Water activity of culture media used in food microbiology

Summary An improved gravimetric method was used to simultaneously determine (in a single assay) the water activity (aw) of 30 solid and liquid culture media covering the entire useful range of aw in food microbiology, including media recommended for osmophilic, halophilic and xerophilic organisms. The water activities of the media ranged between 0.999 and 0.739. As could be expected, the water activities of the solid media were not affected significantly by the presence of agar. No significant differences were observed either between the gravimetric (20°C) and the cryoscopic measurements (below 0°C) of liquid media of very high aw.