Microwave damaged by heat. The microbial lethality is
Microwave heating Microwave heating is a non-thermal mode of food processing which uses the electromagnetic waves of frequencies between 0.3 and 300 GHz in order to generate heat. Rao et. al.(1995) reported that microwave are very short waves of electromagnetic energy that travel at the speed of light (186,282 miles per second). The food placed in a microwave container is placed in microwave and allowed to heat. The food at the periphery of the container heats faster and this develops a temperature gradient between the centre and the periphery. Mechanism :Microwave processing primarily involves two mechanisms : a) Dielectric Interaction- The polar water molecules inside food on absorbing the microwave energy rotate according to electromagnetic field. The water molecule being polar acts as a “dipole”. The dipole has one positively charged end and the other negatively charged end. Since it has two charged ends, similar to magnet, the dipoles align themselves with respect to the electromagnetic field. The rotation of polar water molecules is responsible for generation of heat. b) Ionic interaction- The food that we eat have dissolved salts in it. On microwave heating, oscillatory migration of these ions is seen which then tend to collide and produce heatLandgra and Tassinari (1997) explained theoretically that heat is generated quickly and uniformly by microwave process, but numerous experiments have proven this theory wrong as more research has been done to improve the quality and safety of microwave foods. Microbial inactivation by Microwave heating : Mechanism of microwave heating –The microwave heating produces two types of effects : Thermal effect – On absorption of the microwave radiations by the cell structures, they tend to vibrate much faster which produces general heating in the molecules. Non-thermal effect:- The experiments conducted emphasise on the fact that the microwave heating causes bacterial cultures to be destroyed to a large extent. Effect of Microwave heating – The way irradiation by microwaves affect the microorganism depend on the frequency of radiation and the amount of radiation absorbed by the microorganismWhen microwaves are applied at a particular frequency, with high energy and for a sufficiently long period of time, the thermal effect comes to existence and kills bacterial cells or yeast. The microwave irradiation of the bacterial cultures in a wet environment shows increased killing of bacterial cells as compared to the bacterial cultures in a dry environment. The killing effect of microwave in a dry environment is significantly decreased and takes a prolonged period of irradiation due to decreased transformation of microwave energy to heat energy. Thermal effect- The energy absorption from microwaves can rise the temperature of the food high enough to inactivate microorganisms (Sun et.al., 1988). Heddleson & Doores (1994a) found that the thermal effect on microorganisms lead to potentially irreversible heat denaturation of bacterial enzymes, proteins, nucleic acids and metabolites and co-factors crucial to cellular function may leak through membranes damaged by heat. The microbial lethality is now understood as the result from the penetration of electromagnetic waves into a biological wet material, heating up the intra and extracellular fluids by the transfer of energy from polar water molecules and dissolved ions. This results in the generation of heat within the material itself due to molecular activity (Han, 1996). The studies have done to establish sterilisation effect by minimum dosage of microwaves. In a study done on streptococcus mutans, a bacteria that causes dental caries, the bacteria was exposed to radiations from microwave oven at high power for about five minutes which completely decontaminated the bacteria. Similarly, when the cultures of escherichia coli and spores of bacillus cereus were exposed to the microwave oven at home, they were completely destroyed after two and four minute interval, respectively. Non Thermal effect- Kozempel et.al.(1998) studied another mechanism for inactivation by microwaves involving non- thermal effects like, electroporation, cell membrane rupture, and magnetic field coupling. Milk and Milk products :Fujikawa et al. (1992) studied that the absorption of microwave energy can increase the temperature of foods rapidly, deactivating microorganisms and performing pasteurization or sterilization. The pasteurization of goat milk using microwave, reduced the contaminating microflora significantly (Calvo and Olano, 1992).Heddleson and Doores (1994a) reported the inactivation of Bacillus cereus, Campylobacter jejuni, Clostridium perfringens, Escherichia coli, Enterococcus spp., Listeria monocytogenes, Staph. aureus, and Salmonella by microwave treatment.