Food-processing industry has made large investments in processing facilities relying mostly on conventional thermal processing technologies with well-established reliability and efficacy. Food contains many heat sensitive nutrients which include vitamins, minerals, and nutrients having functional properties such as pigments, antioxidants, bioactive compounds. Many processes during manufacturing of food cause detrimental effects on these nutrients. Therefore, retention of these nutrients in food products requires innovative approaches for process design because of their sensitivity to a variety of physical and chemical factors, which causes either loss of biological functionality, chemical degradation and premature or incomplete release. Alternative methods for thermal processing of food are gaining importance, due to increased consumer demand for new methods of food processing that have a reduced impact on nutritional content and overall food quality. Also because of the increasing consumer demand for minimally processed fresh-like food products with high sensory and nutritional qualities, there is a growing interest in non-thermal processes for food processing and preservation.
Pulsed Electric Field (PEF) is one such widely used novel non-thermal technology for food preservation. PEF is a mild food preservation technique making use of short electric pulses of variable intensities yielding few to no detrimental effects on quality attributes in pumpable foods such as fruit juices, liquid eggs, jams, soups, milk, yogurt etc., but with advancement, this technology have become friendly for solid foodstuff too.
The process is based on electroporation, which generates high voltage (10–80 kV/cm) pulses into foods placed between two electrodes for short time at ambient temperature, then packaged aseptically and distributed in refrigerated condition. Food is exposed to electrical field of treatment chamber leading to the opening of pores in tissue mass. This process attains a 5 log reduction on most pathogenic bacteria by rupturing the cell membranes in liquid media. Inactivation of microorganisms occurs due to generation of shock wave generated by an electric arc that stimulate the formation of highly reactive free radicals from chemical species in food. It causes only minimal detrimental changes to the physical and sensory properties in foods, helping in retaining ‘fresh’ quality and assists in nutrient retention.
Treatment Affecting Factors
- Electric field strength- Strength of electric field depends on type of food and microorganism to be treated.
- Processing time- Varies with microorganism.
- Treatment temperature
- pH-Low acidic foods are preferred but not limited to them
- Ionic Strength
- Conductivity of the medium
- Type of microorganism present in food media
Construction of PEFs machine involves a pulsed power supply, treatment chamber that converts the pulsed voltage into PEF and a control panel
- High voltage Pulse Generator: This unit provides a maximum of 2.5 kV pulses. The instrument consists of a capacitor (7 F), charge and discharge switches, and a wave controller. The wave controller may be connected to the electroporator to improve the discharge pattern. Treatment cuvettes with a 0.1-cm electrode gap and l00 L volume may be used for PEF treatments, which give a maximum intensity of approximately 25 kV/cm. Appropriate voltage and current monitors should be attached to it.The frequency, shape and amplitude of the pulses result in the intensity of the treatment.
- Treatment chamber: PEF treatment chamber may be static or continuous which consists of two electrodes, with insulating materials in between that acts as an enclosure containing food materials. Uniform electric fields can be achieved by parallel plate electrodes with a gap sufficiently smaller than the electrode surface dimension. The chamber also contains a spacer, and two lids. Each electrode is made of stainless steel, whereas the spacer and lids are made of polysulfone. The distance between two electrodes is termed as the treatment gap of the PEF Chamber. A flow channel was provided between the two electrodes to eliminate dead corners as well as to ensure uniform treatment. The electric field may be applied in the form of exponentially decaying square waves, bipolar, oscillating pulses at ambient, sub ambient, or slightly above ambient temperature. Cooling of the chamber is accomplished by circulating water at a selected temperature through jackets built into the two stainless steel electrodes.
- Control System for monitoring Process Parameter: It consists of two major devices:
- Oscilloscope: It measures the voltage across the treatment chamber and shows the output voltage shape.
- Temperature Probe: Sample temperature is measured with a thermal probe placed inside the treatment chamber.
Field of Applications
- Improved quality of Potato chips and finger fries with less fat content: PEF pre-treated potatoes enables cutting at optimum product texture which results in smooth surfaces, reduced starch loss, less damage and less absorption of oil/fat by potato chips and fries.
- Drying: PEF facilitates rapid removal of moisture from cell matrix.
- Wastewater Treatment: PEF burst cells are more susceptible to other biological and chemical processes such as anaerobic digestion, which significantly reduces solid waste content in wastewater. Thus reducing the treatment time for wastewater.
- Efficient Extraction of Fruit Juices: Reports suggests, PEF treated apples and carrots are found to be producing more juice as their cell mass gets disintegrated which is the essential pre requisite for solid liquid separation during juice extraction from fruits and vegetables.
As this process include wide benefits like maintaining the product quality without thermal and mechanical stress, not degrading the nutrient properties, not producing any harmful by product, increasing the extraction yield by softening of tissues, it involves certain limitation such as its limited effects on microbial spores and its ineffectiveness on foods that have higher or variable electrical conductivity. Traditional thermal processing techniques for preserving foods leads to destruction of heat sensitive ingredients such as vitamins, which is helpful in promoting health. Therefore, PEF can be seen as the promising replacement, which allows the preservation of heat sensitive food products for which fresh taste is the main quality parameter, without or with only minor detrimental effects on the quality.