Pulsed Electric Fields, an alternative method to heat-treatment technologies

Parameters influencing the PEF process
Relevant parameters that influence the treatment can be divided in three groups: process, product and micro-biological parameters:

• Intensity of the electric field: the electric field is generated by devices similar to those used in radars; the most frequently used device generates a short square wave and inverts its poles ± also in order to prevent electrodes erosion. However, the cost of bi-polar generators is twice that of mono-polar ones. Among the possible wave shapes, there are sinus-shaped ones that are easier to produce as they use devices similar to common radios, but they reach a power peak only for a short instant and supply less energy per cycle than a square wave.
• The geometry of electrodes, for both static and continuous operation.
• The shape, duration and frequency of pulse repetitions.
• The treatment temperature: the treatment is usually performed at 35-50°C because it has been ascertained that micro-organisms are more resistant to low temperatures. For E.Coli, there is a decimal reduction increase from 1 to 6.5 Log when the temperature raises from 32 to 55°C (4).
• The pH and electric conductivity of the treated liquid product: the lower the conductivity the lower is the required power(see Tab.1).
• The exposure period of time: the electric field is produced at a frequency of 1.000 cycles/sec, and the fluid is exposed to multiple pulses while it flows through different treatment chambers.
• The pressure: the pressure is applied to inhibit the generation of air bubbles, in which electric arcs may occur for fields exceeding 20K V/cm.
• The involved micro-organism strain.
• The synergic effect in combination with moderate temperatures.
• The presence of chemical additives or natural antibiotics: several studies have pointed out a higher effectiveness of the treatment in combination with preservatives and heat.

Application fields
PEF technology has been successfully applied to the treatment of fruit juices, milk, yoghurt, soups and eggs. More precisely, the first fruit juices pasteurised with PEF technology were introduced on the market in 2005, in the northeastern part of the United States, by the Genesis Juice Corp. The major advantage of this application is the preservation of the aroma that is usually lost with the heat treatment. The shelf-life of these juices is of 4 weeks at storage temperatures of 4°C. The limit is given by the presence of air bubbles in the product and by a low electric conductivity. This method has proven to be very effective towards a variety of microorganisms including Escherichia coli, Salmonella and Listeria monocytogenes, for which the maximum inactivation level has reached between 5 and 9 log reductions. In the European HighQ RTE project, by associating the PEF treatment with a short pre-heating process (48°C x 10 minutes) a log 6 reduction of Salmonella in eggs has been achieved, and the product has acquired a “potential shelf-life” of 60days (with storage of the treated product at 4°C). PEF treatment can be used also to improve the effective excretion of compounds from tuberous plants and, for its capacity of braking cell membranes. PEF is also used for reducing the volume of sludge in waste water treatment plants. Critical factors needing more accurate analysis The major technical-scientific and technological obstacles that must be overcome can be summarised as follows:

• the inactivation method by means of electric fields is not clear yet. The effect of involved variables and their contribution to the reached inactivation level must be standardized and adequately quantified.
• Models should be developed and certified allowing a reliable connection between PEF dose applied to food products and inactivation of bacteria, even considering the various organisms that might be normally present in food.
• The synergy of PEF treatment with temperature, as for instance a pre-heating treatment at low temperatures (<50°) and the use of natural antimicrobial agents, requires extensive researches, as well as the effects of PEF on spores and enzymatic activity. New electrode design, suiting the needs of continuous flow treatments, needs more accurate studies.

These critical factors described above and requiring more accurate studies are the major obstacle to a wide-range diffusion of the PEF processing technology.

by Donato Smaldone

Literature
1. H. Doevenspeck,  Influencing cells and cell walls by electrostatic impulses. Fleischwirtschaft, 1961
2. J. H. Sale and W. A. Hamilton, Effects of high electric fields on microorganisms: I. Killing of bacteria and yeasts. Biochim Biophys Acta, 1967
3. W. A. Hamilton and A. J. H. Sale, Effects of high electric fields on microorganisms. II. Mechanisms of action of the lethal effect. Biochim
4. H. Vega-Mercado, O. Martin-Belloso, F. J. Chang, G. V. Barbosa – Canovas, and B. G. Swanson, Inactivation of E.coli and Bacillus subtilis suspended in pea soup using pulsed electric Fields. Journal of Food Processing and Preservation, 1996.
5. Biophys Acta, 1967. Diversified Technologies – www.divtecs.com
6. http://www.foodscience.afi sc.csiro.au/pef-technology.htm
7. Raghupathy Ramaswamy, Tony Jin, V. M. (Bala) Balasubramaniam, Howard Zhang. Pulsed Electric Field Processing, Fact Sheet for Food Processors, Food Science and Technology
8. M. Kempkes, M. Gaudreau, T. Hawkey, J. Petry. Scale up of PEF systems for food and waste streams, Diversified Technologies
9. Peter Clark.,Pulsed Electric Field Processing, Food Technology, www.ift.org
*Pictures courtesy of Diversified Technologies, Inc. Phone 781/275-9444-211; www.divtecs.com