Lee et al. (2000) have observed similar phenomena, and correlated
the DG to the viscoelastic properties. At low DG the viscoelasticity
of the dough is too low, leading to less expansion, while at
high DG the viscoelasticity is too high, which prevents partial collapse
of the bubbles at the outer surface – preventing water vapour
to escape. These pellets show large bubbles with thick cell walls –
which is due to coalescence. At intermediate DG many small bubbles
are formed, where the cell walls have ruptured – giving rise to
total connectivity between the cells – which allows the water vapour
to escape.