Commercial vapor pressure thermocouple psychrometers (hygrometers) are now generally accepted for measuring water activity, aw (water potential) in plants and soils, and commercial instruments are available. We have adapted them for aw measurements in the 0.99 to 0.60 range using a two-step procedure. Water is first condensed on the thermocouple; then the sample is inserted in the thermocouple chamber and the psychrometric cooling of the wet thermocouple measured. The procedure is calibrated with a series of saturated salt slurries of known aw values. Typical aw values (with standard deviations) for a variety of foods were: Cheddar cheese, 0.95 ± 0.03; Parmesan cheese, 0.76 ± 0.03; milk powder, 0.75 ± 0.02; milk chocolate, 0.60 ± 0.04; luncheon meat, 0.96 ± 0.03; bread, 0.95 ± 0.03; dried raisins, 0.82 ± 0.02; corn syrup, 0.60 ± 0.02; and orange juice concentrate, 0.80 ± 0.03. The coefficients of variation ranged from 1.9 to 5.8%. When compared with published values obtained by other methods, these figures were within the standard errors of measurement. The thermocouple detector did not foul since it had only vapor contact with the sample. An economical sample chamber and instrument is described. Analysis time is 4 to 8 min. The procedure is accurate, convenient and rapid.
1Journal Paper No. 2628 of the Utah Agricultural Experiment Station. Approved by the director.
2Current address: Ready Pac Produce, 950 North Fair Oaks, Pasadena, CA 91103.
3Mention of products or companies does not constitute endorsement by Utah State University over products not mentioned.
4Wescor, Inc., 459 South Main Street, Logan, Utah 84321.