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Notes: The objective was to develop a new method for water activity (aw) determination. A reference material (circle of filter paper) of known sorption isotherm was equilibrated to the sample. Sorption isotherm data for this paper were obtained by equilibrating for 24 hr to each of six salt slushes; these data were described well by a linear equation relating moisture content to log (1 − aw). This equation served as the calibration curve. Results showed that this new method (1) does not require a vacuum, (2) gives a linear calibration equation, (3) is applicable over the aw range from 0.40–0.98, and (4) with minor replication can bring the standard deviation of the mean to less than ± 0.01.

Notes: Sorption isotherm data are obtained by instrumental determination of water activity at a known moisture content or determination of moisture content after equilibration against a saturated salt solution. The latter method is simpler and the salt solutions are primary standards. However, the equilibration takes a long time, one to ten weeks, depending upon food composition. Thus, the objective of this work was to devise a method that retains the saturated salt solution but accelerates the rate of equilibration. In an equilibration environment between a food product and a saturated salt slurry, the driving force is the difference in vapor pressure; therefore the faster the vapor space reaches equilibrium with the saturated salt slurry, the quicker the maximum driving force for water absorption will be applied to the sample. It was felt that a reduction in the size of the usual large desiccater to a single sample size would provide the necessary area to volume ratio. The vessel chosen was a small plastic chamber (65 mm/53 mm). In this chamber, the surface area to vapor volume ratio was 0.3101 as compared to 0.0335 for the standard desiccator. The sample was contained in a standard aluminum weighing tray modified by removing a 44 mm diameter circular section from the bottom. This was replaced with a 47 mm diameter circle of Whatman No. 1 quantitative filter paper to support the sample and at the same time allow transmission of moisture. This would allow water molecules to travel in a straight line and thus the shortest distance between the saturated salt slurry and the sample. This small vessel with a single sample supported on a filter paper will be referred to as a Proximity Equilibration Cell (PEC). Using this technique, it was found that a 2/mm deep sample of corn starch required only 6 days for complete equilibration, as compared to 21 days for the conventional desiccator. Thus, the PEC satisfied the objective by reducing time by 70%. In making comparisons with the same salts in conventional and the PEC, it was noted that the end point was higher in the latter. Evidently, absorption in case of the conventional desiccator was so slow at the end that no weight gain could be detected in 24 hr but equilibrium had not yet been attained. In contrast, the PEC equilibrated so rapidly that it allowed a closer evaluation of true equilibrium.