a _w value

The water activity (also a _w value , or A ctivity of W ater) of a food is a measure of the "available" or "active" water in contrast to the mere indication of the water content. The importance of this parameter results from the fact that not only the pure water content is important for the shelf life of food, but also the extent to which the water is bound by the substrate. Water activity influences the growth of microorganisms , the course of chemical processes such as fat oxidation and non-enzymatic browning, the activity of enzymes , and the physical properties of the food.

The a _w value is a thermodynamic quantity that the Australian bacteriologist William James Scott (1912–1993) introduced to food technology in the early 1950s. In a series of fundamental works that appeared in the 1950s, he was able to show that the growth of microorganisms does not depend on the water content, but on the water activity of a food. In the 1960s and 1970s, research also showed the influence of water availability on the chemical, enzymatic and physical stability of food.

definition

The water activity is defined as the ratio of the water vapor partial pressure in the food (p) to the saturation vapor pressure of pure water (p ₀ ) at a certain temperature:

{\ displaystyle a _ {\ rm {w}} = {\ frac {p} {p}} _ {0}}

The water activity is synonymous with the (relative) equilibrium humidity , i.e. the relative air humidity at which the food (again at the given temperature) is in equilibrium with the ambient air , i.e. neither loses nor absorbs water. However, the relative humidity is usually given in percent as an auxiliary unit , so that the relative equilibrium humidity is calculated as:

{\ displaystyle {\ rm {RGF}} = a _ {\ rm {w}} \ cdot 100}

At the same time, the water activity ( ) 0–1 corresponds to a relative humidity of 0–100%. ${\ displaystyle a _ {\ rm {w}}}$

In the simplest case, the water activity is measured by placing a sample of the food in a hermetically sealed container and measuring the humidity in the container with a hygrometer .

Water activity of food

The water activity of pure water is 1, that of completely anhydrous material is 0; in between are the values for substances containing water. The following table gives an example of the average a _w value of some foods:

Food	Water activity
cornflakes	0.20
cracker	0.30
Pasta	0.50
milk powder	0.60
oatmeal	0.65
nuts	0.70
Skimmed milk powder	0.70
honey	0.75
Cereal flour	0.75
salami	0.78
margarine	0.84
Fruit , vegetables (fresh)	0.97

The water activity depends on the water content. For example, whole egg powder with 5% water content has a _w = 0.4; those with a water content of 10% a _w = 0.7. The relationship is usually complicated and depends on the water-binding capacity of the substance. Highly hygroscopic foods show only a small increase in water activity when the water content rises sharply, and less hygroscopic foods a high one. In addition, the course of the water activity is partly different when the water content changes, depending on the direction in which the change takes place, i.e. whether it is a drying or moistening process. The course is indicated by so-called sorption isotherms .

meaning

The a _w value is an important measure with regard to the shelf life of food and influences the occurrence of microorganisms (spoilage pathogens), which have different demands on freely available water. If there is a lack of free water, the growth processes of some water-loving microorganisms are slowed down, sensitive organisms can even be killed, whereas xerophilic organisms grow better when the water content drops.

For most microorganisms, the optimum growth is at an a _w value of 0.98 to 1. However, there are microorganisms that tolerate a significantly lower water activity of up to 0.6 (so-called xerophiles ). Examples are osmophilic (sugar-loving) yeasts or extremely halophilic bacteria.

The adaptation to low water activity takes place through synthesis or uptake of compatible solutes .

literature

Andrew Brown: Microbial Water Stress Physiology. Principles and perspectives . John Wiley & Sons, Chichester 1990, ISBN 0-471-92579-9 .
Martin Weidenbörner: Food mycology in the Google book search

Individual evidence

↑ Jorge Chirife, Anthony J. Fontana, Jr .: Introduction: Historical Highlights of Water Activity Research . In: Gustavo V. Barbosa-Cánovas, Anthony J. Fontana, Jr., Shelly J. Schmidt, Theodore P. Labuza (Eds.): Water Activity in Foods. Fundamentals and Applications . Wiley, 2008, ISBN 0-470-37636-8 , pp. 3 ( limited preview in Google Book Search - online edition).
↑ ^a ^b Herbert Weber (ed.): Microbiology of food . 9th edition. tape 1 : Basics. Behr, 2009, ISBN 3-89947-442-2 , pp. 421 ff . ( limited preview in Google Book search).
^ ^A ^b ^c Rudolf Heiss, Karl Eichner: Preservation of food . 3rd, revised and expanded edition. Springer, Berlin / Heidelberg / New York 1995, ISBN 978-3-662-07664-4 , pp. 32 ( limited preview in Google Book Search - eBook).
^ ^A ^b ^c Waldemar Ternes: Scientific basics of food preparation . 3rd revised edition. Behr, Hamburg 2008, ISBN 978-3-89947-422-0 .

[Water_Activity-1] Jorge Chirife, Anthony J. Fontana, Jr .: Introduction: Historical Highlights of Water Activity Research . In: Gustavo V. Barbosa-Cánovas, Anthony J. Fontana, Jr., Shelly J. Schmidt, Theodore P. Labuza (Eds.): Water Activity in Foods. Fundamentals and Applications . Wiley, 2008, ISBN 0-470-37636-8 , pp. 3 ( limited preview in Google Book Search - online edition).

[Weber_2009-2] Herbert Weber (ed.): Microbiology of food . 9th edition. tape 1 : Basics. Behr, 2009, ISBN 3-89947-442-2 , pp. 421 ff . ( limited preview in Google Book search).

[Heiss/Eichner_1995-3] A ^b ^c Rudolf Heiss, Karl Eichner: Preservation of food . 3rd, revised and expanded edition. Springer, Berlin / Heidelberg / New York 1995, ISBN 978-3-662-07664-4 , pp. 32 ( limited preview in Google Book Search - eBook).

[Ternes_2008-4] A ^b ^c Waldemar Ternes: Scientific basics of food preparation . 3rd revised edition. Behr, Hamburg 2008, ISBN 978-3-89947-422-0 .

a w value