how do you determine the total lethality (in equivalent D250 minutes) of a thermal process?
To determine the total lethality of a thermal process in equivalent D250 minutes, you need to calculate the F-value using the Arrhenius equation and then convert it to equivalent D250 minutes. Here's how you can do it:
1. Gather the necessary information:
- Initial temperature (T1): The starting temperature of the product or process.
- Final temperature (T2): The desired or achieved temperature of the product or process.
- Z-value: The thermal resistance constant, specific to the microorganism of concern. It represents the change in temperature required to alter the lethality rate by a factor of 10.
- D-value at the reference temperature (Dref): The time needed to achieve a 1-log reduction in microorganism population at the reference temperature of 250°F (121.1°C or 394.1 K).
2. Calculate the F-value:
- Convert the temperatures from Celsius (or Fahrenheit) to Kelvin: T1 + 273.15 (or T2 + 273.15).
- Calculate the temperature difference (ΔT) between T1 and T2: ΔT = T2 - T1.
- Calculate the F-value using the Arrhenius equation:
F-value = (ΔT / Z) * 10^((T2 - Dref) / Z)
3. Convert F-value to equivalent D250 minutes:
- Divide the F-value by the natural logarithm of 10 to get the base-10 logarithm: F-log10 = F-value / ln(10).
- Multiply the F-log10 by the Dref to get the equivalent D250 minutes: Equivalent D250 minutes = F-log10 * Dref.
By following these steps, you can determine the total lethality of a thermal process in equivalent D250 minutes.