Given the different nature of the heating processes that take
place under conventional and microwave modes, the temperature
control should not be limited to the initial and the final point of the
process, but the whole temperature history of the product should
be taken into account. In this context, the PU parameter offers the
possibility of evaluating the complete thermal load of the heating
process at any reference temperature, as if it had taken place under
isothermal conditions. This implies that the product is considered
to reach the reference temperature instantaneously (Matsui et al.,
2008), so the effect of processing factors that could be causing
differences in the nature of the heat transfer, such as (i) product
characteristics including consistency, solid/liquid ratio and thermophysical
properties, (ii) sample quantity, and (iii) container type,
size, and shape (Awuah et al., 2007), is avoided