial conditions, should be considered before laboratory testing is carried out.
The impermeability of packaging is measured and controlled in various ways. For tins, the sealed edge (fold) must be regularly checked during production, preferably several times a day. The thickness and height of the fold is measured before it is then cut across so that the cross section can be studied and measured under magnification. Measurement includes the join between tin and lid (Figure).
For plastic packaging, a vacuum or pressure is used to check the seal, but there are also more complicated laboratory methods such as investigating the weld with ultrasound.
