The effect of temperature and time on the degradation
of ascorbic acid is presented in Fig. 12, for conventional
and ohmic heating.
Thermal degradation of ascorbic acid follows first
order degradation kinetics for both conventional and
ohmic heating, in the studied temperature range (60 to
97 8C).
Rate constants (k) for both conventional and ohmic
processes were plotted against the reciprocal of absolute
temperature to determine frequency factor (k0) and
activation energy (Ea). The kinetic parameters listed in
Table 2 were obtained by fitting the data to the Arrhenius
equation (Eq. (4)). The obtained kinetic parameters
were identical for the two types of heating processes
leading to the conclusion that the presence of an electric
field does not affect the ascorbic acid degradation. Lima
(1996) took similar conclusions for orange juice systems.
The measured kinetic parameters are within the
range of the values found in published literature for
other food systems under conventional heating conditions
(see Table 1). The only reference found where
kinetic parameters under ohmic heating conditions were
determined (in orange juice) (Lima, 1996) presented
slightly higher values than the ones obtained in this
work.