Abstract
In light of the paucity of knowledge surrounding the existence of specific microwave effects on bacteria, the aim of the present study was to investigate the effects of high domestic microwave radiation on the safety of raw milk. Forty samples of raw milk were collected from retail shops in El Menofia governorates, Egypt. Milk samples were subjected to conventional heating in water bath to reach temperature of 40°C and 1100 Watt in domestic microwave for 3 minutes in cooled water jacket to reach the same temperature. Counts of different groups of bacteria in microwave treated samples were compared with those treated by conventional heating and non-treated raw milk samples. Additionally, to investigate the minor effects of such type of microwave treatment on the cell permeability to antibiotics, 10 of our laboratory bacterial strains with known antibiotic resistant patterns were treated by microwave and conventional heating. The zone of inhibition for tested antibiotics were measured after each treatment. Of note, the average total aerobic, Enterobacteriaceae, Pseudomonas spp. and staphylococci counts in milk samples were 6.34, 4.20, 3.42 and 3.72 log cfu/ml, respectively. The bacterial counts were nearly similar in samples treated by conventional heating and microwave. Statistical analysis revealed non-significant effect of microwave treatment at sub-lethal temperature as P values were more than 0.05. The diameter of zone of inhibition of tested antibiotics did not change in milk treated by both methods and non- treated samples. In conclusion, this study provide evidence that high domestic microwave irradiation at sub-lethal temperature have no significant effect on the hygienic quality of milk and not provide any degree of safety to the consumer. Additionally, it confirms the hypothesis that microwave radiation under sub-lethal temperature resulting in a reversible microwave-induced poration of the cell membrane that prevent penetration of antibiotics after few minutes.

Key words: Microwave, sub-lethal, milk, hygienic quality