The bulk density of
polystyrene foam did not change because its moisture
content was not affected by relative humidity.
For starch-based foams, this might be reasonably
explained by changes in cell wall thickness and
viscosity caused by extrusion temperature and the
changes in the volume and weight of cells after
absorbing sufficient water. Fletcher et al. (1985)
reported that the bulk density correlated with the
shear viscosity of the dough for a number of extrusion shear rates and temperatures.