Category Archives: activity of water

Why measure water activity ?

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.

  • Microbiology

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.

Microorganismes-Aw

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.

  • Other applications

madeleines

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.

  • Conclusion

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.

 

How to measure water activity?

The devices measuring water activity are called Aw-meters or Water Activity Meters, they generally comprise a dish holder where a sample cup containing the product to be analyzed can be inserted.

  • Should the samples be crushed?

The product to be analyzed can be placed in the cup as it is, but it is often crushed/ground or mixed to make it more homogeneous, which considerably speeds up the measurement time of the sample and allows better measurement accuracy.

There is, however, a controversy as to whether or not to crush samples before measuring water activity. The ISO21807 standard does not recommend grinding because of the potential reheating and water loss grinding may cause. Nevertheless, many industries grind their samples before measurement, as it allows quickly homogenizing a product that is not, such as a freshly made product or one that has just been cooked or dried. This homogeneity allows rapid and repeatable measurements representative of the water activity of a product manufactured and stored for some time. We believe that grinding can be useful as long as certain precautions are duly taken, for example:

  • keep a substantial ratio between the ground sample volume and the grinder’s volume, indeed with a limited volume of air in the grinder, water exchange between the sample and the air remains minimal.
  • only use a moderate grinding so as not to modify the matrix of the product and not to overheat it excessively.
  • put a lid on the cup if the sample can not be measured immediately.

In general, the sampling of a product to be measured requires a certain level of cautiousness. The measurement of a large product such as a ham during drying can produce very different results depending on the sampling area. It is therefore necessary to put together a sample consisting of one or more samples – in the same way as coring is used in geology –in order for this sample to be representative of the product as a whole before proceeding with its grinding.

  • Main technologies

Capacitive sensor: A hygroscopic polymer is used as a dielectric. This polymer absorbs or rejects a certain amount of water in relation to the water activity of the measured sample. The resulting variation in electric capacitance makes it possible to deduce the water activity of the sample.

The main advantages of this technology are its rather low cost and a greater insensitivity to volatile substances such as alcohol and propylene glycol.

The main disadvantages of this technology are the loss of efficiency of the sensor over time and fouling issues. The sensor must be recalibrated and/or replaced periodically. Accuracy is lower and it may present some hysteresis. This technology requires several calibration points for the linearity of measurements to be ensured.

Resistive sensor: This technology is very similar to the previous one, only capacitance measurement is replaced by the measurement of the electrical resistance of a sensor made of hygroscopic salt.
This technology also shows a high degree of similarity with the previous one as regards its advantages and disadvantages.

Dew point by mirror cooled: A mirror, coupled with a thermoelectric module and a temperature probe, is chilled until dew appears in its center. The appearance of dew is detected by means of a photodetector. Controlling the amount of dew to a constant value allows to determine the dew point temperature and to then deduce the water activity of the sample.
The main advantages of this technology include very good accuracy, long lifespan and fast measuring time. It is based on a fundamental principle of hygrometric measurement, its natural linearity does not require several calibration points.
The main disadvantage of this technology resides in its greater sensitivity to volatile substances such as alcohol and propylene glycol, that can generate measurement errors. The chilled mirror must be clean for accuracy to be respected.

 Tunable laser diode sensor: This technology called TDLAS for Tunable Diode Laser Absorption Spectroscopy, is of the absorption spectrometry type where the spectrum is extremely narrow and centered on a wavelength specific to the detection of gas phase water. It is a recent technology offering the main advantage of being the most insensitive to volatile substances. Another positive point, that it shares with the chilled mirror, is that there is no hysteresis phenomenon. Accuracy however is lower than that of the chilled mirror ; it requires several calibration points for the linearity of measurements to be effective. It comes at a higher prices and its reliability in time remains to be proven, as this technology has only been available for a few years.

  • Measurement time

Irrespective of the technology used, the measurement of a product’s Aw is done indirectly. It is actually the air contained in the measuring chamber that is analyzed, that is why the measuring chamber must have the smallest possible volume of air while allowing a maximum exchange surface. Water exchanges between the sample and the air contained take place as soon as the measuring chamber is closed. Only when this air is in thermodynamic equilibrium with the measured product will the measurement of Aw be reliable. Most Aw-meters automatically detect a satisfactory equilibrium state, and stop the measurement as necessary.

The measurement time can vary significantly from one product to another. A fat product can form a lipid barrier that will slow down the migration of water, and thus take much longer to measure than another product. Modern devices offer measurement times of often less than 5 minutes, and yet the possibility of performing long-term analyzes represents a significant advantage in that it allows − beyond the simple Aw measurement − understanding the various water migration phenomena that closely depend on it.

  • General Information

Aw-meters often include a non-contact infrared thermometer to measure the temperature of the placed sample.
Water activity devices can be controlled and calibrated using saturated or unsaturated salt solutions, the various existing solutions cover the entire measurement range.
Some devices offer temperature control to make measurements at a stabilized temperature chosen by the user, for example 25 °C which is the temperature recommended by AFNOR, other devices merely measure at room temperature.