What are xalophilic bacteria and xerophilic bacteria

Aw value, water activity

English: Water activity
French: Activit de l'eau
Italian: Attivit dell'acqua
Spanish: Actividad acuosa

Table of Contents

The aw-Value is the amount of unbound and loosely bound water, i.e. for the availability of water in food or dishes and preparations. The awValue is therefore an important parameter for the shelf life of food. The larger the awValue, i.e. the more unbound water a food contains, the easier it is for it to spoil. Because the freely available water is of crucial importance for the growth and metabolism of microorganisms. It is also required by some enzymes - the so-called hydrolases - for hydrolysis. In other words, it is needed to break chemical bonds through reaction with water.

Calculation of the awValues

Example: clarified butter contains almost no water and can keep for months, whereas butter contains up to 16% water and therefore has a much shorter shelf life.

The awValue is calculated from the quotient of the water vapor pressure above the food (p) and the water vapor pressure above pure water (p0) at a certain temperature.

awValue = p / p0

So it's a number from 0 to 1. Pure water has an aw-Value of 1 while absolutely anhydrous substances (food) have an aw-Have a value of 0. One can also say: 1 means that condensation occurs and 0 is anhydrous.

The water availability of most fresh foods is between 0.98 and 0.99. At the same time, this is the optimum value for the growth of most microorganisms. Measures such as drying or curing reduce the awValue and thus the Water activity of food.

Bacteria, fungi, yeast

bacteria usually place the highest demands on the content of freely available water in food. You need at least one aw-Value between 0.91 and 0.96. All potentially pathogenic, i.e. pathogenic microorganisms, react very sensitively to low water activity. For example, this is the limit value for the water activity of the bacterium Clostridium botulinum is 0.95. The bacterium can be problematic Staphylococcus aureus be viewed, the deep aw-Values ​​between 0.86 and 0.90 can survive.

Yeasts can produce yeasts at lower awValues ​​as bacteria grow. The lower limit for their growth is only between 0.88 and 0.94.
Exceptions, however, are the so-called xerophilic microorganismswhich also tolerate a water activity of up to 0.6. For example osmophile, so sugar-loving yeasts or extreme halophiles, that is, salt-loving bacteria.


Molds require the least aw-Value for your growth. You can still go to aw- Values ​​grow between 0.80 and 0.85. The ability of organisms to survive in areas of life with very little water is also called Xerotolerance designated.

The aw-The value of a food is only an average value. If a food is divided into smaller sections, there is often a significantly higher water activity, especially at the interfaces (for example at cut surfaces) than inside the food. There is an increased multiplication of microorganisms. In addition, there are compound foods with an overall relatively low aw-Value does not yet guarantee low growth of microorganisms. Pores in food can store water as a result of capillary forces despite the surface appearing dry.

Measures to reduce the aw-Values ​​of food

  • Drying - Drying evaporates the water in the food. For example, dried fruit or herbs are preserved.
  • Salt cooking - by salting or curing the water in food is bound and is no longer free. For example, salted herring, gravlax or smoked pork can be preserved.
  • Adding sugar - Adding sugar binds the water in the food to the sugar. For example, candied fruit, syrup, jelly or jam can be preserved.
  • Freezing - Freezing turns the water into solid ice and is therefore no longer active.


  • Frank Brandes, Hermann Grüner, Conrad Krödel, Reinhold Metz, Marco Voll, Thomas Wolffgang: The young cook. Pfanneberg, 2015 »