Effect of Combined Freeze-Thaw and Ferrate(VI) Treatments on Escherichia coli in Phosphate-Buffered Saline

Abstract

The study examined the individual and combined effects of potassium ferrate(VI) and freeze-thaw treatment for the inactivation of Escherichia coli (E. coli) in a phosphate buffered saline (PBS) solution. A low dose (LD) of 0.5 mg/L and a high dose (HD) of 1.0 mg/L of potassium ferrate(VI) were used as a pretreatment and posttreatment to freeze-thaw, at temperatures of −10, −20, and −30°C, for 1, 8, and 15 days completely frozen. The LD did not have a significant effect on E. coli concentrations following a 15-min contact time. However, when the LD was used as a pretreatment to freeze-thaw, E. coli concentrations were reduced to <0.18 MPN/mL, representing by >6-log inactivation, regardless of the freezing temperature or the duration of the freezing conditions. When used as a posttreatment to freeze-thaw, LD improved E. coli inactivation compared to stand-alone freeze-thaw treatments; however, LD was much more effective when used as a pretreatment to freeze-thaw. The HD resulted in up to 6.4-log inactivation of E. coli following a 15-min contact time. When used as a pretreatment to freeze-thaw, E. coli concentrations were reduced by more than 6-log regardless of the freezing temperature or the duration of the freezing conditions, similar to the pretreatment tests with LD. Stand-alone freeze-thaw treatments at temperatures of −10, −20, and −30°C, for 1, 8, and 15 days resulted in 1.4 to 3-log inactivation of E. coli, with no significant difference among the various freezing temperatures and freezing times. The study confirmed the effectiveness of potassium ferrate(VI) and freeze-thaw treatments for the inactivation of E. coli and also demonstrated that relatively low doses of potassium ferrate(VI) are extremely effective when used as a pretreatment, prior to freeze-thaw.