At the beginning of baking, the product is a medium with essentially
closed porosity (gas bubbles are not interconnected), and the
gas mass transfer is off. Mass transfer occurs in the liquid phase. As
the temperature increases and carbon dioxide released, the pore
volume becomes significant and interconnections appear. Transfer
in the gas phase becomes more consistent. To simulate pore opening,
a Heaviside function (a) centred at the pore opening temperature
(Topen) and with a range (DT) (Fig. 4) is used (Vanin, 2010).