Measurement of Water Activity of Salt Solutions and Foods by Several Electronic Methods as Compared to Direct Vapor Pressure Measurement

The Kaymont‐Rotronics Hygroskop DT, the Beckman‐Sina and the Protimeter were tested for their ability to measure water activity (a w ) of five foods over the 0.11 – 0.85 a w range. Each instrument was calibrated at 30 ± 0.1°C against ten saturated salt solutions using a modification of the AOAC Method 32.004‐32.009. Regression of the reading versus true a w , as determined by the vapor pressure manometer (VPM), gave high r 2 values for the saturated salt solutions between 0.32 – 0.85 a w . An error of approximately 0.01 a w units was found using the 1 hr data but no significantly better regression line was found using the 24‐hr data as compared to I hr data. Measurement of the a w of five foods, however, gave values differing by an average of 0.051 a w units as compared to the VPM readings. This study demonstrates justification of the FDA cutoff a w value of 0.85 for low‐acid foods as a margin of safety.