In this method of food preservation, electric energy at low power is collected over an extended period and stored in capacitor. That same energy is discharged, almost instantaneously at high level of power. When exposed to electric field, plasma membrane of cell becomes permeable to small molecules and the permeation causes swelling and eventual rupture of cell membrane. Rupture of cell membrane depends on factors such as intensity of electric field, number of pulses and duration of pulses. The induced transmembrane potential larger than natural potential of cell membrane cause rupture of cell membrane. Generation of pulse electric requires two devices; a) Pulse power supply and b) Treatment chamber.
Pulse power supply
1. Bench top unit: A commercial electroporator can be used for bench top power supply. This unit provides 2.5 KV pulses for maximum. It consists of capacitor, charge and discharge switches and wave controller connected to electroporator to improve discharge pattern. Treatment cuvettes are used for PEF treatments which gives maximum field intensity of 25 KV/cm. This bench top unit provides method for determining inactivation kinetics for selected microorganism in convenient way.
2. Lab scale pulser: Experimental decay pulses could be generated by discharging a capacitor into a chamber containing food. With the given power supplies, capacitors are used to store electric energy that is discharged across metal electrodes creating electric field to inactivate microorganism and enzymes. Mercury ignitron spark gap is used as discharge switch.
Treatment chamber:
Treatment chamber in PEF consists of two electrodes, held in position by insulating material that also forms enclosure for food materials. Uniform electric field is achieved by parallel plate electrode with a gap sufficiently smaller than electrode surface dimension.
Preservation treatments using PEF:
High intensity Pulsed Electric field (HIPEF): This treatment is applied to preheated liquid food which enhance microbial inactivation and shelf life stability. It is possible to achieve five log cycle of microbial count reduction (5 D reduction) of naturally occurring microorganism in orange juice with pulse of 100μs at voltage intensity of 33.6 – 35.7 KV/cm and processing temperature 42 – 65°C. Increase in shelf life from 3 days to 1 week with no sufficient change in color and taste can be achieved by using HIPEF.
ELSTERIL: This treatment is used for sterilization and pasteurization of liquid and electrically conductive media.
ELCRACK: This treatment is used for disruption of cell membrane to release fat from animal cells. This process consist of exposure of slurry of contaminated fish or slaughter house offal to high intensity electric pulses that breakdowns cells.
Application of PEF in food processing:
1. Inactivation of microorganism: Raw and reconstituted apple juice, peach juice, skimmed milk, beaten egg and pea soup can be treated using PEF. coli inoculated in milk can be reduced by 2D when exposed to 60 pulses of 4μs at 37°C.
2. Denaturation of protein: Presence of alkaline phosphatase indicates inadequate pasteurization or contamination with raw milk. In fresh milk alkaline phosphatase is present in association with membrane of fat globule and in skimmed milk, in the form of lipoprotein particle. Inactivation of alkaline phosphatase by PEF is a function of field intensity, fat content of the milk and concentration of alkaline phosphatase. The activity of alkaline phosphatase decreases with an increase in field intensity. A reduction of 43 – 49 % in alkaline phosphatase activity is reported when the enzyme is suspended in 2 % milk and exposed to 70 pulses of 0.4 -0.45 millisecond at 14.8 – 18.8 KV/cm.
PEF can also be used for inactivation of plasmin and protease enzyme from Pseudomonas fluorescence. Inactivation of plasmin activity by 90% can be achieved by 10 – 50 pulses of 2μs duration and processing temperature of 10 – 15°C.
Reference:
Vega-Mercado, H., Gongora-Nieto, M. M., Barbosa-Canovas, G. V, & Swanson, B. G. (2004). Pulsed electric fields in food preservation. FOOD SCIENCE AND TECHNOLOGY-NEW YORK-MARCEL DEKKER-, 167, 783.
About Author
Name : Pratiksha Shrestha
pratiksha.shrestha2001@gmail.com
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.