DETERMINATION OF FACTORS CONTROLLING ACCURATE MEASUREMENT OF A w BY THE VAPOR PRESSURE MANOMETRIC TECHNIQUE

The precision and reproducibility of the VPM method has been improved by the simultaneous measurement of water vapor pressure and temperature of the food sample. It has been shown that evacuation of the system substantially lowers food temperature due to evaporation of water. The drop in sample temperature is dependent on water activity of the material and is large at high water activities. Even at steady readings of the manometer, the temperature of the sample is not equal to room temperature. Therefore, calculation of water activity based on room temperature gives erroneous results which are significantly lower than the results based on the measured sample temperature.