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Two coiled tube reactors were designed to investigate the influence of Reynolds number (Re) and diameter of fluid carrying tube on UV-C inactivation of Escherichia coli W1485 and Bacillus cereus endospores in raw cow milk (RCM) and skimmed cow milk (SCM) at room temperature. UV reactors were constructed using perfluoroalkoxy (PFA) tubing having internal diameters of 1.6 and 3.2 mm and each had a residence time of 11.3 s. Four levels of Re were tested for each milk type, each tube size and each bacteria type. Inactivation efficiency increased as the Re increased in both the reactors for both types of milk. The inactivation of both bacteria was higher in the 1.6 mm UV reactor than the 3.2 mm UV reactor. Maximum reduction of 7.8 log10 CFU/ml of E. coli was achieved in SCM in the 1.6 mm UV reactor corresponding to the Re of 532 and higher, whereas the maximum reduction of E. coli in RCM was 4.1 log10 CFU/ml at the highest level of Re (713) tested. For B. cereus, the maximum reduction was 2.72 log10 CFU/ml in 1.6 UV reactor, in SCM at Re of 1024; whereas the maximum reduction of B. cereus in RCM was 2.65 log10 CFU/ml at Re value of 713. Inactivation efficiency of both bacteria was more in SCM than RCM. The coiled tube reactor design provided adequate mixing and UV-C dosage for efficient disinfection of E. coli cells in milk.