Biomedical Engineering Reference
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equipment has been able to reach pressure of up to 400 MPa, the so-called dynamic high
pressure (DHP) or ultra high pressure homogenization (UHPH) (López-Pedemonte et al .,
2006). Unlike HPP technology, UHPH is a continuous process with treatment time lasting
several seconds, which makes it more suitable for liquid food processing.
The microbial inactivation exerted by UHPH is believed to be due to the mechanical
destruction of cell integrity caused by different mechanisms, including spatial pressure and
velocity gradients, turbulence, impingement and cavitation, which occur in liquids during
high pressure homogenization (Donsì et al ., 2009). Different studies have shown signifi-
cant microbial inactivation of pathogenic and spoilage organisms ( E. coli O157:H7,
L. monocytogenes , S. aureus , Penicillium ssp, S. cerevisiae , L. plantarum and L. mesenter-
oides , among others) by applying pressure up to 400 MPa in a single cycle (Briñez et al .,
2006a , 2006b , 2007 ; Campos and Cristianini, 2007 ) or in consecutive passes (Tahiri et al .,
2006 ; Bevilacqua et al ., 2009 ; Capra et al ., 2009 ; Donsì et al ., 2009 ). Fluid viscosity has
been shown to be a major environmental parameter affecting bacterial inactivation by
UHPH, as opposed to water activity and product composition, whereas high cellular con-
centration seems to exert a protective effect on microbial cells, which are less exposed to
the effects of pressure (Diels et al ., 2005 ; Tahiri et al ., 2006 ). In addition, several authors
observed the lack of sublethal injury after UHPH (Wuytack et al ., 2002 , 2003 ; Diels et al .,
2005a ; Briñez et al ., 2006a , 2006b , 2007 ; Roig-Sagués et al ., 2009 ). As a recent technol-
ogy, little scientific information on the effects of dynamic pressure on quality parameters
and bioactive compounds is found in the literature. The potential of UHPH to inactivate
food quality-deteriorative enzymes is still unknown. Pectinmethylesterase (PME) seems to
be highly resistant to dynamic pressure (25 and 80% inactivation after a single pass at
150 MPa and five passes at 250 MPa, respectively); despite the low inactivation achieved,
the homogenization process seems to decrease the size of juice particles in suspension and
avoid destabilization and precipitation (Welti-Chanes et al ., 2009 ; Carreño et al ., 2010 ).
Regarding sensory properties, several studies have shown the potential of UHPH to pro-
duce milk and soy derived products such as yoghurt, cheese and ice cream with enhanced
sensory and rheological properties (Vannini et al ., 2008; Cruz et al ., 2009; Innocente et al .,
2009 ; Serra et al ., 2009 ).
The UHPH system consists of two positive displacement high pressure pumps in
series with two-stage homogenization valves. The first stage valve, which is made of
ceramic, is used for the homogenization process and usually set at a pressure between 50
and 350 MPa. The second stage pneumatic valve, which is located after the first one, is
used for micronization and usually maintained at 50 MPa. A manometer connected to the
circuit displays the pressure of the system (Kumar et al ., 2009 ). A flow-through
micrometric gap between the valve and the seat causes the acceleration of the fluid in
concurrence with the pressure drop, generating significant mechanical stress, which then
causes a significant temperature increase depending on the fluid viscosity (estimated as
1.5-2.5 °C per 10 MPa) (Popper and Knorr, 1990). A tubular heat exchanger (set at 4 °C)
connected immediately after the homogenization valve is used to minimize the thermal
effects. Several companies in the European Union (Stansted Fluid Power Inc. and GEA
Niro Soavi) commercialize UHPH bench- and pilot plant-scale equipment (up to 400 MPa
and production rates around 200 l/h). Promising results have been achieved using UHPH
pilot plant-scale equipment. However, further research on the design of large-scale
equipment and scientific knowledge on the effect of dynamic pressure on quality
parameters and nutritional content of food is needed for the scale-up of this technology
to industrial production.
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