Increasing the field strength from 28.0 to 33.6 kV/cm had an
insignificant effect on inactivation (P > 0.05) of the E. coli in BPW
at 55? C. This phenomenon has occasionally been observed before
in pulsed electric field processing (PEF) and in radio frequency
electric field (RFEF) processing. In a study investigating RFEF pro-
cessing of apple juice, increasing the field strength from 16 to
26 kV/cm had no effect on the inactivation of E. coli (Geveke and
Brunkhorst, 2004). Likewise, increasing the field strength above
30 kV/cm had no effect on the inactivation of Listeria innocua dur-
ing PEF processing (Wouters et al., 1999). The results in the present
study are favorable because processing at lower field strength
reduces the PEF energy input and cooling water requirement of
the heat exchangers located after each set of treatment chambers
(see Fig. 1), thereby reducing the operating cost. In addition, lower
field strength minimizes the potential for arcing across the treat-
ment chamber electrodes.
In an effort to increase inactivation of the E. coli in BPW, the out-
let temperature of each of the three PEF treatment chambers were
increased from 55.0 to 57.5 ?C. This significantly increased the
inactivation (P < 0.05). As shown in Table 1, at a field strength of
30.0 kV/cm, inactivation went from 2.80 ± 0.42 log cfu/ml at
55.0 ?C to 6.55 ± 0.92 log cfu/ml at 57.5 ?C. This effect of increasing
inactivation with increasing temperature has been nearly univer-
sally observed in PEF studies. Similar to the case at 55.0 ?C, there
was no significant effect (P > 0.05) on inactivation by increasing
the electric field strength from 30.0 to 33.6 kV/cm.
One of the main goals of this research was to determine the
lowest temperature that resulted in a 5 log inactivation of E. coli
in strawberry purée. The preliminary results of the BPW experi-
ments assisted in setting the initial conditions for the strawberry
purée experiments. However, when the outlet temperature was