Water, essential element to all forms of life, is found in varying amounts in the foods we consume ; and while it does not provide energy value, it influences the foods’ properties particularly their proneness to degradation. Water also has an impact on the appearance, texture and flavor of the food.
Water activity is one of the main parameters as regards a product’s shelf life, be it food, pharmaceuticals, cosmetics, or agricultural seeds. Measuring such activity makes it possible to control and optimize the manufacturing and preservation process to ensure mechanical, physical, chemical and microbiological stability. The measurement of water activity is decisive for the quality and health safety of a product, especially if it is not sterilized or stored in a non-sterile environment. Aw measures form part of the calculation of the CSD and MDD of many products.
Although the concept of water activity emerged and started developing in the 70’s and 80’s, the primary means for correcting water activity have been known since Antiquity, and probably before then: drying, or adding an Aw reducing agent such as sodium chloride (salt) for sausages, or sucrose (sugar or formerly honey) for jams.
Water content and Aw
The activity of water or Aw represents the « free » water contained in a product; it is not a measure of water content, also called moisture content, but a measure of the availability of this water. This water, which is not strongly bound to the product from a physico-chemical point of view, directly influences the growth and toxinogenesis of micro-organisms such as bacteria, yeasts and molds… as well as the development of enzymatic reactions and oxidations…
Although water activity is correlated with the water content of a product, the relationship between these two quantities is not linear, and it closely depends on the nature of the product under consideration. The relationship between the water content and the water activity of a product at a constant temperature is called sorption isotherm. Sorption isotherm is made of two distinct lines (a phenomenon called hysteresis). A desorption line if starting from a water saturated product with a drying process. An adsorption line if starting from a dry product with the addition of water. The water present in the product thus binds more strongly with its food matrix during drying than during rehydration. This means that water content is not enough to determine a product’s ability to preserve. In other words, water activity measurement is much more accurate than water content measurement as far as the control of food safety and quality is concerned.
In theoretical terms, water activity can be defined as a ratio of vapour pressures. The activity of water equals the water vapour partial pressure of a wet product divided by the saturation vapor pressure of pure water at the same temperature:
Aw = p(T) / p0(T)
Water activity therefore varies within a range from 0 to 1, pure water having a water activity value of 1. However, Aw is rarely calculated from a gauge pressure measurement, there are several methods and technologies to determine its value.
Water activity is sometimes expressed as a percentage and is then called Equilibrium Relative Humidity (ERH):
HRE(%) = 100 x Aw
It allows direct comparison with the ambient Relative Humidity (RH) commonly/frequently measured and controlled during drying or storage operations.
The development of microorganisms is closely related to the activity of water due to the influence of the osmotic pressure exerted by the environment on the membranous exchanges within the cells..
Most pathogenic bacteria grow at Aw values above 0.91. This is the value set by European directives as the upper limit allowing the conservation of food at ambient temperature. pH also plays an important role and this limit is pushed back to 0.95 when the pH is lower than 5.2. Most molds start growing from 0.80. Water activity measurement makes it possible to predict which microorganisms are potential sources of contamination. A product may be considered microbiologically stable if its Aw is under 0.6. This is the limit that is sought for products that can be stored at room temperature for a long time.
Water activity and growth of microorganisms in food products
Composite foods and water transfers
Water migrates between heterogeneous media due to the different partial pressures of water vapor that may be present. These different partial pressures will tend to equilibrate more or less rapidly depending on the nature of the constituents. Since water activity is representative of the partial pressure of water vapor, it can therefore be used to predict the movements of water inside a food, composed of several layers of different structures, such as a biscuit and a filling. The water will migrate from the part where it is more active to the part where it is less active until a balance is established. It therefore helps to (re) formulate a product so that it does not deteriorate due to the migration of the water it contains.
Water activity measurement can be used to fine tune settings for cooking processes or to improve their reproducibility. The browning reaction, also called Maillard reaction, concerns foods containing amino acids and sugars. These non-enzymatic reactions reach a maximum when water activity is between 0.50 and 0.70.
Controlling the activity of water is also a way to prevent a clustering or caking phenomenon of powders or wet particles, such as spices, flours, cosmetics or pharmaceuticals … It may be useful to control the Aw of raw materials to avoid the clogging of a supply system that could block a production line.
Water activity measurement is critical for the health security of products stored in a non-sterile environment. It can replace or usefully complement the measurement of the water content of a product and it allows optimizing a manufacturing process. The influence of water activity on texture, flavor, microbiology, water migration, enzymatic, browning and oxidation reactions as well as on powder caking demonstrates that its control is highly valuable in numerous instances.