Use of polyols to measure equilibrium relative humidity

Polyols were used to estimate the equilibrium relative humidity (ERH) of foods. Glycerol, ethylene glycol and propylene glycol were found to be suitable for estimating ERH, up to an ERH of 93%, by measuring the refractive index after they had equilibrated in the headspace of a food. The ERH determined by the polyol method agreed with the ERH determined using Sina and Vaisala meters to within 1% ERH for a lithium sulphate slurry, and for foods were usually within 2%. The ERH of prunes determined using the Vaisala meter was about 6% lower than the value obtained by the other methods. Unlike most electronic techniques the polyol method was unaffected by the presence of glycerol, propylene glycol or butylene glycol and so is suitable for measuring the ERH of foods containing these humectants. The polyol method can be used in developing countries where access to sophisticated equipment is limited.