Inactivation of Escherichia coli in milk by high pressure processing at low and subzero temperatures

Inactivation of Escherichia coli strain ATCC 25922 in frozen and unfrozen milk by high pressure processing at low or subzero temperatures was investigated. Frozen (-21 degrees C) and unfrozen (3 degrees C) milk samples were exposed to a pressure of 300 MPa for 0.5-15 min at varying temperatures between -5 degrees C and 5 degrees C. Reduction in the number of E. coli in unfrozen milk was enhanced by pressure treatment at subzero temperatures, whereas no survival was detected in frozen milk samples after a 10 min pressure treatment at -3 degrees C, corresponding to a minimum of 6.8 log cycle reduction in the number of E. coli. A substantial reduction of 3.8 log reduction in number of E. coli was observed after a 30 s pressure treatment at -3 degrees C. Results suggested that the mechanical forces could play a role in enhanced inactivation of E. coli in frozen milk samples.