Dew Point Method for the Determination of Water Activity

UNIT A2.2

The single most important property of water in food systems is water activity (aW; Taoukis et al., 1988). Throughout history the importance of controlling water in food by drying, freezing, or addition of sugar or salt was recognized for preserving and controlling food quality. Water activity is a measure of the energy status of the water in a system (or the degree to which the water is "bound"), and hence of its availability to act as a solvent and participate in chemical or biochemical reactions and growth of microorganisms. It is an important property that can be used to predict the stability and safety of food with respect to microbial growth, rates of deteriorative reactions, and chemical or physical properties (Figure A2.2.1). The water activity principle has been incorporated by various regulatory agencies (e.g., Food and Drug Administration Code of Federal Regulations Title 21) in defining safety regulations regarding growth and proliferation of undesirable microorganisms, potentially hazardous foods, standards of several preserved foods, and packaging requirements (Fontana, 1998).

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

Peta Labuza
Water activity

Figure A2.2.1 Water activity stability map (adapted form Labuza, 1970). A representation of a typical sorption isotherm for food materials and of the effects of water activity on the relative reaction rates of several chemical processes, as well as the growth of microorganisms, in foods are shown.

Vapor Pressure Measurements of Water

Chilled Mirror

Figure A2.2.2 A representation of a modern dew-point chamber consisting of (1) mirror, (2) optical sensor, (3) infrared thermometer, (4) internal chamber fan, and (5) sample cup with sample.

the temperature of a chilled mirror when condensation starts. The water activity of the sample is the ratio of the saturation vapor pressure at dew-point temperature to the saturation vapor pressure at the sample temperature.

In a modern dew-point instrument, a sample is equilibrated within the headspace of a sealed chamber containing a mirror, an optical sensor, an internal fan, and an infrared thermometer (Figure A2.2.2). At equilibrium, the relative humidity of the air in the chamber is the same as the water activity of the sample. A thermoelectric (Peltier) cooler precisely controls the mirror temperature. An optical reflectance sensor detects the exact point at which condensation first appears; a beam of infrared light is directed onto the mirror and reflected back to a photodetector, which detects the change in reflectance when condensation occurs on the mirror. A thermocouple attached to the mirror accurately measures the dew-point temperature. The internal fan is for air circulation to reduce vapor equilibrium time and to control the boundary layer conductance of the mirror surface (Campbell and Lewis, 1998). Additionally, an infrared thermometer measures the sample surface temperature. Both the dew-point and sample temperatures are then used to determine the water activity. The range of a commercially available dew-point meter is 0.030 to 1.000 aw, with a resolution of ±0.001 aw and accuracy of ±0.003 aw. Measurement time is typically less than 5 min. The performance of the instrument should be routinely verified as described in the Support Protocol.

BASIC PROTOCOL

Dew-Point Method for the Determination of Water Activity

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