So it is a number from 0 to 1, Pure water has a value of 1 while aw absolutely anhydrous substances

The aw value is the amount of unbound and loosely bound water, ie for the availability of water in foods or food and preparations. The aw value is thus an important factor for the shelf life of food. The larger the aw value is, so the more free water containing a food, the easier it can spoil. Because the water is freely available, especially for the growth and metabolism of microorganisms is crucial. In addition, it is required by some enzymes, rationaltheater the so-called hydrolases required for the hydrolysis, that is the cleavage of chemical bonds by reaction with water. rationaltheater
The aw value is calculated from the quotient of the water vapor pressure over the food (p) and the water vapor pressure over pure water (p0) at a given temperature. aw value = p / p0
So it is a number from 0 to 1, Pure water has a value of 1 while aw absolutely anhydrous substances (food) have an aw value of 0. You can also say: 1 means that condensation occurs and 0 is anhydrous.
The water availability of most fresh foods is from 0.98 to 0.99. This is also the optimum for the growth of most microorganisms. Measures such as drying or salting reduce the aw value and therefore rationaltheater the water activity of the food.
Bacteria usually make the highest demands on the content of free water available in food. You need to increase at least one aw value between rationaltheater 0.91 to 0.96. All potentially pathogenic, ie, pathogenic microorganisms are very sensitive at low water activity. Thus, for example, is the limit of the water activity of the bacterium Clostridium botulinum is 0.95. The bacterium Staphylococcus aureus can be considered a problem that can survive even low aw values between 0.86 to 0.90.
Yeasts can grow yeast at lower aw values than bacteria. The lower limit of their growth is only 0.88 to 0.94. However, exceptions are the so-called xerophile microorganisms that tolerate a water activity of up to 0.6. For example, osmophilic, ie sugar loving yeast or extremely halophilic, ie salt-loving rationaltheater bacteria. Xerotoleranz
Molds need the lowest aw value for your growth. rationaltheater You can still grow at aw values between 0.80 to 0.85. The ability of organisms in areas of life to survive with very little water is also known as Xerotoleranz.
But the aw value of a food is only an average. If we divide rationaltheater a food into smaller portions so there especially at the interfaces (for example, on-sectional areas) are often much higher water activity than in the interior of the food. There, an increased proliferation of microorganisms takes place. Besides giving particular composite foods with a total of relatively low aw value does not guarantee a low growth of microorganisms. Pores in foods can save despite dry appearing surface due to capillary water.
Measures to reduce the aw value from food drying - drying the water from the food evaporates. For example, dried fruit or herbs are preserved. Cooking salt - Bound by salt or pickle the water in foods and is no longer free. For example, salt herring, marinated salmon or Kassel be preserved. Addition of sugar - sugar by adding the water in the food to the sugar bound. rationaltheater For example, candied fruit, syrup, jelly or jam are preserved. Freezing - By freezing the water to solid ice and thus is no longer active.
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