Fish Products and Byproducts Technology

Unit 3 - Salting and Drying

3.3.2. Water activity and water relationships in fish
Duckworth and Kelly (1973) concluded that in water/solute/polymer systems the minimum a_w at which solvent action (and, by implication Scott’s ‘water competition’ between microbes and solutes) becomes apparent depends upon the solute not the polymer. The amount of water unavailable as a solvent in such a system depends upon the nature of the polymers present. The greater the solute concentration of the surface cells of fish, the more water is held by interaction with ions and polar groups such as (-CO-,-NH₂-,-OH) on other food components and, therefore, the lower is the vapour pressure compared with that of a free water surface at the same temperature. In addition to these, water may be bound into crystals or held by surface tension effects in capillaries both of which would tend to further reduce a_w.