Water activity measurement

Thus, in the observation of the drying process of biological materials, the water activity is at least as valuable as the water content of a material. As water activity measurements are complete in about 3 min, they represent a useful tool for control of the drying process. 

When is equilibrium reached is an important follow-up, timescale question. The water activity measurement involves two interrelated time-scales. The first timescale is related to the nonequilibrium nature of most... 

A variety of measurement methods have been developed for determining the water activity of food materials and are well described in texts such as Rahman (1995), Wiederhold (1997), and Bell and Labuza (2000). In general, water activity is a relatively easy parameter to measure, which can be an advantage, especially for use in the food industry. Depending on the technique selected, the water activity of a food material can be measured in a time frame of minutes (e.g., electronic instrument). In addition, individuals can be trained, with a limited amount of instruction, to make water activity measurements. Consequently, when appropriate, water activity measurements can be made relatively quickly by personnel overseeing a manufacturing line for quality assurance purposes. Measurement protocols, such as calibration procedures and proper temperature control, should be implemented to assure the accuracy of online c/w measurements. 

Friedel, R.R., and Cundell, A.M., The application of water activity measurement to the microbiological attributes testing of nonsterile OTC drug products, Pharm. Forum, 25(2) 6087-6090 (1998). 

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. 

Start the water activity measurement (see Basic Protocol, steps 7 and 8). Make two measurements. 

To obtain accurate and reproducible water activity results with a dew-point instrument, the temperature, sensor cleanliness, and sample preparation must be considered. Temperature influences water activity measurements in two ways one is its effect on the water activity in the sample and the second is the effect on the dew-point measurement. 

Temperature must be defined for water activity measurements. Changes in sample temperature will cause the water activity readings... 

Roa, V. and Tapia de Daza, M.S. 1991. Evaluation of water activity measurements with a dew point electronic humidity meter. Lehensmittel-Wissen-schaft Technol. 24 208-213. 

Stoloff, L. 1978. Calibration of water activity measuring instruments and devices Collaborative study. J. Assoc. Official Anal. Chem. 61 1166-1178. 

The experimental results provide water activity of a given sample at a specified temperature. The water activity measurements may range from 0.05 to 0.99. Isotherms can also be obtained by plotting water activity of a sample against its corresponding moisture content. The resultant isotherms can be modeled by empirical or theoretical equations as described in unit... 

Nunes, R.V., Urbicain, M.J., and Rotstein, E. 1985. Improving accuracy and precision of water activity measurements with a water vapor pressure manometer. / Food Sci. 50 148-149. 

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

Equipment based on the change in electrical conductivity of immobilized salt solution is available from American Instrument Company, Beckman Instruments, Nova Sina AG, and Ro-tronic AG. Equipment based on change in electrical capacitance of polymer thin films is available from WeatherMeasure and General Eastern Corporation. The approximate price range for water activity measurement instruments based on electronic sensors is 6000 to 8000. 

Specific problems arising from the use of a sensor for water activity measurement are... 

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). 

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. 

Canola oil, composition, 474 (table) Capacitance, in water activity measurement, 67-70 Capacitance manometer vs. U-tube manometer, 61,64 Capacitor model in interface studies, 624-625 Carbazole, 737 Carbohydrates... 

Conductivity, in water activity measurement, 67-70 Confocal laser scanning microscopy to characterize lipid crystals, 575-579 description of, 575, 577 Conjugated dienes and trienes, determination of, 515-517 Conjugated linoleic acid (CLA), fatty acid analysis, 437-438, 445-446 Convection oven, gravimetric measurement of water, 7-8, 10-11... 

Electronic sensors, water activity measurement, 67-70 Electronic transitions of amino acids, CD analysis, 234-237. see also Circular dichroism... 

Seeds, extract preparation, 412 Sensors, electronic, for water activity measurement, 67-70 Sensory scores related to TBA values, 558-560... 

Table 6.7 Initial water content of the dry biocatalysts and water activity measured in a closed system obtained after incubating 60 mg of mycelia and 200 mg of Novozym 435 for 24h in 6 ml of toluene.

Favetto, G., Resnik, S., Chirife, J. Ferro Fontan, C. (1983). Statistical evaluation of water activity measurements obtained with the Vaisala Humicap humidity meter. Journal of Food Science, 48, 534-538. 

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