Microbiological quality and survivability of Escherichia coli P511 in microwavable food

Microwave heating utilizes electromagnetic waves of certain frequencies to produce heat energy in the material and is able to achieve high temperature-short
time processing for bacterial destruction while the thermal degradation of the food components is reduced. However, the non-uniformity of microwave temperature
distribution which caused hot and cold spots in the heated product has become its limitation in food processing. Uneven spatial distribution of the electromagnetic field inside microwave has caused temporal difference in heated food products. By using
proper microwave heating techniques, this study tested the effectiveness of microwave heating in inhibition of food-borne pathogens. Different microwave heating treatment at 270 W for 60 s, 950 W for 150 s and 300 s were carried out and small portions of treated food weighed 25 g each was serial diluted and total coliform
counts, aerobic plate counts, presumptive coliform test, enumeration of E. coli and Salmonella were carried out. Potential food pathogens (i.e. E. coli and Salmonella)
were not detected in the food products while Enterobacteriaceae were found in the food samples after enrichment process. Several biochemical tests, DNA extraction, PCR, DNA purification and DNA sequencing were carried out. Klebsiella pneumoniae were identified in the food sample after enrichment. Combination effects of
microwave power and treatment time were also studied in this project and it showed that the microbial count of E. coli P511 can be reduced from initial 9.47 log cfug¯¹ to
2.67 log cfug¯¹ at 600 W for 120 s with average internal temperature of 60.09±0.82 °C. E. coli count was reduced to under detection limits with microwave heating at 600W for 150 s and at 900 W for 90 S with average internal temperature of 68.20±2.92 °C and 65.44±3.56 °C respectively. 300 W of microwave power at 180 s is
not sufficient to inhibit the growth of E. coli and the average internal temperature measured 52.21±3.63 °C.