Apparent density of compressible food powders under storage conditions

The stress-dependence of the apparent density and the wall friction angle of various food powders like salt, sugar, flour, starch and protein powders were studied using a ring shear tester. The approximation of the stress-dependence of the apparent density using a power function showed a very good correlation (r² > 0.97). The two parameters of the approximation function are the apparent density at 1.0 kPa and the exponent, which characterizes the compressibility of the powder. The first correlates very well with the apparent density measured according to ISO 697 or EN ISO 60 (r² = 0.98), while the second shows some correlation (r² = 0.76) with the particle size. Combining the powder properties apparent density, the mass median diameter and the spread of the particle size distribution in a power function allows a reasonably good estimate of the exponent (r² = 0.93). The wall friction angle usually decreases at higher values of the wall normal stress. This dependence can often be approximated using a simple function. However, for powders with a low stress-dependence of the wall friction angle it is better to use a constant average value. Thus, the apparent density of compressible food powders under storage conditions can be described well.