Besides applying different levels of pulse frequency, the number
of pulses exerted on the parallel-positioned carrots (pre-conditioned
at 40 °C) during PEF treatment (fixed electric field strength of
0.6 kV/cm) had influenced the cutting force profile (Fig. 4c). Similar
phenomenon was observed in which the plant cells at epidermis and
cortex regions showed better tissue flexibility towards cutting action
as well as improved cutting accuracy when compared to the reference
(untreated) carrots. PEF-treated carrots (pulse frequency 20 Hz, 500
pulses and specific energy input of 3 kJ/kg) had significantly impacted
the tissue softening of the epidermis in achieving 38% cutting force
reduction. However, the xylem region showed resistance towards the
cutting action led to an emergence of various measured force peaks
with maximal cutting force required was approximately 62 N. In addition,
the zone of appearing force peaks after penetrating through the
xylem was extended for approximately 6 mm as compared to the previous
investigations (Fig. 4a and b). This occurrence could be attributed
to the fact that plant cells at endodermis region were resistant to PEFinduced
cell disintegration and thus build up resistance against the
cutting knife. For the other PEF conditions, increasing the pulse number
to 3000 with the same pulse frequency (20 Hz) and specific energy input
of 113 kJ/kg had improved the tissue softening of the epidermis region;
thereby further reducing the cutting force by 14.5%, i.e. approximately
43 N. The zone of the emergence of maximal force peaks when cutting
from inside (xylem) towards the outside (epidermis) was reduced by
3 mm instead. Therefore, it could be speculated that tissue softening
due to an increased pulse number had enhanced the tissue softening
at endodermis, cortex and epidermis region. Likewise, the application
of fixed pulse frequency at 80 Hz on carrots showed similar characteristics
of cutting force profile to that of carrots subjected to pulse frequency
at 20 Hz while increasing the pulse number from 500 to 3000 (Fig. 4c).
In other words, pulse frequency had significant impact on triggering tissue
softening. For carrot pieces that were treated at 80 Hz for 3000
pulses, cutting through the xylem region had resulted in a higher cutting
force of approximately 43 N. The softening effect appeared to improve
the degree of an approximately 4 mm shorter all the way through the
zone of maximal force peaks as compared to when subjected to 500
pulses with the same level of pulse frequency. Therefore, the zone of
maximal force peaks around 11 mm from the epidermis was measured.
In fact, higher level of pulse frequency applied to carrots led to a smoother
cutting surface on epidermis, cortex, endodermis and xylem. In the
context of increasing pulse frequencies and increasing pulse numbers
from 500 to 3000 pulses and specific energy input from 3 to 113 kJ/kg
resulted in significant softer epidermis solely for 20 Hz but in the case
of 80 Hz there was an improvement in the softening of endodermis
and xylem.