Techniques based on the control of water activity
Water activity (aw) is a parameter that measures the availability of water in fish flesh. It is expressed as the ratio of water vapour pressure in fish/vapour pressure of pure water at the same temperature and pressure. Aw varies from 0 to 1. Water is necessary for microbial and enzymatic reactions and several preservation techniques have been developed to tie up this water (or remove it) and thus reduce the aw. These include drying, salting, smoking, freeze-drying, the use of water binding humectants and a combination of these. Some of these techniques, such as drying, salting and hot smoking, have been used for thousands of years. They can be implemented very simply, e.g. by salting, solar drying, or using fully automated equipment with temperature control, relative humidity, etc.
Techniques based on the physical control of microbial fish loads, its chemical and enzymatic activity
These physical methods use heat (cooking, blanching, pasteurizing, sterilizing), ionizing irradiation (for pasteurization or sterilization) or microwave heating. Cooking or pasteurizing are processes that do not allow complete inactivation of microorganisms and thus often need to be combined with refrigeration to preserve fish products and increase their shelf life. This is not the case of sterilized products and which are stable at ambient temperatures (less than 40°C). These require packaging in metal cans or retortable pouches before the heat treatment, thus the term “canning”.