Biomedical Engineering Reference
In-Depth Information
2007). High temperatures also increase the electrical conductivity of the product, lowering
the maximum electric field strength viable in the treatment chamber before undergoing a
“dielectric breakdown” of the system, evidenced by an electric spark. In addition, air bubbles
or suspended particles may also produce “dielectric breakdown” (Wouters et al ., 2001a ;
Góngora-Nieto et al ., 2002 , 2003 ).
Most of the studies on PEF microbial inactivation agree that high voltage treatment
produces a series of structural and functional changes in the cellular membrane that lead to
microorganism death (Mañas and Pagán, 2005). The well-known theory exposed by
Zimmermann (1986) states that when an external electric field is applied to a system it
induces a potential difference across the cellular membrane. The maximum potential
difference that the membrane can withstand is known as the “critical transmembrane
potential” or “critical electric field” (E c ). When the external electric field exceeds E c ,
membrane breakdown occurs leading to the formation of pores. The magnitude of E c
principally depends on the membrane characteristics of the microorganism (Gram-positive
and Gram-negative), size and shape (smaller cells and rod-shaped cells require higher E c
values) (Qin et al ., 1998 ; Heinz et al ., 2002 ; Toepl et al ., 2007). In general, “reversible”
pores are formed under conditions where the electric field is lower than E c , and the membrane
returns to its initial form after the treatment. These pores may cause certain sublethal
damage, leading to cellular death in stress conditions such as low pH and refrigeration
temperature. On the contrary, if the electric field is much higher than E c , “irreversible” pores
are produced, leading to the death of the microorganism. Besides structural changes in the
membrane, other changes have been observed inside the cell, such as leakage of intracellular
material and alterations in cell proteins (Wouters et al ., 2001b ; Aronsson et al ., 2005 ; García
et al ., 2007). A high electric field and short time treatment (HEST) (30-40 kV/cm for
20-500 μs) combined with mild temperature (40-55 °C) has been proposed as optimal to
achieve the highest degree of inactivation of some pathogen microorganisms, such as E. coli
O157:H7, Listeria monocytogenes , Listeria innocua , S. aureus and different Salmonella
strains (Mosqueda-Melgar et al ., 2008). Each microorganism may behave differently under
PEF treatment. Gurtler and co-workers (2010) conducted a comparison study with 23 strains
of pathogenic and spoilage microorganisms and identified E. coli 35218 as a surrogate for
E. coli O157:H7.
Several studies on inactivation of enzymes during PEF processing have reported
conformational changes and alteration of protein helix alignment due to the movement of
charges produced by the electric field, making difficult the active site-substrate interaction
(Bendicho et al ., 2002 ; Zhang et al ., 1995). PEF processing showed some effects on
bioactive compounds, such as carotenoids, and water-soluble vitamins, such as ascorbic
acid in fruit juices, thiamin, riboflavin, biotin, folic acid and pantothenic acid in milk, other
lipid-soluble vitamins (retinol, cholecalciferol and
-tocopherol) and the fatty acids in milk
(Bendicho et al ., 2002 ; Rivas et al ., 2007 ; Zulueta et al ., 2007 ; Riener et al ., 2008 ; Soliva-
Fortuny et al ., 2009). On the other hand, sensory properties such as flavor, aroma and color
are not significantly affected by electric field treatment in fruit juices (Jia et al ., 1999 ; Min
and Zhang, 2003 ; Min et al ., 2003a , 2003b ). Applications of this technology are mainly
focused on preservation of liquid food. PEF treatment has also been applied to improve the
extraction of different bioactive components from diverse foodstuff by increasing the
permeability of plant cells (Ade-Omawaye et al ., 2001), thereby improving the juice yield
and quality (Guderjan et al ., 2007 ).
Several pilot plant-scale PEF systems have been developed and a great number of
bench-scale equipment is spread around the world in different research groups. The use of
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