Dew-Point Method for the Determination of Water Activity

The Basic Protocol describes the determination of water activity of a product using a chilled mirror dew-point water activity meter. Dew point is a primary measurement of vapor pressure that has been in use for decades (Harris, 1995). Dew-point instruments are accurate, fast, simple to use, and precise (Richard and Labuza, 1990 Snavely et al., 1990 Roa and Tapia de Daza, 1991). In a dew-point instrument, water activity is measured by equilibrating the liquid-phase water in the food sample with the vapor-phase water in the headspace, and then measuring the vapor pressure of the headspace. The basic principle involved in dew-point determinations of vapor pressure in air is that air may be cooled without change in water content until it saturates. The dew-point temperature is the temperature at which the air reaches saturation. It is determined in practice by measuring 

Dew-point measurement is a primary method based on fundamental thermodynamics principles and as such does not require calibration. However, the instrument performance needs to be verified using salt standards and distilled water before sampling (see Support Protocol). To obtain accurate and reproducible water activity results with a dew-point instrument, temperature, sensor cleanliness, and sample preparation must be considered. Equipment should be used and maintained in accordance with the manufacturer s instruction manual and with good laboratory practice. If there are any concerns, the manufacturer of the instrument should be consulted. Guidelines common to dew-point instruments for proper water activity determinations are described in this protocol. The manufacturer s instructions should be referred to for specifics. 

The type (e.g., liquid, solid, powder, gel, syrup, emulsion, granule) and range of food samples (raw ingredients to final products) for water activity measurement are immense. The amount of sample required for measurement is typically 5 to 10 ml. A homogeneous and representative sample should be prepared and placed into the sample cup. For the majority of samples, no preparation is necessary the sample is simply placed into the cup. Multicomponent (e.g., muffin with raisins or pizza) and coated samples (e.g., breaded foods or chocolate-covered bar) may have to be sliced, crushed, or ground in order to obtain a representative sample. If sample preparation is necessary, then a consistent technique must be used with each sample to ensure reproducible results. 

Dew-point water activity instrument (e.g., AquaLab Series 3, Decagon Devices) Disposable sample cups (and optional lids available from the instrument manufacturer) 

Place a dew-point water activity instrument on a level surface and in a relatively temperature-stable environment. Allow instrument to warm up for 15 min after turning on for optimal performance. 

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A2.1 Factors to Consider When Estimating Water Vapor Pressure A2.2 Dew-Point Method for the Determination of Water Activity A2.3 Measurement of Water Activity Using Isopiestic Method A2.4 Direct Manometric Determination of Vapor Pressure A2.5 Measurement of Water Activity by Electronic Sensors... 

There are many measurement techniques for activity coefficients. These include measuring the colligative property (osmotic coefficients) relationship, the junction potentials, the freezing point depression, or deviations from ideal solution theory of only one electrolyte. The osmotic coefficient method presented here can be used to determine activity coefficients of a 1 1 electrolyte in water. A vapor pressure osmometer (i.e., dew point osmometer) measures vapor pressure depression. 

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