3.1. Effect of carrot matrix and piece position on endogenous enzyme
activity
The initial temperature of carrot samples (carrot mash and carrot
pieces) and treatment medium (phosphate buffer) was standardised
by pre-conditioning at 10 °C to attain an isothermal condition prior to
PEF treatment. In this study, PEF treatments were conducted at temperatures
below 25 °C to ensure the final temperature of carrots and medium
after treated with PEF at varying pulse frequencies did not exceed
25 °C. The results have shown that both the semi-solid (carrot mash)
and solid (carrot pieces) food matrices resulted in different effects on
AAO and POD inactivation. It was found that carrot mash (semi-solid
matrix) had higher residual AAO and POD activity (20 to 30% increment)
as compared to intact carrot pieces (solid matrix) when treated
with an electric field strength of 0.8 kV/cm and specific energy input
of less than 50 kJ/kg (Fig. 2). Previous work on PEF-treated apple and
orange juice has also shown increase in polyphenoloxidase and
pectinmethylestease activity, respectively (Van Loey, Verachtert, &
Hendrickx, 2002). The activation of enzyme activity is probably due to
cell permeabilisation effect of PEF that release intracellular or “free” enzymes
to the liquid portion (juice and phosphate buffer); thus lead to
increase in residual enzyme activity detected in carrot mash sample.
Consequently, most of the endogenous enzymes in the semi-solid matrix
(carrot mash) system would be easily extracted due to enhancement
of PEF-induced electroporation. This phenomenon was not
observed with carrot pieces. Treating the whole carrot could have limits
or protects the release of enzymes, possibly due to high cellular integrity.
Additionally, the lower conductivity of the solid carrot pieces of approximately
b 1 mS/cm (Palaniappan & Sastry, 1991) has hindered
the charged molecules to conduct the applied pulse electric current
within the rigid cell walls. Thus the energy input applied on carrot
pieces will give less impact on plant cells and enzymes due to higher
resistance. A contradictory phenomenon was observed when PEF treatment
was applied to carrot mash system due to higher conductivity of
approximately 7.8 mS/cm surrounding the suspended plant cells.