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

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Pulsed electrified field or PEF processing is a new technology used in the food industry for heat pasteurisation and sterilisation of liquid products by exposing them to a high electrical field (between 20 and 80 KV/cm) for a very short period to inactivate microorganisms whilst leaving the overall quality unaffected

Food industry has always used heat processes for the stabilisation and preservation of food products. Traditional food processing technologies are mainly based on heat treatments for the inactivation of enzymes, micro-organisms and pathogens, for extending the shelf life of food products whilst leaving the overall quality and safety unaffected in accordance to food regulations. PEF or Pulsed Electric Field technology offers a new method for the pasteurisation and sterilisation of liquid products exposed to a high electric field (between 20 and 80 KV/cm) for a short period, inactivating micro-organisms whilst leaving the overall quality of the treated product unaffected. The most common methods for reducing the microbial charge are pasteurisation and sterilisation. The main scope of pasteurisation consists in reducing pathogens; next to pathogens, it inactivates even many Gram negatives and vegetative forms of Gram positives, whereas this treatment is not effective towards bacterial spores and often the sexual spores of Eumycetes. Sterilisation, on the other hand, is a physical treatment at temperatures exceeding 100°C in order to kill all kinds of micro-organisms, spores included. These heat treatments often have negative effects on nutritional content and functional features, as they may affect the sensory features of food ingredients. The use of heat generally causes a thermal damage like chemically induced changes of the nutritional features of the treated product, loss of vitamins and of thermolabile compounds, the release of undesired substances such as Malliard compounds or carcinogenic substances like acrylamide. Consumers are more attentive to their own health and well being, correct life style and information on food nutritional values. Nonetheless, consumers are not ready to give up naturalness and taste while protecting their health. The research for new processing technologies is triggered by the growing need for high-quality food products and guaranteed safety parameters. These products must combine different properties that are often impossible to combine from the technological point of view, such as safety, freshness, sensorial features, convenience, nutritional features, next to a shelf-life that complies with distribution needs in order to reach the end-consumer. This is the scope of the European Project HighQ-RTE – www.highqrte.eu (Innovative non thermal processing technologies to improve the quality and safety of ready-to-eat meals) involving 15 partners (industries and research centres) of 8 different countries for the study and implementation of innovative 4 non-thermal processing technologies (photosynthetising, Pulsed Electric Fields, High Homogenization Pressures, High Pressures with CO2) and of the European project Europeo NovelQ  (http://www.novelq.org/IAP/IAP_index.aspx), the overall aim being to improve the nutritional and structural quality and safety of food . Furthermore, in July 2009 the University of Milan (Università di Milano Bicocca), in co-operation with the Italian Institute of Nuclear Physics of Milan Bicocca, received a special mention for the project on Advanced Physical Technologies applied on food safety, aimed to the development and technological transfer of PEF technology, presented by the Provincia di Milano with the job advertisement of the Expo 2015.

PEF Technology
The development of unconventional preservation and processing technologies, non involving heat, such as PEF (Pulsed Electric Field) technology, is one of the most interesting aspects of among the most recent innovations in the food processing industry. Although the first studies on PEF were conducted in 1960 by two research groups (Doevenspeck [1], Hamilton and Sale [2] [3]) who described the first lethal effects on bacterial cells exposed to a high-frequency electric field, only in the second half of the 1990s the effectiveness of PEF was demonstrated for the scope of microbial inactivation and food preservation. Additional steps forward allowed the generation of high-intensity electric pulses that can be applied for continuous processing. Cold pasteurisation of liquid food products and increased outputs of vegetable raw materials are an important promise for the scale-up of industrial applications. PEF is a non-thermal pasteurisation process, as the treated product is not heated. After processing, the product is healthy whilst keeping its original features in terms of taste, colour, antioxidising capacity and texture. When a micro- organism is exposed to a pulsed electric field, the permeability of its cell membrane increases considerably: inside its phospholipidic structure, new pores open up or existing ones expand in the area where proteic receptors are located. These openings can be permanent or temporary, according to treatment conditions. When the number and size of these pores increases, the cell brakes, the cell content spills out or the external liquid enters into the cell causing its death. The process is applied to liquids or fluids that can be pumped into a treatment chamber, where they are exposed to a very short high-voltage pulse (1 μs – 20 μs). The high voltage generated through the liquid product (approx. 35-50 kV/cm) kills all micro-organisms by electroporation, i.e. rupture of the cell membrane. The treatment chambers (see picture 2) are separated one from the other by a ceramic or polymeric insulator. Each chamber features two electrodes that can be conductors or semi-conductors. Different electrodes designs have been proposed over the years, but the most used are still those with parallel plates and coaxial cylinders. Square-shaped pulses have established themselves as the most effective ones, thus suggesting the use of pulse generators based on real transmission lines or with finite elements. Electric components exposed to more than 35,000 V must be kept in oil bath for insulation and cooling. Presently, most researches and applications on PEF make use of electric fields within 25-50 kV/cm, for pulses of 1-5 μ seconds each, and repetition frequency between 1 and 1000 Hz. The number of pulses achieving the highest inactivation level usually does not exceed 500, and the treated flows range from 10 L/h (in test laboratories) to 10000 L/h (in industrial plants). With a multi-chamber system, from 5 to 9 decimal reduction can be achieved, similarly to those obtained with a conventional pasteurisation treatment.

1 COMMENT

  1. Interesting technology and very good information in this report. I work on a food company and we would like to try if something like this could work for our processes. We need to reduce microbial contamination of a liquid stream. Which commercial brands are manufacturing these kind of equipment? Can you send the contacts to me?
    Kind regards,
    Juan

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