INTRODUCTION

Preservation of food had always played an important role in civilization of mankind. Nowadays, consumers have a tendency to buy a foods which are fresh, natural, free from chemical preservatives and other harmful additives, minimally heat processed, contain less salt or sugar and at the same time have a reasonable shelf life with assured quality. Thermal processing has always been regarded as one of the most reliable and effective methods for extending shelf life of foods. Among the emerging non-thermal processing technologies, high intensity pulsed electric field is considered as one of the most promising technologies due to its potential to inactivate microorganisms without any drastic change in sensory and nutritional properties of food.

GENERAL BACKGROUND

The first reported investigation on the bacterial effects of direct and alternating current is attributed to Prochownick and Spaeth (1890) and the “Electropure” process using alternating current applied to milk pasteurization between the 1920s and 1950s (Toepfl, Heinz and Knorr, 2007). In this electropure process milk was heated to 70°C by passing through carbon electrodes in an electric chamber to inactivate Mycobacterium tuberculosis and Escherichia coli thereby extending the shelf life of milk. Here the applied electric field was very small (220 Volt AC) and non-pulsed, and the inactivation of microorganisms was essentially achieved by thermal means. This system was popular in USA and near about 50 plants used it till 1950s. After that these plants are replaced due to their high energy costs and also due to the availability of alternative processing viz. Ultra High Temperature (UHT) system which is a comparatively milder thermal milk preservation technique.

A German scientist namely, Doevenspeck, engaged in PEF Research for more than 50 years. He designed PEF equipment for food applications. His work has been followed up by Thyssenn Krupp in the 1980s, where a group including Heinz Doevenspeck and Werner Sitzmann installed first industrial scale systems in fish meal industry. In the 1990s the technology was investigated in a number of research projects in the EU and the US.

The first papers on potato treatment have been published by Angersbach in 1997. Angersbach suggested the use of PEF to accelerate drying of potato cubes as well as to improve starch extraction.

Another method known as “electrohydraulic” process developed in 1950s which uses pulsed discharge of high voltage electricity between two electrodes (submerged in liquid medium within a pressure vessel) for microbial inactivation in liquid food. Electric arcs generated by high voltage pulses from transient pressure shock waves upto 250 MPa and UV light pulses. Formation of highly reactive free radicles due to shock waves and UV rays are assumed to be responsible for microbial destruction. However, the process cost disintegration of food particles and erosion of electrodes which could contaminate food. This drawback hampered the development of the process in food sector. But it proved to be helpful in the treatment of waste water. Later on, it was reported that the application of short and homogeneous pulses without arcing doesn’t cause much adverse changes in food due to secondary which led to the development of “Elcrack” process for fish and meat sectors and “Elsteril” process for liquid media decontamination. Both of these processes with an electric field strength upto 30 kV/cm were also used for inactivation of vegetative cells in milk and fruit juices, but limitations in the form of high energy dissipation and high operational costs restricted their uses. At present four pilot scale systems of these processes are being operated in USA and Europe.

WHAT IS PEF?

Pulsed Electric Field processing involves application of high voltage pulses (usually 20-80 kV/cm) for less than 1 sec to food placed between two electrodes to reduce microbial load in the foods to a considerable extent. The process is carried out at ambient, sub-ambient or slightly above ambient temperature for every short duration to reduce energy loss due to heating of foods. Although a little heat may be generated in the food, microorganisms are essentially inactivated by non-thermal means due to electric field. However, it is widely believed that a combination of heating (40-50°C) and PEF is often responsible for the preservation effect.

MECHANISM OF MICROBIAL INACTIVATION BY PEF

Two mechanism namely electroporation and electrical breakdown as discussed below have been put forward by the scientists to explain microbial inactivation by PEF technology.

ELECTROPORATION

Electroporation or electropermeabilization is a microbiological technique in which an electric field is applied to cells in order to increase the permeability of the cell membrane, allowing chemicals, drugs or DNA to be introduced into the cell. Electroporation works by passing thousands of volts across a distance of 1-2 mm of suspended cells in an electroporation cuvette (1.0-1.2 kV, 250-750 V/cm). Using of PEF for killing of microorganisms in liquid foods is a promising new non-thermal food processing and preservation technology. However, to implement and optimize this technology, a good understanding of the actual mechanisms that govern microbial inactivation by this technique is required. Here, fundamentals of cell electroporation, which is considered as underlying phenomenon of food processing technology, are discussed. The whole process of the cell electroporation (food processing) by PEF is divided into the following four main stages-1) buildingthe transmembrane potential up by the applied external electric field, 2) creation of small meta stable hydrophilic pores, when the transmembrane potential has been built up, 3) evaluation of pore population – the change in the number and sizes of pores – during an electric treatment, 4) post- treatment stage consisting of the processes that take place after the electric treatment (leakage of intracellular compounds, pore shrinkage and disappearance etc.).

ELECTRICAL BREAKDOWN

Electrical breakdown or dielectric breakdown is occur when current flows through on electric insulator and when the applied voltage across it exceeds the breakdown voltage. This results in the insulator becoming electrically conductive. According to this concept, bacterial cell membrane is considered as a capacitor filled with dielectric material of low electric conductance and a dielectric constant in the range of 2. The normal potential difference across the membrane (about 10 mV) is increased when an external electric field is applied. The increase in potential difference reduces the thickness of membrane. Large pores are formed in the membrane when the potential difference across the cell membrane reaches a critical level of about 1 V. Formation of large pores causes discharge of cell material and further damage of membrane. Reversible membrane breakdown occurs when the pores are small in relation to the total membrane surface but the cells are destroyed when the pores are large and covers a large membrane area.

FARMERS PROSPERITY

India produced 165.4 million metric ton of milk during 2016-17 and the country ranks number 1 in the world in milk production. However, due to lack of proper preservation facilities, considerable quantity of milk gets spoiled everyday in our country. This sometimes compels the farmers to sale their milk in distress. Non-availability of feasible technologies at under rural conditions for preservation of milk solids aggravates the situation further. Commonly available preservation techniques involve thermal treatment of milk which not only are expensive but also reduces the nutritional values of dairy products. An innovative non-thermal technology such as PEF would not only provide an affordable method for preservation of milk solids at low cost, but would also minimize damage to the quality of dairy products. This will enable the farmers to reduce dependence on middlemen and to a large extend do away with the distress sale of milk. The farmers are expected to get remunerative price for milk which will improve their socio-economic condition through tangible gain from dairy business.

NEED OF NEW TECHNOLOGY

Mainly PEF used in food preservation (non-thermal). High voltage PEF treatment is a potential non-thermal food decontamination technique to replace by thermal processes. Microorganisms in foods can be inactivated with PEF. Inactivation kinetics of Listeria innocuadone by PEF technique. Inactivation of Salmonella senftenberg, Bacillus subtilis, Listeria monocytogenes, Salmonella typhimuriumalso done by high intensity of PEF technology. Effect of growth stage and processing temperature on the inactivation of Escherichia coli by PEF occurred.

PEF processing of white grapes after crushing was studied on pilot-plant scale, to investigate the effects of the treatment on wine composition, wine color and wine aroma compounds. In biotechnology, medicine and food processing, simple and reliable methods for cell membrane permeabilization are required for gene delivery into the cells or for the inactivation of undesired microorganisms.

Processing foods with high intensity PEF in a new technology to inactivate microorganisms and enzymes with only a small increase in food temperature. That’s why new techniques are needed in the food industry.

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APPLICATIONS OF PEF TECHNOLOGY

PEF processing has clearly demonstrated its effectiveness against microbial inactivation. Effect of PEF processing on some of the foods are narrated below

MILK

Shelf life of skim milk to two weeks during storage at 4°C when treated with PEF(40kV/cm of 30 pulses)for 2 µs. PEF processing of skim milk clearly inactivate coliform organisms but couldn’t kill naturally occurring microbes. During PEF treatment applied high voltage damaged the electrodes and caused deposition of solids.

LIQUID WHOLE EGG

Effect on colour of liquid whole egg due to PEF processing was found to be insignificant and the shelf life of the product was about 25-28 days at 4°C. Aseptic packaging of PEF treated liquid Whole egg has been recommendedto increase keeping quality.

SOUP

PEF processing didn’t change physico chemical and sensory characteristics of the product during storage of 4 weeks at refrigerated temperature. Effect of PEF on pea soup was limited when the temperature of the bulk soup was <50°C.

BAKING PRODUCTS

Decrease in water loss in wheat dough treated by PEF at 50kV/cm for 20 min was noticed which lead to an increase in the keeping quality of the bread.

CONCLUSION

PEF is basically a nonthermal and energy efficient pasteurization process to reduce microbial load in food without significantly altering its quality. It is believed that combination of PEF and mild heat might help to inactivate enzymes to a greater extent and prevent refrigerated storage of food. PEF can be considered as a mild preservation method and alternative to conventional heat treatment methods for food preservation. The effectiveness of PEF processing for microbial and enzyme inactivation have been well documented which has created keen interest among food processors to realize its commercial potentiality in food system for enhanced stability during storage. There is, however, a need for synchronization and comparison of data available from various research laboratories to help in the efforts to launch PEF processed products in market as early as possible.

Article Written By

Ishika  Ghosh

B-Tech  Dairy Technology

West Bengal University of Animal  and  Fishery  Sciences

Email id- ishika.ghoshdt@gmail.com