Biomedical Engineering Reference
In-Depth Information
could notice the “carbonation” effect of HPCD processing was 230ppm of CO
2
in the
product (Kincal
et al
., 2006). An enhanced cloudiness in citric and carrot juices has been
observed, due to both precipitation of calcium as calcium carbonate and reduced particle
size by pressure, which delays the destabilization of colloidal suspension (Park
et al
., 2002 ;
Boff
et al
., 2003 ; Kincal
et al
., 2006 ; Lim
et al
., 2006 ; Zhou
et al
., 2009 ).
The HPCD continuous system is more suitable for liquid food processing and industrial
scale-up. It is designed to continuously mix and pressurize a stream of product with carbon
dioxide. The product feed tank is connected to a pump and the pump stroke length is adjusted
based on desired pressure, CO
2
:product (w/w) ratio, and fluid flow rate. A reciprocating
intensifier pump is used to pressurize CO
2
. Proper mixing of CO
2
and product is achieved by
using a reactor tank or a membrane reactor. The CO
2
/product mixture is held in a tube for a
particular residence time. Thermocouples are used to measure temperature in the holding
tube. The system also has equipment for depressurization and separation of CO
2
and product
(Kincal
et al
., 2006). Praxair Inc. (USA) has designed HPCD units for processing at 37, 74,
and 148 l/min continuous product flow rates (Damar and Balaban, 2006; García-González
et al
., 2007). Further developments in large-scale high pressure equipment manufacturing at
a lower cost and development of niche products for this application are challenges for the
commercial success of HPCD technology.
13.2.4 Pulsed Electric Fields (PEF)
For many years high intensity electric fields have been applied to induce electroporation, a
phenomenon used to promote bacterial DNA interchange by perforating microbial
membranes (Zhang
et al
., 1995). Recent advances in the design of the treatment chamber
led to the development of a new processing technology known as pulsed electric fields
(PEF) (Vega-Mercado
et al
., 1997 , Yin
et al
., 1997) and radio frequency electric fields
(RFEF) (Geveke and Brunkhorst, 2003). The principle of PEF technology is the treatment
of a biological material or food placed between two electrodes installed 0.1-1.0 cm apart in
a treatment chamber separated by an insulator with short pulses (1-10 μs) that are generated
by a high voltage (5-20 kV) pulse generator. A power source is used to charge a capacitor
bank and a switch is used to discharge energy to the treatment chambers (Zhang
et al
.,
1995). Two principle types of chambers are used in the PEF technology: continuous
chambers for pasteurization of liquid food and static chambers more suitable for studies in
solid food (extraction of bioactive compounds) (Barbosa-Cánovas
et al
., 1998 ). New designs
for uniform electric field distribution and geometry, such as the co-field continuous chamber,
applying the electric field in the same direction as the fluid flow (Yin
et al
., 1997 ), have
made it possible to apply a uniform electric field in large-scale equipment. New manufacturing
materials such as graphite are much more resistant to electrolysis and metal migration
(Toepfl
et al
., 2007 ; Huang and Wang, 2009 ).
The main processing parameter is the electric field (kV/cm), which is defined by the
relationship between the electrical potential difference applied to two electrodes and the
distance between them. The main pulse waveform used in a PEF treatment is the square
wave, which can be applied in monopolar or bipolar mode (Ho
et al
., 1995 ). Treatment time
(μs) is defined by the pulse width (μs), the pulse frequency (Hz), flow rate and chamber
dimensions. An oscilloscope is used to monitor voltage, current and pulse width during the
treatment. PEF treatment produces a temperature rise due to ohmic heating and a cooling
system is used to minimize thermal effects. Increase in temperature generally produces a
synergetic effect on microbial and enzyme inactivation (Wouters
et al
., 2001a ; Toepfl
et al
.,
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