Measurement of Water Activity by Electronic Sensors

Water activity of foods is an important thermodynamic property affecting stability with respect to physical, chemical and microbiological changes. Water activity, av, is the ratio of the vapor pressure of water in a system, (Pl)sy, to the vapor pressure of pure water, (Pv)w, at the same temperature. It is equal to the equilibrium relative humidity (ERH) established in the surrounding air. Thus  

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. 

10 g saturated salt solution (Table A2.3.1 and Table A2.3.2) or standard supplied with the equipment from the manufacturer 5 g sample from bulk food, retaining its original state (e.g., solid with original structure liquid, or ground sample) 

Instrumentation for measuring water activity by electronic sensors (American Instrument Company, Beckman Instruments, Nova Sina AG, and Rotronic AG) Plastic sample holder with lid (diameter -4.0 cm height 1.0 cm usually supplied with instrument) 

Turn on the instrument and set it at a specified temperature (that at which water activity of sample needs to be measured), and then keep it at that temperature for at least 20 to 30 min to allow stabilization. 

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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... 

Figure A2.5.1 Schematic diagram of setup for water activity measurement by electronic sensor.

Water activity measurement by electronic sensor gives the water activity value within the range 0 to 1 (up to three decimal points) as a function of sample water content and measurement temperature (25° to 50°C). 

Traditionally, ferrocene derivatives [8-10] and quinones [11, 12] have been used as mediators. For these small molecular weight mediators, the electron transfer from GOD to the electrode is achieved by the diffusion of the mediator molecules to the electrode. If the mediator molecules are fixed by inclusion into the polymer gel in a manner similar to enzymes, it is possible for these mediator molecules to leak into the measurement solution after repeated use. This phenomenon can occur even if these molecules are somewhat water insoluble, which leads to the gradual reduction of electrode activity. In particular, for the development of implanted sensors, the fixation of mediator molecules onto the electrode is a serious problem. 

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