Disinfectants, be they heat or radiation or chemicals, usually kill a constant proportion of the cells present per unit time if the cells are all equally susceptible. Hence, the term ‘log kill.’ The probability of death is constant over time given a ‘single hit.’ The more cells present at the start, the greater the number killed per unit time and the longer it will take to kill them ‘all.’ One decimal reduction is one log place (exponent). Log Nt = Log N0 – t/D. D is the time required for a log kill. A suspension of 105 CFU/mL of Salmonella is treated with bleach, and 90% of the cells are dead after 10 minutes. How many viable cells per mL would you expect after 30 minutes? Yes, the 30 minutes here implies 3 logs of killing. This is an excellent problem to use with the Death Equation because you can reason it through first, and then solve it using the equation.
Disinfectants, be they heat or radiation or chemicals, usually kill a constant proportion of the cells present per unit time if the cells are all equally susceptible. Hence, the term ‘log kill.’ The probability of death is constant over time given a ‘single hit.’ The more cells present at the start, the greater the number killed per unit time and the longer it will take to kill them ‘all.’ One decimal reduction is one log place (exponent).
Log Nt = Log N0 – t/D. D is the time required for a log kill.
A suspension of 105 CFU/mL of Salmonella is treated with bleach, and 90% of the cells are dead after 10 minutes. How many viable cells per mL would you expect after 30 minutes? Yes, the 30 minutes here implies 3 logs of killing. This is an excellent problem to use with the Death Equation because you can reason it through first, and then solve it using the equation.
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