Agriculture Reference
In-Depth Information
the inner zones was negligible. Fruits that had been frozen
by HPSF method maintained an acceptable microstructure
supported by SEM micrographs. The cells are arranged ad-
jacently without clear breakage, demonstrating that this is
the best freezing method for preserving fruit microstruc-
ture.
20
10
Compression
0
LIQUID WATER
Cooling
-10
Freezing
PULSED ELECTRIC FIELD PROCESSING
The potential to commercialize the nonthermal PEF tech-
nology as a new method to preserve food products has
caught the attention of the food industry that wishes to
fulfill consumers' demands for fresh products (Wouters
et al., 2001). The pulse electric field (PEF) treatment, or
high-intensity pulsed electric field (HIPEF), is one of the
best suitable techniques for fruit processing, and the tech-
nique has emerged as a promising alternative to conven-
tional pasteurization (Sizer and Balasubramaniam, 1999;
Toepfl et al., 2007). It is expected that application of PEF
treatment would be less detrimental than heat treatment for
plant tissue ingredients like pigments, vitamins, and fla-
voring agents. This process has been studied as a nonther-
mal treatment for food pasteurization (Eshtiaghi and Knorr,
2002). However, the PEF technology is mostly suitable for
liquid foods to increase their shelf life while maintaining
the sensory attributes.
During the past few years, a significant effort has been
made to use this technology on a commercial scale for
pasteurization of food (Huang and Wang, 2009). The PEF
treatment of liquid foods is based on the application of
high-intensity electric field (typically 20-80 kV/cm) to the
food product as it flows between two electrodes. Gener-
ally, PEF treatment systems consist of (1) a pulse gener-
ator, (2) treatment chambers, (3) a fluid-handing system,
and (4) monitoring systems (Rivas et al., 2006). The PEF
treatment chamber is used to house two electrodes and
deliver a high voltage to the food material. A schematic
of PEF treatment of fluid food is illustrated in Fig. 5.5.
The design of the treatment chamber is one of the im-
portant factors in the development of the PEF treatment
for nonthermal pasteurization, as it should impart uniform
electric field to foods with a minimum increase in tem-
perature, and the electrodes should be designed to mini-
mize the effect of electrolysis (Toepfl et al., 2006). The
PEF may be applied in the form of exponentially decaying,
square wave, bipolar, or oscillatory pulses and at ambient,
subambient, or slightly above ambient temperature. Dura-
tion of pulses is in seconds. The key variables involved in
PEF are electric field strength (
E
), pulse duration or pulse
width (
τ
), treatment time (
t
) pulse repetition rate (
f
), wave-
form of the pulse, and treatment temperature. Huang and
ICE I
-20
Pressure release
-30
0
50
100
150
200
250
Pressure (MPa)
Figure 5.3.
High-pressure shift freezing process
(broken line, process path; solid line, pure water) P-T
phase diagram.
2 sec, and the freezing process is completed at atmospheric
pressure, lowering the refrigeration temperature down to
-25
◦
C. Since the product has already been supercooled to
almost -21
◦
C, the sudden change in pressure causes an
instantaneous conversion of the supercooled sensible heat
to latent heat, resulting in instantaneous ice nucleation.
Approximately 20% of the free water gets instantaneously
converted to ice nuclei.
Otero et al. (2000) studied modifications on the mi-
crostructure of peach and mango produced by HPSF. The
high level of supercooling leads to uniform and rapid ice
nucleation. The HPSF produced no appreciable damage at
the surface of the sample for the surface zone of peach
frozen by high pressure (Fig. 5.4), and also, damage in
Figure 5.4.
SEM micrograph of peach frozen by
HP-shift freezing (source: Otero et al., 2000).
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