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 r² 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.