III.2. Sterilisation

The heat treatment required for sterilisation of the product is determined by the Ph of the content as follows:

- Acid products (i.e. pH less than 4.5) such as fish packed in tomato sauce require very little processing as the heat-resistant, spore-forming pathogenic bacterium, Clostridium botulinum cannot survive under acid conditions. For these products, the centre of the container should be held at 100° C for about 5 minutes.

- Medium to low acid foods (i.e. pH 4.5 to 5.3) will support pathogens such as Clostridium botulinum and must be fully heat processed to eliminate all spores. In this case the centre of the container should be held for a period equivalent to 10 minutes at 121° C.

- Low acid foods (i.e. pH higher than 5.3) will support both Clostridium botulinum and the highly heat-resistant spoilage bacteria Bacillus stearothermophilus. Spores of the latter only germinate and grow at temperatures greater than 37° C, and thus the product should be stored at temperatures lower than this. It is not possible to ensure that all spores of these bacteria are destroyed by heat as the severe processing required would result in a total breakdown of the product.

In practical terms, fish canning normally involves holding the product in a retort (steam pressure vessel used for thermal processing of food) for up to an hour at a temperature of 115-116° C under pressure. The process includes a pre-heating stage since the centre of the product is the slowest to attain the temperature of the retort. Tanikawa (1971) gives an excellent review of canning and processing times for fishery products, and a recommended international code of practice for canned fish is available from FAO (See Appendix I).