Water is a fundamental component in food, both in terms of maintaining food quality, its preservation and minimising microbial spoilage. One term frequently quoted in food science is water activity which has come from basic thermodynamics in chemistry. It is simply the ratio of vapour pressure of water in a system such as a food, to the vapour pressure of pure water at the same temperature. Another expression for water activity is the equilibrium relative humidity of the air surrounding the food at the same temperature. The higher the value the more susceptible a foodstuff is to spoilage or microbial fermentation. It’s a measure most usefully needed by microbiologists to help them assess the risk posed by growth of pathogens for food poisoning.
A variety of methods exist for measurement of water activity. They include:-
– the method of vapour pressure measurement
– the isopiestic method or equilibrium sorption rate method
– the hygrometric instrument method
Other techniques have used the change in the freezing point of water to estimate water activity. Generally, an accuracy up to 0.02 water activity units is needed.
A great many reviews discuss the meaning of water activity and its impact on food quality and most food science texts examine its role. Labuza at the University of Minnesota, USA has written extensively on the subject and his seminal studies provide much of our understanding of the relationship (Labuza, 1980; Rahman and Labuza, 2007; Barbosa-Canovas et al., 2007) and it’s worth just checking out the various lecture notes in his profile. Troller and Christian (1978) provide extensive coverage with large portions of this work freely available on the Internet. There is also excellent coverage provided by the research institutes. One to look at is CSIRO’s coverage (CSIRO, 2011) where the concept was initially explored.
Barbosa- Cánovas, G.V., Fontana, A.J., Schmidt, S.J., Labuza, T.P. (2007) Water Actvity in Foods: Fundamentals and Applications. Blackwell Publ. IFT Press
Labuza, T.P. (1980) The effect of water activity on reaction kinetics of food deterioration Food Technol., April 34(4) pp. 36-41,59
_______________ also FScN 4312 Food Analysis http://www.ardilla.umn.edu/Ted_Labuza/FScN%204312/4312-default.html
Rahman, M. and Labuza, T.P. (2007) Water activity and food preservation. Chapter 20 In: Handbook of Food Preservation, 2nd Edition. M. Rahman (ed.). Taylor and Francis, Boca Raton, FL. USA.
Troller, J.A., Christian, J.H.B. (1978) Water Activity and Food. Publ. Academic Press, Inc. (London) ISBN 0-12-700650-8 (UK & USA printed versions)