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.