What is Water Activity?

Water activity is expressed as aw, which is the ratio of the vapor pressure of water in food to the vapor pressure of pure water at the same temperature. It shows how the water in the food depends on the structure, and the degree and status of its use for microbiological activities with some chemical and enzymatic reactions. In other words, to summarize briefly, to control the water activity value of the product and to protect the product from further quality losses. Foods with the same water content may vary in strength. Correlations between food processing and storage degradation and quality loss are best expressed by water activity. Preserving food has existed since the existence of human beings. For this reason, knowing the water activity of the food shows how you can process the food. Generally applied methods are drying, salting, freezing, sugar addition etc. (Fontana, 2000).

Dried vegetables, cake mixes, crackers 0,2-0,3
Dried eggs, cocoa 0,4
Chocolate, confectionery, honey 0,5-0,6
Dried fruits, corn syrup 0,6-0,7
Salted fish, dried figs (soft) 0,7-0,8
Hungarian salami, jams 0,8-0,9
Fresh meat, vegetables and fruits, orange juice concentrate 0,9-0,99

At temperatures below freezing point, water activity varies only with temperature and is not affected by the amount of solute present. At temperatures above the freezing point, the water activity varies depending on the temperature and composition of the substance.

Some of the water in the food is bound to substances such as protein, sugar, fat and some of it is free. Connected water is described by some researchers as the amount of water that does not freeze even at very low temperatures. In general, 5-10% of the cell water remains frozen. However, after a certain temperature, the amount of freezing water decreases. This is because many components of food are dissolved.

Microbial growth

Microbial growth is the event in which water activity is effective. There are limit values ​​of water activity that microorganisms can develop. Water activity refers to the amount of water available for microbial growth. Although the amount of water in the product is high, if the energy level is low for microorganisms, it will not be effective in their development. In this case, creating an osmotic imbalance between the organism and the environment prevents the development of microorganisms and causes their death.

Apart from the development of microorganisms, another important event related to the value of water activity is the formation of secondary products produced by these organisms that may be harmful to humans (Hussein et al., 2001).


Fontana, A.J., 2000. Water Safety’s Role in Food Safety and Quality, Second NSF International Conference on Food Safety, October 11-13, 2000 Savnnah, GA, USA.

Hussein, H. S. and J. M. Brasel, 2001. Toxicity, metabolism, and impact of mycotoxins on humans and animals. Toxicology 167, 101 - 134.


Great info.
Aw of 0.60, mold and yeast action occurs, and with Aw of 0.85 bacteria can act as well. Meat has a high water activity of 0.95. Interestingly corn syrup have low amount of water that one might predict, and it flow as well, but contain high level of glucose forming hydrogen bonds with the water molecules. Drying is applied to preserve or alter aw of food.
Thanks for the good learning points.