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ABSTRACT Seven methods of water activity (a w ) measurement were tested in a collaborative study between three laboratories. These methods included the direct measurement of vapor pressure, an isopiestic technique and several electronic devices. Both saturated salt solutions and foods were measured. When compared to standards, the vapor pressure manometric (VPM) technique gave the best results. However, the absolute values of the standards are questionable. The results of the comparison of the values for foods showed a range of ##0.02 a W units. Thus, one can question the validity of literature values reported to three decimal places or the absolute values for limits on microbial growth. This research suggests that some standards must be set for all research groups and shows that overall, the VPM is probably the best and most accurate method
LABUZA et al. (1976) studied this question.