Agriculture Reference
In-Depth Information
tivity decreased significantly as
E
and
t
increased. Bipolar
pulses led to a greater enzyme inactivation than monopo-
lar pulses. The highest PME inactivation occurred during
PEF treatments was about 80%, and this inactivation level
was achieved when orange juice samples were processed at
35 kV/cm for 1,500
μ
sec with 4-
μ
sec bipolar pulses at 200
Hz without exceeding 37.5
◦
C. The higher enzyme inacti-
vation achieved when the pulse frequency increased, and
the pulse width was longer for a constant electrical energy
density input.
Experimental values of PEF inactivation of orange
juice PME with processing parameters were fitted to a
first-order fractional conversion model (equation 5.1a) and
a Weibull's distribution function (equation 5.6) to obtain
kinetic models:
Connected to high
voltage pulser
Insulation
Electrode
Fluid in
Fluid out
Electrode
Treatment region
Baffles
Figure 5.5.
Schematic of a continuous pulse electric
field for liquid food processing.
Wang (2009) reviewed various PEF designs for liquid food
pasteurization.
−
RA
∞
RA
0
−
RA
∞
RA
ln
=−
k
E
t
(5.1a)
where RA is the residual enzyme activity, RA
0
is the initial
residual enzyme activity (100%), RA
∞
(%) is the residual
enzyme activity after a prolonged time of treatment (stabi-
lization value), and
k
E
is a first-order rate constant (
μ
sec
−
1
).
The first-order fractional conversion model
(equation 5.1a) fitted well the inactivation of orange
juice PME by PEF. Kinetic rates (
k
1
) were not statisti-
cally influenced either by the pulse polarity or by the
E.
Reaction kinetics parameters for PME inactivation are
presented in Table 5.5:
PEF treatment for fruits
Over the past 5 years, PEF treatment has received a great
deal of attention towards fruit processing. The process re-
tains quality attributes (color, vitamins, and flavor com-
pounds) with mild thermal effect and ensures product safety
by inactivating micro-organisms. Most of the research
works on PEF treatment of fruit products are comparative
type. Post-PEF products are routinely compared with con-
ventional high-temperature short-time (HTST) pasteuriza-
tion to ensure it is as safe as HTST-processed products. PEF
technology has been integrated with the aseptic process-
ing and packaging technologies to process fruit products
through a continuous flow processing line. A combination
of mild heat treatment and PEF are found to be effective.
Application of PEF is restricted to low electrical conductiv-
ity food products that can withstand high electric fields. It
is also important that product does not entrap bubbles. The
particle size of the liquid food is an application limitation
for this technology. Several theories have been proposed to
explain microbial inactivation by PEF—the most studied
are electrical breakdown and electroporation.
Application of PEF to tropical fruit products is elab-
orated
exp
t
α
γ
RA
=
RA
0
.
−
(5.6)
where
is the shape pa-
rameter (dimensionless). The experimental values of RA as
a function of treatment time were adjusted to the Weibull
model (equation 5.6). The computed parameters (scale fac-
tor,
α
is the scale factor (μsec) and
γ
) of the fitted model and the tested
conditions are given in Table 5.5. The Weibull model fits
well the experimental data. Both
α
; shape parameter,
γ
resulted in being
significantly nondependent on
E
and pulse polarity.
Simplified Hulsheger's model (equation 5.7) and Fermi's
model (equation 5.8) were tested to describe the enzymatic
inactivation by PEF related to the electric field strength:
α
and
γ
for
important
quality
attributes
and
microbial
inactivation.
(
E
−
E
c
) (5.7)
where
E
is the electric field strength (kV/cm),
E
c
is the
critical electric field strength obtained by the extrapolated
value of
E
for RA
=−
RA
0
.
b
E
.
RA
Inactivation of enzymes
Inactivation of PME of orange juice processed by PEF at
different processing conditions (electric field strengths [
E
]:
5-35 kV/cm; total treatment time [
t
]: 1,500
μ
sec; pulses
width: 4
μ
sec; frequency [
f
]: 200 Hz; applied mode: mono-
and bipolar) was studied and modeled by Elez-Martınez
et al. (2007). For both mono- and bipolar pulses, PME ac-
=
100%, and
b
E
is the model constant
(cm/kV):
RA
0
1
+
exp
E
−
E
h
k
F
RA
=
(5.8)
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