Non Thermal Preservation of Food – Oscillating Magnetic Field (OMF)

Oscillating magnetic field (OMF) is generated with electromagnet of alternating current. The intensity varies periodically according to frequency and type of wave in the magnet. OMF is applied in the form of constant amplitude or decaying amplitude sinusoidal wave. The magnetic field may be homogenous or heterogeneous. In homogenous field, field intensity (B) is uniform in the area enclosed by the magnetic field coil. In heterogeneous field, field intensity (B) is non – uniform with the intensities decreasing as distances from the center of the coil increases.Preservation of food with OMF:This involves food in plastic bag and subjecting it to 1 – 100 pulses in a OMF with a frequency between 5 – 500 MHz at temperature in the range of 0 – 50°C for total exposure time ranging from 25 100 millisecond. Frequency > 500 KHz are less effective for microbial inactivation and tend to heat the food material. Magnetic field treatment is carried out at atmospheric pressure at moderate temperature. Exposure to magnetic field causes inactivation of growth and reproduction of microorganism. Inhibition of growth of microorganism exposed to magnetic field may be result of the magnetic field themselves or the induced electric field.
Inactivation of microorganism with OMF is achieved in milk, yogurt and orange juice. Only one pulse of OMF is adequate to reduce bacterial population between 102 and 103 cfu/g. The intensity of magnetic field required is 2 – 25 T in the frequency range of 5 – 500 Hz. Efficiency of OMF depend on magnetic field intensity, number of pulses, frequency and properties of food (thickness, resistivity, electrical conductivity).

Mechanism of microbial inactivation:
Weak OMF could loosen the bond between ions and proteins. Many proteins like enzymes, vital for cell metabolism contains ions. Effect of OMF on calcium ions bond in Ca+ binding proteins such as Calmodoulin, magnetic field causes plane of vibration to rotate that loosens the bond between Ca+ and Calmodoulin. OMF may couple energy into magnetically active parts of large critical molecule such as DNA. 5 – 50 T magnetic field intensity couple to 1 dipole energy per oscillation in DNA generate 10-2 to 10-3 ev energy. Several oscillations and collective assembly of enough local activation may result in breaking down of covalent bonds in DNA molecule and inhibition of growth of microorganism.
Factors affecting microbial inactivation:
1. Magnetic field:
Magnetic field may stimulate inhibit growth and reproduction. Single pulse of 5 – 50 T and frequency of 5- 500 KHz reduce microorganism by 2 log cycle. Magnetic field can also effect membrane fluidity and other properties of cell. However, it is impossible to clearly state the microbial inactivation effect of magnetic field.
2. Electrical resistivity: To apply OMF, food need to have high electrical resistivity. The applied magnetic field intensity depends on the electrical resistivity and thickness of food. With larger magnetic field intensities, products with large resistivity and thickness is recommended.



About Author

Name : Pratiksha Shrestha

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.