Water Activity of Flour at High Moisture Contents as Measured with a Pressure Membrane Cell


  • Contribution from the Department of Food Science, University of Illinois, Urbana, Illinois. Based on a Ph.D. thesis by G. Gur-Arieh, October, 1963.

  • The authors are indebted to the Pillsbury Company, Minneapolis, Minnesota, for sponsoring this research, furnishing the materials, and making particle-size determinations.


A method was developed for the determination of water activities of flour at high moisture contents. In this method a flour sample was allowed to equilibrate with water contained in a porous membrane while under a certain constant mechanical pressure. The cell was fitted with a porous membrane which allowed the transfer. of moisture from and to the sample. After equilibrium was achieved the sample was analyzed for moisture content. A thermodynamic relationship enabled calculation of the water activity corresponding to the pressure used in the cell. Results obtained with this method were in agreement with those obtained with the constant-relative-humidity desiccator technique in the over-lapping range of water activities. The two methods were also in agreement that the water activity of flour was not affected by the particle size distribution.