Fish Products and Byproducts Technology

Unit 3 - Salting and Drying

3.3.1. Water activity and Microbial spoilage

Micro-organisms compete with solute molecules for the water they require for growth over the entire range of aw. This, of course, is an oversimplified view in that it assumes that equilibration of aw has taken place throughout the food is being dehydrated or rehydrated, salted or desalted. Mossel (1975) questioned the choice of the term ‘water activity’, in the context of microbial spoilage prevention, on the grounds that deterioration was always controlled by a number of factors, of which aw was only one. Furthermore, it only had a direct influence on spoilage rate when water content was so low that the mobility of reactants was severely reduced.
At a range of aw levels, they are viable, knowledge of a food product’s water activity, amongst other factors, can give an indication of its preservation status. Table 2.1 gives an idea of the limiting water activities for the growth of various specific micro-organisms with examples of foodstuffs in which one would except to find this aw.
Table 2.1 Limiting water activities and examples of appropriate foodstuffs for the growth of various specific micro-organisms