Measurement of Water Activity

Many methods of measuring water activity have been developed by researchers. These include direct vapor pressure measurement, equilibration with a stable hygroscopic substance that has a known sorption behavior, and various types of hygrometers (Doe, 1998). 

The Aqualab CX2 water activity meter (Decagon Devices Inc., Pullman, WA) detects water condensation on a chilled mirror (dew point temperature). The instrument is sensitive to water activity units of 0.001. Readings take 5 min or less and are accurate to 0.003 water activity units. 

Care must be taken with any measurement of water activity to ensure that the sample is representative of the food under test. Dried fish, for example, will have moisture and salt contents, and thus water activity, varying widely from thin, exposed flesh to the relatively moist interior. If the worst-case scenario for the growth of potentially toxic or spoilage organisms is of interest, the sample of flesh for water activity determination should be excised from the thickest, most moist region of the fish. 

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The practical measurement of water activity nowadays is usually performed by dew point determination or a change of electrical properties of the material depending on the relative humidity. 

This now forms fhe basis of fhe experimental measurement of water activity. 

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... 

A2.3 Measurement of Water Activity Using Isopiestic Method A2.3.1... 

Figure A2.3.2 An apparatus with circulated air chamber and in situ weighting system for measurement of water activity (aw). Adapted from with permission Igbeka and Blaisdell (1982).

Vos, P. and Labuza, T.P. 1974. A technique for measurement of water activity in the high aw range. J. Agric. Food Chem. 22 326-327. 

Lewicki, P.P. 1989. Measurement of water activity of saturated salt solutions with the vaporpressure manometer. J. Food Eng. 10 39-55. 

Zanoni, B., Peri, C. Giovanelli, G., and Pagliarini, E. 1999. Design and setting up of a water vapor pressure capacitance manometer for measurement of water activity. J. Food Sci. 38 407-423. 

Sood, V.C. and Heldman, D.R. 1974. Analysis of a vapor pressure manometer for measurement of water activity in nonfat dry milk.. /. Food Sci. 39 1011-1013. 

A multilaboratory, multi-instrument study of the use of electronic sensors applied to a cross-section of commodities and reference standards demonstrated that measurements of water activity by the described method (with instruments using immobilized salt solution sensors) can be made with an accuracy and precision within 0.01 water activity unit, provided there are no interactions between the instrument and the commodity under investigation (Stoloff, 1978). Commodity-instrument interactions did exist with a number of products, especially those containing highly volatile compounds. 

Smith, PR. 1971. The determination of equilibrium relative humidity or water activity in foods A literature review. The British Food Manufacturing Industries Research Association, U.K. Stekelenburg, F.K. and Labots, H. 1991. Measurement of water activity with an electric hygrometer. Ini. J. Food Sci. Technol. 26 111-116. Stoloff, L. 1978. Calibration of water activity measuring instruments and devices Collaborative study. J. AOAC 61 1166-1178. 

In view of the difficulties mentioned above it is not surprising that statements such as acidity function failure have appeared in the literature in recent years. However, Cox and Stewart (1976) have delineated the conditions which are necessary for use of acidity function techniques for pAa determination. Also the recent measurement of water activities in aqueous DMSO mixtures at 25° (Lam and... 

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