Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Nutrition, Dietetics, and Food Sciences

Department name when degree awarded

Nutrition and Food Sciences


Gary H. Richardson


The Wescor Dew Point Microvoltmeter, Model HR 33T, together with a Wescor C-52 sample chamber, was used to measure water activities between aw 1.0 and aw 0.60. Because of temperature gradient problems, the sample chamber was mounted in a styrofoam insulation box constructed so that the chamber could be opened and the sample replaced while it remained inside the closed insulation box. The standard psychrometric procedure was used for samples with a w between 1 and 0.94. A modified procedure was used for samples dryer than a 0.94. The sample chamber was first loaded with water, and water was condensed on the thermocouple. The water was then replaced by sliding the sample into the chamber, and the psychrometric cooling of the thermocouple measured. The procedure was calibrated with a series of salt solutions and saturated salt slurries of known aw, and was used to measure aw of a variety of foods. Four subsample determinations were made on from 8 to 23 separate days of the foods or standards.

Typical water activity mean and standard deviation values were : milk chocolate bar .60 ~ .035; corn syrup sol ids .60 _I_ .019; nonfat dry milk .7 5 + .017; cashews . 75 ± .021; sunflower seeds .75 ± .022; orange juice concentrate .80 ± .026; whey concentrate . 83 ± .030; chocolate fudge sauce . 85 ± .028; Cheddar cheese .95 ± .026; salami .96 ± .019; Swiss cheese .9 6 ± .019; processed cheese spread .96 ± .023; Gouda cheese .99 ± .041; and soy sauce .98 ± .019. The coefficient of variations ranged from 1.9 to 5.8%. Accuracy of the instrument was good in that the a values varied only s = ± .03 for the literature values for various foods. No fouling of the detector occurred when powders were evaluated. Analysis time was 5 min for samples over aw 0.94 and 10 min for low aw samples. The Wescor instrument appeared to be rapid, economical, and to have good repeatability.



Included in

Nutrition Commons