Fish Preservation - Control of Water Activity

Control of Water Activity

The water activity, a_w, in a fish is defined as the ratio of the water vapour pressure in the flesh of the fish to the vapour pressure of pure water at the same temperature and pressure. It ranges between 0 and 1, and is a parameter that measures how available the water is in the flesh of the fish. Available water is necessary for the microbial and enzymatic reactions involved in spoilage. There are a number of techniques that have been or are used to tie up the available water or remove it by reducing the a_w. Traditionally, techniques such as drying, salting and smoking have been used, and have been used for thousands of years. These techniques can be very simple, for example, by using solar drying. In more recent times, freeze-drying, water binding humectants, and fully automated equipment with temperature and humidity control have been added. Often a combination of these techniques is used.

• Women drying fish, 1971

• Dry fish market at Mohanganj

• Drying stockfish in Iceland

• Fish barn with fish drying in the sun – Van Gogh 1882.

• Platforms, called fish flakes, where cod dry in the sun before being packed in salt

• Remains of Roman fish-salting plant at Neapolis

• Reconstruction of the Roman fish-salting plant at Neapolis

• Drying salted fish at Malpe Harbour

• Salt fish dip at Jakarta

• Ruins of the Port Eynon Salt House – seawater was boiled to extract salt for preserving fish